Reference literature and physics textbooks report that sound waves propagate adiabatically (or approximately adiabatically) because the rapid compressions and rarefactions occur without allowing heat to escape.
Causality, a cornerstone of physical laws, fundamentally links a system's structural characteristics to its wave interaction properties, such as the minimum thickness of acoustic absorbers required for specific absorption spectra. Traditional causality principles for sound absorption are derived under the assumption of adiabatic sound propagation. In this Letter, we propose a generalized causal framework that incorporates isothermal processes, accounting for nonslip boundary conditions at the fluid-solid interface. These conditions introduce velocity and temperature gradients, challenging the conventional adiabatic assumption. To validate our framework, we analyze two distinct absorber types: a metamaterial with multiresonant units and a metafoam with multilayer double-porosity structures. Our theoretical and experimental studies reveal that absorber thickness can exceed the adiabatic limit, being instead governed by isothermal constraints. This paradigm shift deepens the understanding of sound absorption mechanisms and paves the way for designing high-performance acoustic devices that approach fundamental performance limits.
Dalton and others had endeavoured to measure directly the rise of temperature produced by the compression of a gas. Dalton had observed a rise of 50° F. in a gas when suddenly compressed to half its volume, but no thermometers at that time were sufficiently sensitive to indicate more than a fraction of the change of temperature. Laplace was the first to see in this phenomenon the probable explanation of the discrepancy between Newton’s calculation of the velocity of sound and the observed value. The increase of pressure due to a sudden compression, in which no heat was allowed to escape, or as we now call it an “adiabatic” compression, would necessarily be greater than the increase of pressure in a slow isothermal compression, on account of the rise of temperature. As the rapid compressions and rarefactions occurring in the propagation of a sound wave were perfectly adiabatic, it was necessary to take account of the rise of temperature due to compression in calculating the velocity. To reconcile the observed and calculated values of the velocity, the increase of pressure in adiabatic compression must be 1.410 times greater than in isothermal compression.
Turbulent processes in the convective envelopes of the sun and stars have been shown to be a source of internal acoustic excitations. In single stars, acoustic waves having frequencies below a certain cutoff frequency propagate nearly adiabatically and are effectively trapped below the photosphere where they are internally reflected. This reflection essentially occurs where the local wavelength becomes comparable to the pressure scale height. In close binary stars, the sound speed is a constant on equipotentials, while the pressure scale height, which depends on the local effective gravity, varies on equipotentials and may be much greater near the inner Lagrangian point (L_1). As a result, waves reaching the vicinity of L_1 may propagate unimpeded into low density regions, where they tend to dissipate quickly due to non-linear and radiative effects. We study the three dimensional propagation and enhanced damping of such waves inside a set of close binary stellar models using a WKB approximation of the acoustic field. We find that these waves can have much higher damping rates in close binaries, compared to their non-binary counterparts. We also find that the relative distribution of acoustic energy density at the visible surface of close binaries develops a ring-like feature at specific acoustic frequencies and binary separations.
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