ROOM ACOUSTICS

ROOM ACOUSTICS

Sound Indoors – Acoustics of Room
• Sound in an enclosure radiates out from the source until it hits a surface that reflects or absorbs it
• If the source is continuous, a state of equilibrium will be reached
• Levels are constant throughout most spaces except at points very near the source (for a given source, the built-up or reverberant levels will be highest in a space with a few absorptive surfaces – lobby with marble walls/floors; in a space with large areas of sound absorbing materials, the levels will be lowest
1. Sound Absorption
• Materials varying sound absorption characteristics
 Some absorb low frequency energy
 Some absorb high frequency energy
 Others absorb energy equally over a broad spectrum
• Mechanism of Absorption: 3 families of devices for sound absorption; all types absorb sound by changing sound energy to heat energy
 Fibrous materials (Porous materials): absorption provided by a specific material depends on thickness, density, porosity, resistance to air flow (e.g. materials must be thick to absorb low frequency sounds effectively; suspending an acoustical ceiling tile a foot below the structure results in better broad band absorption than cementing the tile directly to the structure; “acoustic” paints with sand or walnut shells are useless as sound absorbers
 Panel resonators: built with a membrane (thin plywood, linoleum in front of a sealed airspace); panel is set in motion by the alternating pressure of the impinging sound wave; sound energy is converted into heat through internal viscous damping; used where efficient low frequency absorption is required/ mid and high frequency absorption is unwanted; used in recording studios
 Volume resonators
• Coefficient of Absorption (α): Knowing the amount of absorption at different frequencies for each material in a room is essential in designing the room acoustics
 The absorption coefficient is the fraction of incident energy that is absorbed by a surface expressed as
α = Ia
Ii
Ii = incident energy, watts/cm2
Ia= absorbed energy, watts/cm2
α = absorption coefficient (no units)
 Total absorption (A) provided by a surface (S) is expressed in sabins
A = Sα
A = Total absorption, sabins
S = Surface area, sq ft
α = Coefficient of absorption

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Coefficients of General Building Materials and Furnishings
Coefficients for absorption in auditorium is shown for both empty and occupied seating conditions


• Many products are prefabricated for sound absorbing treatment (suspended ceilings/wall mounted treatments
• 3 kinds of custom-designed treatments that are often used in auditoriums/churches (the deep air space insures adequate low frequency performance


2. Noise Reduction by Absorption
• Noise levels in a room are highest for a given source if the room’s surfaces are primarily sound reflecting; lowest if there are large areas of sound absorbing materials (e.g. sound absorbing ceilings
3. Reflections
4. Reverberation