= Peter H. Dahl
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<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.<Applied Acoustics Lectures 12–14 by Peter H. Dahl>[Lecture13 pp.6–8 and lecture14 pp.1–3]{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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