Sound interference homework problem

[r_maths]

Really stuck at this question.

In an experiment on sound interference, two sources A and B are placed 2m apart. As a girl walks from X to Y she hears a point of maximum loudness ar point P and the next at point Q. Using information from the diagram below find distances AQ and BQ.
http://img102.imageshack.us/img102/6814/capturewiz006qq6.jpg

Any help would be welcome. Thanks.

PS. Answers = AQ- 12.4m BQ- 13m

 

[berkeman]

Welcome to the PF. One of our rules here is that you must show us some of your own work and thoughts in order for us to help you.

What can you say about constructive interference? What relationship must the two sound waves have at P and again at Q in order for there to be maximum constructive interference?

 

[r_maths]

Welcome to the PF. One of our rules here is that you must show us some of your own work and thoughts in order for us to help you.

What can you say about constructive interference? What relationship must the two sound waves have at P and again at Q in order for there to be maximum constructive interference?

Constructive interference, waves meet in phase.
At P would the zero order maximum.
At Q would be first order maximum.

I tried the similar triangles but that didn't lead to finding the lengths...
http://img19.imageshack.us/img19/4990/capturewiz007my4.jpg

 

[f(x)]

take AQ as x and BQ as $x+ \Delta x$
Apply equations for Constructive Interference and solve.
Fringe width is PQ.
BTW, you mat want to refer to Youngs Double Slit experiment.