British Bats
Why use ultrasound?
Bats use high frequency sounds known as ultrasounds with which they echolocate. These calls are in the range of 20 and 120 kHz. Humans can only hear sounds with a frequency of up to 20 kHz, sounds above this level are inaudible. There are many disadvantages to bats in using sounds that are so high. Not only are they very costly to produce, requiring a lot of energy, but because they are at a high frequency they are quickly degraded by the environment. This means they are unable to travel over long distances. Sounds lower in frequency travel much further before disappearing. The problem is compounded because in echolocation the sound must travel both from the bat to the object it wants to detect, and then back again, thus having to cover double the distance. This means that echolocation can only really be used over very short distances and for objects that are close to the bat. Normally echolocation calls can only discriminate objects no more than 1 or 2 meter's away from the bat. As an example of how sounds of different frequencies travel we only need to compare the Noctule and the Pipistrelle and their calls. The calls of the Noctule are fairly low in frequency, being only about 20 kHz, these travel about 25 meter's before being degraded by the atmosphere and disappearing, the calls of the Pipistrelle however, are higher in frequency typically being about 45 kHz, these calls only travel about 10 or 15 meter's before being destroyed.
Why then do bats use such high frequency sounds? There are a number of reasons but the main one is that by using calls with a high frequency allows a good resolution of even very small objects. When a high frequency of sound is used in echolocation the wavelength of the sound is shorter. As the frequency increases the wavelength of sounds decreases. Sounds with shorter wavelength allow smaller objects to be detected. As bats catch insects, which tend to be very small in size, it makes sense to emit calls that have short wavelengths so that the insects can be detected. In the complex, cluttered environment in which bats live, detailed pictures of their immediate surroundings are essential, and therefore calls with high frequencies which allow this level of detail must be used.