Frequencey response on soundcards

What's the frequency response on soundcards. I usually says 20-20k but I guess it's as untrue as for mics... What are the theoretical frequecy limits for A/D converters, based on 24/96? I can see the upper limit being dependent on the sample rate but why would there be a lower limit? Is the upper limit by any chance (sample rate)/2 as you would need two samples to record the voltage top and bottom of the shortest possible wave.

Thanks

/Ola
 
The frequency response depends on the sampling rate of the soundcard... if it can sample at 48khz (most soundcards), then the highest frequency it can record is 24 khz. If it's 96khz, then it can record up to 48khz... your right, the theoretical (and generally the real world) limits of the soundcard are based on that sampling rate / 2 formula.

The lower limit doesn't really have a specific formula, it's more due to the physical limitations... electrical circuits 'leak', and at lower frequencies, the leak is larger than the sound wave.

A (bad) comparison: pumping up a tire with a hand pump. If you pump it quickely, you'll put more air in than will escape out the pump (higher frequency sounds). If you pump it really slowly, more air will get out than gets pumped in (lower frequency sounds). In this case, putting air in is equivilent to having a larger waveform, and loosing air is equivilent to having a nice flat waveform... the low frequencies are lost.

Well, hopefully this makes SOME sense :)

William Underwood
 
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