Originally published in IAR Hotline!
Issue 56,1989 by J. Peter Moncrieff.
Posted with permission.
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Optimizing ASC Tube
We're pleased to see the worldwide
acceptance that Tube Traps have gained, in the four years
since IAR's pioneering review (see Hotlines 39 & 40).
Virtually all major reviewers and manufacturers use them as
a key element of their listening rooms, which they rely upon
for evaluating all the audio components you will buy. When
reviewers publish plans of their listening rooms, most show
Tube Traps in the corners.
On the other hand, we're disappointed to see that most people
are realizing merely a fraction of the sonic benefits that
they could obtain from Tube Traps, given all their power and
flexibility. Most people apparently don't fully understand
all the sonic improvements that are achievable with Tube Traps,
and they don't know how to optimize the use of Tube Traps
for their listening room. The purpose of this article is to
help you redress this injustice to your listening room, this
shortchanging of you and your enjoyment of music. It is intended
as a practical, nuts and bolts guide to using and adjusting
Figure 1 shows the typical Tube Trap arrangement that many
people are using. The solid black rectangles represent the
speakers, and the X represents the listener. The circles represent
Tube Traps, with one column in each corner.
What's wrong with this typical arrangement? In the first
place, it only achieves bass control, working on the room's
bass resonance modes. This arrangement does absolutely nothing
to exploit the power of Tube Traps in controlling wall reflections.
It provides none of Tube Traps' sonic benefits in many other
areas: in eliminating the dreaded mud factor (especially warmth
and lower midrange regions), in reducing glare and colorations
(especially midrange and upper midrange regions), in improving
articulation and transparency, in improving stereo imaging
and ambience, and in making the room's reverberation characteristics
smooth and well behaved. These aspects are discussed below,
under Perimeter Reflection Control.
BASS CONTROL --
Furthermore, this typical arrangement is not even the optimum
setup for controlling bass. A room's bass resonances occur
between opposite parallel walls. Putting Tube Traps only in
the corners does little to control the resonance that develops
between the broad center section of opposite
walls. In order to control this, you should place some bass
absorbing Tube Traps at the center position of some walls.
Referring to Figure 2, adding the bass Tube Trap in the center
of the back wall (in back of the listener, i.e. at the bottom
of the room plan as shown) is particularly crucial to controlling
the front-to-back bass resonances along the presumed long
dimension of the room.
Conversely, the two typical Tube Traps at the room's front
corners (the speaker end, i.e. at the top of the room plan
as shown) are not really that important. Why not? Because
a bass resonance requires a pair of opposite
walls to develop (it takes two to tango). Consequently, damping
just one of the opposing walls is sufficient
to control most bass resonances. You can still use Tube Traps
in the front corners of the room, but they should generally
be selected to be tuned to your speakers, not tuned to your
room's bass resonances.
What then is the optimum setup for controlling a room's bass
resonances? Figure 2 shows our suggestion. As with all our
suggestions, you should regard this as a starting point, not
as final gospel. Every room is different, every listener's
taste and sensitivities are different, and every model speaker
The most important Tube Traps in Figure 2 are the three bass
trap columns at the back of the room. They work to control
the lowest frequency bass resonances of the room, which occur
along the room's longest dimension. Note that we are presuming
an oblong room, and a listening setup along the longer dimension
this room's plan. If your room is close to square in plan,
or (worse yet) cubic in volume, then additional control is
necessary, and some of the following steps will become mandatory
instead of optional. If your listening setup is across the
room's longer dimension, then the bass Tube Traps should be
along a side wall, with adjustments in the midrange Tube Traps
along the opposite side wall in order to preserve symmetrical
These three bass Tube Traps columns should be fat enough
models to absorb the lowest frequency, fundamental bass resonance
of the room. What is this frequency for your room? It's the
half wavelength frequency for the speed of sound travelling
this longest dimension. To figure this out, simply calculate
550 (which is half the speed of sound) divided by the longest
dimension in feet. For example, a room with a longest dimension
of 15 feet would have its fundamental resonance at 550/15=
37 hz. To control 37 hz, you need to use the 16 inch Super
Tube Trap model, which operates down to 35 hz.
The fatter the Tube Trap model, the lower in frequency its
absorptive capabilities extend. The manufacturer's specs list
the bass "cutoff"of the different models as follows:
Super 16 inch (diameter) goes down to 35 hz; 16 inch goes
down to 55 hz (as do the 16 inch half and quarter rounds);
Super 11 inch goes down to 70 hz; 11 inch goes down to 85
hz (as does the 11 inch half round); and 9 inch goes down
to 110 hz (as does the 9 inch half round).
Incidentally, the Super models get their extra bass extension
by employing a low Q, anharmonic, eccentrically built Helmholtz
resonator, which is used to shift the phase and thus increase
the pressure gradient at very low frequencies, thereby improving
effective absorption at these very low frequencies.
A little arithmetic shows that most people are not using
fat enough Tube Traps to control the bass resonances of their
listening room. If your longest room dimension is merely 11
feet (or more), then your room's fundamental bass resonance
is 50 hz (or lower), and so you need to use the Super 16 inch.
You shouldn't use the next cheaper model, the regular 16 inch,
unless your room is less than 11 feet long.
One commonly sees rooms treated with the corners containing
one 11 inch Tube Trap plus one 9 inch Tube Trap. This typical
setup is only effective down to 85 hz, which would control
the room's fundamental bass resonance only if the room's longest
dimension were less than 6.5 feet! Thus, using the typical
11 plus 9 combination is false economy for bass control.
That's why ASC introduced the quarter round 16 inch model,
as a better choice for people on a budget (see discussion
below for Figure 4). The quarter round 16 incher achieves
bass absorption down to 55 hz, though not as much absorption
as the full round 16 inchers (it's analogous to a small speaker
system, which can be engineered to put out as deep bass as
a bigger system, but not as loudly). Incidentally, way back
in Hotlines 39 and 40 we did recommend the 11 incher as being
better for bass control than the 9 incher (which it still
is), but of course at that time the 11 incher was the fattest
Tube Trap available.
So far, then, we have three columns of bass Tube Traps along
the back wall of the room. If this controls the room's lowest
bass resonance to your satisfaction, fine. If your room still
sounds too soggy in the lower bass, then you should try standing
more bass Tube Trap columns along the back wall (if this doesn't
help, then the sogginess is probably from the room's side
to side or even its floor to ceiling resonant mode).
If you decide that you do want additional bass Tube Traps
along the back wall (in addition to the three basic columns),
then we suggest hanging them from the ceiling, in a horizontal
orientation, along the junction of the ceiling and the back
wall. This is easily accomplished with large eye screws. In
Figure 2, these horizontally hung Tube Traps are shown as
a rectangle filled with wavy lines. These horizontal Tube
Traps should be full round models (not the economy quarter
round), because, as we will discuss later, rotating them is
critical for optimizing stereo imaging and ambience.
Installing Tube Traps horizontally at this junction of ceiling
and back wall provides further sonic benefits. It helps control
the room's bass resonant modes from the floor to ceiling dimension.
And it terminates a kind of surface acoustic wave that propagates
along the ceiling (which is hard surfaced in most homes),
from the speaker end to the listener end of the room.
Thus, even if you find that you do not need more low bass
control after installing the three bass columns along the
back wall as discussed above, we would recommend that you
still install this horizontal row of Tube Traps along the
rear ceiling to wall junction, in order to obtain these other
sonic benefits. In this case, don't use fat low bass Tube
Traps, but instead use the 9 inch or 11 inch full round models
(calculate the vertical mode from the floor to ceiling dimension,
and select the appropriate model, depending on whether you
want to absorb the fundamental of this vertical mode or let
We have now dealt with the fundamental bass resonance mode
along the longest room dimension, which is front to back in
our example. The three tall columns and the horizontal row
at the back wall also tame the room's vertical resonant mode
somewhat. Now, how about the fundamental bass resonance mode
that occurs between the room's side walls?
