Recording - March 2018 - 52

Beyond the

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

us only the presence of sound energy,
but nothing at all about the sound itself (other than its volume at a given
moment in time). It is used to indicate the
presence of sound reflections arriving
at the measurement microphone certain
amounts of time after the initial impulse.
We're mostly interested in what's happening between the initial impulse and
about 30 milliseconds out from there.
There's an interesting phenomenon that
occurs around 30 ms; reflections arriving
sooner than that aren't perceived as distinct
echoes, but rather as the same sound
arriving from different locations. This phenomenon is called the precedence effect.
These sounds are below our echo
threshold, since we don't perceive them
as separate. There is some variability
with exactly where these thresholds lie.
For instance, in short, staccato sounds
(like a drum with no reverb) it's easier
to hear echoes at lower time thresholds,
but for longer, more legato sounds (like
a violin) the thresholds are much longer,
sometimes as much as 100 ms.
Remember that sound takes time to
travel through the air, with a certain
velocity. Various factors can influence
the speed (air temperature, humidity,
etc.) but it's around 1100 feet per
second, or 1.1 feet per millisecond. So
in Figure 6, the two strongest reflections
occur at 3 and 22ms. At 1.1 feet per
ms, it indicated sound that has traveled
3.3 feet and 24.2 feet further than the

direct sound from the speakers directly
into the measurement mic. From there
you can work out which surfaces are
causing the reflections. In this case it
was reflections from the console/desk
for the 3 ms reflection, and the rear wall
for the 22 ms reflection.
Once these paths are identified, they
can be treated, usually with absorption
(as thick as possible) placed at the
reflection points, or at least somewhere
in the path the reflected sound takes to
get to the listening position. These treatments will reduce the size of the spikes,
since it eliminates (or greatly reduces)
these early reflections. It will bring us
closer to the ideal impulse response,
which would basically be a diagonal
line pointing evenly downward as it
decays smoothly into silence, with no
noticeable spikes out to 30ms or so.
Figure 7's response graph, again taken
from a different, highly treated room,
gets us closer to the mark.
Notice in this graph, there is only one
reflection within -25 dB of the initial
impulse, and only a handful within -30
dB. This is in contrast to the untreated
room depicted above, which has several reflections within -20 dB. The treated
room will have a lot more clarity, since
there are no strong reflections present
within the echo threshold, and no sounds
competing within the precedence effect
to confuse our psychacoustic perception
of spatial locations. In other words, the

I t i s i m p o rta n t t o
r E a l i z e t h at w e c a N
o n ly m e as u r e o u r
r o o m r e s p o n s e at
one point in the room
at a n y g i v e n t i m e .
52

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