Experiments in InfraSonics Notebook
Measurements on Soundcards:
The expected frequency response of a soundcard would be limited at the high end by the maximum sampling rate (max. sampling rate 44100Hz) - about 20kHz (upper limit of human hearing) by nyquist theorem. The low frequency end can be expected to be not lower than about 20Hz (the lower limit of human hearing).
Feeding a wideband noise signal into the microphone inputs of a PC soundcard and then the microphone input of a laptop produced the following results.
The PC soundcard frequency response:-
From this you can see that the low end frequency is 3dB down at about 14Hz.
The laptop soundcard frequency response:-
Here the low end frequency response is 3dB down at about 8Hz.
On the output side of the equation the PC soundcard output is 3dB down at about 5Hz as measured on and oscilloscope. I did not measure the output frequency response from the laptop.
From these results it can be reasonably expected that the PC and laptop soundcards can be used for reception and transmission of ultrasonic signals down to about 15Hz.