Recording - December 2013 - 58

Taming The Low

Fig 1

Frequencies
cavity built into a wall or other room surface that
absorbs sound above a frequency for which the
trap depth is a quarter wavelength.
This means it works as a lowpass filter, where
sound above a certain frequency is absorbed and
sound below is not. So a bass trap doesn't discriminate between frequencies that should be absorbed
and those that should remain. And it misses everything below its cutoff that may need attenuation.
These also require a substantial amount of
depth behind a wall or other surface to provide
a space to put the trap in. For example, the
quarter wavelength for 80 Hz is roughly 31/2 ft.,
and is longer for lower frequencies. You need a
lot of space to stash that puppy, and its opening
needs to be flush with the room surface (known
as its "boundary" in tech lingo)-not really
workable for a home studio.
Another low-frequency absorber comes from
manufacturers that provide interestingly sculpted
foam thingies that you put in a room's corners.
The shapes are visually pleasing, but you need
to remember that they don't really absorb much
in the low frequency range. Here's why.
The problem is that foam is a resistive
absorbent that works by resisting air particle
motion/velocity. Fair enough, but at a room surface, where it's easiest to absorb the lows, particle velocity approaches zero while pressure goes
to its maximum. So what you really need to have
effective low-frequency absorption is a device
that reacts to pressure, not particle velocity. [See
our interview with Peter D'Antonio, October
2013, for a deeper discussion of this.-Ed.]
Getting in tune
This brings us to resonant absorbers. These react
to a particular frequency band, like the bellshaped curve on a console's equalizer, and so
need to be tuned to a frequency. And they thrive
in high-pressure situations, if you'll excuse the pun!
What I don't like about bass traps (my definition, above) is that they affect all the low frequencies, while resonant absorbers allow you to focus
on your problematic frequencies. In other words,
a bass trap is like a shelving bass control, turning
down both the problems and the frequencies you
want to keep with an indiscriminate hand. On the
other hand, resonant absorbers can be tuned to
the problem frequency alone, like a band-reject filter, leaving the other "good" frequencies that
should be left untouched. The result is a flatter lowfrequency region (accomplished acoustically, not
electronically) and "tighter bass" from attenuating
the extended decays.
One version of a resonant absorber uses a
wood or metal panel, or flexible membrane,
mounted over an airtight cavity so that it vibrates
freely to dissipate the sound. The panel or membrane must be exposed to the room, and needs
58

RECORDING December 2013

A typical waterfall plot, showing an extended decay ridge that is
acoustical energy stored in a room resonance.
to be a bit on the large side. My problem with this is that it puts a large reflector in your sonically sensitive space, which can cause issues from the early
reflections off it. You also need to use them symmetrically in the room, with two
panel absorbers mirroring each other, on the two side walls or either side of
center on the back wall. Asymmetry will make the sound from your speakers
change as you move your listening position from left to right. In my book, strike
one-it limits how you can implement it.
With a membrane absorber, you also have to figure out how tightly to tension
the membrane, along with a host of other things that affect its tuning. A difficult
prospect at best, although some commercial products have successfully tackled
this. But this involves a complicated calculation, so in my book, strike two.
My fave
Another type of low-frequency absorber is the Helmholtz or volumetric resonator. There are several types of these, but my favorite looks like a ported

Fig 2

The black boxes shown back in the mix room corners are fabric-covered
Helmholtz resonators at MAX Post, a Fremantle Media Company,
located in Burbank, CA.

Recording - December 2013

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