Robert Harley moderates a discussion on room acoustics, equalization,
and DSP-based room correction with
TAS Senior Writer Robert E. Greene, Peter Lyngdorf of TacT
Audio,
and Art Noxon of Acoustic Sciences Corporation.
REPRINTED FROM tHE ABSOLUTE SOUND MAGAZINE, OCTOBER/NOVEMBER,
2004
Robert Harley:
I think everyone would agree that room acoustics dramatically
affect the quality of reproduced sound. Given how important
room acoustics are to sound quality, why have acoustics been
generally overlooked by audiophiles, when they’ll worry
about things like the purity of the rhodium plating on binding
posts?
Art Noxon: Air is free. Sound
is free. Once generated, sound is free—from the speaker
to your ears. You can’t tax it or insure it. It’s
taken for granted, and there are no bells and whistles, and
you can’t plug it into the wall like you can a piece
of equipment.
RH: You seem to be saying that
because air moving in a room is intangible, it’s overlooked.
AN:
Absolutely. It’s something that’s taken for granted
and it’s assumed everything is fine, unless you stop
to think about it. Or if you’ve got great components
that don’t sound great and you’ve exhausted all
other options, then you may then consider that the room is
at fault.
Robert E. Greene: I think another factor is that people don’t
realize how badly the room is behaving. They buy all the nice
equipment, set it up, and start to listen. They hear some
problems, especially in the lower frequencies, say, below
500Hz where the room really begins to have a dramatic effect,
but they assume that’s okay. I’ve had the experience
over and over of going to someone’s house with some
measuring equipment and showing them that there’s a
giant hole at 200Hz and a big boom at 80Hz, and they’re
shocked. They’ve just been assuming that was what the
recordings sounded like.
RH: They don’t have a
reference.
REG: They don’t know
that the distorted tonal balance—mainly too much bass—isn’t
in the recordings. I think it’s one reason why audiophiles
are obsessed with female vocals. It’s because they know
what the human voice sounds like in that range. That range
is not terribly affected by the room.
AN: One of the easiest ways
to hear the room’s effect is to compare the sound of
speakers in a room with a good set of headphones. Just play
anything over headphones and listen to the musical clarity.
Forget about imaging. Just listen to the musicality of what
you’re hearing over quality headphones. That’s
an easy, cheap way to get in contact with the phenomenon we’re
talking about.
RH: Peter, you have extensive
experience with demonstrating DSP room-correction systems,
showing people at hi-fi shows what music can sound like with
the room-effects removed.
Peter Lyngdorf: I certainly have, and I’ve set up in
a huge number of rooms over the years. The fact of the matter
is that below roughly 300 or 400Hz, the room is the overwhelming
factor in reproduced sound, and below 300Hz is where you have
about 90 percent of the energy in an average piece of music.
Below middle C on a piano, the sound is totally controlled
by the room.
Most of the tonality of music is destroyed by
an average room, and it is not only the tonality but the delays.
Every time you have a resonance in the room, you also have
a delay, which means that every time you extend the frequency
response downwards with larger speakers, you will hit lower
and lower resonances. Every one of those resonances is accompanied
by a delay. If you have a resonance at 30Hz of 10dB, you are
actually accumulating ten times the energy at that frequency,
which means that the average delay at 30Hz then would be about
0.6 seconds. Consequently, the energy at that frequency comes
almost at the next beat of the music.
So we have two issues: one is the tonality,
the other is the timing. And timing is totally out the window
if you do not have your room/speaker interface under control,
and I believe that’s one of the reasons that most musicians
seriously hate hi-fi. The music is so totally out of beat
that they can’t stand it. And the bigger the speaker
and the more powerful the system, the more they hate it.
Another thing which is quite interesting and
actually quite funny is that a lot of manufacturers are consistently
saying that their equipment needs to break in for two weeks
or three weeks or four weeks, and I believe that is almost
entirely nonsense, because very few products change dramatically
over a few weeks. I think what they wait for is that the customer
runs through all of his CDs one more time and finds another
two or three tracks that sound pretty good in his room.
RH: On the face of it, equalizers
appear to be a tempting solution to fixing these problems,
but audiophiles have abandoned them. Is equalization fundamentally
flawed, or has the problem been in the execution of the products?
