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.
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
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?
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
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.
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
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
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
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
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
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. ¦