3.7 Placement of absorbers and diffusers

Acoustic treatments like absorbers and diffusers are crucial for controlling sound in architectural spaces. Absorbers reduce sound energy by converting it to heat, while diffusers scatter sound in various directions. Their placement is key to achieving optimal room acoustics.

Proper placement of absorbers and diffusers affects reverberation control, sound isolation, and specific room requirements. Absorbers on room boundaries control reverberation, while diffusers on reflective surfaces reduce strong reflections. The choice and placement depend on the desired acoustic characteristics and room type.

Types of absorbers and diffusers

• Absorbers and diffusers are two main types of acoustic treatments used to control sound in architectural spaces
• Absorbers reduce sound energy by converting it into heat through friction and viscous losses in porous materials or resonance in membrane and Helmholtz absorbers
• Diffusers scatter sound energy in various directions to avoid strong specular reflections and create a more diffuse sound field, using surface irregularities or volume diffusion

Absorption vs diffusion

• Absorption reduces the overall sound energy in a room, lowering reverberation time and noise levels, while diffusion redistributes sound energy without reducing it
• The choice between absorption and diffusion depends on the desired acoustic characteristics of the space, such as clarity, intimacy, spaciousness, and reverberation time
• A balance of absorption and diffusion is often necessary to achieve optimal room acoustics, with absorbers controlling excessive reverberation and diffusers enhancing spatial impression and sound diffusion

Placement for reverberation control

Absorbers on room boundaries

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• Placing absorbers on room boundaries (walls, ceiling, floor) is an effective way to control reverberation time and reduce overall sound energy
• Absorbers should be placed on surfaces that receive direct sound from the source and contribute significantly to the reverberant field, such as the rear wall and ceiling
• The amount and placement of absorbers depend on the desired reverberation time and the room's volume, with larger rooms generally requiring more absorption

Diffusers on reflective surfaces

• Diffusers can be placed on reflective surfaces to scatter sound energy and reduce the strength of specular reflections, which can cause echoes, flutter echoes, and comb filtering
• Diffusers are often used in combination with absorbers to maintain a balance between clarity and spaciousness, especially in performance spaces and recording studios
• The placement of diffusers should consider the incident angle of sound and the desired scattering pattern, with different types of diffusers (Schroeder, primitive root, skyline) offering various scattering properties

Placement for sound isolation

Absorbers in cavities and partitions

• Placing absorbers in wall and floor/ceiling cavities can improve sound isolation by reducing the transmission of sound energy through the structure
• Porous absorbers (mineral wool, fiberglass) are commonly used in cavities to dissipate sound energy and reduce resonances in the air space
• The effectiveness of cavity absorption depends on the thickness and density of the absorber, as well as the cavity depth and the presence of decoupling elements (resilient channels, isolation clips)

Absorbers for noise reduction

• Absorbers can be used to reduce airborne noise transmission between rooms by placing them on the walls and ceiling of the source or receiving room
• Porous absorbers are effective at reducing mid and high-frequency noise, while membrane and Helmholtz absorbers can target low frequencies
• The placement of absorbers for noise reduction should consider the path of sound transmission and the critical frequencies to be attenuated

Placement for specific room types

Absorbers in recording studios

• In recording studios, absorbers are used to control reverberation time, reduce standing waves, and minimize unwanted reflections that can affect the clarity and accuracy of the recorded sound
• Absorbers are typically placed on the rear wall, ceiling, and corners to control low-frequency modes and create a neutral acoustic environment
• The placement of absorbers in recording studios should consider the position of the microphones and monitors, as well as the desired frequency response and stereo imaging

Diffusers in performance spaces

• Diffusers are used in performance spaces (concert halls, theaters) to enhance spatial impression, create a more immersive sound field, and improve the distribution of sound energy
• Diffusers are often placed on the side walls and ceiling to scatter lateral reflections and increase the apparent source width and listener envelopment
• The placement of diffusers in performance spaces should consider the audience area, stage, and the desired balance between clarity and reverberance

Absorbers and diffusers in classrooms

• In classrooms, absorbers and diffusers are used to control reverberation time, improve speech intelligibility, and reduce background noise levels
• Absorbers are typically placed on the rear wall and ceiling to reduce late reflections and control excessive reverberation, while diffusers can be used on side walls to scatter early reflections and improve sound distribution
• The placement of absorbers and diffusers in classrooms should consider the room geometry, seating arrangement, and the location of the teacher and students

