Sound and Communications - February 2008 Issue

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to move up and down slightly
differently from others. In particular,
the 250Hz bar graph seemed to be
quite predominant and did not
appear to decay as fast as its neighbors.
The question was, however,
was this as it should have been (i.e.,
was the display just following the
music?) or was this an indicator of
the potential problem?
Now, as any self respecting
acoustic consultant, sound operator
or musician with a physics degree
will immediately know, there is a
note (B) at 247Hz that it could have
been that I was seeing, but I don’t
think so. Of course, also within the
limits of the 250Hz 1/3 octave band
are “A” at 220Hz and “C” at 262Hz.
So, without a reference, in this case
the musical score, I could not tell if I
should be hearing/seeing a note or
not. Clearly, there was a need for
some objectivity and/or a defined
reference to work with.
Between shows, there was a 15-
minute window when I could make
some quick measurements after the
cleaners (and vacuum cleaners) had
departed and before the next audience
arrived. Time, therefore, for a
blast of one of my favorite references:
pink noise. This, indeed,
showed that the overall response
was rather “middy,” and that a peak,
indeed, did occur in the 250Hz 1/3
octave: at 233Hz, to be precise.
The problem was that it didn’t quite
happen physically where it should
have, but pretty close by. With five
minutes to go, but with the audience
already starting to enter, time for
another reference sonic delight: a
STIPA signal that enabled me to verify
on the spot the potential intelligibility
at two of my observation positions.
This immediately showed that,
indeed, there was potentially a
noticeable difference in intelligibility
between the positions as subjectively
experienced.
The other test that I really didn’t
have time to do (but did anyway
despite a chorus of protests from the
ushers and audience) was to record
a reference sine sweep at two of the
identified locations. This was then
later deconvolved (signal processing
technique to extract time domain
(impulse response) information from
the frequency sweep) to extract the
impulse response and detailed
frequency response information.
Now, of course, it would have
been nice to have been able to take
a complete measurement rig with
me and make extensive, non-invasive
measurements during the show.
Nice idea in theory, but not a
practical proposition for a first-time
visit to look at a vague, ill-defined
problem of unknown prominence.
However, the brief wander with
the real-time analyzer and real-time
assessment of the STI gave me an
immediate handle on the problem.
As the audience filed in, I got the
sound operator to apply a narrow,
233Hz, 5dB cut to all of the front of
house main speakers.
So, did I hear the problem during
the second show? No—but then I am
not sure if I heard it during the first
one, either! Was the sound still
generally middy? Yes. Was I happy
that I had cracked the problem? No.
Did I think I now knew what the
problem was? Yes. Did I know what
was causing it? No. Did I know how
to cure the problem? No.
The next step was to return the
following day, when I could have the
auditorium to myself for several
hours and “measure the heck out of
it.” That enabled me to find the
problem, after several hours and
with the assistance of three different
acoustic analysis software platforms
and 30 years of experience.
What did I learn? Quite a lot,
actually, but, primarily, it reinforced
my never-ceasing wonderment of the
vagaries and infinite variety of problems
associated with sound and, of
course, the need for a reference,
whether music, speech, pink noise or
a modulated STIPA signal. It also
provided me with a new consulting/
problem-solving acronym, WWLL:
Wander, Wonder, Look and Listen!
But what was the cause of the problem
you ask? Well, that’s another
story, but I blame the architect; but,
then, I would say that wouldn’t I? ■
16 Sound & Communications
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