Oscillation / Soundwaves Question - Phase difference?

[LadyMario]

An observer stands 3 m from speaker A and 4 m from speaker B. Both speakers,
oscillating in phase, produce 170 Hz waves. The speed of sound in air is 340 m/s.
What is the phase difference (in radians) between the waves from A and B at the
observer’s location, point P?

And I have no idea how to solve this :uhh:

Basically looks like:

P---------B (4m away)
|
|
|
|
A (3m away)

 

[tms]

Think about what what a phase difference means, and then think about wavelengths and distances, and how all three are related.

 

[LadyMario]

Think about what what a phase difference means, and then think about wavelengths and distances, and how all three are related.

I get that graphically a phase shift means it's literally shifted across the x-axis. But I can't think of an equation that would relate this an the distance.

 

[tms]

If you draw two sine waves, one shifted a bit relative to the other, on the same graph, a distance should jump out at you.

 

[LadyMario]

If you draw two sine waves, one shifted a bit relative to the other, on the same graph, a distance should jump out at you.

But we don't know the phase...

I get that they're 1m apart and it covers 340 m every second so the phase would be out of sync by 1/340.. do I convert that to radians then?

 

[tms]

The 340 m/sec is the speed of sound. You need to know the wavelength of the emitted sound, and compare that to the distance between the speakers.

 

[LadyMario]

The 340 m/sec is the speed of sound. You need to know the wavelength of the emitted sound, and compare that to the distance between the speakers.

Okay, so it travels 170 waves / s and one of the speakers is 1m further so it's out of phase by 170 waves (not sure how to get that into rads) but I feel like we need the speed of sound in there somewhere..

 

[tms]

Okay, so it travels 170 waves / s and one of the speakers is 1m further so it's out of phase by 170 waves

You are assuming that the wavelength is one meter; that is something you need to calculate using the wave speed and the frequency.