sound meter

[Adam Glatt]

Earl Haury wrote:

> Generally even the sound meter calibrators are +- 0.5 dB. Makes the 
> flight zero for >94 (F3A) really a tough rule (even flying behind the 
> flight line only zero's the maneuver)! Albeit, a fail isn't official 
> until verified by a second meter - but generally both are calibrated 
> with the same device, so the calibrator offset will be present at both 
> meters. Calibrator equipment itself should be re-certified pretty 
> often (yearly), want to bet when the Nats equipment was last certified?
>  
> The only way to be sure is to use known accuracy equipment and target 
> a number that allows for equipment variance. The air density at the 
> time you make measurements will vary the results also, both from an 
> engine output (more/less rpm) and a sound transmission standpoint. 
> This can amount to a couple of dB, and the Muncie site is usually 
> better air density than soggy Houston - so the noise measured is 
> higher in Muncie. Again, allow for this in your noise plan.
>  
> The good news is that the Nats meters seem to be on the liberal side 
> and the display is analog - making it difficult to discern the 
> difference between 94 and 94.xx. Also the 96 dB AMA limit is much 
> easier to attain and the penalty for missing a little not too bad.
>  
> Earl

Good idea, but terrible rule.  They should give a 5% score penalty to 
the noisiest 10% of planes in a class (either by ear or by sound meter), 
imo.  Measuring a plane on the ground, with someone holding it, people 
standing around, wind blowing, and the prop a few inches from the 
ground, and from a distance that doesn't represent the problem of noise 
(i.e. Shulman's electric being one of the loudest planes on the meter 
due to low clearance and gear box noise, but one of the quietest in the 
air) is a dumb way to do it.  The only noise that matters is what we 
hear while the plane is in the air.

It was obvious in Poland the FAI officials knew the sound rule was 
implemented wrong, as well.
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