How Far Must Point P Be from Speaker A for Constructive Interference at 400 Hz?

[Firben]

Homework Statement

Two speakers A and B, sends out sounds at 400 hz

In what distance x between speaker A and the point P is it constructive interference ?

http://s716.photobucket.com/user/Pitoraq/media/Fys2121_zps9a68fef2.png.html

Homework Equations

λ = 2ΔL/(2p-1),

v = 340 m/s

f = 400 Hz

The Attempt at a Solution

λ = 2ΔL/(2p-1),

λ = 340/400 = 0.85 m <==>

lowest constructive interference p = 1

0.85 m = ΔL <==>

0.85 = y - x, where y = (2.5)^2+x^2=y^2 is the distance between P and B.

Is this right ?

 

[olivermsun]

That should get you a constructive interference point (node).
I don't think the nearest node to A (i.e., smallest x) occurs for p = 1, though.