In a message dated 10/1/01 12:02:09 PM Eastern Daylight Time,
mattm@optonline.net writes:
> Or get a Radio Shack sound meter. Here the state fairgrounds require sound
> levels of 95dbA at 100 feet as an absolute max, and that's what we use,
>
I have some acoustics experience. Noise can be a tricky and emotional
subject and tends to be measured in an overly simple manner.
"A weighting" passes the frequencies above about 1000 Hz without change while
progessively rolling off the frequencies below that point. Sorta like if you
turned down the bass knob on a receiver and then sent the remaining signal to
the meter needle. The idea is to allow for the fact that the human ear is
most sensitive to sounds in the roughly 2-5 Khz range. Very deep sounds are
pretty much ignored when the A scale is used. You can have a deep sounding
car do way over 100 dBC yet way below 95 dBA. C weighing is essentially
flat. Note that the deep sounds that are rolled off on a meter set to dBA
actually carry the furthest distance to the neighbors through the air
compared to higher freqs. A flowmastered camaro that measures 95 dBA
locally might sound louder at a much greater distance to a human than an
equally loud (dBA) small motored revvy car. The low pitched sounds
attenuate more slowly with distance than the higher freqs. The sound of
thunder and boomer stereo cars carries a long distance. An apartment
neighbor with a loud bassey stereo or home theater can measure fine next door
dBA yet very high on dBC. Then there is the noise versus music subjective
factor - we enthusiasts love the engine "music" yet others might not like the
sounds of power. So nonenethusiasts could get pissed faster than
enthusiasts at a given sound pressure level. Also the speed of rev changes is
another subjective factor. An s2000 can sound faster to others than a
larger engined car. Nomotor sounds such as tire squeal, brakes, PA systems
etc can also cause dBA scale havoc as they are fairly high in freq and so
well represented on the meter.
Stan
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