chapter 03 - Sound Reflection Diffraction Diffusion - Color
Chapter 3: Architectural Acoustics
3) Sound Reflection, Diffraction and Diffusion
3.1 The Boundary Phenomena
Incident sound energy when sound interacts with enclosure boundaries.
Reflected sound energy divided into three parts:
Reflected energy returns to the space.
Absorbed energy is converted into heat by the material.
Transmitted energy passes through the boundary element.
Energy balance equation: ( P = \rho + \alpha + \tau = 1.0 )
3.2 Absorption Coefficient of Sound
Absorbed and transmitted sound energy is grouped together as energy lost from the enclosure.
Reflection and absorption are characterized by: ( \rho + \alpha = 1.0 )
3.3 Sound Diffraction
Sound diffraction is the ability of sound to bend around obstacles.
Without diffraction, sound would travel straight, creating shadows behind obstacles, similar to light.
3.3.1 Wavelength of Sound and the Size of Reflector
Degree of diffraction is dependent on wavelength (frequency).
Low-frequency sounds (long wavelength) bend more than high-frequency sounds (short wavelength).
Small elements relative to sound wavelength lead to significant sound diffraction, creating minimal shadows.
Larger elements compared to sound wavelength result in reflection, with slight diffraction.
3.3.2 Experimental Observations
For effective sound reflection, a plane panel must have dimensions at least 5λ (lambda).
Example: A 3 m x 3 m (10 ft x 10 ft) panel is required for reflecting 500 Hz sound (λ ≈ 0.6 m).
Sound reflecting panels are useful in auditoriums to direct sound towards the audience.
3.4 The Relevance of Acoustical Shadows
Acoustical shadows negatively affect hearing and listening in performance venues.
Formed under deep balconies, affecting high frequencies more than low frequencies.
Low-frequency sounds can diffuse into shadow areas; high frequencies cannot, leading to tonal coloration of music.
3.5 Acoustical Transparency of a Screen
3.5.1 Factors Affecting Acoustical Transparency
Acoustical transparency depends on visual transparency, sound frequency, and void distribution in the screen.
Smaller, closely spaced voids allow more sound diffusion compared to fewer large voids.
Lightweight, knitted or perforated fabrics enhance acoustical transparency.
3.6 Diffuse and Specular Reflections
3.6.1 Specular Reflection
Occurs when a smooth reflecting surface creates directed reflections.
"Smooth" means surface irregularities are smaller than the wavelength of sound.
3.6.2 Diffuse Reflection
Heavy texture on walls required for diffuse reflection; surface should have irregularities comparable to sound wavelength (e.g., 0.3 m for 1 kHz).
3.7 Sound Diffusion
A perfectly diffuse sound field is where sound reaches listeners equally from all directions.
Important for musical performance spaces to avoid harsh reflections (acoustic glare).
Excessive diffusion can hinder clarity in speech-focused spaces.
3.7.1 Effect of Room Geometry and Size on Sound Diffusion
Poor diffusion typically occurs in rectangular rooms with flat walls.
Non-rectangular designs favor better sound diffusion.
3.7.2 Effect of Sound Absorption on Sound Diffusion
Reflective surfaces improve diffusion, while sound absorption materials reduce it.
3.7.3 Interior Ornamentation
Ornamental features like pilasters, balconies increase sound diffusion.
Extensive ornamentation contributes to the acoustics of older symphony halls.
3.7.4 Diffusion and Convex Reflectors
Convex surfaces scatter sound and improve diffusion.
Concave surfaces can focus and concentrate sound, leading to poor acoustics unless designed to scatter sound effectively.
3.8 Sound Diffusers
Reflective surfaces with irregularities comparable to wavelength serve as diffusers.
Randomness in surface irregularities enhances diffusion performance.