Recording - October 2013 - 56

Peter D’Antonio
We do these tests all the time at the Cleveland Institute of Music, where I teach. The
effect is so audible! Somebody’s negotiating over a tweak of 1 dB at some frequency, and they’re ignoring the fact that...
...that the console’s completely screwing them up?
Right! [laughs]
So if you’re trying to control reflections, where do you stop doing harm and
start doing good? What’s the rule of thumb?
Thin foam panels absorb high frequencies, and because thin foam doesn’t have any
contribution down in the middle or low frequences it can do more harm than good.
Let’s say you’re trying to control reflections by putting a 1" absorber on the wall. What
happens then is, you’re introducing a high-frequency filter, because a thin absorber is
only going to affect the very high frequencies and not the low or mid frequencies.

Dr. D’Antonio measures a new
diffusor’s properties in the lab.
So you’re essentially eq’ing your room?
Precisely! It’s true, you won’t hear flutter echo any more, which people often equate with
good acoustics, but you’re basically putting an eq in your signal path that you can’t
unplug. So the rule of thumb is: whatever acoustical material you add, whether it be
absorption or diffusion, it should be at least 2" thick (and I say that graciously!) or more,
preferably 4". You want your absorber to behave like a fader, not an equalizer. People
trying to do everything with foam often have a very limited budget, which is not a crime,
but foam certainly isn’t a panacea! If you’re going to use it, use it properly. (See Figure 3.)

Diffusion
Is this frequency dependence a problem for your last type of treatment, the
diffusor?
As for diffusors, a properly designed diffusor’s effect doesn’t vary much depending on the frequency of the sound, nor by the angle at which sound hits it, nor by
the angle at which sound coming off of it is heard.
Which leads to your famous diffusor designs with the carefully calculated shapes...
That’s correct.
...or the poor man’s equivalent, the bookshelf on the rear wall, loaded with
books of varying sizes and shapes.
There’s nothing wrong with that!
Now, before we go on, let’s talk about that rear wall. Why is it so important? When
control room design went through its metamorphosis from reflective front/absorptive rear
to absorptive front/diffusive rear, our contribution was the initial idea of a diffusive rear.
If a rear wall’s not absorptive, what can it be? You definitely don’t want it to be
reflective! Those reflections will be fairly strong; they’ll compete with your direct
sound and compromise your spatial soundstage and spectral content.
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RECORDING October 2013

Consequently, the only other thing you can
do is to make the wall diffusive; when the
sound hits the rear wall, it’s not absorbed, it’s
being redistributed uniformly so that only a
fraction of the sound reaches back to the mix
position to compete with direct sound. It will
also scatter sound to hit the side walls, and
then comes back to the listener. This gives a
sense of “envelopment” that’s a really key
aspect of room sound.
Most of the early rooms lacked diffusion; we
developed the Reflection Phase Grating (RPG)
diffusor. In retrospect, treating the rear wall with
these RPG surfaces was so valuable because
they basically introduced ‘passive surround
sound’. We all know how much more enjoyable
sound is to listen to when you’re immersed in it,
as opposed to listening to a mono playback.

Figure 3. Sound absorption vs. the thickness of an
absorber. These numbers are for fiberglass, but you’ll
hear similar results for foam... as thickness increases,
so does effectiveness at lower frequencies.

We created the idea of a spatio-temporal
reflection-free zone in the front of the room;
after the direct sound there are no first-order
reflections (spatial) until you hear sound coming from the back wall (temporal), with a time
delay dependent on how far away the back
wall is. Then the rear diffusion creates a sense
of ambience and “surround sound” that’s passive, it doesn’t come from speakers.
That relies on a specific room shape and
design, as shown in the ray-tracing diagrams in Figure 4; sound that doesn’t come
directly from the speakers to the listener’s
ears is bounced around the mix position
into the back of the room, where it gets
diffused before it can return to the listener
and create this sense of ambience. But a
lot of it is the shape of the room, rather
than added treatment...
When we did Peter Gabriel’s studio with
Harris/Grant Associates, they requested that no
porous absorption be used at all. Everything had
to be created through geometry, in a fashion similar to what’s shown in the diagrams in Figure 4.
But if you don’t have the option of building
a room with a specific shape or knocking
holes in the walls to soffit your speakers,
can you also create a reflection-free zone
using absorption at the front of your room?

Recording - October 2013

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