Hello Koky,
I think the best solution for you might be to go with several M series boards. If the largest frequency you will measure is 20KHz, then according to the Nyquist theorem, you will need to sample at least twice that frequency (i.e. at least 40KS/s). However, this will not give you good shaping of the waveform. Morever, you have to multiply 40KS/s by 16 for your 16 microphones and we are now at 640 KS/s because the M series boards have to multiplex all 16 channels. You could go with the S series boards which do simultaneous sampling but they are more costly. The data sheet for any DAQ board will give you more information on sampling frequency, range, and interchannel delay so you can check and see which boards work best for you. I would recommend giving your technical representative a call at (888) 280-7645. They will be able to help you design a system and give you a price quote as well.
Take care,
E.Lee
Eric
DE For Life!