written for the first Hong Hong HiFi Show, 1993
Translated and published in Chinese
by Arthur M. Noxon, PE, MSME, Physicist
The Chain is as Strong
as Its Weakest Link
The electronic components and interconnects available in
today's hi end audio are superb. Yet there is always room
for improvement and changes in both the technology and style
of audio. Playback audio will continue to be improved in every
detail but if the minute improvements are not perceptible,
they will not be heard and appreciated. New equipment that
presents no audible difference from older equipment will not
sell so easily and without good sales, further product development
cannot be supported. The evolution of the product line slows
to a halt and sales become driven by marketing hype, not performance.
The Listening Room
If a technical improvement is real it should be audible,
but if it cannot be heard it becomes useless, or at best a
curiosity. As long as an improvement is within the realm of
human perception, it can be heard except for only one factor
- the noise floor. Whenever improvements are buried in the
noise floor of a system they cannot be heard. Only when the
noise floor is lowered can the hidden, inner detail become
The electronic "hiss" noise floor had been a problem
in audio for many years. More recent improvements in signal
processing have driven the electronic noise floor down to
almost the threshold of hearing. The noise floor now is more
mechanical - transport decks, air conditioning and outside
noise intrusion. But there is one less obvious noise floor
and it is currently the single most significant sound barrier
to better listening. It is the self noise of the listening
room acoustic. Lingering sounds from one musical moment become
the masking noise floor for the next musical moment. The chain
is as strong as its weakest link and for today's audio, the
weakest link is the listening room acoustic.
There are two phases of listening room acoustics and both
must be understood and properly handled in order to reduce
the room acoustic noise floor. We begin by recalling the piano
keyboard. It is divided in half with notes belonging to the
bass cleft to the left and the treble cleft at the right.
For all practical purposes, it is sufficient to divide the
room acoustic noise floor into these same two regions, bass
When most people envision sound, they think of how the sound
in the treble cleft behaves. It bounces around the room as
if it were a ball and that is something with which we all
some experience. It is because we are familiar with bouncing
balls that we have an intuitive feeling for sound in the treble
cleft. The bass cleft is quite different. With bass, sound
doesn't beam forward and bounce around, bass goes in all directions
equally - as easily backwards as forwards. Place a speaker
outside, in the yard and play it while you stand behind it.
What do you hear? Bass without treble. Despite this most obvious
fact, the behavior of bass continues to elude people. Most
of us do not have experiences that lead us to an intuitive
understanding for sound in the bass range.
Old Wives' Tales
The first idea the hifi person usually has about bass these
days is the idea of room modes or room resonances. There have
been articles about this in the press. The basic expectation
is if the room has the correct ratio it will sound great.
Another popular idea is to build a room that has no parallel
walls, even the ceiling could be at an angle. This is supposed
to eliminate standing waves. Another often heard notion is
that of killing resonance by adding bass traps or an equalizer.
Amazingly, all of these too common ideas about bass and rooms
are simply incorrect. Not one thing about these notions is
true, but the people are not at fault. They have been taught
these ideas by the sci fi writers of the hifi press whose
imagination exceeds their training or experience in acoustics.
The search for the perfect room dimension is a very old one
and it is based on the good idea of evenly distributing the
resonances. This means each room resonance is as evenly spaced
away from adjacent resonant frequencies as possible. Originally
this concept belonged to the art of building reverberation
chambers. Reverb chambers are used to test acoustic properties
of materials and they need to have evenly spaced resonances
to give good results. Reverb chambers have their speaker located
in one of the tri-corners (floor/wall/wall) and the microphone
in the opposite tri-corner (ceiling/wall/wall). Every room
mode can be stimulated by the speaker in the tri-corner and
the mic will hear every mode loudly. This very unique property
only belongs to the corners of the rooms.
There are certain room ratios that give evenly distributed
resonant frequencies. However, the hifi listening room is
not an acoustic test chamber. Hifi speakers are almost never
sitting in a corner except for Klipsch and the listeners are
surely never sitting in a tri-corners of the room. In a hifi
room the speaker is located away from the corner and from
there it cannot stimulate all the room resonances. Which room
resonances are stimulated depends on where the speaker is
located in the room. Only modes stimulated by the speaker
need to be well spaced apart. There is absolutely zero guarantee
that a room with reverb chamber quality well spaced modes
will also have well spaced modes when the speaker is moved
out of the comer.
