Recording - April 2020 - 22

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various room modes at work. The response shows substantial modal
buildups at both 70Hz (Axial Height Mode) as well as 140Hz
(where the Axial Height, Width, and Lengths stack up). There are
also severe nulls in the response around 85Hz and 170Hz. The
peak at 140Hz is nearly 85dB in volume, but the nulls fall below the
bottom edge of the graph at 35dB; there's more than a 50dB swing
between the loudest frequencies and the softest!
How do you think this booth would sound? Apart from the
boxiness in the mids/highs from the powerful early reflections,
the bass will be boomy and inarticulate. I'm less concerned
about the problems under 100Hz though, since those usually
get filtered out of vocal tracks anyway. Still, the jagged response
above 100Hz will undoubtedly affect the sound of the vocal
track, in the range where most vocal fundamentals are. At the
very least, these tracks will need some serious surgical EQ repair
work in a mix to become usable, and will never be optimal.

Dimensional shift

We can improve things considerably by building our booth a
bit larger, and by avoiding making any two dimensions the same
or multiples of one another. Figure 2 shows a 5'x7'x8' room.
There are still some big problems with the response, but the gap
between the tallest peak and the deepest null has improved by
more than 20dB-just from the room geometry in play.
This larger booth has more space for acoustic treatments. Figure 3 shows what happens if we make all the surfaces in the
booth (except the floor) 50% absorptive. This response is much
smoother and more usable down below 60Hz. Tracks recorded
here should be quite dry and focused, and additional EQ will likely help tune it in and level
things out even more. There will be less peaky
comb filtering, which usually gets exacerbated
with compression or saturation plugins.
This is the smallest booth I'd recommend,
and it'll need extensive treatment on the inside. My go-to for inside vocal booths is at
least 4" of broadband absorption covering
the interior to deaden it down to 100Hz and
below for starters.

Build considerations

With the dimensions sorted out, how do we
build the thing? Detailed plans are beyond
the scope of this article, but here are some
things we need to consider.
22 RECORDING April 2020

Airtightness

A soundproofed booth needs to be airtight. If air can get in
or out, so can sound. Construction tolerances have to be tight,
and we'll probably want to line the interior of the booth with
acoustic caulk. We'll need to pay attention to sealing up the
door, and getting a mic cable and a headphone cable at the
very least into the booth. We'll also need to think about how to
get fresh air in and out of this airtight booth. How long do you
think oxygen lasts in a tiny, airtight room with a belting singer
giving it their all in a magical performance? Opening the door
between takes, just to breathe, gets old quickly.

Mass and wall damping

The walls need to have an adequate mass to stop sound. Using multiple layers of sheetrock is an economical and effective
way to accomplish cheap mass. Performance gets even better using a constrained layer damping compound like Green
Glue between the layers. All of this depends on how much
soundproofing is required, plus how much weight can be added to the structure. A small booth built with multiple layers of
sheetrock weighs a lot! If you're coming up from a concrete
slab (like in a basement or garage) you'll be OK, but if this
is an attic studio then you'll want to get a structural engineer
involved for obvious reasons-and this is literally a matter of
life and death.
The bottom line: if the build is planned carefully, cleanly constructed, and properly treated, it can be a game changer since
now you'll be able to safely record without concern for outside noise. And with the proper dimensions and treatment, the
resulting tracks will be quite nice sounding.

FIGURE 2

Modal distribution and frequency response
improves significantly in a slightly larger 5'x7'x8' vocal booth

Recording - April 2020

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