Final project thoughts: How do Materials absorb sound?

or maybe "Modeling of sound absorption by porous materials"

Questions and Inspiration:

As part of a possible research project I want to find out which parameters to look for in the creation of possible dynamically tunable acoustic treatment system or absorption materials. How could mechanically compressing (for example) foam change its transmissibility of sound? At what point does absorption switch to reflection? Is the impact of controlling the density/porosity of a foam higher than changing its shape?

It might become apparent that it would be better to stray from material based absorption into the design of resonant panels or structures later.

Interested in:
  • Creating a model in which I can play around with (hopefully the smallest set of) parameters to see how transmisibillity of sound is impacted by for example size of pores in a foam.
  • I would like to understand if I could make a material which transitions from absorption to reflection at a certain rate of mechanical compression.

I'm not sure yet how to attack these questions. Defining the boundrary conditions for the wave equations might be the right way to go even though it most probably will be heavy on computational power.

General thoughts:

All materials absorb, reflect, and transmit sound in their own particular way. Sound absorbing materials are usually used to change the aural identity of an environment, either by quieting a source of noise or enhancing intelligity of sound.

"Acoustic absorption refers to the process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body. The energy transformed into heat is said to have been 'lost'." (Absorption (acoustics) - Wikipedia))

Porous absorbers

Porous absorbers fall into three categories:

  • Cellular
  • Fibrous
  • Granular

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Image credit: Arenas & Crocker

Absorptivity of porous materials is primarily related to porosity and flow resistivity. Porosity is defined by the fractional amount of air volume within the material. Pores should open and connected. Flow resistance is a measure of the resistance experienced by aor when it passes through open pores in a material (analogous to electrical resistance).

Important parameters of pores for absorption:

  • Open or closed cell walls
  • Shape
  • Length
  • Tortuosity

Some practical uses of sound absorbing materials:

  • Reduce noise / sound pressure
  • Soundproofing
  • Reduce reverberation
  • Eliminate echoes
  • Improve speech intelligibility

Absorption coefficients are usually measured in real physical settings in either large Reverberation Chambers (room scale) or smaller Impedance Tubes.

References (so far):

1) Arenas, Jorge & Crocker, Malcolm. (2010). Recent Trends in Porous Sound-Absorbing Materials. Sound & vibration. 44. 12-17. )

2) Egab, Laith & Wang, Xu & Fard, Mohammad. (2014). Acoustical characterisation of porous sound absorbing materials: A Review International Journal of Vehicle Noise and Vibration. 10. 129 - 149. 10.1504/IJVNV.2014.059634.

3) Adams, Tyler Sound Materials - A Compendium of Sound Absorbing Materials for Architecture and Design

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