This side to side mode has also been partially tamed by the
two tall columns in the corners of the back wall. But, just
as we needed a third back wall column at the center of the
back wall, by the same reasoning we should add bass Tube Trap
columns near the center of each side wall. This is shown as
the two circles midway along the side walls in Figure 2. Using
the calculation discussed above, determine from the room's
side to side dimension which model Tube Trap is required,
to absorb the fundamental bass resonance of the room's side
to side mode.
When do you stop adding low bass Tube Traps? That is, how
do you know when you have achieved adequate control of low
bass, of the room's fundamental resonance modes? The ideal
lower bass of an audio system (which includes the listening
room) should be powerfully impactive for the initial transient,
but then tight and dry for the decay tail immediately thereafter
(as a test, listen to a bass drum, kick drum, or plucked bass).
This is the best quality lower bass transient response for
your system and room (see IAR Journal 3 for discussion of
bass transient response). If there's excessive lower bass
resonance, the tail will sound dull and soggy (it doesn't
sound boomy per se; audible boominess as such occurs from
upper bass resonances).
As you add more low bass Tube Traps, the low bass should
lose its dull soggy quality, and become tight and dry. As
you progress, low bass quality should improve.
At some point, it will sound right for your taste, given your
room and speakers. If you go beyond this point by adding more
low bass Tube Trap absorbers, you will start reducing the
initial bass transient impact, reducing the quantity
of low bass; this you probably don't want to do.
When listening for the correct point for your taste, be careful
to pay attention only to the lower bass quality, that soggy
vs. tight dry sound on bass transients. Ignore any upper bass
boominess you might hear. There might be some upper bass boominess
remaining after you have reached the optimum point of controlling
lower bass. That upper bass boom relates to the harmonic overtones
of the room's resonance modes (and to speaker placement);
this will be dealt with by separate tactics (below).
If by chance our suggestion of three fat (low bass) Tube
Trap columns along the back wall already goes beyond the optimum
point of absorbing low bass for your taste, then feel free
to employ fewer bass Tube Traps. When we talk of a column
of Tube Traps, we always mean a full height column from floor
to as close to the ceiling as you can get. You could lessen
the amount of low bass absorption by using half height columns.
Alternatively, if your room dimensions are small enough to
allow using the regular 16 incher instead of the Super 16
incher, then you could lessen the amount of absorption by
using quarter round 16 inchers in the corners instead of full
round 16 inchers.
Note that some listeners actually enjoy lower bass ringing
resonance, from a room's fundamental modes, because it gives
them added room and stomach shaking, perhaps also compensating
in a crude way for their speaker's rolloff in the lower bass.
Technically, a resonance in the lower bass is not correct.
But this dull sogginess in bass quality doesn't bother some
listeners, and they are happy to endure it, in order to get
the illusion of more quantity of low bass that comes from
the ringing resonance of the tail. If you are such a listener,
then you might not want to control your room's fundamental
bass resonance modes at all. If so, then don't take any action
based on this section, and act only on the suggestions in
the next section, in which we control your room's harmonic
The second harmonics of the room's fundamental bass resonances
might need further quelling, even after the fundamental has
been controlled to your satisfaction (whatever degree that
happens to be). There are several reasons for this. The harmonics
are in the upper bass region, which is far more audible to
human hearing than lower bass, thus making even a slight remaining
resonant boom more objectionable. Also, most music and most
loudspeakers put out far more energy in the upper bass than
the lower bass, thus triggering even a slight remaining upper
bass room resonance mode more frequently and more severely.
Is there a way to control upper bass boom independently of
lower bass sogginess, so you can optimize each to your taste?
Yes there is, thanks to the power and flexibility of Tube
Traps. How? Actually, there are two basic strategies, which
you can combine, to yield a variety of specific tactics.
If you want to control upper bass boom without further affecting
low bass, one strategy is to select skinnier Tube Trap models,
which have a higher frequency bass cutoff. They would control
the second harmonic modes of the room, without affecting the
fundamental modes. To select the appropriate Tube Trap model
for controlling only the second harmonic mode (and higher
order modes) of a particular room dimension, pick the model
whose cutoff frequency is lower than the second harmonic frequency,
but higher than the fundamental frequency. The second harmonic
frequency is simply twice the fundamental frequency (or you
can simply change the above calculation formula to 1100 divided
by the appropriate room dimension in feet).
This strategy of selecting- skinnier Tube Traps, to affect
only the upper bass, is only partially effective. The bass
rolloff of a Tube Trap's absorption capability is gradual,
rather than a sudden cutoff. Thus, Tube Traps selected for
an upper bass cutoff would still absorb some lower bass. (Note,
incidentally, that you can use this fact to advantage if you
are on a budget; if you want to absorb say 50 hz, you can
cheat a bit by using a Tube Trap rated at a 55 hz cutoff,
instead of having to spend more for the Tube Trap with the
35 hz cutoff.) Also, the second harmonic of the longest dimension
room resonance mode might be very close in frequency to the
fundamental of a shorter dimension room mode, so a Tube Trap
selected to absorb the upper bass second harmonic of one mode
might also absorb the upper bass fundamental of another dimension's
mode (although this might be desirable, given that both are
contributing to the undesirable upper bass boom you hear and
want to control).
The second strategy overcomes these shortcomings, and allows
you to precisely select only the second harmonic (and higher
order) resonant modes, in just those dimensions you wish to
control. This strategy relates to placement of the upper bass
control Tube Traps.
If you place bass Tube Traps along one wall, then they will
absorb the fundamental and all harmonics of the room resonance
mode that occurs in that dimension (with that one wall being
the stopping end of that dimension). For example, placing
bass Tube Traps along the back wall as discussed above will
absorb the fundamental and harmonics of the room resonance
mode occurring between the front and back walls, along the
front to back dimension. But if you place these same Tube
Traps at the halfway point along this same
dimension (e.g. halfway from the front to the back wall),
then they will absorb only the second harmonic
(and all higher even order harmonics), while leaving the fundamental
Why this magic property of selective absorption? The fundamental
mode (and all harmonics) have their pressure maxima at the
two opposing walls which create and sustain the resonance
mode. Tube Traps work by absorbing when there is acoustic
pressure. The fundamental resonance mode, being a half wavelength
mode, has zero acoustic pressure at the halfway point between
the two opposing walls creating and sustaining that particular
mode. Thus, if you put some bass Tube Traps at the halfway
point for a particular dimensional mode, there's no acoustic
pressure there from the fundamental for the Tube Trap to absorb,
and so it does not affect the fundamental low bass mode. Meanwhile,
the upper bass second harmonic has full acoustic pressure
at this point, so it is maximally absorbed.
Thus, by placing bass Tube Traps appropriately, you can selectively
control and absorb upper bass boom, without further affecting
lower bass quantity or quality (which you have already optimized
to your taste using the low bass Tube Traps along the walls,
as discussed above). The overall tactic is simply to place
those bass Tube Traps, which you intend to control the second
harmonic of a bass resonance mode but not absorb the fundamental,
at the halfway points of the room dimension causing that mode.
This is shown in Figure 2 as the two columns of Tube Traps
located at the center point of each side wall; these work
to control the second harmonic (upper bass boominess) of the
room's front to back mode, without affecting the fundamental.