The non-parallel wall is another magic carpet concept in
audio. It is true that non-parallel walls keep the flutter
echo down but that is a treble effect. Sound is energy. Put
this energy into a room and it stays there until one of two
things happen. Sound can be absorbed by bass traps and wall
friction or it can be leaked out of the room. Sound energy
is stored in the room in either of two forms, organized or
disorganized. Disorganized sound dies out faster in rooms
than does organized sound. Organized storage of acoustic energy
is a room resonance. Because the room resonance is so efficiently
stored, it is the sound in the room we mostly hear. It causes
"room boom" and we wish it would go away. No matter
what shape the room has, the room still has room resonances.
If a room has walls it has resonances. The only effect the
shape of a room has on resonance is which frequency it is
and how the resonance is laid out in the room. Resonance is
not eliminated by changing the angle of walls.
An equalizer is irrelevant to room resonances. What is more
is that they are almost never used in high end audio. A parametric
equalizer changes the loudness of sound at a specific frequency.
If a room resonance is "equalized", then what used
to sound loud now sounds normal but the room is physically
the same. The long decay of the resonance still fills the
room - it is just not as loud.
Bass Traps actually improve bass in rooms. Designer built,
pro sound recording studios have the bass traps usually built
into the walls and they are not usually visible. Only in hifi
and the smaller project or midi studios are bass traps being
set up in front of the existing walls and corners of the room.
Here, bass traps have become quite visible. Regardless of
their location, bass traps can never "kill" room
resonances. Properly designed bass traps do not weaken bass
in the room. They actually increase the bass power delivered
to the listener. Bass traps do not effect the direct wave
from the speaker. They do reduce the strength of the room
resonances that interfere with the direct wave. Bass traps
reduce the sound of the room and let you hear more of the
sound from the speaker.
Rooms have resonances because rooms have walls, floor and
a ceiling. This is a fact of nature. Bass traps do change
the sharpness or "Q" of the room resonance, no different
than adding resistance to a resonant electrical circuit. No
matter what the resonance, there will always be an intense
sound pressure in the corners of the room. That is why pressure
zone bass traps loaded into the corners of the room are so
effective. By converfing 1% of your listening room volume
into highly efficient corner loaded bass traps, an amazing
conversion, a stability in the room acoustic sets in. Bass
trap all four vertical corners and the ceiling perimeter comer
with a soffit bass trap. This is the best way to control the
behavior of bass in the room and causes as little visual impact
Dimensions and Floorplans
Rooms are being built exclusively for listening and home
theatre more often in recent years. People always ask what
are the best dimensions. It is better if the room is designed
to fit the sound system, letting the dimensions fall naturally
into place. Speakers are usually 7 to 8 feet apart and the
listeners in biff like to be 8 to 12 feet away from the speakers.
Speakers should be about 3 to 4 feet off the sidewalls and
4 to 6 feet off the front wall. The listener is best located
3 to 5 feet off the back wall. This puts room dimensions to
be 13 to 16 feet wide and 15 to 23 feet long. If ratios are
desired, any combination of 7, 9 and 13 works well, such as
7 to 9 or 7 to 13 or 9 to 13. Standard cone drivers should
never be located 25% of the width away from the side walls.
The better dimension is 29 to 32% of the width from the side
walls. The distance from the cone to the rear wall is best
set within 10% at 1.4 or 2.4 times the distance from the cone
to the side wall.
With dipoles and towers there is a strong need for a good
length/width ratio and less of an issue exists about the height
ratio of the room. Tall speakers stimulate strongly the front
to back and side to side modes of the room, but not too much
in the vertical direction because the speaker drivers line
up most of the room's vertical height. If ratios are preferred
only the width/height ratio is important. The speakers create
ringing in the front of the room, "head end ringing".
