Constructive Interference of Sound Waves

[cjavier]

Suppose two loudspeakers are separated by three meters and are being driven by exactly the same sinusoidal signal at a frequency that can be varied. If you are located four meters away directly in front of one of the speakers, determine three lowest frequencies for which you would hear constructive interference. (Assume speed of sound to be 346 m/s) Explain your reasoning carefully.

Through my professor's book, I know that constructive interference happens when waves are in phase. I believe one of the equations I should be using is total phase difference = phase difference of the sources + 2∏Δr/λ. Not sure where to go from here.

 

[CWatters]

It's a 3,4,5 triangle so the two distances are 4 and 5 meters. For constructive interference the difference (eg 1m) must be a multiple of what?

Once you know that apply v=fλ.