REG:
Everybody in the consumer world has got it in his head that
equalization adds phase shift, but, of course, it’s
really just the other way around. If you equalize the resonances
out of a room, the correct timing is restored. The resonances
themselves are generating phase shift. The resonances are
technically known as “minimum phase.” When you
take away the resonance with equalization, the timing—the
phase—also corrects itself.
AN: Equalization has disappeared
from high-end audio. All the image-distorting widgets, which
include equalizers, were taken out of high-end systems, just
about the same time that the [Dahlquist] DQ-10 appeared and
speaker designers realized that they had to get their speakers
phase-aligned. I’m surprised to see equalization back
in high-end audio—if it is back at all.
REG: I don’t actually
agree with that, at least in the bass. I think the phase shifts
in the bass are generated by the room and eliminated by the
EQ.
PL: Well, my view is that equalizers
work, but it’s important to have a good room to start
with. So I don’t think we are in total disagreement
with Art. It’s just that at the lower end of the frequency
response, where you have the most amount of energy, it’s
extremely difficult to remove those big fundamental resonances
by acoustic means alone.
REG: Even just a few adjustable
parametric equalizer coefficients will enable you to get control
of the room’s major disasters. It’s a little on
the crude side compared to the automated DSP systems, which
as you know have scores or sometimes hundreds of parametric
coefficients. But even analog equalizers with adjustable center
frequencies let you attenuate those two or three discrete
frequencies that are going “boom-boom.”
RH: That leads me to the next
topic, DSP room correction. Is DSP room correction a panacea
that solves the problems we’ve been talking about?
REG: My experience with the
DSP devices is that you can do amazingly effective things
with them. When set up right, it’s amazing how well
they work.
RH: And what’s also amazing
is how dramatically different the sound is, when those resonances
are removed. It’s not a subtle effect.
PL:
It’s certainly something that anyone can hear. But some
audiophiles are scared about this kind of equalization because
it implies that you’re shifting bits and doing all kinds
of things to the signal, when everybody has been trying to
keep the signal as pure as possible. What a lot of audiophiles
still do not realize is that with digital technology you can
do an awful lot that doesn’t really change the musical
information, add distortion, or create noise as the old EQs
did. Room correction can precisely hit the room problems without
adding distortion or noise. That, I think, is why the early
equalizers did not work—they could never exactly hit
the problem.
RH: Peter raised an interesting
point about audiophiles having an aversion to signal processing.
In the digital age, do we still want to keep the signal path
as simple as possible, or does digital technology open up
new frontiers that weren’t available in analog audio?
[This topic will be the subject of a future TAS Roundtable-RH]
PL: The goal here is to make
a piano sound like a piano and a cello like a cello, with
all the oomph and body you get from the real instrument. I’ve
heard so many high-end audiophile systems where the midbass
is simply removed. And then a lot of audiophiles are clapping
their hands because it is oh-sodetailed, and the female voice
is beautiful, and it’s clean. The only problem is that
the instruments don’t sound like actual instruments
anymore. And that’s where the combination of sensible
room treatment with sophisticated room correction can make
the instruments sound like the real things. A piano is one
of the most difficult things to reproduce, and with the proper
EQ and proper setup of speakers you can really make a piano
sound like it is in front of you. And I don’t think
that is possible unless you use both a sensible room treatment
and very sophisticated DSP equalization.
RH: So we still need acoustic
treatments and correct loudspeaker placement—DSP room
correction isn’t the magic bullet.
PL: If you have the chance
to separate your low-frequency devices from your mid/high-frequency
devices [i.e., a subwoofer with small loudspeakers], you should
place your mid/high frequency devices where they give you
the best possible imaging and the bass units all the way into
the corner of the room. If the bass units are in the corner
of the room the whole initial sound from the woofer is minimum
phase. It is all going in the right direction at once. Whereas
if you try to reproduce the low frequencies from your main
speaker standing freely in the room, then much of the bass
will go back into the corner of the room, reverse, and then
come dripping back to you in the next 4, 5, 6, 10 milliseconds.
AN: That was the phenomenon
the TubeTrap was designed to fix back in ’83. It’s
a bass trap placed in the corners behind the speaker with
a treble-range diffuser that faces forward into the room.
