Sound absorption

Acoustic absorption refers to a material, structure or object absorbing sound energy when sound waves collide with it, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted.

The energy transformed into heat is said to have been ‘lost’.
When sound from a loudspeaker collides with the walls of a room part of the sound’s energy is reflected and part is absorbed into the walls. As the waves travel through the wall they deform the material thereof (just like they deformed the air before). This deformation has mechanical losses which convert part of the sound energy into heat through acoustic attenuation, mostly due to the wall’s viscosity.

The same attenuating mechanics apply for the air and any other medium through which sound travels. The fraction of sound absorbed is governed by the acoustic impedances of both media and is a function of frequency and the incident angle. Size and shape can influence the sound wave’s behavior if they interact with its wavelength, giving rise to wave phenomena such as standing waves and diffraction.

Acoustic absorption is of particular interest in soundproofing. Soundproofing aims to absorb as much sound energy (often in particular frequencies) as possible converting it into heat or transmitting it away from a certain location.

In general soft, pliable, porous materials like cloths serve as good acoustic insulators absorbing most sound. Whereas dense, hard, impenetrable materials like metals reflect most.

How well a room absorbs sound is quantified by the effective absorption area of the walls, also named total absorption area. This is calculated using its dimensions and the absorption coefficients of the walls. The total absorption is expressed in Sabin and is useful in for instance determining the reverberation time of auditoria. Absorption coefficients can be measured using a reverberation room, which is the opposite of an anechoic chamber.

Electrical and Mechanical Analogy

The energy dissipated within the medium itself as sound travels through it is analogous to the energy dissipated in electrical resistors or that dissipated in mechanical dampers. All three represent the resistive part of a system of resistive and reactive elements. The resistive elements dissipate energy (irreversible) and the reactive elements store and release energy (reversible). The reactive parts of an acoustic medium are represented by its bulk modulus and its density, analogous to respectively an electrical capacitor and an electrical inductor, and analogous to respectively a mechanical spring and a mass.

Note that since dissipation solemnly relies on the resistive element it is independent of frequency. In practice however the resistive element itself varies with frequency. For instance when the vibration of matter interacts with its physical structure and changes physical properties thereby changing the ‘resistance’. Additionally the cycle of compression and rarefaction exhibits hysteresis which is a function of frequency, although for every compression there is a rarefaction, the total amount of energy dissipated due to hysteresis changes with frequency. Furthermore some materials behave non-Newtonian causing their viscosity to change with the rate of shear strain experienced during compression and rarefaction, in other words change with frequency. Gasses and liquids generally exhibit little hysteresis (sound waves cause adiabatic compression and rarefaction) and behave Newtonian. Combined, the resistive and reactive properties of an acoustic medium form the acoustic impedance. The behavior of sound waves encountering a different medium is dictated by their acoustic impedances.

As with electrical impedances there can be matches and mismatches depending on intended purpose, energy can be transferred for certain frequencies whereas for others it could be mostly reflected.

In amplifier and loudspeaker design electrical impedances, mechanical impedances, and acoustic impedances of the system have to be balanced such that the frequency and phase response least alter the reproduced sound across a very broad spectrum whilst still producing adequate sound levels for the listener. Modeling acoustics as electric circuits gives designers a powerful design tool.

When a sound wave strikes a rigid surface some of it is absorbed by the friction on the surface and the remaining sound generated is absorbed in four ways

  • Absorption in the air – The absorption by the air particles is mainly due to the friction between the oscillating air particles when air travels in air. However this value of absorption is extremely small.
  • Absorption by the audience – Sound energy is usually absorbed by the clothing of the people present in the room. This value can change in winter due to heavy clothes.
  • Absorption by furniture and furnishings – Furniture, curtains, carpets, etc. have the ability to fairly absorb sound energy to a good extent.
  • Absorption by boundary surface – When sound waves strike boundary surfaces like walls, floors, ceilings, etc. absorption takes place due to the following factors
    • Penetration of waves in porous materials
    • Resonant vibration of panel materials
    • Molecular damping
    • Transmission through structures
Sound in interiors
Acoustic design of a hall

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