The 13/7 ratio is usually best for height but the other ratios
can be used. Room height can vary between 7 feet and 9 feet
but heights over 10 feet should be avoided except with tall
The higher the ceiling, the lower the floor to ceiling resonant
frequency. An 11 or 12 foot ceiling has a fundamental resonance
of about 50 Hz and is easily stimulated because most bass
drivers and especially subwoofers are located near the floor.
Probably the worst ceiling to use is a peak or vaulted ceiling.
The peaks reach substantial heights and their megaphone effect
is not fun to listen to. If angled ceilings are used, they
should be low over the speakers and open up behind the listener.
Another listening room floor plan is coming into vogue. It
is the short, wide room. These rooms can be 15 or 16 feet
deep and 24 or more feet wide and the listener is against
the back wall. The room is so wide the bass build up at the
back wall is not as bad as one would expect. The two wings
become ambience storage chambers and the sound has a strong
image presence with a strong lateral ambience. This, like
all listening rooms, needs 1% volume dedicated to bass traps.
The width/depth ratio of 3/1 must be avoided by 15%. The speaker
separation must be 15% more or less than 1/3 the width and
definitely not 50% of the width.
Another listening room arrangement that some people try to
work with is to set the axis of the system down a diagonal
of a square room. Huge bass traps are needed in the corner
between the speakers and the corner behind the listener. The
walls behind the listener should be pretty absorptive to keep
imaging clear. This room can be frustrating to set up but
it seems to have some ambience and stage depth properties
that keep people trying to make it work.
When dedicated rooms for hifi are not available then a portion
of the house is used for playback. The first rule for set
up, above all others is acoustic symmetry. Without acoustic
symmetry the imaging and fidelity will be highly distorted.
Probably the most common asymmetric room is an "L"
shaped listening room. One speaker has its back to the corner
and the other speaker is against the wall in the center of
the room. The corner loaded speaker requires heavy use of
bass traps to balance out the bottom end so both speakers
begin to sound the same. Sometimes furniture can be rearranged
to create reflections that mimic the presence of a wall for
the speaker that is out in the open. A cabinet can be moved
in against the back wall, just to the outside of the speaker
to simulate a wall corner. A cleverly positioned lamp shade
can reflect the treble. Work to get symmetry in the acoustic
Absolutely the worst bass performance occurs when there is
a room next to the speakers and the doorway is open. This
coupled resonant cavity puts such a "double kick"
on the bass transients it is nearly unlistenable. Close the
room up or at least put heavy drapery to lower the strength
of the report from the coupled cavity. Opening doors to adjacent
rooms behind the listener can add to low frequency ambience
which can be desirable.
When building or remodeling a listening room, the walls,
floor and ceiling materials become particularly important.
Today's audio gear is powerful and can deliver real punches
to the surfaces of the room. Although acoustic pressure maybe
quite small in a mechanical sense, when spread out over the
surface of a wall, real force can be developed. At the very
least the head end (speaker end) of the room needs to be mechanically
inert. That means when you thump the walls or ceiling with
the heel of your fist, not much happens. Concrete is inert
but many homes are built out of lighter weight materials such
as wood studs and sheet rock. The wood stud acts like a spring
and the sheet rock acts like a weight connected to the spring.
The result is "wall/stud resonance", a sympathetic
resonance of the walls at about 70 Hz. This drumhead effect
can drive anybody in audio completely mad. Ceiling hop occurs
at lower frequencies for the same reasons. Good audio rooms
will have all surfaces mechanically inert.
The wall cavity can be filled with insulation. It is always
a good idea but doesn't help the wall/stud resonance. Some
people wish to pour sand into their walls to add mass and
establish a damping action inside the wall. The big problem
with this is that sand settles, gradually increasing pressure
on the lower wall until the sheetrock wall bulges, wedged
apart by compacted sand. The best method is to use constrained
layer damping in the wall construction while suspending the
sheetrock off the studs. There is an item used to increase
the isolation between walls of apartments. It is a resilient
metal fir strip often called "Z-metal" "RC-1".
This is installed onto the studs then the sheetrock is screwed
to the it. The result is that the spring of the studs become
disconnected from the mass of the wall. Now there is no spring/mass
to vibrate and the 70 Hz wall boom problem is gone.
