I should also point out the loading of the transducer on the output will also effect the 40KHz 555 frequency setting, as we dont know the transducer spec', the transistor buffer also isolates the transducer from pin #3.
EDIT:
Compare the two version, buffer, no buffer.
I have had to adjust the timing on the unbuffered version to get the frequency back to 40KHz, also note now the Mark/Space ration is no longer 1:1.
Also observe the power difference in the 470 load, the difference increases as the load resistor value is reduced.