Recording - December 2013 - 56

By Bruce Black
The importance of having neutral acoustics in a sonically sensitive
space is simple-it is important to hear exactly what is in the track, so
that when it plays back in the rest of the world it matches as closely
as possible to what you intended it to be.
With the advent of the Internet, music projects can be very collaborative. In post production sound-where much of the work is also
done in small, often repurposed rooms-projects are even more collaborative, with lots of different people working in lots of different
rooms. The variety of rooms a single project may be played in ranges
from small edit suites to large mix stages. The acoustic signature of
each room affects the creative choices being made, so it is important
that each decision maker in each room is hearing the same thing, as
closely as is possible.
This can be accomplished by making each room as neutral as possible, where what you hear is what is on the tracks and with little or
no change to the sound caused by the room acoustics. A difficult goal
to reach, to be sure, but very important indeed to the quality of the
end product and the level of confidence people have in the facility.
Now, nearly all of us build a studio by repurposing an existing
room. It most often is a spare bedroom, but it could also be a basement, the attic, or perhaps even a small closet. Regardless, we're
stuck with whatever characteristics were built into the room when it
was constructed. While not literally cast in stone, it's not likely that
they'll be changed-altering the physical space is expensive. So the
room is what it is.
Because the low frequencies heavily influence a room's "sound", it
is important to pay attention to them acoustically. Nearly all the
room's characteristics affect what happens in the low end-the dimensions, the construction materials, and even how airtight the room is.
Given that you can't rebuild the room or start from scratch, and yet
its characteristics affect what we hear in the low end, what are our
options to work toward neutral acoustics?
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RECORDING December 2013

Opening the bag of tricks
One popular fix is to use room equalization. This is a
poor choice, and here's why.
Every enclosed space has resonances whose frequencies correspond to the room dimensions. These are
also known as standing waves, modes, eigentones, and
the like. In different positions in the room, the peaks and
dips in the level of each of the resonances will be different, and the way they interact will change. So the
sonic problems change depending on where you're
standing. A good example of this is the increase in
boominess that happens as you move closer to a wall.
Equalization changes the electrical signal exciting the
room. Regardless of where you're listening, these changes
remain the same. But the way the room's characteristics
change the acoustic signature is different depending on
where you are listening. In other words, room EQ only
makes the proper corrections for one listening position. As
an example, if you were to EQ a room with your measurement mic close to a wall, other positions in the room will likely sound deficient in the low end, because bass frequencies
become more prominent as you get closer to room surfaces.
In the final analysis, room EQ is good to flatten a speaker's
response and to make fine tweaks to the room's response,
but not to "make a room sound more neutral".
So while equalization will change things, you're
applying corrections to the wrong thing-the electrical
signal. The sonic issues caused by resonances, vibrating room surfaces, and even little holes in the wall are
problems with the room acoustics, not with the signal
being played. Sometimes the innocuous, simple things
have more of an effect on a room's acoustic signature
than we might think.
Here's an example from my own work. A major post
house in Hollywood called me in to figure out some
problems, one of which was a lack of low end and
"punch" in the larger of their two mix rooms, while the
tracks mixed there sounded boomy in other rooms.
After examining the room, I suggested they close the
door to the hallway. The problem was gone; a simple
but surprising solution. What I saw was that the open
door created a Helmholtz resonator. The hallway and
rest of the building acted as the air volume or "cabinet",
while the open door acted as the port, demonstrating
that in acoustics, everything makes a difference. So:
rather than using EQ, to really fix the problem, you
need to apply solutions to the physical room.
This brings up an issue that I almost never hear in conversations about acoustics. Sound energy can also be
stored and then released after the initial sound stops,
causing an extended decay at a given frequency. The
sample of a waterfall plot shows this as "ridges" extending beyond the decay or fall-off of other frequencies.
(See Figure 1)
This can happen with a vibrating wall, floor, or ceiling panel that takes extra time to release the acoustical/kinetic energy stored in it. Energy can also be
stored in the room's resonances. As the other sound
decays quickly, the stored energy lingers longer, adding
a coloration similar to turning up an equalizer at that
resonant frequency.
So what are the choices?
Some people refer to any device that absorbs low frequencies as a "bass trap". I like to differentiate a bit
more than that. To me, a true bass trap is an absorptive

Recording - December 2013

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