If you need even more control of the second harmonic of this
front to back mode, we suggest a horizontal row of Tube Traps
hung from the ceiling across the room at the midpoint; this
is also shown in Figure 2 (as the mid-room rectangle with
the wavy lines). There might be circumstances in which you
would want to install this horizontal row first, before trying
the vertical columns at the midpoints of the side walls; that's
because the side columns affect also the room's side to side
resonance mode, and affect its fundamental. whereas the horizontal
row affects this resonance mode less (while affecting the
top to bottom resonance mode more). See immediately below
for discussion of multiple mode considerations.
Naturally, the Tube Traps placed at midpoints, to control
just second harmonics, do not need to be as fat as the end
wall Tube Traps that you intend for control of the fundamentals
as well. The bass Tube Traps for the second harmonic should
be selected so that their bass cutoff frequency is lower than
the frequency of the second harmonic for the appropriate room
dimension, but it doesn't have to be lower than the room's
fundamental frequency for that dimension (if it is lower than
the fundamental, no harm is done, since its midpoint location
won't affect the fundamental anyway -- but you may have wasted
money on a larger Tube Trap, unless you are also using that
same midpoint Tube Trap for controlling the fundamental
of the room's resonance in a perpendicular
In some circumstances, a listener might want to select and
install bass Tube Traps to control only the room's upper bass
harmonics, leaving the fundamental resonance modes alone.
One example would be if relatively small speakers were used,
with their resonance being slightly higher in frequency than
the room's fundamental resonance modes. In this case, the
room's fundamental resonances could be used to effectively
extend the bass response and apparent bass power of the speaker
(though the quality of this resonant ringing bass would be
Note in this example that placement of the speakers in the
room also plays a role in extending their bass capabilities,
and in exciting the room's various fundamental modes. A whole
treatise could be written on the compromise tradeoffs of speaker
placement: keeping them far enough away from all walls (and
the floor) for best stereo imaging and least colorations,
yet close enough to these walls to pick up a loading advantage
at bass frequencies from them, yet far enough away from these
walls so as not to excite room resonance modes excessively.
A second example would be if very small speakers were used,
with such a high frequency bass cutoff that they cannot even
excite the room's fundamental resonance modes. If the fundamental
modes are never excited, then you don't need to select or
place bass Tube Traps to control the fundamental modes (other
than perhaps the floor to ceiling mode, since this typically
occurs along a short dimension and hence at an upper bass
frequency). You can concentrate on controlling just the upper
bass, second harmonic modes.
A third example would be applicable for many listeners of
rock music. Rock recordings are typically EQ'd to boost their
upper bass, but without enough lower bass in comparison. And
a little extra low bass overhang can add visceral excitement
to the rhythm, without hiding the music (as upper bass boom
does). In this example, the listener might want to utilize
the fundamental resonance modes of the room to enhance the
low bass excitement, and control only the upper bass boom
of the room's second harmonic modes.
A fourth example, mentioned earlier, is one in which you
the listener simply prefer all the low bass quantity you can
get, even if it is a bit soggy in quality. It's primarily
upper bass boom that bothers you, and which you wish to control.
In these four examples (and perhaps other cases as well),
it is desirable to control the far more audible upper bass
boom, primarily from the second harmonic modes, and to leave
the fundamental modes pretty much alone. How can you accomplish
this? Interestingly, the best tactic is precisely the reverse
of the typical bass Tube Trap setup. The typical setup places
the bass Tube Traps only in the room corners. But the best
tactic for controlling only the second harmonic upper bass
boom is to place no bass Tube Traps in the corners at all.
Instead, the four bass Tube Trap columns should be moved from
those typical corners to the center point
of each of the four walls. This center wall position is the
one that controls the second harmonic mode the best while
affecting the fundamental modes the least.
I have yet to see anyone try a bass Tube Trap installation
with bass columns only at the center walls and none at the
corners, but this tactic would be sonically preferable to
some listeners, and appropriate to systems with smaller speakers.
Note again that the correct bass Tube Traps for dealing with
the second harmonic can be smaller models than the correct
ones for dealing with the fundamental, thereby saving you
An ordinary rectangular room has of course not one dimensional
mode, but three (rooms with alcoves, etc. have even more).
So the above considerations, calculations, and placement tactics
should be executed three times, one for each of the dimensions.
The optimum tactics for dealing with one dimensional mode
might conflict with the optimal tactics for another dimensional
mode. In some instances this conflict is easy to resolve.
For example, the second harmonic of the length or width of
your room might be very close in frequency to the fundamental
of the height. But, since both are likely contributing to
an upper bass boom, you can easily kill two birds with one
stone; simply select bass Tube Traps with the appropriate
cutoff frequency, and then run a horizontal row across the
ceiling, at the midpoint of the dimension whose second harmonic
you wish to control. In Figure 2, the horizontal row across
the middle of the room works to control the second harmonic
of the front to back dimension, while it also is at the end
of the floor to ceiling dimension, thus helping to control
On the other hand, placing the same Tube Traps in vertical
columns at the midpoint of the side walls (also shown in Figure
2) places minimal Tube Trap area at the ends of the floor
to ceiling dimension, so it hardly affects the vertical room
modes at all (meanwhile, however, it affects the fundamental
of the side to side room mode more than does the horizontal
row of the same Tube Traps hung from the ceiling).
Once you have understood the basic strategies of bass Tube
Trap selection and placement, it becomes pretty easy to place
each set of bass Tube Traps so that it controls the one room
mode dimension the way you want it controlled, and then affects
the room modes of the other two dimensions maximally or minimally,
as you wish. Just remember that, because you're dealing with
a three dimensional room, each bass Tube Trap location will
inevitably affect the two other dimensions besides the one
you're focussing your attention on. By intelligent placement,
you can make that Tube Trap affect the second dimension minimally
and the third dimension maximally, or vice versa as you choose,
while still accomplishing the desired control for the first
dimension. Remember throughout that the fundamental modes
can be damped by bass Tube Traps along just one of the two
opposing surfaces causing that mode; you don't need to damp
both ends, and this will afford you more flexibility in placement.
We have concentrated on discussing control of the fundamental
and second harmonic modes of the room. What about higher frequencies?
As noted, bass Tube Traps at the end of a dimension act to
absorb not only the fundamental and second harmonic, but also
the third harmonic and all higher harmonics. The bass Tube
Traps at the midpoint of a room dimension act to absorb not
only the second harmonic (while leaving the fundamental alone),
but also all higher even order harmonics. However, in most
rooms the third harmonic, and certainly the fourth harmonic,
will already be higher in frequency than the upper bass, and
will be in the warmth region. In this frequency region we
have to start worrying about reflection control (see section
below), which involves considerations far beyond room resonance
modes (for example, it involves the typical distance of speakers
and listener to the side walls, etc.). Thus, we recommend
that you worry only about the low bass and upper bass when
paying attention to room resonance modes.
Additional considerations apply to the bass Tube Traps in
the front corners of the room (the speaker end, shown as the
top of the plan in Figure 2). These Tube Traps should actually
be selected to match not the room modes, but rather the bass
characteristics of your speaker. The bass control of these
Tube Traps should be adjusted to match the bass cutoff of
your speaker, its bass Q, its need for bass boost from nearby
surfaces or corners, and the distance from the speaker back
to the corner. These factors might well call for the use of
skinnier Tube Traps than the room mode dimensions would suggest,
thereby saving you money (this is why we show these front
corner Tube Traps as being possibly smaller circles).
