Jay:
So if we want to be cautious, perhaps we should be using "C." Well
transmitted low frequencies (as well as high freqs.) could annoy the locals.
True? Relying on a neighbor who can only hear 1k to 3k might be a bit
risky. :^)
Matt Murray
-----Original Message-----
From: Jay Mitchell [SMTP:jemitchell@compuserve.com]
Sent: Tuesday, October 02, 2001 11:09 AM
To: Matthew D. Murray
Cc: evolution-discussions
Subject: Re: Re: Fairfield County Sports Car Club Event
Matt Murray wrote:
> As someone on the other lists, specified dBa and dBb are
* somewhat unreliable in fulfilling our needs in the noise
* abatement issue. I think it was mentioned that dBc is
* the closest match to "what we hear."
Actually, "A" weighting produces the best match to the
sensitivity of human hearing. The A weighting filter is a
bandpass with -10 dB points at 200 Hz and 20kHz. The frequency
range that receives the greatest weighting is 1-3 kHz, which is
the frequency range to which normal_ human hearing is generally
most sensitive.
> I don't believe the Radio Shack meters have the dBc filter (or is
that a
> non-filter?).
Actually, they do have C weighting, which is close to unweighted.
A car with lots of low frequency output (big-ass motor) will read
louder "C" weighted than "A," whereas an F Mod car (or an
unmuffled rotary) will tend to get higher readings if the "A"
weighting is used. IME, "A" weighting correlates well with the
relative "annoyance factors" of the types of engine noise.
> For this event the "if it sounds loud" guideline is workable.
All of the weightings are intended to model some aspect of human
hearing and/or susceptibility to hearing damage. Obviously,
"sounding too loud" to folks who are not at the event is what
you're trying to avoid, and measuring _any_ parameter onsite,
regardless of how accurately you accomplish the measurement, may
not give a reliable indication of this condition.
> FCSCC did use the RS meter when we ran at Southern CT State
University in the
> early nineties. That was what I mentioned in the previous report.
As one who owns and uses acoustic test of nontrivial cost and
sophistication instrumentation on a daily basis, I'm impressed
with the accuracy of the RS meter (the earlier mechanical-meter
unit, I don't own a newer digital-display type). For noise
testing of the sort that SCCA regions need to do, there's no
point whatever in spending more money than the RS unit costs. The
real difficulty in determining what's too loud to people offsite
has nothing to do with the meter used or the weighting, it's
caused by the relatively poor correlation between what you
observe at 100 ft. from a noise source and what's audible 1/2
mile away.
Jay
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