Recording - March 2017 - 56

a second leaving the resonances particularly exposed, and then
roughly levels out at a third of a second.
The ear does not respond quickly enough to discern all this so
quickly, so in addition to noting a low-frequency deficiency, we
hear the level of the low frequency relative to the rest of the spectrum
jumping around: it's equal-it's lower-it's equal-it's higher. This
rapid change is too fast for the ear but possibly not the brain,
leading to a certain amount of "I don't know... the bass just sounds
weird." Well, yeah! When the variations between the frequencies
whipsaw around like that, in a time frame that short, it's going to be
disorienting and confusing.

Fine, fine. Too much science. Fix it.

Now that we know the cause, let's make the problem go away
and the LF come back.
Two things will help. First, making the wall heavier (adding mass)
will shift the resonant frequencies of the panels down, and require
more acoustical energy to drive them to a given vibration level.
Second, making the wall stiffer will also require more energy to

Facts Or It Didn't Happen

set it into motion, and raise the Q (bandwidth) of the absorption.
You can achieve all this by adding a layer of drywall on top of
the existing one. But to make it perform properly, you need to do
it right. And don't forget the ceiling.
You want to use 5/8 in. or thicker drywall. 3⁄4 in. is better, and
if you want to splurge on QuietRock™, well, more power to you.
But you're not done yet. There are three things the installer must do
to make it right.
1. Generously glue the second layer to the first with a
high-quality construction adhesive.
2. Drywall screws must be no further apart than 5 inches.
3. Caulk all the seams with acoustical caulk like Henkel/OSI
SC-175; "mudding" the seams with drywall compound isn't
enough to seal them.
If your chosen drywall professional balks at any of the above,
find a new one, or watch their work carefully. You don't want any
of this omitted while your back is turned.
You may be tempted to do this work yourself, but unless you're
an experienced drywaller, I don't recommend it. They have specialized tools and experience that make it happen quickly and correctly.
For example, their drywall screw gun has settings to make sure it
drives deep enough, but not overly deep, it properly sets the screw
head into the drywall surface, and provides a dimple for quick and
easy finishing-it isn't your garden variety drill motor. Some of them
look like they came straight out of a Rambo movie. There are also
different types of drywall compound for different uses; select the
wrong one and it could give you problems for years to come.
This may seem like a lot of work and expense, but in the end,
your bass is worth it!
Bruce Black (black@recordingmag.com) is a recording and
post-production engineer now working as an acoustics consultant
with Media Rooms Tech in Ventura, CA. Learn more at
mediaroomstech.com and read Bruce's blog at manofacoustics.com.

These projects really brought home to me just how important it is to gather real data about a room if you want to make
any real improvements to it. There are tons of small but important things that even a well-trained and experienced ear
just cannot detect.
Walking into a room and clapping your hands doesn't tell you anything except it's a little reverberant, or moderately
reverberant, or terribly reverberant. That's not really any useful information. I'm sure the scientists at the Manhattan
Project were not saying, "Well, was that a little boom, a medium boom, or a really big boom?"
Over time I've found that the majority of the time, the culprit in a "bad sounding room" is related to low frequency in
one way or another (it seems our perception of sound quality is primarily in the low frequencies). It could be time-related
like this, or accentuated resonances, or buzzing panels and light fixtures, or a number of other problems. These are things
that you cannot diagnose with a hand clap or a dog clicker... it takes carefully collected and analyzed data.
So if someone walks around your room, clapping their hands, clicking Rover's attention-getter, or things like that, don't
expect any effective solutions to come out of it. In my humble opinion, that's the mark of someone who doesn't really know
what they're doing.-BB

56

RECORDING MARCH 2017


http://www.mediaroomstech.com http://www.manofacoustics.com

Table of Contents for the Digital Edition of Recording - March 2017

Contents
Recording - March 2017 - Intro
Recording - March 2017 - Cover1
Recording - March 2017 - Cover2
Recording - March 2017 - 1
Recording - March 2017 - 2
Recording - March 2017 - 3
Recording - March 2017 - 4
Recording - March 2017 - 5
Recording - March 2017 - Contents
Recording - March 2017 - 7
Recording - March 2017 - 8
Recording - March 2017 - 9
Recording - March 2017 - 10
Recording - March 2017 - 11
Recording - March 2017 - 12
Recording - March 2017 - 13
Recording - March 2017 - 14
Recording - March 2017 - 15
Recording - March 2017 - 16
Recording - March 2017 - 17
Recording - March 2017 - 18
Recording - March 2017 - 19
Recording - March 2017 - 20
Recording - March 2017 - 21
Recording - March 2017 - 22
Recording - March 2017 - 23
Recording - March 2017 - 24
Recording - March 2017 - 25
Recording - March 2017 - 26
Recording - March 2017 - 27
Recording - March 2017 - 28
Recording - March 2017 - 29
Recording - March 2017 - 30
Recording - March 2017 - 31
Recording - March 2017 - 32
Recording - March 2017 - 33
Recording - March 2017 - 34
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Recording - March 2017 - Cover3
Recording - March 2017 - Cover4
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