Phase constant problem, two trumpets

[Daniiel]

Two trumpeters are standing 3m apart and blowing in the same not at 932Hz. A listener stands directly infront of one of the trumpeters. 10m from the wall. Take the velocity of sound to be 340m/s.

i) what is the phase difference between the two waves when they reach the listener?
ii)what frequency closest to 932Hz would produce a minimum intensity at the listener?

I think i have part i down
what i did was (making p = phase constant and w wave length, dl change in distance)

p=2dLpi/w
v=fw
so
p = (2pi dL f) / v
and i got 52.4

then for part ii I am not sure what to do, the equation for intensity involves power in my book
im not sure how to find power
thanks

 

[merryjman]

well, all you really need to do is think about phase difference. What would the phase difference be in order to produce minimum intensity? If you know that, then you can work backwards to find the required frequency, in a similar manner to how you used a given frequency to find phase difference at 932 Hz.

 

[Daniiel]

ohh
so would that be when the phase difference is like
180? pi?
thanks

 

[Daniiel]

or would it be like 90 degrees? pi/2