Young's double slit experiment?

[dawn_pingpong]

Two small loud speakers A and B are positioned 1.5m apart in a large room, connected to the same signal generator. The frequency emitted is 3400Hz . A microphone is placed equidistant from the 2 loudspeakers. The perpendicular distance between the speakers and microphone is 9.0m. Calculate the distance between the original position of the microphone and the first maximum. (ie microphone is moving parallel to the 2 loudspeakers.

Formulas:
X=λ*D/d
V=fλ
Speed of sound 340

Solution
By v=fλ,
λ=0.1m
then use X=λ*D/d
x=0.6m
However the answer key is 0.3m, and say that youngs double slit formula cannot be used. It's something like.

0.5△x=λD/a,which I don't really understand what it means...

Thank you!

 

[jbsteele]

The Young's double slit formula that is being used is X = λD/d, where λ is the wavelength of the sound, D is the distance between the two speakers, and d is the distance from the microphone to one of the speakers. Using this formula, we can calculate the distance between the original position of the microphone and the first maximum as 0.3m.