Absorbent Material

A material which absorbs energy from an incident sound wave and therefore can be used to reduce the amount of noise reflected from walls or roofs. There are three basic forms of sound absorbent materials:

  • Porous materials (e.g. mineral wools, felt). Consisting of networks of interconnecting pores. In the narrow channels and cavities viscous loses turn some of the acoustic energy into heat. The absorption characteristic is a strong function of frequency - low at low frequencies and increasing with the thickness of material. The low frequency absorption can be improved by mounting the material away from the wall/panel. Rockwool and cork are two commonly used absorbent materials. Absorbent materials e.g. bonnet pads effect only the higher frequencies, for instance alternator noise, because such materials can only absorb frequencies which have a wavelength equal to or less than twice the materials thickness.
    Fibrous material
  • Conventional acoustic lining covering a frequency spectrum down to around 100 Hz consumes as much as 1/3 of the room’s raw volume.

  • Mass/spring (e.g. panel mounted away from a backing, skinned foam) - the panel vibrates under the influence of the incident sound and dissipates some of the incident sound energy into heat. Absorbers of this type are very efficient at low frequencies.

  • Cavity resonators (e.g. Helmholtz resonators) - act most efficiently over a narrow frequency range. May be in the form of perforated panels with a lattice work behind forming many resonators tuned to slightly different frequencies resulting in attenuation over a wide frequency range.
    Perforated panelHelmholtz resonator
The position and distribution of absorbent material within a volume is important.

Area Effect - Acoustical materials spaced apart can have greater absorption than the same amount of material butted together. The increase in efficiency is due to absorption by soft exposed edges and also to diffraction of sound energy around panel perimeters.
There are 6 basic parameters that are used when modelling an absorbent material:

Porosity The ratio of the volume taken up by connected pores to the total volume of the material.

Tortuosity A measure of pore or streamline curvature.

Characteristic viscous length A geometrical parameter determined by the viscous interactions between fluid and structure and indicative of the narrowest pore dimensions.

Characteristic thermal length A geometrical parameter similar to hydraulic radius that can be determined from the internal surface area per unit volume.

Fow resistivity The ratio of the static pressure gradient across the sample to the resulting flow velocity.

Thermal permeability Related to diffusion and trapping of air molecules at pore walls.

Reference

ASTM C522 Airflow Resistance of Acoustical Materials

See also: Absorbent Ducts, Absorption Coefficient, Acoustic Pack, Adsorbent, Sound Absorption.

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Subjects: Materials Noise & Vibration