Frequency of sound detected by microphone

[desmond iking]

first, i find the wavelength of sound = 330/1100= 0.3m

then, I find the frequency of rise and fall of intensuty of sound = 3/0.3 = 10hz..
but the ans is 20hz. which part is wrong?

 

[Doc Al]

How does the intensity of the sound vary along the line between X and Y? Where are the peaks?

 

[NascentOxygen]

first, i find the wavelength of sound = 330/1100= 0.3m

then, I find the frequency of rise and fall of intensuty of sound = 3/0.3 = 10hz..
but the ans is 20hz. which part is wrong?

I reckon the part that's wrong might be right near the start, when you told yourself that this is too straight-forward to bother sketching any diagram.

How about drawing the two speakers and what you're expecting to find between them?