Peter H. Dahl develops physical accounts of acoustic radiation and measurement. His applied-acoustics lectures derive how an extended source's geometry determines its directional sound wave field and distinguish hydrodynamic near-field effects from geometric near/far-field approximations.[ref]
The far-field approximation drops quadratic path-length phase terms only when their phase error is small. This explains why a receiver being many wavelengths away is not always sufficient for a large aperture: source size also enters the criterion.
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