How is it that they receive and transmit ultrasonic frequencies with a computer sound card? Is there not a low-pass filter at around 22KHz on inputs and outputs of all sound cards?
To get up to 23kHz I used a sample rate of 48kHz.
It's not a low-pass filter per se, the limit is imposed by the Nyquist–Shannon sampling theorem (http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_samplin...), which essentially limits the frequency spectrum to 1/2 the clock rate. Good-quality audio from a sound card is clocked at 44 KHz, which means the audio range extends only to 22 KHz.
But you can clock a modern sound card at a rate much faster than 44 KHz, so the above might be only the normal limit, not the extreme.