There are no simple rules for optimizing these front corner
bass Tube Traps. The basic goal is to keep this corner from
excessively talking back from the speaker's bass rear radiation
(speakers are omnidirectional in the bass). Excessive talk
back will cause an amplitude hump plus a ringing boom at some
frequencies (depending on the speaker to corner distance)
and a cancellation at other frequencies, with bass phase shift
at the frequencies in between (see Hotlines 39 and 40 for
discussion). But your speaker might have been designed to
depend upon some bass support from nearby (i.e. three to five
feet away) surfaces or corners, especially if it's a small
or low Q bass system. If this is the case, you'd want to damp
corner talk back above the speaker's resonance frequency,
but not below, so select Tube Traps for the front corner whose
bass cutoff is just a bit higher than your speaker's bass
Incidentally, if your speaker needs corner boost for bass,
and still sounds lean in the warmth region, you might try
an even skinnier Tube Trap in the front corners, one having
a bass cutoff that is not slightly higher but rather much
higher than your speaker's bass resonance frequency. There
should be some sort of Tube Trap in the corner to at least
control corner talk back in the lower midrange, where it could
cause bad honking colorations (the Tube Traps that we will
place directly in back of the speakers are even more important
in this regard; see below).
Smaller speakers, those requiring a stand, can also benefit
from your using a full round, 2 or 3 foot high Tube Trap as
a speaker stand. Select the Tube Trap model whose bass cutoff
will or won't attenuate your speaker's fundamental resonance,
as you wish. Line up the seam with the front of the speaker.
On the other hand, if your speaker is a large system designed
for full bass in a free field, then it doesn't need any help
from a nearby surface or corner, and indeed it will probably
sound too bass heavy and boomy unless you do fully damp the
nearby corner, down to below the speaker's resonance frequency.
In this case, select front corner bass Tube Traps with a cutoff
frequency at least as low as your speaker's bass resonance.
It's obvious that a lot of experimentation and listening,
with various Tube Trap models, is necessary if you are going
to truly optimize the bass performance of your room for your
taste and your speakers. Hopefully, your ASC Tube Trap dealer
will be prepared to help you, perhaps lending you additional
bass Tube Traps so you can try different configurations and
discover which you prefer. If you only buy a limited number
of bass Tube Traps, you might well never discover whether
bass control can get even better in your room with just one
more bass Tube Trap column in some location, say some center
Scientifically speaking, you can never know whether you have
optimized bass performance of your system (including your
room) unless you go beyond the optimum for your taste, and
then back off. This fact necessarily implies that you should
try listening to more bass Tube Traps than you will wind up
buying. And that in turn implies that an intelligent ASC dealer
should be prepared to lend you extra bass Tube Traps to try
(it's a wise business move for him too; you will surely be
buying more bass Tube Traps than otherwise, once you hear
the sonic improvement from that extra bass control from additional
bass Tube Traps).
If perchance you are not near an ASC dealer, we suggest you
follow the various considerations discussed with respect to
Figure 2. You don't need to emulate all the bass Tube Traps
shown in Figure 2 unless you have large speakers and like
high quality bass. You can, if you wish, emulate only those
parts of Figure 2 that pertain to you, as indicated by the
PERIMETER REFLECTION CONTROL--
The above sections dealt with bass control and room resonance
modes. These phenomena take place between pairs
of room surfaces (again, it takes two to tango).
But that is only one facet of the sonic magic that ASC Tube
Traps can achieve for your listening room and your musical
enjoyment. Tube Traps can improve many other sonic aspects.
These other sonic aspects relate to reflections. Reflections
occur from single walls, and do not really depend upon a pair
of opposing walls. Reflections therefore occur wherever there
is a single wall or surface in your listening room. Consequently,
in order to control these reflections, you must use Tube Traps
wherever there are room surfaces, in other words, along all
walls and the ceiling.
If you control reflections correctly in your listening room,
whole new worlds of sonic magic will open up to you. The typical
application of Tube Traps, for just bass control, is but a
small slice of the rich pie representing the multifaceted
sonic capabilities of Tube Traps. What are some of these other
sonic capabilities? They were discussed extensively in Hotlines
39 and 40, but for your convenience we'll briefly go over
the highlights again.
The most unique sonic capability of Tube Traps is in controlling
the dreaded mud factor. The mud factor is an echoey blur your
room imposes on music, primarily in the warmth and lower midrange
regions. It's too high in frequency to be heard as an upper
bass boom, but too low in frequency to be heard as a midrange
honk or upper midrange glare. In fact, it's hard to hear its
presence at all. But everyone can easily hear its absence.
When you remove the dreaded mud factor from a room, music
becomes dramatically clearer, more dynamic, and more alive
and sparkly. Gone is that sensation of needing to keep turning
up the volume, trying in vain to get the music to sound clearer
and more dynamic. The music sounds as if it had been literally
lifted out of a sea of obscuring mud and rinsed clean, when
you use Tube Traps to control the mud factor.
This sonic capability is unique to Tube Traps because only
Tube Traps can absorb sound below 400 hz, covering the critical
mud factor region of 100 hz to 400 hz. Wall panels of acoustic
foam or fiberglass are ineffective below 400 hz, so they cannot
control this dreaded mud factor.
Tube Traps also are superb at eliminating hot spot wall reflections
that cause obscuring smear plus tonal colorations in the midranges.
These colorations are chiefly audible as a midrange honk and
upper midrange glare. They might sound as though they are
originating in your speakers, but often it is your room surfaces
that are the culprit. Using Tube Traps to control these hot
spots makes music sound more neutral. It also makes music
sound again much clearer and more dynamic, as it is freed
from an obscuring sea of smearing midrange echoes.
This Tube Trap control of midrange hot spot reflections will
also dramatically improve all aspects of stereo imaging. That's
because these same hot spot reflections represent a temporally
coherent packet of energy that arrives at the ear/brain from
a different direction than the direct sound from the loudspeaker,
and at a different time. This degrades stereo imaging, especially
if this hot spot coherent packet arrives within 15 milliseconds
of the direct sound (which it will, unless your room is so
large that both speakers and listener are more than 10 feet
away from the nearest wall or ceiling surface).
The research by Haas and Damaske demonstrated that stereo
imaging (and musical transparency) can be enhanced if reflected
energy heard by the ear/brain is incoherent ,
nondirectional (coming from many directions at once and not
concentrated from any one direction in particular), and arrives
more than 15 ms after the direct sound. Reflected sound that
fails to meet these criteria is detrimental to stereo imaging,
and can even blur and obscure the original music.
Therefore, you don't want to eliminate reflected sound entirely.
An acoustically dead room will not support a rich stereo image
from a two speaker stereo system. Indeed, some reflection
from virtually all parts of the room's surfaces is vital to
achieving a uniform yet randomly incoherent reverb decay characteristic
for your room, which enhances both stereo imaging and the
rich heft of dynamic sonority. Thus, you want to control reflections
throughout the room surfaces, but not totally eliminate them
The secret to Tube Traps' success in accomplishing this desideratum
is twofold. First, they are very effective absorbers where
they are located, which allows you to leave wall area bare
between them, thus obtaining some full spectrum reflection
from all areas. In contrast, surface mounted absorbers of
foam or fiberglass are less effective absorbers, so you need
to cover more wall area for the same absorption control, thereby
leaving less area to support those enhancing reflections.
Second, Tube Traps allow tunable reflection directional control,
which is vital for optimizing the various aspects of stereo
imaging (see discussion below). In contrast, surface mounted
absorbers are not tunable for direction.
To obtain all these additional sonic benefits available from
Tube Traps, the basic tactic is simplicity itself (which should
be a welcome relief from the complex considerations involved
above for bass control). You can accomplish reflection control,
for the entire perimeter of your room, by simply placing a
9 inch Tube Trap column (the skinniest and least expensive)
every 3 feet or so around the perimeter of your room (except
for the locations where you have already installed bass Tube
Traps for optimizing bass control).
