How to resolve two different equipment measurement results.

[Sliderule]

I have conlicting results with two measurement systems. I have multiiple measurements of a panel speaker made with REW and Moto M2 audio interface with an ECM8000 calibrated mic. The speakers measured show a rapid drop-off above 12KHz. The attached screen shot (63) shows the drop-off smoothed to 1/3 and 1/6 octave smoothing. The same measurements made using a Crown RTA-2 do not show the same drastic frequency dropoff. Using REWs's cursers I estimated the 1/3 octave values at 8KHz and 16KHz. With REW the drop-off was 11.4dB. Estimating the difference in the 8 and 16KHz 1/3 octave readings on the Crown RTA-2 was only 4dB. My REW mic is calibrated and the unclaibrated values are +/- 2dB 20KZ to 20KHz. The Crown doesn't appear to allow for microphone calibration so that is a possible source of difference. The REW drop-off results have been consistant over 200+ readings.

Questions:
1. Does anyone have experience with the RTA-2 and it's idiosyncrancies?
2. Any ideas on the cause of the level difference?

TIA David White

 

[Wayne A. Pflughaupt]

Wow, that’s an antique! What does it date to, the 1980s or earlier? All the RTAs I’m familiar with from the 1990s were two-space components.

Anyway – the thing is, these old hardware RTAs were all delivered with what they called a “calibrated mic.” But the mics themselves weren’t calibrated - they had piles of the same make and model at the factory. Since no two mics are exactly the same, what was actually calibrated was the RTA itself. IOW, each unit was calibrated for the specific mic it was shipped with.

Thus, you can’t randomly plug a mic into a hardware RTA and get accurate results.

Another consideration is the mic orientation you are using. It’s popular these days to measure with the mic upright, pointing at the ceiling. Such measuring requires a 90° calibration file to compensate for the mic’s reduced off-axis high frequency response.

However, any calibration a hardware RTA came with assumed the mic would be pointing directly at the speaker, or perhaps no more than 20° off axis. Thus, you don’t want to use 90° orientation with any hardware RTA, no matter the mic in use.

Regards,
Wayne

 

[Sliderule]

wow, the RTA-2 was introduced in 1978. I downloaded the manual and found there was no way to calibrate the RTA-2. What I did suggest to the user was to connect the pink nosie output to the mic input and monitor the response. it is an easy tool for production people to monitor quality(?).

 

[Tim Dawson]

I would rerun the test using the same mic for both, and without moving it. Sure, cal may not be there, but typically (at least with a good mic) those factors are not drastic, and I would expect to see the same thing. It's possible that the mic was just in a null, directional, or some other factor for the one dropping at 12k. You should also be able to validate that both instruments are reading correctly with a decent pink noise source.