There was no such thing as a hi-fi subwoofer back then. With
full-range speakers, the wavefront would expand back into
the corner and fold back out. Trapping that bass with a TubeTrap
put this company on the map and has kept us alive for twenty
years so far. Our approach is to absorb that bass so it doesn’t
get reflected back into the room, and to backscatter the mids
and highs with a polycylindrical diffuser. That rear bass
energy you’re talking about is the same thing we’ve
been addressing with Tube Traps for more than two decades.
REG: I’d like to introduce
a slightly heretical thought here. If you have a resonance
which you then remove by precise equalization, you’ve
actually made your speaker in some sense happier. It does
not have to work nearly as hard. The frequency range that
had a 10dB boom has now become a range where basically the
speaker can produce the required level with 10dB less output.
Corner placement is actually nice for speakers because it
insures maximum coupling to the room and minimum power input
required to generate the bass you want. But, of course, you
have to add some time delay if you’re going to put the
woofers in the corners.
AN: When you build a reverb
chamber for acoustic testing, the classic speaker position
to stimulate all the room’s resonant modes is in the
tri-corner [the point where two walls and the ceiling or floor
intersect]. This is a room made from concrete two-feet thick
coated by two inches of polished marble, and that has a reverberation
time of fifteen seconds. The classic position for the measurement
microphone to pick up all those resonances is also in a tri-corner.
The goal of speaker placement is to avoid stimulating
room resonances, so I’m pointing out an inconsistency
here with regard to corner placement of a subwoofer. The typical
position to avoid stimulating resonances is 29 percent of
the room’s dimension off the floor, 29 percent in from
the sidewall, and 29 percent in from the end wall. That is
the most neutral position possible. I was recently reading
a 1974 AES paper where the fellow produced a very smooth response
in the room simply by moving the subwoofer around.
But now we have the opposite proposal of putting
the subwoofer in the corner. It may have the benefits discussed,
but it stimulates all the resonances. We have two diametrically
opposed ideas. One is to put it at the minimum resonancestimulating
location in the room and achieve fairly flat response, and
the other is to put it in the corner and apply DSP room correction
to correct for all the distortions that are introduced by
stimulating the room modes. I think it’s important that
we be clear about these opposing perspectives.
PL: I agree they are opposing,
but try to look at it this way: corner placement of the woofer
will give you more total energy. Why will it give more total
energy with the same amount of excursion from the cone? Simply
because less of the energy generated by the speaker will be
canceled by out-of-phase components interfering with in-phase
components. So once you do precision equalization, you will
get better timing of the signal. Really, you can say that
the more SPL [sound-pressure level] you get at the listening
position from a certain placement of the woofer system the
less the signal you are receiving is out of phase and canceling.
Of course, in most cases you end up getting too much energy,
but that is so easy to take away with good DSP room correction.
So that’s my very simple argument for corner placement.
AN: If you have no acoustic
treatments in the room and you have a DSP processor, what
happens to the articulation in the room? You’re still
injecting energy into the room. You still have reverberation
time. You still have the lack of intelligibility that you
had before. You still have bass energy circulating in the
room. People confuse DSP with intelligibility. Intelligibility
is the ability of a room to rapidly respond to the dynamic
changes of sound. We want a fast build-up and a fast decay.
DSP doesn’t address the decay rate factor of rooms,
and neither does equalization. There is no electronic sound
absorber for sale.
REG: None of us wants to try
to correct your marble reverberation chamber with DSP—in
the mids and highs. But in the bass, room ringing really can
be cancelled out, at least for one listening position. The
adapted DSP filter rings, too, but in reverse phase to the
room ringing, so the combined result is that ringing is gone.
RH: I’d like each of
you to comment on what you think is the single most important
thing that audiophiles can do to improve the sound of their
rooms.
REG: My feeling is damping
the first reflections off the ceiling, floor, and sidewalls
makes a fantastic difference. I have a preference for speakers
with narrow radiation pattern, but that’s a long, complicated
subject. Most audio systems have way too much energy in the
mid and higher frequencies bouncing around the room and not
nearly enough of the first arrival. The first arrival is,
among other things, where the imaging information is.
