In a message dated 9/30/00 5:15:13 PM Central Daylight Time, Mark Sirota
wrote:
<< To the regions that actively do sound control at autocrosses:
How loud are street tires? I remember many years ago in Finger Lakes
Region, we started doing sound control. I believe our limit was
95 dBA at 50 feet, and nobody had much trouble getting their exhaust
systems under that limit (though a bunch of folks had to spend money,
and many got pretty creative).
But as I recall, the squeal of street tires was louder than that.
Can anyone quantify how much louder? >>
Well, I have not actually measured sound levels at an autocross, but being a
sound tech for a living, I can shed some light on the odd readings.
The spec for noise is always measured in db A weighted. This shifts the
sensitivity to the upper frequencies. Human hearing is more sensitive and
suceptible to damage at these higher frequencies, so OSHA uses the narrower A
weighting to determine what is safe to be exposed to. A loud low frequency
boom may not even show on a A weighted meter. Engine noise is reletively low
frequency, tire squeal is high pitched, right into where A weighting shows it
up.
If you are using the Radio Shack world reference sound meter (Even Lucasfilm
uses these things), try hitting it over to C weighting for a few runs and
watch the engine noise peg the meter well over 100 db all the time. 105 db A
is seriously annoying, while 105 db C is just moderately loud music with
decent bass. The spec for the main channels in a movie theatre is a reference
level of 85 db C and a max level of 105 db C. The Sub Woofer is set up for 95
db C as a reference and must produce to 115 db C without distortion. Measured
on db A the sub barely reads, and the main channels are under 75 db A.
I'll have to bring my meter to the event and measure a few cars for grins.
Gary M.
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