Figure 3 shows such an installation, including the bass Tube
Traps from Figure 2, plus the added 9 inch Tube Trap columns
for reflection control around the perimeter of the room. If
by chance you did not fill all of the locations of Figure
2 with bass Tube Traps for bass control, then these locations
should be filled now with 9 inch Tube Trap columns for reflection
control (that includes at least the two horizontal rows across
the ceiling, which is also a surface requiring reflection
In fact, Figure 3 shows the general Tube Trap layout in the
plan of our lab's master listening room, which is 30 feet
long (front to back) by 25 feet wide, with a 14 foot cathedral
ceiling. Most other listening rooms are smaller, so they would
require fewer columns of 9 inch Tube Traps for adequate perimeter
Basically, we recommend that you place reflection control
Tube Trap columns every 3 to 4 feet around the perimeter (use
whatever distance within the 3 to 4 foot range that works
out to evenly divide into your room's exact dimensions).
On the front wall, in back of the speakers, we recommend
you place these columns every 2 to 3 feet; or perhaps, just
in back of each speaker, place an extra column or shorten
the distance between columns somewhat. The reason is that
energy from the speaker going straight back to the front wall,
and then reflected straight back toward the listener, is especially
detrimental in terms of time smearing and degrading spatial
We use only two horizontal columns across the ceiling, because
our ceiling is very high and pitched, so it does not create
severe reflection problems. But a low and flat ceiling would
create severe reflection problems, and so would require more
horizontal rows, perhaps even a row every 3 to 4 feet, spaced
to match the columns along the side walls.
In addition, we have found that it helps coherence and stereo
imaging to have a pair of columns placed physically halfway
between the speakers, about 6 inches from each other, as shown
in Figure 3. This finding owes a debt of thanks to Monster
Cable, who marketed an acoustically absorbent dividing screen
for just this purpose.
Our pioneering article on ASC Tube Traps in Hotlines 39 and
40 introduced this concept of perimeter reflection control,
and discussed its importance. Even the manufacturer of Tube
Traps had not recognized the virtues of using his product
in this manner. Since then, a number of high end listening
rooms have employed this strategy, with impressive sonic results.
In fact, a room so completely treated with Tube Traps for
perimeter reflection control has become known in the industry
as a Moncrieff room.
On a recent visit to Italy's top high end dealer, Absolute
Sound, we witnessed such a room, and with some fine tuning
using our trained ears, we were able to obtain superb sonics
and imaging from Goldmund's premier system set up in this
room (the room did have a low ceiling, and did not have the
required horizontal rows on the ceiling to control this, but
otherwise had excellent perimeter reflection control).
Placing 9 inch Tube Trap columns every 3 feet or so is necessary
to control the dreaded mud factor. Why? Recall that the mud
factor predominates from 100 hz to about 400 hz (where surface
mounted absorbent foam or fiberglass is totally ineffective).
The wavelength of 500 hz is about 2.25 feet. Thus, placing
perimeter control 9 inch Tube Trap columns every 3 feet, leaving
2.25 feet of blank wall space between columns, breaks up every
other half wavelength up to about 500 hz. A half wavelength
is the shortest amount of an acoustic wave that can reflect,
at the pertinent frequency.
It is undesirable to allow several half wavelengths to accumulate
along a reflective wall; this would create a coherent reflected
packet of sound from that segment of the wall (or ceiling).
One example is shown as the arrow in Figure 3, coming from
the left speaker and reflecting from the side wall toward
the listener (such coherent reflection packets would occur
from all surfaces around the perimeter, unless they are controlled).
This coherent reflected packet would be heard as a distinct
sound source, which smears the music signal both temporally
and spatially, creating obscuring mud and degrading stereo
imaging. Recall that incoherent random reflections are valuable
sonic assets, but coherent directional energy packets of reflected
sound are detrimental.
Is a 3 foot spacing close enough? It seems to be. Breaking
up every other half wavelength seems adequate to insure that
no large coherent packet of sound will be reflected from any
one spot around the room's perimeter. This spacing seems adequate
to control the mud factor, between 100 and 400 hz.
Above 500 hz the wavelengths are short enough so that the
blank wall space between columns would reflect some coherent
packets. But two other factors come into play to alleviate
potential problems above 500 hz. First, these shorter wavelengths
tend naturally to bounce all over the room more randomly,
hence to become randomly incoherent by themselves. Second,
the round shape and 9 inch dimension of each Tube Trap column
begin to aid in this random scattering of the shorter wavelengths
above 400 hz, because this is the frequency where the Tube
Trap's reflective side begins to reflect, and also begins
to act as a cylindrical diffuser (this diffusion takes full
effect at all the frequencies above where the 9 inch diameter
is itself equivalent to half a wavelength, which is 550/.75
= 733 hz). Thus, frequencies above 400 hz, and certainly above
733 hz, are randomly scattered into incoherent reverberant
energy which helps the music and the stereo imaging, rather
than harming them.
In sum, it is both necessary and sufficient to place Tube
Traps every 3 feet or so around the room (and perhaps across
the ceiling), for adequate perimeter reflection control of
frequencies above 100 hz. Incidentally, we recommend leaving
the room walls absolutely bare behind the Tube Traps, so they
are fully reflective of higher frequencies wherever the Tube
Traps aren't. If the resulting room acoustic is too bright
for your taste, with all Tube Traps in place, then you can
hang some attractive soft surface absorbers on the walls between
(and behind) the Tube Trap columns.
--Validating Perimeter Control
We've written before about the amazing power and sensitivity
of Tube Traps in sonically affecting all those sonic aspects
that benefit from proper perimeter reflection control. We
can often hear the difference if one Tube Trap column is moved
a fraction of an inch, or rotated a fraction of an inch.
But many of you are probably looking with trepidation at
all the Tube Traps involved in emulating Figure 3 for your
room. The budget requirements are not inconsequential. But
think of all the money invested in your library of music recordings
and in your stereo system. What's it worth to hear all this
investment at its proper potential for a change? It might
also seem strange to be surrounded by a veritable forest of
Tube Traps. But our listening room has the comforting feel
of an open Greek temple, with the columns around the perimeter.
Still, you might be doubting that all those Tube Trap columns
are really necessary. So we conducted a measurement experiment,
to prove to you that every one of those Tube Trap columns
is necessary, to achieving adequate perimeter reflection control
for the room. The proper scientific method for proving this
thesis is to fully treat a room, and then remove just one
Tube Trap column. This should make a very small difference,
but just enough of a difference to test whether every one
of those Tube Trap columns is truly necessary.
So that's the experiment we performed. We removed just one
9 inch Tube Trap column used only for perimeter reflection
control (the one shown with stripes) from the complete room
setup shown in Figure 3. We set up the measuring microphone
along the listening path, between the wall reflection point
shown and the listener. It's an omnidirectional microphone,
so it would be measuring the total room behavior, as heard
along this path.
When the one Tube Trap column is removed, there is one 5.25
foot gap between Tube Traps, with all other gaps being 2.25
feet as recommended; when it is replaced, this 5.25 foot gap
is eliminated, and is replaced by two of the recommended 2.25
foot gaps. Would there be a measurable difference in the room
from the elimination of just one Tube Trap column from the
total set of 33 columns we had installed for total perimeter
If there was to be a difference, it would probably show up
in that dreaded mud factor region, somewhere between 200 hz
and 400 hz. That's because we would be allowing one unprotected
gap in the room's entire perimeter, a gap of 5.25 feet to
be precise (6 feet minus the 9 inch diameter of the columns
themselves). That gap would have the capability of sustaining
a coherent reflection packet of two half wavelengths, or three,
four, etc. Two half wavelengths within 5.25 feet calculate
out as equivalent to 210 hz. Thus, by exposing an unprotected
gap of 5.25 feet, we would expect to see some room troubles
somewhere above 210 hz, troubles which would be cured by controlling
that gap and narrowing it to the recommended 2.25 feet. If,
that is, every single Tube Trap column (with 2.25 foot gaps)
were truly important to adequate perimeter reflection control
in this room.
