The sound wave bifurcates at the juction of the straight and the rectangular parts. The wave through the straight part travels a distance `p_(1) = 10 cm ` and the wave through the rectangular part travels a distance `p_(2) = 3 xx 10 xm = 30` before they mwwt agian and travel to the receiver. The path difference between the two wave received, therefore.

`Deltap = p_(2) - p_(1) = 30 cm - 10 cm = 20 cm`

The wavelength of either wave is `(v)/(upsilon) rArr (400 m//s)/(upsilon)`. For constructive interference, `Deltap = nlambda`, where `n` is an integer.

or, `Deltap = n.(v)/(v) rArr v = (n.v)/(Deltap)`

`rArr v = (400)/(0.1) = 4000 n`

Thus, the frequencies within the specified range which cause maximum of intensity are

`4000 xx 1 Hz, 4000 xx 2 Hz`