When an echolocating bat approaches a target, its outgoing sounds return as echoes, which are Doppler shifted upward in frequency. In certain species of bats, which produce constant frequency (CF) echolocation calls, the bats compensate for the Doppler shift by changing their call frequency as they change speed towards a target. This keeps the returning echo in the same frequency range as the normal echolocation call. This dynamic frequency modulation is called the Doppler shift compensation (DSC), and was discovered by Hans Schnitzler in 1968.[1]
CF bats employ the DSC mechanism to maintain the echo frequency within a narrow frequency range.[2] This narrow frequency range is referred to as the acoustic fovea. By modulating the frequency of the outgoing calls, the bats can ensure that the returning echoes stay nearly constant within this range of optimal sensitivity. Ultimately, by keeping the echoes in this optimal range, the bats can quickly ascertain certain properties (such as distance and velocity) about the target.
This behavior appears to have evolved independently in several species of the Rhinolophidae and Mormoopidae families.[3] The common features shared by bats with DSC are that they produce CF sounds, and that they have a specialized cochlea that is adapted to receiving a narrow range of frequencies with high resolution.[3] DSC allows these bats to utilize these features to optimize the echolocation behavior.