FIFTH, prevention of Self-Defeaning.

Since ultrasound is so high-pitched, have you wondered why do bats never go deaf? There are two different ways in which bats conduct echolocation. This categorisation is based on the percentage of time the bat spends emitting ultrasounds. 

Low duty cycle bats have a situational alteration of the length of each call. When the bats are searching for aerial prey, low duty cycle bats release short bursts of ultrasounds that last between 5-20 ms, 10 times per second.

As it centres on a particular target, it goes into the approach phase, where frequency of the sounds emitted are higher while pulse duration decreases even more. 

As it gets even closer to the target, it enters the terminal phase where each pulse shortens even more to a fraction of a millisecond.The time taken for the sound to reach its target and reflect back into the bat’s ears decreases as the distance needed to be covered by the sound wave decreases.

However, these bats are unable to handle pulse emitting and echo listening simultaneously as they contract their middle ear muscle when emitting ultrasound to prevent self-deafening but need to relax it in order to listen out for returning echoes. As the middle ear muscle contracts to separate the three bones there--the malleus, incus and stapes, or hammer, anvil and stirrup - reducing the hearing sensitivity. The middle ear muscle relaxes 2 to 8 ms after the contraction. The ear is then ready to receive the echo of even an insect one meter away.




Here is a video for a concise summary about low duty cycle echolocation! :)

High duty cycle bats however, emit long pulses of ultrasound that last from 10 ms to 50 ms, and with duty cycles that can exceed 80% of the time. Their ears are attuned to a specific frequency range that does not include its own call, preventing self-deafening. However, due to shift in frequency of the echoes arising from the Doppler effect caused by its own motion, they are able to hear the echoes. Bats thus have to be able to handle the fact that the Doppler shift occurs on part of both the movement of its target and also its own flight, having the ability to change the frequency of their pulse with respect to their flight speed in order to nullify the Doppler shift effect from its motion such that the echoes remain in their hearing range.

Here is a video for a concise summary about high duty cycle echolocation! :)