cstevec
I’m not at all sure what the comparison of dB meters and infra-red sensors has to do with this discussion. They use entirely different components and techniques to make their measurements that have no relationship to each other. Anyway, it’s just not relevant to this discussion.

Yes, the meter you purchased is what is basically suggested by the AMA test procedures. These procedures are essentially those developed by the pattern groups for their purposes and then adopted by AMA for lack of anything else. The following are quotes from IMAC (that uses the same procedures) documents concerning sound level measurements as performed using the method you referenced;

“So what we are left with are commercially available consumer retail units from Radio Shack (RS). Initially we tested various RS units in the lab (at General Dynamics) and found them to be within one dB of each other. So we quite happily employed them in the field as Pattern has done in years past. Unfortunately, as is the case with many systems, once employed in the field the inaccuracies started cropping up. It was not uncommon to find the RS meters consistently 2dBA apart on readings, and in many cases we have found units to be as much as 3dBA apart! When one considers the fact that the intensity of sound doubles for every 3.01 dBA, the use of the RS meters can (and has) given widely varying readings during the ground test.”

Would you accept a class of instruments that measure 2X range for the same inputs?

If, using your meter, you measure your sound level as 96 dBA, and at a meet are told your sound level is 99 dBA and unacceptable, how would you feel? Is a test reasonable if you can’t reliably determine a repeatable value from one tester to another?
Further, IMAC doesn’t even use the ground level test as the final arbiter, they require the judges to evaluate the sound level of the contestant and give them a subjective loudness score that can result in point deductions. Why bother with the SPL measurement if ultimately you’re going to revert to subjective measurements anyway.

Also from the same IMAC document;
“So you ask, knowing all of this what do I as a CD do with the Ground Test at a contest? First keep in mind that we do not intend it to be the "end all test." Folks come to fly at our events, and we want to give them every reasonable opportunity to do so. After all, if someone is Too Loud, the In-Flight Judging Criteria will "catch" them. So in answering this question, we should again consider the inherent inaccuracy of the RS sound meters. Since there can be as much as a 3dBA difference in readings from meter to meter, you as a CD don’t know if your unit is on the high side or the low side. In my view, no one should be turned down from flying if they test within 3dBA of the limit. When this problem first surfaced IMAC began recommending that the use of ATV was appropriate for the ground test. Therefore use the Ground Test for it’s true purpose, giving competitors an expectation of what the plane will do in the air. Our recommendation: Never ground a competitor unless they are substantially (3dBA) over the limit.”

If you’re going to fudge the number by +3dBA why not set the absolute limit to 99dBA and be done with it. And the IMAC document goes on to say that ground level measurements have no relationship to sound levels generated while in the air.

As to the RS meter, according to the specs on your model are ± 2 dB at 114 dB SPL. From the following chart you can see that this meter is not acceptable for even general noise surveys as accepted by standards bodies.

International Standard Classes of Accuracy: Almost all SLM’s are designed and specified to perform to one of four internationally standardized levels of accuracy:
Class Sound Level Meters Purpose Calibrators
0 +/-0.4dB Lab reference standard +/-0.15dB
1 +/-0.7db Precision SLM +/-0.3dB
2 +/-1.0db General purpose SLM +/-0.5dB
3 +/-1.5dB Intended for noise surveys --------------------
(Table showing permitted tolerances as defined by the IEC 60651 and ANSI S1.4-1983.)

I’m saying that the hobby’s approach to sound level determination is extremely poor and promotes a procedure and instruments that result in measurements that have essentially no value in repeatability (which is a must if the measurement is going to be used for anything other than its feel good value) or comparability. Just understand, your measurements don’t mean much.

By the way, you shouldn’t have to take your meter in for calibration, the calibration devices are quite portable, why not borrow it and calibrate just before you make your measurements, which is what all test procedures (except AMA/IMAC) I am familiar with require anyway.
Sincerely,
Robert Camp