Placement for modal control

Absorbers at room modes

• Room modes are standing waves that occur at specific frequencies determined by the room dimensions, leading to uneven sound distribution and frequency response
• Placing absorbers at the pressure maxima of room modes can effectively reduce their amplitude and minimize their impact on the sound field
• The placement of absorbers for modal control requires knowledge of the room dimensions and the calculation of modal frequencies using the room mode formula: $f = \frac{c}{2} \sqrt{(\frac{n_x}{L_x})^2 + (\frac{n_y}{L_y})^2 + (\frac{n_z}{L_z})^2}$

Diffusers to scatter modal energy

• Diffusers can be used to scatter the energy of room modes and reduce their coherence, leading to a more diffuse sound field and a smoother frequency response
• The placement of diffusers for modal control should consider the wavelength of the targeted modes and the scattering properties of the diffuser
• Diffusers with a deep well depth and a wide range of well widths (quadratic residue diffusers) are effective at scattering low-frequency modal energy

Placement for early reflections

Absorbers for reflection control

• Early reflections are sound waves that arrive at the listener's position within the first 50-80 milliseconds after the direct sound and contribute to clarity, localization, and spatial impression
• Placing absorbers on surfaces that generate strong early reflections (side walls, ceiling) can help control their amplitude and direction, improving speech intelligibility and reducing comb filtering effects
• The placement of absorbers for early reflection control should consider the critical distance (distance at which direct and reverberant sound energy are equal) and the desired ratio of direct to reverberant sound

Diffusers for spatial impression

• Diffusers can be used to scatter early reflections and create a more spacious and immersive sound field, enhancing the listener's perception of the room's size and the source's width
• The placement of diffusers for spatial impression should consider the direction and timing of the desired reflections, as well as the scattering properties of the diffuser
• Diffusers with a wide scattering range (hemispherical diffusers) and a combination of diffusers with different scattering patterns can create a more natural and enveloping sound field

Placement for late reflections

Absorbers for reverberant energy

• Late reflections are sound waves that arrive at the listener's position more than 50-80 milliseconds after the direct sound and contribute to the perception of reverberation and spaciousness
• Placing absorbers on surfaces that contribute to the reverberant field (rear wall, corners) can help control the reverberation time and reduce the overall sound energy in the room
• The placement of absorbers for reverberant energy control should consider the desired reverberation time and the frequency-dependent absorption coefficients of the materials used

Diffusers for diffuse sound field

• Diffusers can be used to scatter late reflections and create a more diffuse and uniform sound field, reducing the perception of discrete echoes and improving the listener's envelopment
• The placement of diffusers for a diffuse sound field should consider the room's volume, the desired scattering coefficient, and the frequency range to be diffused
• A combination of diffusers with different scattering properties and a random or pseudo-random arrangement can help achieve a more isotropic and diffuse sound field

Placement considerations

Aesthetic integration

• The placement of absorbers and diffusers should consider the aesthetic integration with the room's architecture and design, ensuring a visually pleasing and coherent appearance
• Custom-designed absorbers and diffusers can be used to match the room's style and color scheme, while preserving their acoustic performance
• The use of acoustically transparent materials (fabric, perforated panels) and hidden placement (behind walls, above ceilings) can help maintain the room's aesthetic integrity

Fire and safety regulations

• The placement of absorbers and diffusers must comply with local fire and safety regulations, ensuring the use of fire-resistant and non-toxic materials
• The placement of acoustic treatments should not obstruct emergency exits, sprinkler systems, or other safety features
• The use of fire-rated materials and the proper sealing of penetrations can help maintain the room's fire resistance and prevent the spread of smoke and flames

Maintenance and durability

• The placement of absorbers and diffusers should consider the ease of maintenance and the durability of the materials used, especially in high-traffic areas or rooms with demanding environmental conditions
• The use of moisture-resistant and cleanable materials can help prevent the growth of mold and bacteria and extend the life of the acoustic treatments
• Regular inspection and cleaning of absorbers and diffusers can help maintain their acoustic performance and visual appearance over time.