[sdiy] Audio test generator and mesurement tool - can anyone suggest a good set?

[rsdio at sounds.wa.com]

On Apr 22, 2013, at 10:07, cheater00 . wrote:
> Also: are distortion analyzers usually limited to 20 Hz - 20 kHz, or
> are there ones that go below 1Hz and high above?


I just attended an Audio Engineering Society talk/lecture titled  
"Audio, Radio, Acoustics, Signal Processing—the Way Forward," by J.J.  
Johnston, where a very important point was made: noise measurements  
are completely meaningless unless the spectrum is considered (i.e.  
SNR is pointless w/o spectrum). That's because our hearing is very  
frequency dependent, and signal-to-noise ratios as poor as 6 dB can  
be fine because the noise (distortion) is inaudible. Meanwhile, a 60  
dB signal-to-noise ratio might be insufficient if the harmonic  
distortion or other noise is at a very sensitive frequency like 1.5  
kHz. Not all THD is the same!

Distortion is basically a subset of noise, and if you can't hear the  
distortion then there's little need to measure it. Below 20 Hz,  
you're maybe going to feel something, but you can't really hear it.  
It's difficult to calculate amplitude for extremely low frequencies  
anyway, so I don't see the point of measuring distortion below 20 Hz.  
Any significant amount of harmonic distortion will involve harmonics  
within the 20 Hz to 20 kHz band, so those would be measured. Don't  
forget that if you're controlling the test signal frequency, then you  
can still measure the *audible* distortion from signals between 1 Hz  
and 20 Hz, but the THD factor will only include the harmonics we can  
hear.

As Neil mentioned, there is certainly measurement available above 20  
kHz, so the argument about what can be perceived above that doesn't  
need to be settled before you can measure it.

Brian Willoughby
Sound Consulting