Sound Level meter requirements

[tony at radiosouthrc.com]

John:

Yes, it is very tricky.  The problem is finding the known reference.  That's why we have to re-calibrate so often even with a "quality" meter.  AMA told me at the time that the calibrator device itself was about $500.00...  Of course, it was designed specifically to fit over the mic of that particular brand of meter, and may not be very accurate with another meter unless a custom head was built to hold the calibrator exactly the same distance from the mic while blocking all outside noise sources...


Tony Stillman
Radio South
3702 N. Pace Blvd.
Pensacola, FL 32505
1-800-962-7802
www.radiosouthrc.com
  ----- Original Message ----- 
  From: John Pavlick 
  To: NSRCA Discussion 
  Sent: Wednesday, February 16, 2005 7:31 PM
  Subject: RE: Sound Level meter requirements


  Tony,
   This is all very interesting. Even in the short time I've spent looking at these meters I noticed some questionable areas. That's why I asked if there was a more specific set of test criteria / procedures. It seems to me, that even under controlled conditions, a GOOD meter might not produce repeatable results. It is of course very important to calibrate to a reference (i.e. 94dB / 1KHz), then measure. Without doing that you can't take accurate absolute measurements. Also, Unless the spec. is written to allow for tolerance of the meter (most of the ones I've seen are +/- 1.5dB), sometimes you'll read (too) high and sometimes you'll read low, even with the same source and a calibrated meter. It's probably more important to have an accurate means of re-calibrating the meter rather than having a meter that has a tighter tolerance. Taking reasonably accurate measurements relative to a known reference should be acceptable, I would think. That's why a band tunes to the keyboard if they don't have an electronic tuner handy. A modern electronic keyboard can be considered an accurate reference for pitch. 
  John Pavlick
  http://www.idseng.com
    


   -----Original Message-----
  From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On Behalf Of tony at radiosouthrc.com
  Sent: Wednesday, February 16, 2005 7:25 PM
  To: discussion at nsrca.org
  Subject: Re: Sound Level meter requirements


    Del:
    The problem with all meters I know of is that they all change!  I was the first one in the USA to work with the FAI in establishing how a db check would actually be done in the field.  I did the first ones at the 1983 US Team Selections in Rough River, KY right before the 1983 World Champs.  As I was the assistant CD for the WC, AMA wanted me to gather the data and learn how the system the FAI had designed would really work in the field.  We found that even expensive db meters developed drift.

    The expensive versions used by AMA have to be adjusted about every 4 hours, as they can drift 1/2 to 1 1/2 db in that period of time due to temperature changes.  That is the problem with any meter.  Unless you have a calibrator to adjust the meter, it is really pretty worthless in getting an accurate measurement.  As long as + or - 5 db is OK, you don't need to worry.  However, if you are trying to meet any model noise requirement out there, your really spinning your wheels.  

    Tony Stillman
    Radio South
    3702 N. Pace Blvd.
    Pensacola, FL 32505
    1-800-962-7802
    www.radiosouthrc.com
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