What is the typical maximum
speed of such a device,
'twould seem that anything that goes through an analog/voice phone line
should work with acoustic coupling
OTOH, if you want to do FSK (Frequency shift keying), and want to do it by
counting cycles to differentiate the different tones, then think about how
many cycles you would need to identify a tone. If it is equal to, or more
than 1, then you're not going to get more bits per second than the lowest
tone's number of cycles per second. Therefore, you need to play games
with more than one bit per audio tone to get past about 1200 bits per
second.