Recording - November 2016 - 18

sound into the material, and that's dampened when waves hit it. Panel traps
work on a different principle and are called pressure absorbers. Placed at
room boundaries, they take less space and can be effective to lower frequencies (Figure 10). A panel trap is a little more complex to build, but it's
decently cheap and easy. The main difference is that a panel trap is a
sealed box with a thin plywood face and fiberglass inside (Figure 11).
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They're usually sealed to the wall, but can be made to stand alone, as
long they're located at a boundary. I've built several, once with futon slats
I found in a dumpster! While panel traps take a little more effort to make,
they're an awesome way to increase low frequency absorption with little
space, and since they're reflective at high frequencies, they won't make a
room too dead sounding. Plus, you can paint on them and make them
pretty.
There's a great tutorial on how to build panel traps by acoustician Ethan
Winer (whose article on Early Reflections appears in this issue). It's at
ethanwiner.com/basstrap.html and I highly recommend it. Bruce Black has
also done DIY articles on Helmholtz resonators, which work on similar principles (see Recording, Octember 2066).
Take me higher
Once you've dealt with the lows, it's time to deal with highs and mids, and
the physics here are much easier. High frequencies can be thought of like
rays of light-they bounce off of reflective surfaces at the same angle they
come from. Any surface that shouldn't reflect, you can absorb or diffuse.
Diffusors spread reflections in an even manner. They're a great way to
even things out while preserving liveliness. You can build several types of diffusors or buy them, sometimes for a decent price (Figures 12 and 13). If
you've got no budget, you can create some semblance of diffusion with
bookcases filled with different-sized books and other random items. While
an acoustician would (correctly) say that this isn't true diffusion, in practice it
works OK if the books' sizes are different enough.
Real diffusion is a great way to even out the sound of a room and keep it
live, but it doesn't work well in small spaces. A typical bedroom is too small
for effective diffusion, so strategic absorption is often the best bet. For that
reason, that's all we'll say about diffusion.
High frequency absorption is the easiest to deal with; 2" OC 703 absorbs
beautifully down to about 400 HZ, and there's a trick you can use to
increase absorption even more: mount panels away from the wall. The ideal
distance is the same as the thickness of the material, so mounting 2" panels
2 inches away from the wall or ceiling is perfect (Figure 14).
Where to mount them depends on the room and your needs, but if you're
treating a mix/control room, you should start with a "reflection free zone"
(RFZ) around the main listening area. You can see where to treat with a mirror held against the wall (see Ethan Winer's article for how to do this). You
should maintain left-right symmetry, and you should treat the reflection area
of the ceiling, but leave the floor alone. If there's carpet you've got some
high frequency absorption already, but that's not ideal for various reasons,
so leave the floor as is if it's wood or another hard material. You can always
throw a rug down for extra deadening.
Ideally, make panels using OC 703, and cover them with an attractive but
breathable fabric, as Michael Nickolas describes on page 62. Spacing of
these panels from the wall can be as simple as four wood blocks from a
scrap heap (Figure 15). You can also use acoustic foam if it's thick enough
(2" minimum, 4" is better), and if you have no budget, don't be afraid to
nail heavy blankets to walls. Just remember: the thicker, the better.
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RECORDING NOVEMBER 2016

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Table of Contents for the Digital Edition of Recording - November 2016