Calculating Relative Maximum for Sound Waves from Speakers Separated by .7m

[sun]

Homework Statement

A pair of speakers separatd by .700m are driven by the same oscillator at a frequency of 690Hz. An observer originally positioned at one of the speakers begins to walk along a line perpindicular to the line joining the speakers. How far mus the observer walk before reaching a relative maximum in intensity

Homework Equations

v(air)=345m/s

r2-r1=n(lambda)

The Attempt at a Solution

How do i figure out the r values when only one side is given? I know i should use the pythagorean theorem here. I also know a relative maximum is constructive interference so i will use a full integer for n. r2-r2=1(lambda). I'm not sure what I'm missing here.

i have:
f=690Hz
lambda=.5m
T=.0015s

suggestions would be appreciated.
thanks

 

[rl.bhat]

Let A an B are the positions of speakers and C is the position of the observer. When observer listenes maximum intensity, the path difference CB -CA = lamda = 0.5 m. According to pythagorean theorem CB^2 - CA^2 = AB^2. Solve these equations and find the distance CA of the observer.