Question: Compare the result you found in Exercise to the result of Eq. The requirement D >> d is not fulfilled in this case, but the result of Eq. is surprisingly accurate.
Exercise: Assume the two speakers are a distance d = 4m apart and emitting a pure tone with λ = 0.34 m. For D = 12m at what y positions on the vertical line do observers hear the loudest sound? What frequency corresponds to this wavelength?
dsinΘ = nλ