However, a new problem will have developed. We now have a
large freely suspended sheet of gypsum board, a wall that
can make quite a sound of its own. If visco damping material
is applied between the Z-metal and the sheetrock, damping
of the wall panel will occur. The next improvement is add
a second layer of sheetrock with visco elastic material sandwiched
between the layers. The wall really becomes inert. The ceiling
can be treated exactly the same way. An alternative to the
thin 1/16" thick double sided adhesive visco-elastic
sheet, would be to 100% glue both sides a layer of sound board
(firtex or celotex) between the two layers of sheetrock. Constrained
layer damping means extra labor and cost but the unmuddled
sound in the room is well worth the extra effort.
Once the surface of the room has been rendered inert and
the strip type corner loaded bass traps have been installed
in the four vertical corners and the ceiling perimeter corners,
the listening room is ready for occupancy and detailing. It
is also at this point that a number of regrets become realized.
The size and placement of windows and doors can cause untold
grief if they were planned by the architectural standards
that apply to residential rooms and not hi end audio playback
Doors should be located on the side walls and definitely
nowhere near the speakers because they rattle. Locate doors
behind the listener but not flush to the corner. At least
2 feet of solid corner wall should always be maintained. A
door on the back wall is a lessor choice due to the intense
bass pressures on the back wall. The back wall door should
not be located in the center or in either corner. It should
begin about 2 feet from a corner. One of the best set ups
possible is to have open french doors or archway directly
behind the listener. This is a classic set up. It eliminates
problems and adds benefits. Adjust heavy drapery for best
Windows are another item that commonly occupy a position
in a room. Large windows are to be avoided, they are too tympanic.
Tall, narrow windows are the choice. Use laminated glass,
the type used for glass shelving in stores. Windows are best
located behind the speakers on the side walls but not close
to the corners. They can also be located to either side of
the listener, preferably slightly behind the listener, but
slightly in front of the listener is okay too. One of the
biggest mistakes is thinking a thermal window is good for
acoustics - it isn't. It's terrible. The glass is very thin
and the air space is negligibly small for acoustic purposes.
Standard sliding glass doors are never to be used in the hi
end room as they resound with tympanic thunder.
Windows allow distracting outside noise into the room as
well as let sound leak out. To minimize sound transmission
problems, use a double window. The two sheets of glass should
be separated at least by 4 inches of air space. The glass
sheets ought to be of different thickness. The space between
the glass is to be vented into the wall cavity. Frame work
and trim molding need to be sealed with expandable foam and
acoustical caulking. Set all glass into a bed of visco-elastic
Speakers don't play by themselves, they require equipment
to drive them. Power amps are best left near the speakers
with short connecting cables. The preamp and all other equipment
is best positioned slightly behind the listener, against the
wall. Some people are building equipment closets, much like
the equipment rooms of recording studios. A glass door and
remote control work well together.
There is a tradition in mid fi that tends to creep into the
hi end. The electronics are piled into a custom built wood
and glass equipment cabinets centrally located on the wall,
directly behind the speakers. All cabinets, or shelving in
hi end audio must be acoustically porous, not reflecting.
Any kind of cabinetry or equipment stacked up between the
speakers locates it directly in the middle of the sound stage.
The last thing any of us really want is their sound stage
filled up with sound reflecting equipment, blinking lights,
and hollow resonant boxes. The sound stage needs to be absolutely
free from visual or acoustic distraction. It needs to be an
acoustically quiet backdrop upon which sub fie images play
out their action.
Lighting in the playback room needs to be versatile. Bright
lights are needed during set up, adjustments or upgrade installs.
But for playback and imaging, it is best if there are no strong
visual distractions in the front of the room. We want our
eyes to be relaxed, wide open but without anything to catch
our visual attention. Visual activity competes with acoustic
imaging. It is best if the front of the room appears to be
nothing more than vague visual shadows in a dim grey fog.
The lights for playback are best located behind the listener
and they should be diffuse. Hollow can lighting is attractive
but they make a lot of noise. You will hear them in a good
room. You can also hear singing metal waste paperbaskets in
a rood room, (use wicker). Best light has a ceiling bezel
and lens of thick slightly rounded glass. A dimmer is appropriate
but many of the electronic dimmers create a buzz of the filaments
when dimmed down and we prefer to avoid introducing noise
into a playback room. A variable transformer voltage control
for lights does not produce noise. Low voltage lighting should
be considered. There may be electronic dimmers that are quiet
but be sure to test them before installing them.
