1.   It is a dilemma inherent in the dramatic difference in intensity between outgoing sound and returning echo, a difference that is inexorably imposed by the laws of physics.

2.   The radar signals were sent out in necessarily very powerful pulses, which might have damaged the highly sensitive aerials waiting for the faint returning echoes.

3.   Then they relax so that the ear returns to maximal sensitivity just in time for the returning echo.

4.   By comparing the outgoing frequency with the frequency of the returning echo the police, or rather their automatic instrument, can calculate the speed of each car.

5.   Actually some bats play a trick that is more interesting than simply emitting hoots of constant pitch and measuring the pitch of the returning echoes.

6.   The devices send acoustic pulses straight down into the water, and the intensity of the returning echo is used to calculate the abundance of fish.