# Doppler effect with 2 sound sources

• papanik
In summary, The fly is moving at 3m/s, the bat is chasing it at 5m/s, and is emitting a sound at 50kHz. They are both moving towards a source emitting a sound at 57Hz. The frequency of the sound the fly is hearing is calculated using the Doppler effect equation and is found to be approximately 50.3kHz. The fly will also hear a sound from the source at 57.5Hz. The length of the fly is not needed in the calculations. The equation for f1 should have (v-vs) in the denominator. The frequency of the total sound may actually be 57kHz instead of 57Hz.

## Homework Statement

A fly is moving with a speed vl=3m/s and a bat is chasing it with vs=5m/s. The bat is emitting a sound with fs=50kHz. They are moving on the same line towards point C which is a source emitting a sound at f'=57Hz. Find the frequency of the total sound the fly is hearing. The fly has a length of 2mm.

## Homework Equations

Doppler effect equation

## The Attempt at a Solution

The fly is hearing from the bat a sound at a frequency f1=(v-vl)*fs/(v+vs)=50.3kHz approximately
The fly is also hearing a sound from source C f2=(v+vl)*f'/v=57.5Hz
where v=343m/s speed of sound
The frequency of the sound the bat is emitting is not influenced by source C and vice versa.
Since f1>>f2 the fly will hear two different sounds with discrete frequency
The length of the fly i assume that it is not needed in the calculations

Hi,
Since the bat is moving towards the fly, in the equation for f1 the denominator should be (v-vs). Other than that everything is OK..

Thank you

papanik said:
They are moving on the same line towards point C which is a source emitting a sound at f'=57Hz. Find the frequency of the total sound the fly is hearing.
Are you sure that's not 57kHz?