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The Nyquist theorem says that with a sampling rate of F Hz you can faithfully reproduce signals with a frequency up to F/2 Hz. So for example a CD is sampled at 44 kHz and it reproduces sounds up to 22 KHz. This is hi-fi sound quality.
I found a Honeywell CVR/FDR manual which states that the CVR inputs are sampled at 8 or 16 KHz and thus can record sounds up to 3.5 or 6 kHz. (This gives some margin to the "Nyquist limit"). This is "telephone line" sound quality. I was slightly surprised that the sampling rate is so low. Probably a legacy from times when solid state memory was much more expensive.
Edit: I might add that in order to avoid sampling errors like you describe (aliasing), almost all digital systems use techniques such as a low-pass anti-aliasing filter before the digital sampler, or use oversampling i.e. a sampling rate several times the signal frequency.
Last edited by snowfalcon2; 23rd Jan 2015 at 07:27.