4.1 Sound reflection, absorption, and diffusion

Sound reflection, absorption, and diffusion are key concepts in room acoustics. These processes determine how sound waves interact with surfaces, shaping the acoustic environment. Understanding them is crucial for controlling reverberation, managing echoes, and creating balanced sound spaces.

Proper application of reflective, absorptive, and diffusive materials can optimize a room's acoustic properties. By strategically using these elements, we can tailor spaces for specific purposes, from clear speech in classrooms to rich, immersive sound in concert halls.

Sound reflection, absorption, and diffusion

Principles of sound reflection

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• Sound reflection occurs when sound waves encounter a surface and bounce back into the space
  • Follows the law of reflection where the angle of incidence equals the angle of reflection
• The amount of sound reflection depends on the acoustic impedance of the surface material relative to the medium the sound is traveling through
  • Hard, dense surfaces (concrete) reflect more sound compared to soft, porous surfaces (curtains, carpets)
  • Acoustic impedance is determined by the density and speed of sound in the material

Sound absorption mechanisms

• Sound absorption is the process by which sound energy is converted into heat as it interacts with and propagates through a material, reducing the amount of sound reflected back into the space
• Porous absorbers (fiberglass, foam, fabric) allow sound waves to enter the material where friction converts some of the acoustic energy to heat
  • Effectiveness depends on the thickness, density, and porosity of the material
  • More effective at absorbing high frequencies than low frequencies
• Resonant absorbers use a combination of a flexible membrane and air cavity behind to absorb sound energy at specific resonant frequencies
  • Resonant frequency is determined by the mass of the membrane and the stiffness of the air cavity
  • Examples include panel absorbers and Helmholtz resonators

Sound diffusion principles

• Sound diffusion is the scattering of sound waves in many directions by an irregular surface, reducing distinct echoes and creating a more even spatial distribution of sound in the room
• Diffusers are characterized by an uneven surface with wells of varying depths or shapes to scatter sound waves in different directions based on their frequency
  • Examples include Schroeder diffusers, pyramid diffusers, and barrel diffusers
• Diffusion helps to reduce strong specular reflections and flutter echoes in a room while still maintaining the acoustic energy and liveliness of the space
  • Specular reflections are mirror-like reflections from flat surfaces that can cause distinct echoes
  • Flutter echoes occur between parallel surfaces and result in a rapid series of echoes

Surface materials and sound propagation

Reverberation time and room materials

• The reverberation time (RT) of a room is the time it takes for sound to decay by 60 dB after the source stops
  • Depends on the volume and total absorption of the room
  • Rooms with longer RTs (concert halls) are considered more "live" while shorter RTs (recording studios) are described as "dry" or "dead"
• The choice of surface materials in a room, characterized by their absorption coefficients (α), determines how much sound energy is absorbed versus reflected at each frequency
  • Materials with higher α values (acoustic foam) absorb more sound
  • Absorption coefficients vary with frequency, so materials may absorb certain frequencies more than others

Surface geometry effects

• Concave surfaces (domes, barrel vaults) can cause sound waves to converge at a focal point, leading to uneven sound distribution and echoes
  • Convex or irregular surfaces help to scatter sound and reduce focusing effects
• Parallel surfaces in a room can cause flutter echoes and standing waves at specific frequencies related to the distance between the surfaces
  • Results in uneven frequency response and coloration of the sound
  • Can be mitigated by using non-parallel surfaces or adding absorption/diffusion
• The placement and orientation of absorptive materials in a room affects which sound rays are intercepted and attenuated as they propagate through the space
  • Treating specific surfaces (first reflection points) can help control distinct echoes or sound foci

Diffusion and scattering

• The use of diffusers on walls or ceilings helps to scatter sound waves and create a more diffuse, even sound field in the room
  • Reduces the negative effects of strong specular reflections or flutter echoes
  • Maintains the acoustic energy and spaciousness of the room
• Diffusers can be designed to scatter sound waves in specific patterns or directions based on their shape and dimensions
  • Quadratic residue diffusers (QRD) use a sequence of wells of varying depths to scatter sound based on mathematical principles
  • Primitive root diffusers (PRD) are similar to QRDs but use a different mathematical sequence for the well depths

Room acoustics optimization

Design considerations

• Room acoustic design involves selecting and placing surface materials and geometries to achieve the desired reverberation time, frequency response, and spatial distribution of sound for the intended use of the space
• The optimal reverberation time for a room depends on its volume and the type of sound source
  • Larger rooms and lower frequencies generally require longer RTs
  • RT should be balanced across frequency bands to avoid excessive warmth or brightness
• The placement of absorptive and diffusive materials in a room should be based on an analysis of the sound propagation paths and the desired acoustic response
  • May involve using ray tracing or modeling software (CATT-Acoustic, ODEON) to predict the behavior of sound in the space

Acoustic treatment strategies

• Absorptive materials are used to control the overall RT of the room and to attenuate specific reflections or echoes that may be problematic
  • Porous absorbers (acoustic panels) are effective at high frequencies
  • Resonant absorbers (membrane absorbers, Helmholtz resonators) can target lower frequencies
• Diffusers are used to scatter sound waves and create a more even, diffuse sound field in the room
  • Often placed on the rear wall or ceiling to maintain acoustic energy while reducing distinct echoes
  • Can be used in combination with absorbers to control the balance of clarity and spaciousness
• In some cases, active acoustic treatment (electronic reverberation, active noise control) can be used to supplement passive treatment and fine-tune the room response
  • Electronic reverberation systems can add adjustable reverb to a room
  • Active noise control uses microphones, speakers, and signal processing to cancel out unwanted noise

Application examples

• Recording studios and control rooms
  • Require shorter RTs (0.2-0.5 seconds) and neutral frequency response for accurate monitoring and mixing of audio
  • Use a combination of absorbers and diffusers to control reflections and create a diffuse sound field
  • Isolation from external noise is critical, so heavy walls, floating floors, and sound locks are used
• Performance spaces (concert halls, theaters)
  • Aim for longer RTs (1.5-3 seconds) to enhance the richness and envelopment of the sound, but not so long as to reduce clarity and intelligibility
  • Use diffusers and reflective surfaces to create a spacious, immersive sound field
  • Absorbers are used to control excessive reverb and echoes, particularly at low frequencies
• Classrooms and lecture halls
  • Require a balance of clarity for speech intelligibility and a moderate RT (0.7-1.2 seconds) for natural sound reinforcement
  • Use absorbers to control excessive reverb and echoes, particularly at high frequencies
  • Diffusers can be used to improve sound distribution and reduce the effects of flutter echoes between parallel walls