On speed, the two transmitter "carrier" frequencies in use today, of which I am aware, are 50 and 200 kHz and transmitted by a percussive instrument (the pizoelectric crystal in your transducer), meaning sound waves. Electrical waves make it vibrate sending out sound vibrations and the received sound vibrations (coming back) make electrical signals. That's also the reason you can mount your transducer inside a metal boat that would block the signal if it were radio waves like you get over the air (TV, Internet, Satellite).
The reason for the different frequencies is probably so that you can use two different instruments on one boat without interference, or reduce the interference that might be experienced in a fleet of boats, since one is tuned to receive echos from one freq. and the other the other. Also in deep water applications, the lower frequency may provide better results due to electronic characteristics of the wave, or just do better in recognizing certain sized/density objects. Just a guess....never researched the reason. However sound travels at about 600 mph or 880 ft/sec, (your car travels at 88 ft/sec at 60 mph) regardless of the electrical carrier frequency. Just a tad faster than your boat. So the better responses we get today are just due to technology improvements and boy those things are happening so fast you can't keep up with them.
Didn't have anything else to do this morning so I thought I would exercise my jaw.
Mark