Wall reflections and ceiling reflections obscure imaging
detail, stage depth, smooth lateral positioning and musical
accuracy. This is because the early, first few reflections
are still strong and arrive within the sound fusion time period.
Our listening process correlates all the early reflections
that arrive within 20 milliseconds of the direct signal. What
we think we hear is really the sum of the direct and early
reflected signals. This is primarily a treble range effect.
The single worst reflection is the crosstalk reflection. This
occurs for example when the left speaker plays off the right
wall and we hear the reflection in our right ear. Just after
our right ear had received a signal from the right speaker
it is fed another signal from the left speaker. The result
is essentially the loss of stereo and we recognize it because
of the "hole in the middle" soundstage. If there
is but only one reflection to control, it is the crosstalk
Placing absorption at the reflection point will reduce the
problem of reflections. The question becomes, where is the
reflection point? The crosstalk reflection is on either side
wall, at ear level and located about 1 1/2 feet behind, towards
the listener from the halfway point between the speaker and
A very accurate and easy method to locate the reflection
point does exist. It is an optical alignment technique known
as "ray tracing" and it is learned by all students
of acoustics. Tape up a strip of mylar reflecting plastic
sheet on the wall or have someone hold a mirror fiat against
the wall. Use a strong, narrow beam flashlight and hold it
near your head and shine the light on the mirror. Observe
where the light lands. Adjust the mirror and light beam until
the light lands on the midrange speakers and tweeters. Mark
the spot on the wall where the reflection occurs because that
is where your wall acoustic treatment will be placed.
The wall absorption should be effective at least through
200 Hz in order to cover the treble cleft. There is one problem
with simple, commodity type absorption. They usually do not
absorb in the lower treble octave and even worse these office
panels, which are fabric covered noise control panels or blocks
of corrugated foam, leave a "black hole" in your
acoustic periphery. This is very distracting for those of
us who worked hard to develop a sensitivity for sound and
imaging. Wall absorbers do need to absorb the early reflection
but they can also be fit with reflectors. Sound that bounces
off the rear wall, behind the listener will graze back up
the side walls, impact the reflectors and backscatter towards
The best of both worlds is now attained. Early reflection
absorption is followed by time delayed, low level ambient
reflections and all from the same spot on the wall. This technique
of using time delayed backscatter sound absorption produces
the very desirable effect of lateral ambience recovery. It
is used in properly designed recording studios and it can
also be set up at home. The important feature in hi end sound
absorbers is the presence of some sort of reflective quality.
It may seem like a contradiction of terms but it is really
a matter of refined taste.
There is another reflection in the room that is particularly
hard on the listener. It is the rear wall bounce. Sound passes
by the listener only to hit the rear wall and bounce back
past the listener again. If the positive phase of a sound
wave is bounding back just when the negative phase of the
wave passes the listener - they cancel. This occurs when the
round trip distance between the listener and the back wall
equals 1/2 wavelength. For example, sitting 4 feet from the
wall means cancellation occurs for a 16 foot wavelength, a
70 Hz frequency. This is not a room resonant problem. The
low frequency "cold spot" depends only on the distance
the listener is from the rear wall. Bass traps behind the
listener against the wall must be used to control this problem.
Remember, treble range reflectors across the face of any bass
trap keeps the ambience from feeling dead. The goal in hi
end acoustics is of course acoustic control, but control done
in such a tasteful way that you can never hear where the control
The Last Link
By following the above details, the perfect room will still
not be realized but the room you have will be so good that
you will be very happy with it for a long time. You will recognize
it is a good room because the speakers will seem to completely
disappear. They become silent projectors of an acoustic holograph
that appears in the front of your room.
Just remember, acoustic modifications, detailing and upgrades
are every bit a part of today's hi end audio system as the
speaker. Modern hi end audio electronics are of such quality
that the room acoustic simply has to be improved in order
to even begin to hear the hi end part of hi end audio. The
audio chain can be only as strong as its weakest link, which
for now is the playback listening room.