Recording - March 2018 - 44

Beyond the

Clap Test
E va luat e Yo u r R o o m
BY JAMES LINDENSCHMIDT
In Part 1 of this series (November 2017), we
looked beyond the simple evaluation of the "clap test"
to identify more characteristics of good-sounding rooms
than the simple reverb time it reveals, and began to
discuss ways to evaluate the sound quality of a given
room. We introduced the fantastic Room EQ Wizard
software (henceforth REW, see roomeqwizard.com
for details), going over many of the basic features
included in the software. We talked about how to get
the software working with your computer and sound
system, its powerful signal generator functionality,
and its Real-Time Analyzer (RTA).
But these features are only the beginning! The real
power of REW lies in its ability to measure and provide
extraordinary detail on what the acoustics are doing
in your room. Last time we did not get into this kind of
room testing with REW, promising that it would come
in the next part of this series. And here it is!

HOW DOES REW WORK?

To test the sound of a room, you need a loudspeaker (or a
set of speakers) and a measurement microphone connected to
the computer. REW sends out a test signal (a "logarithmic sine
sweep"), and then listens back to the signal through the microphone. By comparing what it hears through the microphone
to the original signal it sent out, it can derive a staggering
amount of information about the sound: frequency response,
phase response, impulse response, and more.
This is derived mathematically with a Fast Fourier Transform
(FFT). An FFT is a mathematical algorithm that can take a
complex signal (such as the audio heard by the microphone
during the test), and divide it into content that can then be
usefully organized and displayed in graphs.
Since we're not mathematicians, we don't really need to
understand the details of the algorithm. We just need to know
how to use it to generate useful tests, and then look at the
results so we can interpret what's happening to sound in our
rooms. Let's walk through a typical test setup where we wish
44

RECORDING March 2018

PART 2: WE KNOW WHAT TO
MEASURE AND WHY-HERE'S HOW
to evaluate a listening environment, such as the rooms many
of us are using for audio production and mixing.

GET READY TO MEASURE

Before we take a measurement, make sure the speakers and
the microphone are both connected to the computer in the
appropriate manner, either through an interface with a mic
preamp as you would normally do when recording, or perhaps with a USB measurement microphone. To be sure things
are running as expected, open the 'Preferences' window and
choose the 'Soundcard' tab. There are several configurations
you can make in this window (see Figure 1).
Sample rate should be set (44.1 is fine for most systems), and
make sure the input and output devices are set to where the
speakers and microphone are connected. Buffer settings can be
the default settings. On the input you have the ability to invert
the polarity should you need to, as well as apply a high pass
filter at 2 Hz, which probably isn't necessary unless your input
signal has a DC offset that needs to be dealt with. These
features are more for measuring equipment than room acoustics.
We are able to calibrate our measurements, which means
that the frequency response anomalies of the soundcard or
audio interface we are using can be taken into account so
as to not affect our measurement data. This feature is useful
to ensure the highest possible accuracy, but I rarely use this
feature for quick room measurements because most modern
audio interfaces have a very flat frequency response, certainly
in comparison to the rooms we are measuring. If you are
motivated to, you can make your own calibration file for your
interface with the 'Make Cal' button; you will need to patch
the output of the interface back into its own input, allowing
REW to calculate the difference between the two signals.
Then, we must set levels for the measurement-both the volume
in the room, as well as the input gain from the mic preamp.

http://www.roomeqwizard.com

Table of Contents for the Digital Edition of Recording - March 2018