Help w choppy distortion graph

[holthillaudio]

Hello all - I am attempting to use REW to estimate THD on a Klipsch midrange horn. I am using REW v 5.19, JRE 1.8.0_162 64 bit on Mac OS X 10.15.6
I am using the sweep function because it's a little faster than the stepped analysis, and I have a number of drivers to measure.
UMM-6 Mic with calibration file loaded. Mic is positioned in line with the driver, but at the edge of the horn profile.
The sweep is set from 700-8,000 Hz, 1M length. I have tried taking 1-4 sweeps per measurement and the results are the same. The distortion graph shows "choppy" and then cuts out completely after 4kHz.

I have tried adjusting both the reference signal levels and the output levels. I get the same results.

Any reason for this? Settings I need to change? Any help would be appreciated. Thanks, Brian

 

[John Mulcahy]

The "Mask harmonics below noise floor" option greys out anything that lies below the noise floor of the measurement, which is nearly everything for those measurements. Probably have to drive the horn harder to see its distortion, or use the stepped measurement which has a much lower noise floor. Since you ended the measurement at 8 kHz the 2nd harmonic ends at 8 kHz / 2 = 4 kHz, the 3rd ends at 8 kHz / 3 = 2.67 kHz etc.

 

[holthillaudio]

The "Mask harmonics below noise floor" option greys out anything that lies below the noise floor of the measurement, which is nearly everything for those measurements. Probably have to drive the horn harder to see its distortion, or use the stepped measurement which has a much lower noise floor. Since you ended the measurement at 8 kHz the 2nd harmonic ends at 8 kHz / 2 = 4 kHz, the 3rd ends at 8 kHz / 3 = 2.67 kHz etc.

Got it. Thanks John.