Having been looking for said transducers, I've come across both receive and transmit versions of seemingly similar products. I would've thought that due to reciprocity, a so called transmitter could receive just as efficiently.
Could somebody enlighten me on the difference please? All I can gather really is slightly wider receiver bandwidths.
Examples
Receiver: http://uk.farnell.com/prowave/400sr100/receiver-ultrasonic-40khz-10mm/dp/1007342
Transmitter: http://uk.farnell.com/prowave/400st100/transmitter-ultrasonic-40khz/dp/1007341
Best Answer
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