Sound Level meter requirements

[Ed White]

I work for a large aerospace company in structual dynamics (shock, vibration & acoustics) so I was able to bring my RS sound meter into work and check it against a NIST traceable calibration source.  The calibration source produced calibrated amplitude tones at 1-octave intervals.  Checked on the A-scale, slow, the RS meter was off by up to 3-4 dB on both the high side and the low side at various frequencies.  The only calibration adjustment in the meter is an overall level and overall, it was about even error on the high side and the low side.  The error in the RS meter for model airplane engine noise covering a large frequency band will depend on the frequency content of the sound.  High amplitude sound at frequencies where the meter is off will lead to lots of error.  Sound at frequencies where the error is both on the high side and the low side will tend to even out and the error will be less.  Based on my one single check (at room temperature) I would think around 2 dB error
 could be expected for an RS meter.  
 
We also found that different RS meters tended to read pretty close to one another (generally within ~1 dB) for the same conditions.  Good enough for a club to use to control its sound levels on a comparison basis or to see if your plane is close to the limit for the Nats.  But it will get laughed out of a court of law.
 
Ed W

Bob Richards <bob at toprudder.com> wrote:
Good point.
 
For my use (making comparisons) the RS meter is fine. The club I formerly belonged to also used them for enforcing a club-imposed noise limit.
 
However, if it comes down to enforcing a local noise ordinance, a calibrated meter traceable to NIST would probably be required. I'm not sure, but I don't see why you would not be able to have a RS meter calibrated, although I am sure the measurement uncertainty would not be very good. I'm sure the calibration would probably cost more than the RS meter.
 
What are/were the circumstances of "needed in a court of law" are you referring to?
 
Bob R.

Lance Van Nostrand <patterndude at comcast.net> wrote:
Earl/John,
Our club bought a RS meter and I remember Gray taking it to a contest where 
either Earl or Mike Harrison had their Extech. The RS meter was 2 db off 
and had no calibration. We then bought the Extech and found that the 
calibration ability and the specs are needed in a court of law.
--Lance


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