Incidentally, because this is a huge room, the effects of
removing one column out of 33 should be much smaller upon
the huge room than the effects upon a smaller room of removing
one column out of say 20. So the results we prove here will
be even more relevant to smaller rooms.
The standard time honored test for echoes, smearing, and
resonances is the tone burst test. ASC makes available a convenient
cassette, which contains a range of tone bursts spanning the
relevant frequencies of 20 hz to 755 hz (technically, the
tape contains a sine wave that slowly slides in frequency,
and then is chopped into 8 tone bursts per second). The perfect
tone burst response should look like a solid rectangular block
of full amplitude sine waves, followed by a period of silence
(of the same duration as the rectangular block, on this tape),
followed by another rectangular block of full amplitude sine
waves, etc. Figure 15 shows a pretty good looking example.
Each picture show 4 tone bursts, with 4 supposedly silent
periods. Note that each of the 4 tone bursts in a picture
will be slightly different, since each represents a slightly
different frequency from the sliding sine wave tone.
Incidentally, don't pay attention to the irregularities you'll
see in what should ideally be a flat top and bottom of each
tone burst's rectangular block; they are mostly random artifacts
of measurement in a reverberant room. Also, you should naturally
expect to see some reduction in the amplitude of every main
tone burst when we add the absorbent Tube Trap in the reflective
What then do you look for in this tone burst measurement?
The most important thing to examine is how well the room falls
to silence after each rectangular block of tone burst. The
tone burst itself mimics a musical transient or any piece
of musical information. After that musical transient or piece
of information is over, the room should not continue with
too much energy as smearing echoes. If it does, these smearing
echoes of the room will form a sea of mud, the dreaded mud
This sea of mud will obscure the end of the musical transient
(and the subtle after resonances that tell you about the texture,
timbre, and material of the musical instrument); it will obscure
the next transient or piece of musical information; and it
will generally blur all the distinctions among various musical
transients and pieces of information, as the music vainly
struggles to rise above the sea of mud. The music's temporal
coherence and even basic clarity will be lost in this sea
of mud. You'll want to turn up the music's volume for more
clarity, but this won't help, since the sea of mud will also
rise. In addition, if the smearing echoes are from a coherent
reflection packet (as is the case here), it will degrade stereo
imaging and cause tonal colorations.
You'll see this sonic description of a sea of mud vividly
illustrated in some of the measurements. In some cases, the
sea of mud rises so high relative to the music (represented
by the rectangular tone burst) that it will be difficult to
even see where each tone burst supposedly stops and restarts
(for example the 3rd and 4th bursts of Figure 10).
Sometimes, as the sea of mud between rectangular tone bursts
rises, you'll also see the amplitude of the rectangular tone
burst itself diminish. This might be caused by cancellations
occurring, where the undesirable packet of coherent reflected
energy interferes with the direct sound from the speaker.
Sometimes the main tone burst is so diminished, or the sea
of mud after the tone burst rises so high, that again you
won't be able to see where each tone burst itself is supposed
to begin and end (for example Figure 16). That is true room
garbage, playing just as loudly as the original music signal.
The sea of mud has risen to swamp the music. When you see
this happening to our lab room from just the removal of one
Tube Trap column out of 33, you'll realize graphically just
how important each and every Tube Trap column is for adequate
perimeter reflection control, for giving you clear music instead
ASC calls this tone burst tape the Music Articulation Test
Tape (MATT), with good reason. You can use this tape yourself,
even without our fancy lab test equipment; all you need are
a set of headphones and your ears. Borrow this tape from your
ASC dealer, and simply listen to the test tone bursts first
through headphones and then through your speakers. Through
the headphones you will hear clear bursts of tones. Through
your speakers you will hear smearing echoes after each burst,
getting much worse at some frequencies. What you are hearing
is the sea of mud contributed by your room. Your room is doing
that to all your music. And you can fix it with Tube Traps,
using them for full perimeter reflection control.
Now on to the measurements. As expected, removing the single
Tube Trap column caused more problems at some frequencies
than at others. Here is a selection of the most illustrative
examples. We predicted that, if the perimeter reflection control
thesis were correct, we should see the first problems somewhere
above 210 hz. Sure enough, the first major difference cropped
up around 250 hz (all frequencies are approximate, because
of the sweeping nature of the test tone).
Figure 7 shows the room's performance around 250 hz with
all Tube Trap columns in place; Figure 8 shows exactly the
same signal, measured at the same point, with just the one
Tube Trap column removed out of 33. Figure 8, without the
single Tube Trap column, shows a lot of smearing echo energy
lingering after each tone burst stops, with only a brief moment
of relative quiet before the next tone burst begins. The tail
of smearing mud is so large in its beginning that you cannot
even tell where the original tone burst itself stops. With
the controlling Tube Trap column restored to its place, Figure
7 shows that the smearing echo is reduced, and there is a
much longer, better quiet period before the next tone burst
begins. Of course, it's still not perfect (you can see two
small single slaps of energy left in the time domain after
the tone burst), but there's an easily visible difference.
The fact that some imperfect echo smear is seen even with
the Tube Trap in place simply means that our room is still
not perfect. The important point is that there is a clear
difference, a clear degradation, when just one column of a
complete 33 column perimeter reflection control setup is removed.
Figures 9 and 10 show a difference evident around 300 hz.
In Figure 10 (without the Tube Trap column) you can see a
large lingering echo smear. For the 1st and 2nd bursts, the
smear begins after a small gap of quiet following each tone
burst. For the 3rd and 4th bursts, the smear is congruent
with the tone burst (so you can't even see or hear where the
burst or musical transient is supposed to stop), and the smear
itself is in the shape of a decaying triangle. Figure 9 (with
the Tube Trap column) shows a smaller, better controlled version
of this echo smear.
Figures 11 and 12 show a difference found around 315 hz.
Figure 12 (without Trap) shows a big, blotchy echo smear;
for the 4th burst, the echo smear is again congruent, and
has that decaying triangle shape. Figure 11 (with Trap) shows
a smaller echo smear, which is also more even and well behaved
(more like the random incoherent reverberation background
noise that is actually desirable).
Figures 13 and 14 show a difference found around 420 hz,
at the upper edge of the mud factor region. Figure 14 (without)
shows a fat decaying triangle of echo smear, while Figure
13, though still not perfect, shows a much smaller decaying
triangle of this echo smear.
Figures 15 and 16 show a difference around 600 hz, which
may be a harmonic of the trouble seen at 300 hz. Figure 15
(with Trap) shows a pretty decent signal, while Figure 16
dramatically shows total degeneration without that one Tube
Trap column. The main tone burst signal collapses down to
noise level in the middle of itself, probably due to some
path length cancellations. It's hard to tell which is the
inter-burst smear and which is the mid-burst collapse.
Figures 17 and 18 show a difference around 630 hz, which
may be a harmonic of the trouble seen at 315 hz. Figure 17
(with Trap) shows pretty good silence in between tone bursts,
with the visible residual noise perhaps representing just
the desirable random incoherent room reverb. Figure 18 (without
Trap) shows a larger amplitude of echo smear, in the form
of a triangular decaying tail.
These measurements provide clear, indeed dramatic visual
proof of the sonic importance of even one Tube Trap column
to total perimeter reflection control in your room. They show
the mud factor already rising, in the predicted warmth and
lower midrange regions, when just 1 out of 33 perimeter control
columns is removed. They show the importance of placing a
Tube Trap column every 3 feet or so, as recommended, instead
of wider spacing. We rest our case.
