Chapter 10 — Attenuation and the nonlinear edge
Where the small-perturbation theory breaks.
The wave equation we derived in chapter 4 was linearised. It ignored viscous dissipation, thermal conduction, molecular relaxation, and all terms quadratic in the perturbations. For most of acoustics this is excellent — losses are slow and amplitudes are small. But the corrections matter:
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Over long distances, even small absorption rates compound to substantial signal loss.
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At high frequencies, atmospheric absorption rises sharply, shaping outdoor acoustic propagation.
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At very large amplitudes, nonlinear effects produce shock waves and bridge to the planned Cavitation book.