Glossary

8.1 Sound absorption mechanisms and materials

2 min read•july 24, 2024

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

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 , essential for acoustic design.

Sound Absorption Mechanisms

Mechanisms of sound absorption

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  • occur when air particles rub against material fibers converting sound energy into heat through friction
  • 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

  • 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
  • consisting of thin panels or sheets mounted over an air cavity effectively absorb low frequencies by converting sound energy into

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
    • 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

  • (α\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 () or impedance tube method ()
  • Calculate : αavg=S1α1+S2α2+...+SnαnS1+S2+...+Sn\alpha_{avg} = \frac{S_1\alpha_1 + S_2\alpha_2 + ... + S_n\alpha_n}{S_1 + S_2 + ... + S_n} where SnS_n is surface area and αn\alpha_n is absorption coefficient
  • Determine (NRC) by averaging absorption coefficients at 250, 500, 1000, and 2000 Hz: NRC=α250+α500+α1000+α20004NRC = \frac{\alpha_{250} + \alpha_{500} + \alpha_{1000} + \alpha_{2000}}{4}
  • Apply Sabine formula for reverberation time: T=0.161VAT = \frac{0.161V}{A} where TT is reverberation time, VV is room volume, and AA is total absorption

Key Terms to Review (18)

Absorption Coefficient: The absorption coefficient is a measure that indicates how much sound energy is absorbed by a material as sound waves pass through it. This value, which ranges from 0 to 1, helps to quantify the effectiveness of different materials in reducing sound reflections in a given space, ultimately impacting sound quality and clarity.
Average absorption coefficient: The average absorption coefficient is a measure that quantifies how much sound energy is absorbed by a material compared to how much sound energy strikes it. This coefficient, typically ranging from 0 (no absorption) to 1 (total absorption), is essential for understanding how different materials affect sound behavior in various environments. It plays a critical role in assessing the acoustic performance of materials used in spaces such as concert halls, recording studios, and other environments where sound quality is paramount.
Helmholtz Resonator: A Helmholtz resonator is a type of acoustic resonator that consists of a cavity connected to the outside air through a neck or opening. It is designed to resonate at a specific frequency, absorbing sound energy at that frequency and thereby reducing unwanted noise in an environment. This principle plays a key role in sound absorption mechanisms, as it allows for targeted reduction of certain sound frequencies.
High frequencies: High frequencies refer to sound waves that have a high pitch and short wavelengths, typically in the range of 2,000 Hz to 20,000 Hz. These frequencies are significant in various acoustic applications, as they are often absorbed more readily by materials than lower frequencies, affecting sound clarity and overall acoustic performance in different environments.
ISO 10534: ISO 10534 is an international standard that specifies methods for measuring the sound absorption of materials in a laboratory setting. This standard plays a crucial role in assessing how different materials can reduce sound energy, helping in the selection of effective sound-absorbing materials for various applications, such as acoustical treatment in buildings and soundproofing.
ISO 354: ISO 354 is an international standard that outlines the method for measuring the sound absorption of materials in a controlled laboratory environment. This standard is crucial for assessing how different materials interact with sound, which directly influences acoustic performance in various applications, including architecture and environmental noise control. It provides a systematic approach to evaluate sound absorption, helping to ensure consistency and reliability in measurements.
Low frequencies: Low frequencies refer to sound waves with a frequency range typically below 250 Hz, characterized by longer wavelengths and a deeper tone. These frequencies are significant in sound absorption and active noise control as they require different strategies for effective management, impacting how materials and systems are designed to mitigate unwanted noise or enhance sound quality.
Mechanical Vibrations: Mechanical vibrations refer to the oscillations of an object or system around an equilibrium position, caused by forces acting upon it. These vibrations can occur in solids, liquids, and gases and are fundamental to understanding how sound waves are generated and propagated in various materials. The nature of mechanical vibrations influences how sound is absorbed, transmitted, and reflected by different materials.
Membrane absorbers: Membrane absorbers are sound absorption devices that utilize a flexible membrane, which vibrates in response to sound waves, effectively dissipating sound energy. These absorbers function primarily through mass-spring-damper systems, where the mass of the membrane and the air cavity behind it create a resonant absorption effect, allowing for targeted sound frequencies to be attenuated. By adjusting parameters such as the tension and mass of the membrane, these devices can be optimized for specific acoustic applications.
Mid frequencies: Mid frequencies refer to the range of sound frequencies that lie between low and high frequencies, typically encompassing frequencies from about 250 Hz to 2000 Hz. This range is crucial in determining the tonal quality of sounds, including speech and musical instruments, making it essential in sound absorption mechanisms and materials.
Noise Reduction Coefficient: The Noise Reduction Coefficient (NRC) is a numerical value that measures the sound absorption performance of a material, indicating how much sound is absorbed versus reflected when sound waves hit it. NRC values range from 0 to 1, where 0 means no sound absorption and 1 indicates total absorption. Understanding NRC is essential for selecting materials that can effectively minimize unwanted noise in various environments.
Nrc - noise reduction coefficient: The noise reduction coefficient (NRC) is a numerical value that indicates the effectiveness of a material in absorbing sound. It ranges from 0 to 1, where 0 means no sound absorption and 1 means total sound absorption. The NRC helps to evaluate how different materials will impact the acoustic environment of a space, which is essential for managing sound levels in various settings.
Porous Absorbers: Porous absorbers are materials designed to reduce sound energy by allowing sound waves to enter their structure, where they are converted into heat through friction and other mechanisms. This process occurs as sound waves penetrate the interconnected air-filled pores, causing air particles to collide with the internal surfaces of the material, leading to energy dissipation. Porous absorbers are significant in controlling reverberation and improving acoustics in various environments.
Reverberation Time: Reverberation time is the duration it takes for sound to decay by 60 decibels after the source has stopped producing sound. It is a crucial factor in understanding how sound behaves in a space, affecting clarity, intelligibility, and overall auditory experience. The measurement of reverberation time is often tied to decibel levels, sound absorption characteristics of materials, and design principles specific to different environments.
Sound energy: Sound energy is a form of mechanical energy that travels through a medium as a wave, resulting from the vibration of particles. It can be perceived by the human ear as sound, and its characteristics are influenced by factors such as frequency and amplitude. Understanding sound energy is crucial in analyzing how sound interacts with different materials, leading to various applications in sound absorption mechanisms.
Structural losses: Structural losses refer to the loss of sound energy due to the physical properties of materials and structures that cause sound waves to dissipate rather than reflect or transmit. This concept plays a crucial role in understanding how different materials absorb sound, impacting overall acoustics in a space. By recognizing how structural losses occur, we can better select and utilize materials that effectively manage sound energy, enhancing acoustic performance.
Thermal losses: Thermal losses refer to the energy loss that occurs when sound waves interact with materials, resulting in a transfer of thermal energy. This process is significant in understanding how sound absorption works, as it explains how certain materials can dissipate sound energy into heat, thereby reducing the intensity of sound waves. These losses are a key factor in evaluating the effectiveness of sound absorption materials in various acoustic environments.
Viscous losses: Viscous losses refer to the energy dissipated in a medium due to the frictional effects encountered when sound waves propagate through it. These losses occur because of the interaction between sound waves and the particles of the material, leading to a conversion of acoustic energy into thermal energy. This phenomenon is crucial in understanding how sound absorption materials function and their effectiveness in reducing sound transmission in various environments.
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