# Doppler together with beat

1. Jun 25, 2012

### IIK*JII

1. The problem statement, all variables and given/known data
In the figure, a motionless observer stands between sound sources A and B, which oscillate at 338 Hz and 342 Hz, respectively. The observer hears a beat. Next, when the observer begins moving at a constant speed on the straight line connecting A and B, the beat is no longer heard. Here, the speed of sound in air is 340 m/s

2. Relevant equations
fb=|fB-fA|....(1)

Doppler equation
f'= f($\frac{v\pm v0}{v\mp vs}$)

v0 = observer's speed

3. The attempt at a solution
1st, I assume that observer walking towards A in order not to hear beat frequency.

Thus, f$^{'}_{A}$=fA($\frac{v+v0}{v}$) (2)
f$^{'}_{B}$=fB($\frac{v-v0}{v}$) (3)

from (1); fb = |338-342|=4 Hz
so |(2) - (3)| = 4 Hz and I got only 1 unknown to solve
∴v0 = 4 m/s
but the answer is 2 m/s toward A, Oh!! Did I do wrong way or my assumption was wrong??
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jun 25, 2012

### Yukoel

Hello IIK*JII
When the observer doesn't hear beats its frequency should be zero or the two frequencies should be equal ,right?I am getting 2m/s as well if my calculations are not wrong. Maybe you have messed up your calculations somewhere by the looks of your method I suspect you have inverted the situation and still getting 4 beats .
Rest is all correct .Even if you had assumed that observer walked towards B you'd getting the same answer in negative.
Correct me if I am wrong.
regards
Yukoel

3. Jun 25, 2012

### IIK*JII

Yukoel !!!!

Okay, I understand it because I inverted the situation..

Thanks a lot :)