AN: We do rooms all day, every
day, all over the world, for twenty years, and the first reflections
are not what we fix first. The first thing we attack is something
we call “head-end ringing.” We shorten the vertical
and lateral reverberation time in the bass by treating the
space to the side of, and behind, the speakers. It’s
one of our trade secrets. By cleaning up head-end ringing,
we dramatically expose low-level detail in the midrange and
treble. I agree that absorbing first reflections improves
imaging, but without treating head-end ringing, there’s
this slug of vertical and lateral shaking of air that oozes
past the listener about a twentieth of a second after the
direct signal and blurs musical detail and imaging. After
we’ve addressed head-end ringing, then we control first
reflections.
PL:
I’ve done a lot of testing on the effects of reflections
in rooms, and there was a big, big project in Denmark about
twelve years ago, with a lot of companies involved in investigating
effects of reflections in rooms. I had the pleasure of being
a test person, where we could actually simulate the audible
effect of the floor reflection, sidewall reflection, ceiling
reflection, and so on independently. The single most disturbing
reflection in the room is the floor reflection. That is what
makes the speaker sound like a radio and not like the actual
event. The second worse reflection is the ceiling reflection.
Sidewall reflections, if they are sufficiently delayed (more
than about five milliseconds) and are left/right symmetrical,
can be actually beneficial to the sound. But if your speakers
are very close to the sidewalls, you have to kill the side
reflections. But do not be too concerned about the sidewall
reflections. The floor reflection absolutely must be handled,
followed by the ceiling reflection, either by absorption or
diffusion.
AN: Well, I’ve been there,
and diffusion blurs imaging. Ceiling diffusers blur imaging,
and they add an artifact because they’re a bunch of
resonators. All of our ceiling traps have reflectors in them,
but the reflectors are always offset and point to the back
of the room. Diffusers are energy storage devices, are tonal
in nature, and create incoherent reflections, which mask the
perception of coherent reflections.
PL: I’m sorry. I didn’t
express myself properly. I was not thinking about diffusers
as the devices you can buy. I’m thinking something on
the ceiling that will prevent the first reflection from reaching
you directly.
RH: I’d like to conclude
by adding my own comment about the single most effective technique
for improving the sound of your room: loudspeaker placement.
Through loudspeaker placement you can control the amount of
bass, overall tonal balance, image specificity, and soundstage
width. Speaker placement has its limitations, and the sound
will still be greatly influenced by the room, but I suspect
that most readers’ systems could be improved by better
speaker placement. ¦
TAS
ROUNDTABLE
AN:
Absolutely. It’s something that’s taken for granted
and it’s assumed everything is fine, unless you stop
to think about it. Or if you’ve got great components
that don’t sound great and you’ve exhausted all
other options, then you may then consider that the room is
at fault.
REG:
Everybody in the consumer world has got it in his head that
equalization adds phase shift, but, of course, it’s
really just the other way around. If you equalize the resonances
out of a room, the correct timing is restored. The resonances
themselves are generating phase shift. The resonances are
technically known as “minimum phase.” When you
take away the resonance with equalization, the timing—the
phase—also corrects itself.
PL:
It’s certainly something that anyone can hear. But some
audiophiles are scared about this kind of equalization because
it implies that you’re shifting bits and doing all kinds
of things to the signal, when everybody has been trying to
keep the signal as pure as possible. What a lot of audiophiles
still do not realize is that with digital technology you can
do an awful lot that doesn’t really change the musical
information, add distortion, or create noise as the old EQs
did. Room correction can precisely hit the room problems without
adding distortion or noise. That, I think, is why the early
equalizers did not work—they could never exactly hit
the problem.
PL:
I’ve done a lot of testing on the effects of reflections
in rooms, and there was a big, big project in Denmark about
twelve years ago, with a lot of companies involved in investigating
effects of reflections in rooms. I had the pleasure of being
a test person, where we could actually simulate the audible
effect of the floor reflection, sidewall reflection, ceiling
reflection, and so on independently. The single most disturbing
reflection in the room is the floor reflection. That is what
makes the speaker sound like a radio and not like the actual
event. The second worse reflection is the ceiling reflection.
Sidewall reflections, if they are sufficiently delayed (more
than about five milliseconds) and are left/right symmetrical,
can be actually beneficial to the sound. But if your speakers
are very close to the sidewalls, you have to kill the side
reflections. But do not be too concerned about the sidewall
reflections. The floor reflection absolutely must be handled,
followed by the ceiling reflection, either by absorption or
diffusion.