Secondly as for the movements of the bats it is

2.2. Doppler effect
The Doppler effect Simvastatin the change in frequency of the periodic event when an observer moves relative to its source. Given the Doppler effect, the perceived sound frequency can change due to the relative motion between the observer and the source. Compared to the sound frequency itself, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession (Young, Freedman, & Ford, 2011).