OK now you've done it
You've put data before the enginerd and now it's time to pay (no good deed goes unpunished)
But more seriously if you have the time I have a few questions as my curiousity is whetted.
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Originally Posted by LRSLA
I have posted a copy of one of the sound traces that I captured this season.
These traces are take at A and some of the reading are taken at C scale.
I do this to show the level of low frequency sound that is not captured on the A scale measurment.
This test showed a Max A weighted level of 84.9dB
and a peak C weighted level of 106.4dB, the trace was taken
over a 32 second period as the boat passed at about sixty MPH+
I was two hundred feet from the source as it passed.
So add 6dB to 100feet and 6dB to fifty feet for an add of 12dB.
You get an A rated reading at 50 feet of 96.9dB then add ten
10db to bring your to 106.9. Only .5dB off what a C scale reading would be.
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I think I was right with you until the 10 dB part I've underlined above. What's that for ? There are 3 very similar traces (yellow, reddish and magenta) that I understand to be A weighted measurements. I would ask if these are 3 passes made by the boat or 3 different time averages or outputs from 3 different meters or ??. If it's the first then it's nice and consistent ! Good to see that in a field test. I understand the green trace to be a C weighted measurement (not sure what the jagged blue one is). It's interesting (to me
) that the aspect angle to the boat doesn't affect the measurement more. Was your technique to pan the mic as the boat passes or leave it aimed "straight out" ? I'm making an assumption hear that the boat's course was more or less a straight line and not a curved trajectory.
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Originally Posted by LRSLA
I do not have info on how the 75db level was arrived at.
It is restrictive. I think that it is the most restrictive in the nation.
I have some responses to the use of the 75db rule.
I have taken numerous readings of normally exhaust ported outboards,
IO's and even older thru the hulls exhausted craft.
What i have witnessed is that 99% of all craft will pass the 75db at fifty feet.
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Reason I asked was that in looking for some sound data on boats I came across an old (1995 I think) set of measurements done by NJ authorities. Some of the normal boats would have had trouble with a 75 dB limit. I suspect your average family runabout has gotten quieter since then.