What is the difference between ultrasonic receive and transmit 40khz transducers?
A similar question piqued my own interest when I was building a Time Difference of Flight system.

There is a difference in the impedance of the components at the exact frequency for which it was designed. Low impedance on the transmitter allows it to be driven at high power, high impedance on the receiver turns the received signals into a high voltage.

I wish I had a better reference, but the answer by Tracy Allen explains it well at: Source