If your room is smaller than ours, you won't need as many
Tube Traps for a complete perimeter reflection control system.
But even this might be out of reach of some of your budgets.
If so, don't give up. Some Tube Traps are better than none.
But which are the most essential Tube Traps for a minimalist
economy system? Our recommendations are shown in Figure 4.
Let's start again with controlling the bass fundamental.
The three bass Tube Trap columns shown at the back of the
room (bottom of Figure 4) accomplish this. These should probably
be 16 inch models, depending on your room length, with the
center rear being perhaps a Super 16.
We've shown 16 inch quarter round models in the rear corners;
they are much less expensive, while still offering good absorption
down to low frequencies. You can save more money by using
only a single Tube Trap at each location instead of a stacked
set making a full floor-to-ceiling column, but of course this
will not absorb as much energy (which might sound better for
your taste in low bass).
Bear in mind that the more economical half and quarter round
Tube Trap models come only in 5 foot high sections, while
the full round models (preferable because they offer rotational
control of perimeter reflections) come in your choice of 2
or 3 foot high sections.
If your taste or circumstances suggest that you do not want
to control the lowest bass resonance fundamental at all, then
you can eliminate these three back wall bass Tube Trap columns
entirely. But then you should still retain some absorption
directly in back of the listener for the warmth region and
higher frequencies. The most economical model for this would
be the 9 inch half round, which is a single 5 foot high unit
(however, you might still want to retain a bass Tube Trap
at this center rear position for controlling the upper bass
boom of the room's second harmonic side to side mode; see
Note that the half round models are available in three versions:
with the high frequency reflective strip in the center, or
offset to one side, or none at all. In this back wall location,
use the version with no reflective strip.
Figure 4 also shows bass Tube Trap columns at the midpoints
of the side walls. These are to offer additional control of
upper bass boom, beyond that afforded by the three bass columns
at the back of the room. These columns can be a step down
from the models at the back of the room, since they absorb
the second harmonic rather than the fundamental of the front
to back room mode (on the other hand, you might want to use
these side columns also for more control of the fundamental
of the room's side to side mode, in which case you should
select the Tube Trap model with a bass cutoff low enough to
include the frequency of the room's side to side fundamental).
If you don't want more control of upper bass boom beyond
that accomplished by the three bass columns at the rear, then
these midpoint columns can be changed to the more economical
9 inch half round model (use the version with the offset reflective
strip, and orient the Trap so the reflective strip points
toward the back of the room).
If you have decided to eliminate control of the front to
back fundamental, by deleting the three bass Tube Traps at
the rear, then it will surely be important to retain these
side midpoint Tube Trap columns as bass units, in order to
control the upper bass boom of the second harmonic of the
front to back mode. Similarly, it would then also be important
to retain the center rear column as a bass Tube Trap, in order
to control the upper bass boom of the second harmonic of the
side to side mode (you might even want to add a bass Tube
Trap column at the center front wall of the room to help in
this; such help may be needed because you have eliminated
the rear corner bass Tube Traps, which worked to control the
side to side second harmonic as well as the fundamental).
So much for bass control; now, on to reflection control.
It's important to provide reflection control directly in back
of the listener (as already discussed) and also the speakers.
These are straight reflection paths which would degrade temporal
coherence of the music, ruin imaging, cause tonal colorations,
and contribute to the mud factor. These reflective paths should
be short circuited for the full spectrum, from the warmth
region up. To accomplish this, use 9 inch half round models
(preferably full ceiling height columns, but it would suffice
to use single 5 foot high units, which are as tall as the
speakers in front and as the seated listener's ears in back).
Use the version with no reflective strip.
As we saw in the measurements above, it's also important
to control the angled primary reflection paths from the speaker
to the side wall to the listener. This is accomplished by
using two half round 9 inch models per side, as shown in Figure
4. Use the version with the offset reflective strip, and orient
the Tube Traps so that the reflective strip points toward
the back of the room (bottom of the picture). The reason for
this orientation is that we want these reflection control
Tube Traps to fully intercept and break up the sound coming
directly from the speaker, which will arrive at the front
side of the half round. But the rear side of the half round
encounters random incoherent reverberant energy (Damaske effect)that
has already been around the room and so is significantly delayed
(Haas effect); this energy we want to encourage, by diffusing
and reflecting it further.
Note again that, if you have chosen to eliminate the bass
Tube Traps at the midpoint of the side walls, then these too
should be convened to the 9 inch half round model, with the
offset reflective strip pointed toward the back of the room.
This would make three half rounds along each side wall, unless
you have a small room, in which case two on each side might
do (a handy rule of thumb is this: start just forward of the
speakers along the side walls as shown, then space them every
3 feet or so, and stop before you get to the side of the listener,
Figure 4 does not show any Tube Traps hung horizontally across
the ceiling. If your ceiling is flat, it may be important
to also include some rows across the ceiling. The model you
select depends upon the problem being addressed. If there
is some upper bass boom from the floor to ceiling mode, then
a row of bass Tube Traps hung across the center of the room
will help. If there is still too much mud factor, then some
half round rows across the room should help; mount them to
the ceiling just where you've placed the half rounds along
the side walls (with their reflective side also pointed towards
the back of the room).
This economy Tube Trap setup won't sound nearly as subtly
refined as the full fledged setup of Figure 3, but it will
provide most of the basic sonic benefits to some degree. That's
a.lot better than the typically seen setup of just four Tube
Trap columns in the corners.
--Alignment of Tube Traps
ASC Tube Traps are designed to absorb the full frequency
spectrum (down to their bass cutoff) on one side, but to reflect
and diffuse most of the frequency spectrum above 400 hz from
their other side. That engineering feature becomes a very
powerful and flexible tool for precisely adjusting perimeter
Recall that we spaced Tube Trap columns about 3 feet apart
to control the mud factor, between 100 hz and 400 hz. The
entire spectrum above 400 hz, including the midrange and trebles,
is a different matter entirely. It consists of short wavelengths
which are directional, and which can be directionally steered
to sonic benefit, rather than simply absorbed everywhere.
By simply rotating the cylindrical Tube Traps, you can determine
how all frequencies above 400 hz will be steered in your room
(with the half round model, you are more limited, and you
must preselect and orient the reflective direction if any).
The subtlest change in rotation of any cylindrical Tube Trap
in the room can make a significant difference in truly optimizing
the sound of your system (that's why we recommend the full
round rather than half round models, if you can afford them).
Rotating Tube Traps to absorb energy (above 400 hz) on short
primary reflection paths (within the Haas window) can work
wonders to improve stage imaging and coherence, and eliminate
glary hot spot colorations. And rotating Tube Traps to reflect
and diffuse later reverberant energy on long secondary, tertiary,
etc. paths can work wonders to improve imaging ambience and
The basic strategy is shown in Figure 5. The line radiating
from the center of each circle represents the seam sewn along
the length of each Tube Trap; this is the direction of maximum
high frequency absorption. The setup shown in Figure 5 will
give decent performance, but it really intended just as a
starting point, from which you should further fine tune by
ear. If you have also installed any horizontal rows across
the ceiling, they should be rotated to point similarly into
the room as the equivalent Tube Trap columns at that point
along the side wall, and then fine tuned from that starting
The rationale for the rotation orientations shown in Figure
5 is as follows. The Tube Traps immediately surrounding each
speaker are set for maximum absorption at all frequencies.
Here the primary reflected path is short (well within the
Haas window), so reflected energy would degrade coherence,
smear the music, cause tonal colorations, and ruin stereo
imaging. The same is true for the Tube Traps immediately in
back of the listener.
