The lousy old 741 opamp in the receiver has hardly any gain (about 20) at 40kHz. Also many 741 opamps do not work when the supply is as low as 9V. Instead, use a TL071 opamp that has a gain of about 90 at 40kHz and is guaranteed to work with a supply as low as 7V and higher.
The receiver is a poor design because it is inverting that has a very low input resistance (only 1k ohms) that shorts the output of the transducer and causes its most sensitive frequency to be reduced.