As sound travels at a constant speed in air, bats can measure how far away an object is by determining the difference between the time at which the call was emitted and the time at which the echo returns from the surface of the object. If an object is far away, the echo takes longer to return to the bat than if an object is nearer.
One of the fundamental reasons bats use ultrasound concerns the size of the insect prey. The intensity of the returning echo falls rapidly when the emitted wavelength is greater than the width of the insect. In order for the bat to detect an insect, the wavelength of the call must be equal to or less than the size of the insect. For example, if a bat is trying to catch an insect with a wing length of 1cm (0.01m), the emitted wavelength has to be less than 1cm, so the frequency it emits has to be at least 330/0.01 = 33000 Hz (or 33kHz) - since frequency = speed of sound / wavelength. This will be a minimum frequency. At higher frequencies the bat obtains more detail or resolution.
Sound pulses may be of higher or lower amplitudes (loudness) or the amplitude may vary with time. They can also vary in duration or frequency. Bats combine these variations within their echolocation pulse to create different “call shapes”. More on the next page….