Recording - March 2018 - 48

Beyond the

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

good, press the 'Start Measuring' button.
I will usually put a Start Delay of about
5 seconds before the test begins, long
enough for me to get well out of the way.
Human bodies absorb sound, so being
in the room will affect the test results,
especially if you are in close proximity
to the microphone or the speakers. If
possible, put a delay long enough for
you to exit the room and close the door
while the test proceeds.
Once the test begins, you will hear
the sine sweep come up from 20 Hz into
the audible range. The frequency will
continue to rise until it gets to the End
Freq, 20 kHz in this case. When the
sound stops, congratulations-you've
made your first room measurement!

THE FIRST GRAPH: SPL

With the first measurement made,
let's take a look at the data we've just
generated. We will begin on the left
with the 'SPL & Phase' tab, which shows
one measurement at a time. The next
tab, 'All SPL', shows all the measurements superimposed over one another,
as in the case of Figure 3, a measurement of left and right speakers made in
an asymmetrical room.
There are several things we should
look at in order to see the data clearly
to make sense out of it. Notice how
the visible graph shows the lower axis
up to just over 400 Hz, and down to
the lower range limit of the speakers,
about 40 Hz in this case. The vertical

RECORDING March 2018

axis of the graph shows a very wide
range of about 80 dB, with the lower
limit at about 10 dB and the upper limit
at about 90 dB. As we observed in Part
1 of this series, it shows quite a range
of loudness at different frequencies!
Furthermore, the left and right speakers
in this case show completely different
responses. The left speaker, shown in
the red curve, is about 4-6 dB louder
from about 70 Hz up to just over 100
Hz, but then from 160 Hz up to about
300 Hz the right speaker, shown in
the green curve, is louder by about the
same amount. Also, there are two very
deep nulls that coincide for both speakers,
just under 300 Hz and just over 400
Hz. These nulls are probably related to
the test microphone position since they
are the same in both speakers. If your
graph doesn't show this scale, you can
adjust the scale by hovering in the top
left (for the vertical axis) or lower right
(for the horizontal axis) and clicking on
the zoom controls that appear.
In addition, I nearly always keep
smoothing turned off, by clicking on
Graph > Remove Smoothing. Turning
smoothing off makes the graphs look
less pretty and more jagged, but they
show more accurate info overall, especially when we are looking at the bass
range. When you add smoothing,
it averages the data within a certain
bandwidth, selectable in fractions of
an octave (i.e. 1/48, 1/24, 1/12, etc.).
The wider the bandwidth the smoother
the resulting graph will be. Consider
the range from 150 Hz to 300 Hz, a
range of one octave. If 1/1 smoothing is
chosen, then all the energy in this range
will be averaged out, and the numerous
peaks and nulls in that range will
disappear! Smoothing can be useful at
higher frequencies, where measurements
are even more jagged due to the comb
filtering that occurs at higher frequencies.
As you can see, the frequency response graph of this room shows problems an order of magnitude more severe
than the worst electronic gear available
these days. Imagine a mic preamp with
a more-than-30 dB variance in frequency
response within one octave, as this room
shows! We'd throw it away as unusable.
By testing our rooms this way, we
can improve things by changing setups

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