The other Tube Traps along the back wall, off to either side
of the listener, are more flexible. They are shown pointing
toward the listener, but you might prefer the sense of added
hall ambience you might get by rotating them somewhat away
from the listener, or even with the seam pointing directly
into the wall. Likewise, the Tube Traps along the front center
wall (off to the side of the speakers) are flexible in rotation.
We prefer them pointed straight into the wall as shown; the
reflected diffusion this offers seems to help the center fill
and liveliness of the stereo stage.
The pair of Tube Trap columns precisely between the speakers
is also flexible in orientation. We prefer them pointing directly
at each other, for minimum upper frequency absorption and
maximum reflected diffusion.
The sets of Tube Trap columns along the side walls that are
off to the side of the listener are an interesting case study.
The Damaske effect says that you can best hear the hall ambience
in a recording when randomly diffuse incoherent information
is presented at the side of the listener's head. Thus, all
Tube Trap columns along the side walls (and rows across the
ceiling) are rotated to optimize the following two factors.
Absorb those unwanted coherent packets
of energy coming along the short reflected path from the speakers
to the side walls to the listener (especially those that come
at the listener from his front). And meanwhile reflect and
further diffuse reverberant energy that has come on a long ,
multiple reflected path and is already incoherent
and delayed (especially that which will come at the listener
from his side).
Meeting these two desiderata simultaneously yields the rotational
alignments seen in Figure 5. If you look closely, you'll notice
that the seams of the Tube Traps near the speaker along the
side walls point almost perpendicularly into the room. Then,
as you go backward in the room toward the listener, the rotational
alignment of each successive column progresses toward having
the seam point parallel to the side wall, and toward the front
of the room.
This progression is in part a natural consequence of having
each column point directly at the nearest speaker, for maximum
absorption of the short path energy. But it also serves to
present to the rear part of the room, and to the side of the
listener, a progressively increasing amount of reflection
and diffusion of the reverberant long path energy in the room.
By the time we arrive at the column directly to the side of
the listener, the seam is pointing directly into the wall,
for maximum reflection and diffusion of reverberant sound
A further subtlety. You'll notice that some of the side columns
are shown with two lines radiating from them. This indicates
that the bottom Tube Trap is rotated to have its seam pointing
more perpendicularly into the room, while the top Tube Trap
has its seam pointing more parallel to the side wall, or more
into the side wall. If your ceiling is tall enough so that
you are using a stack of three Tube Traps per column, start
off with the middle Tube Trap oriented the same as the bottom
Why do this varying rotation ? Because the speakers and the
listener's ears are closer to the room's floor than to the
ceiling. Thus the unwanted short path that we want to absorb
is handled by the bottom Tube Trap. Meanwhile, the top Tube
Trap sees mostly long path energy that has already been through
multiple reflections, which we wish to encourage. So we orient
the bottom Tube Traps for more absorption, and the top Tube
Traps for more reflection and diffusion. Feel free to explore
the possibilities of setting other columns with different
rotations for the bottom and top Tube Traps.
Fine tuning of Tube Traps must be done by ear, not by formula.
It requires a highly trained, sensitive ear, of someone who
knows what to listen for, how to interpret it, and what corrective
adjustments to make. Unfortunately, that is something we cannot
teach you in a written article. Some professional consultants
in the field have a lot of experience with Tube Traps, and
you might wish to hire them for final fine tuning. If you
can't afford that, here are some general hints to guide you.
Think of this fine tuning procedure not as serious business,
but rather as an exploratory experimental game. Make only
one type of change at a time, and learn what each change in
itself sounds like. Learn what the sonic difference is when
you rotate one Tube Trap (symmetrically on both sides of the
room) by 45 degrees, then 10 degrees, then I degree if you
can learn to hear it. Try moving the location of columns 2
inches one way along a wall, then half an inch another way.
The fine tuning of Tube Traps in the back (listener) half
of the room affects mostly the sound of hall ambience from
the recording, while the front (speaker) half affects mostly
the imaging characteristics of the stage where the musicians
In general, you'll find that rotating Tube Traps so that
the seam points more into the wall (giving more reflection
and diffusion into the room) increases ambience and generally
improves imaging. But if you do this too much, you might pick
up hot spots of midrange glare, some loss of musical coherence,
and some unevenness in what should be a continuous curtain
of sound across the stage width and depth -- so listen carefully
for any degradation in these qualities.
Conversely, fine tuning only for the smoothest sound of the
room can easily go too far and make the room too dead, thereby
degrading ambience richness that comes from random incoherent
long path reflections -- so listen carefully for any degradation
in this aspect.
The ideal balance is a room that is neither too live nor
too dead. But it would be a sad oversimplifying mistake to
think that all you are trying to balance is liveness vs. deadness.
You are really trying to make the room dead
for short path coherent reflected packets
and live for random incoherent long
path reverberation, at all frequencies. And even that is only
the beginning. The other factors pertain to the manifold intricacies
of optimum stereo imaging, as follows.
As you play with the Tube Traps along the back wall, listen
for the changes in the sound of hall ambience (from a recording
that is rich in this), especially the ambience that seems
to come from the back of the hall. This is indicated in Figure
6 as rear ambience fine tuning. Play especially with those
Tube Traps along the rear wall that are not directly in back
of the listener. We suggest that you try pointing the top
Tube Trap seam of each column toward the back wall, while
leaving the seam of the bottom one pointing toward the listener
(for the short path vs. long path reasons discussed above).
The Tube Traps along the rear half of the side wall also
affect the perceived hall ambience, especially how large the
hall sounds from side to side; this is labelled as side ambience
in Figure 6.
The Tube Traps along the front half of the side wall affect
the perceived width of the stage, and also how seamlessly
continuous the solid curtain of music sounds across this entire
stage width. With speakers that image well, it should be easy
to have the perceived stage extend beyond the speaker width,
and even beyond your room side walls (because the absorption
of these Tube Traps can help these side walls to effectively
disappear for early reflections).
Listen carefully to all factors when doing this fine tuning.
Too little absorption along the front half of the side walls
will create discrete secondary sources at the side walls,
which might seem to enlarge stage width, but won't do so in
a seamless fashion, for there will be hot spots, with too
much musical energy seeming to be located inside the speakers
and right at the side walls; you want a continuous, seamless
curtain of music across the stage, with instruments located
at all points in a good orchestral recording.
The seamless stage curtain is also affected by fine tuning
the Tube Traps along the center part of the front wall. These
affect mostly the center fill of the stage, as shown in Figure
6. Rotating these Tube Traps with their seam toward the front
wall, so they are more reflective into the room, will solidify
the musicians located center stage (including center stage
rear), instead of leaving them sounding somewhat ghostlike.
It will also allow you to move the speakers farther apart,
to obtain better stage width without creating a hole in the
middle. Beware though of making the Tube Traps at the rear
of the speakers too reflective, for that will cause midrange
honk or glare, with loss of stage depth.
The Tube Traps in the front corners can be rotated to affect
the perceived depth of the far corners of the stage. Again,
be careful not to allow so much reflection that you pick up
midrange honk or glare from the room corners.
Throughout this exercise, the goal is to hear less of your
listening room walls, while hearing more of the recording
stage, hall walls, and hall space.
Finally, the pair of columns in between the speakers can
be fine tuned to affect the 3D projection of front center
stage soloists. This pair of columns is very sensitive to
the slightest change in rotational alignment and in location,
relative to the speakers and relative to each other. When
optimized just right, you can achieve a spooky magical quality
in which center soloists are not only tactilely solid, but
also are holographically projected as 3D entities in this
space between the speakers. You can also try extending this
line of columns toward the listener, with three or more columns.
Well, there you have it. I've helped you get the best sound
out of your room as much as I can -- short of personally setting
up and fine tuning your listening room with Tube Traps. Happy