tuning forks for pure sine waves?

[scott wurcer]

Originally posted by jcx
loudspeakers could be acoustic filtered as mentioned above, indirect measurement of microphone IMD products in the sound feilds of 2 speakers driven with different frequencies could avoid the harmonics in each speaker's output

Shocked again, I was working out the best way to do this just last night. I was going to try to exercise the diaphram with a large lowish frequency tone (B&K mentions 90Hz in their handbook) and then using a lower level one at a higher frequency look for the IMD tones on the high frequency one with FFT's. I figure with enough averaging one could resolve pretty far down in the noise. Just a couple of power levels about 20dB apart should get some kind fit to theory for say a transducer with small 2nd and 3rd order non-linearity. Sort of indirectly trace the transfer function.

Has this ever been written up anywhere? Going up to 140dB on
the 90Hz might require the rest of the households cooperation.

[jcx]

a question occurs to me:
How does Doppler effect enter in this setup? - any high frequency sound reflected from the low frequency driver will be modulated - can the Doppler products be separated by phase? or do you just have to make them small (driver directivity?) and accept the residual as a resolution limit?

[Iain McNeill]

If I understand you correctly, you're using the intermodulation components to test the microphone distortion. The doppler components will be sidebands of the HF signal. Can't you just arrange the IM components to be away from the HF sidebands?

...or am missing something?

[scott wurcer]

Originally posted by jcx
a question occurs to me:
How does Doppler effect enter in this setup? - any high frequency sound reflected from the low frequency driver will be modulated - can the Doppler products be separated by phase? or do you just have to make them small (driver directivity?) and accept the residual as a resolution limit?

I was thinking maybe a shaped burst of 90Hz might help, but I think separation and directivity would be enough. Also prefiltering out the low tone would make the post processing easier, at least dynamic range wise.