8.1 Sound absorption mechanisms and materials

Sound absorption is crucial for controlling acoustics in spaces. Various mechanisms like viscous, thermal, and structural losses convert sound energy into heat. Different absorber types target specific frequency ranges, from porous materials for high frequencies to membranes for low frequencies.

Choosing the right absorbing materials involves considering the noise frequency, environmental factors, and aesthetics. Absorption coefficients quantify a material's effectiveness, ranging from 0 to 1. These values help calculate average absorption and reverberation time, essential for acoustic design.

Sound Absorption Mechanisms

Mechanisms of sound absorption

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• Viscous losses occur when air particles rub against material fibers converting sound energy into heat through friction
• Thermal losses result from heat exchange between air and material surface during compression and rarefaction of air molecules
• Structural losses happen as material fibers or structures vibrate dissipating energy through internal friction

Types of sound absorbers

• Porous absorbers like open-cell foam, fiberglass, and mineral wool effectively absorb mid to high frequencies through viscous and thermal losses
• Resonant absorbers such as Helmholtz resonators and perforated panels are tuned to specific frequencies utilizing resonance to dissipate sound energy
• Membrane absorbers consisting of thin panels or sheets mounted over an air cavity effectively absorb low frequencies by converting sound energy into mechanical vibrations

Material Selection and Absorption Coefficients

Selection of absorbing materials

• Consider frequency range of noise to be absorbed:
  • Low frequencies require membrane absorbers or thick porous materials
  • Mid frequencies benefit from porous or resonant absorbers
  • High frequencies are best absorbed by thin porous materials or perforated panels
• Evaluate environmental factors including humidity resistance, fire resistance, and durability
• Assess aesthetic requirements such as visible surface finishes and integration with existing architecture
• Account for space constraints including available thickness for absorber installation and weight limitations of supporting structure

Calculation of absorption coefficients

• Absorption coefficient ($\alpha$) represents ratio of absorbed to incident sound energy ranging from 0 (perfect reflection) to 1 (perfect absorption)
• Measure absorption coefficients using reverberation room method (ISO 354) or impedance tube method (ISO 10534)
• Calculate average absorption coefficient: $\alpha_{avg} = \frac{S_1\alpha_1 + S_2\alpha_2 + ... + S_n\alpha_n}{S_1 + S_2 + ... + S_n}$ where $S_n$ is surface area and $\alpha_n$ is absorption coefficient
• Determine Noise Reduction Coefficient (NRC) by averaging absorption coefficients at 250, 500, 1000, and 2000 Hz: $NRC = \frac{\alpha_{250} + \alpha_{500} + \alpha_{1000} + \alpha_{2000}}{4}$
• Apply Sabine formula for reverberation time: $T = \frac{0.161V}{A}$ where $T$ is reverberation time, $V$ is room volume, and $A$ is total absorption