Recording - November 2017 - 28

Beyond the

Clap Test
E va luat e Yo u r R o o m

diaphragm condensers, available for
under $100, as well as much more
expensive options. There are also USB
test microphones available; these can be
quite handy, as they allow room testing
without a mic preamp.

ROOM EQ WIZARD:
THE BASICS

Opening the program (Figure 2)
shows a variety of options, but the first
thing we want to do is to make sure the
sound hardware is available to REW.
We need to choose an output device,
and wire the outputs to our monitoring
system. The dropdown menu should
allow you to select the output device.
Similarly, you must also choose the input
device for the microphone, so that the
software can listen back for measurements. You can also specify other
parameters like sample rate, buffer
size, and which speakers are used to
generate test signals (useful for multichannel systems with subwoofers).
There is also a calibration area,
useful if you require ultra-accurate
measurements, where you can "measure"
the soundcard's response so it doesn't
affect your results. While useful, this
feature isn't really necessary for room
measurement. Most sound interfaces
these days are much flatter than the
rooms we will measure, not to mention
the microphones we use to take
the measurements.

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28

RECORDING November 2017

There are also some handy level
meters in place, that allow you to get
the levels dialed in. Usually the sound
in the room should be around 80 to
90 dB SPL, more than enough to get
a good measurement without being
deafened by the test tones in the room.
The mic preamp gain should be set for
a clean signal. There is no benefit to
push the input gain here, though REW
is very forgiving. Room acoustics are a
linear phenomenon, so for testing purposes the inaccuracies of sound remain
consistent and proportional at various
volume levels.
Once you are passing audio out to
your playback system and getting it
back in via the microphone, you can
actually start using REW. The initial
screen is sparse, mostly empty space
with a few icons at the top. You can
make a new measurement or open an
existing measurement, which we'll take
a closer look at next time. But to the
right of those icons are other very
useful tools: an SPL meter, signal
generator, level meters, RTA (real time
analyzer), EQ controls, and the Room
Sim functionality we explored in the
March 2017 issue.
We'll take a look at how some of
these tools can help us evaluate a
room's sound, before we explore full-on
room measurement next time.

GENERATOR

The generator function (Figure 3) is
quite versatile. The drop-down
menu in the top left has many
types of tones that can be generated. I use Sine Wave and Pink
Noise most often by far, but the
others are also useful. With the
sine wave generator, you can
specify the exact frequency it
generates, along with a few options to tweak it (such as adding
dither or harmonic distortion).
This is useful to excite a single
specific frequency in the room,

so that you can hunt down resonances
or modal ringing frequencies.
For instance, suppose you have
noticed that you have trouble hearing
80 Hz in your listening environment.
You can generate an 80 Hz tone to
listen more carefully to how that frequency behaves in the listening room.
If you can't hear it well in your listening
position, chances are good that 80 Hz
is much more audible in other areas
of the room. This allows you to place
bass traps (whether tuned to the specific
frequency or broadband) in those locations where the frequency is excited, to
maximize their performance at that
frequency and improve the response at
the listening position.
Similarly, you can generate pink
noise, and then tune it to a specific
frequency range. My favorite way
to use this test is to use a custom frequency range, from 20 Hz to 400
Hz as shown in Figure 3. This means
that all the bass frequencies are playing at once. With this filtered noise
tone playing through your system, it
gives you a chance to walk around
the room, sticking your head into the
corners, nooks, and crannies of your
room to find bass pressure zones. You
can hear the bass get louder, or use
an SPL meter (a dedicated unit or a
phone app) for more accurate data.
This is also a great way to place bass
traps in a room; when you put a bass
trap in a bass hot spot, there is more
bass present for it to absorb and you
therefore increase its effectiveness.

RTA

The RTA (real-time analyzer) functionality shows a frequency response graph
of what the microphone is hearing in
real time. (See Figure 4) You have a variety of controls that set the range and the
responsiveness of the RTA. Most often I
use 1/3 octave mode with an 8192 FFT
Length (for faster response), with other
settings usually at their default settings.
This is fast enough to be useful in real
time, though RTAs never give the depth/
detail of information in a full test.
An RTA has a number of uses, but
my favorite way is to look in real time
as I move something in the room that
affects response. A subwoofer playing
Table of Contents for the Digital Edition of Recording - November 2017
