# Problem on Doppler effect and sound level

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1. Jul 5, 2016

### Soren4

1. The problem statement, all variables and given/known data
A source emits a spherical sound wave at frequency $f=110Hz$ with power $P=1 W$ in a solid angle of $2 \pi sr$. An observer moves with velocity $v_{oss}=108 km/h$ towards the source. Determine the sound intensity level received by the moving observer.

2. Relevant equations
Doppler effect and sound intensity level

3. The attempt at a solution
I just do not know how to approach the problem because it is very different from a common Doppler effect problem.

The frequency perceived by the observer is $f'=f(\frac{c+v_{oss}}{c})$ but this does not give me the sound intensity level

To find the sound intensity level means to find the intensity. Now, the intensity of a sound wave is $I= 2 \pi^2 f^2 A^2 \rho$.

Plugging $f'$ in the formula for $I$ does not give the correct result and there must be something that I am missing. How to do such exercises?

2. Jul 5, 2016

### Dr. Courtney

You need to define all the variables.