Sound waves: In sound waves, the velocity magntiude (or the Mach number) is very small, although the unsteady term is now comparable to the other leading terms in the equation. Thus neglecting all quadratic and higher-order terms and noting that in the same approximation, is a constant (for example, in polytropic gas ), we haveLamb (1994) §287, pp. 492–495.
Sound waves: In sound waves, the velocity magntiude (or the Mach number) is very small, although the unsteady term is now comparable to the other leading terms in the equation. Thus neglecting all quadratic and higher-order terms and noting that in the same approximation, is a constant (for example, in polytropic gas ) we haveLamb (1994) §287 pp. 492–495