Theory 057/Engineering & Materials /Acoustics
Absorbers Peak at a Depth Ratio
Absorbers are chosen by material and by density. The thickness may be the whole story.
The absorption peak of a porous absorber is set by the ratio of its thickness to the wavelength rather than by its flow resistivity, so the same material can be tuned across a band by thickness alone.
01The evidence
Absorption spectra of porous materials show peaks that move with thickness, and thin layers absorb poorly at low frequency regardless of material.
02The tension
Material selection dominates specification, with flow resistivity as the headline property. Thickness is treated as a space constraint rather than as the tuning parameter.
03The mechanism
A porous absorber works by converting particle velocity into heat through viscous and thermal losses. The velocity profile inside the layer depends on how its thickness compares with the quarter wavelength of the sound.
That makes the thickness-to-wavelength ratio the controlling variable, and predicts that the peak frequency should scale inversely with thickness across materials.
04What this is not
Material selection ranks by resistivity. This ranks by geometry, which predicts peak frequency from thickness.
05Predictions
- P1Peak frequency should scale inversely with thickness.
- P2Peak absorption should collapse onto the ratio.
- P3Material should set breadth, not peak location.
06Refutation
Peak frequency varying with material at fixed thickness would restore material-led selection.
07What it changes
It would make absorber design a geometric calculation.