# Homework Help: Sound waves prob

1. Feb 21, 2005

### kv2

A bat flies toward a wall, emitting a steady sound of frequency 2.00 kHz. This bat hears its own sound plus the sound reflected by the wall. How fast should the bat fly in order to hear a beat frequency of 10.0 Hz?
Give your answer to two significant figures. Take the speed of sound to be 344 m/s.

i know it's like a doppler + beat frequency problem and the equation to use is basically v=freq*wavelength and substitude lots of stuff in. but i get confused... someone help plz. thanks.

2. Feb 22, 2005

### Andrew Mason

Use the doppler shift formula for moving source and stationary observer to find the frequency at the wall. The wall then reflects that sound back to the moving bat, so it acts like a stationary source for the reflected sound. Use a second doppler shift this time for stationary source and moving observer using the first doppler shifted frequency. This will give you the final reflected frequency heard by the bat in terms of the speed and the original frequency. Set v so that the result is a difference of 10 hz

AM

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