We want a sound wave with a wavelength of 0.52 meters or a natural fraction thereof. We'll work in MKS.
w = 0.52/n
That's length. We have speed 344 meters/second so w corresponds to a frequency of
f = 344 / w = n (344/.52)
f = 661.5 n Hertz
I don't really agree with how they're saying it, but all the fundamental talk is probably trying to tell us n=1,
Answer: 661.5 Hertz
Any multiple of that will also produce constructive interference; we can go to about n=30 before we're out of the audio range.
