Chapter 18: SOUND IN ENCLOSED SPACES
Summary
This chapter addresses the propagation and distribution of sound in enclosed spaces—a subject known as room acoustics. The primary acoustical design intent in most enclosed spaces is to maintain and enhance the intelligibility of audible information. Sound from a source may be absorbed, reflected, or transmitted as it interacts with room enclosure elements. The two primary acoustic characteristics of a space are its absorption and its reverberation.
Sound absorption plays an important role in room acoustics. Coefficient of absorption (which varies with frequency) is an indicator of a material’s sound absorption characteristics. The total absorption of a given amount of material is proportional to its area and absorption coefficient—and is expressed in sabins (square feet or square meters). Acoustic (absorptive) materials come in a number of forms: fibrous materials, panel resonators, and volume (Helmholtz) resonators. Patterns of performance for absorptive materials are summarized. It is emphasized that the means of installation will affect the performance of acoustic materials; typical installation options are discussed and performance patterns summarized.
Room acoustics are discussed in terms of reverberation and sound fields. Reverberation is defined as the persistence of a sound after the sound source has ceased. Reverberation time is presented as a measure of reverberation. A means of estimating reverberation time is given. Reverberation is related to sensations of acoustical liveness or deadness. Articulation is noted as the converse of reverberation or reverberance.
Sound fields in an enclosed space are defined as the free (or far) field, the near field, and the reverberant field. The relationship between sound power level and sound pressure level in a space is established and defined mathematically. Directivity factor and room constant play a role in this relationship. Sample calculations are used to illustrate this relationship as a means of obtaining noise reduction via absorption. It is noted that a doubling of absorption only reduces noise level by 3 decibels (the law of diminishing returns). Noise reduction coefficient (NRC) is introduced and critiqued.
Reverberation criteria for speech spaces are discussed and an equation for calculating optimum reverberation time is given. The effects of echoes are discussed, including their effect on intelligibility and directivity. Criteria for music performance are also addressed. Numerous acoustic characteristics such as fullness, clarity, and brilliance are explained. Sound paths are discussed with respect to specular reflections, echoes, flutter, focusing, creep, and standing waves. Resonance is defined. Ray diagrams are introduced as a useful design tool. Auditoriums are presented as a particular room acoustics challenge—with guidelines and sample calculations. The components (input, amplifier, and loudspeakers) and objectives of sound reinforcing systems are introduced. Important characteristics of such systems are noted.
Chapter Outline
18.1 Sound in Enclosures
ABSORPTION
18.2 Sound Absorption
18.3 Mechanics of Absorption
18.4 Absorptive Materials
18.5 Installation of Absorptive Materials
ROOM ACOUSTICS
18.6 Reverberation
18.7 Sound Fields in an Enclosed Space
18.8 Sound Power Level and Sound Pressure Level
18.9 Noise Reduction by Absorption
18.10 Noise Reduction Coefficient
ROOM DESIGN
18.11 Reverberation Criteria for Speech Rooms
18.12 Criteria for Music Performance
18.13 Sound Paths
(a) Specular reflection
(b) Echoes
(c) Flutter
(d) Diffusion
(e) Focusing
(f) Creep
(g) Standing waves
18.14 Ray Diagrams
18.15 Auditorium Design
SOUND REINFORCEMENT SYSTEMS
18.16 Objectives and Criteria
18.17 Components and Specifications
(a) Input
(b) Amplifier and controls
(c) Loudspeakers
18.18 Loudspeaker Considerations
References
Key Concepts
· behavior of sound in enclosed space (as opposed to free field behavior)
· intelligibility (as a primary design intent)
· room acoustics (involving design for sound within a space)
· room characteristics (relative to absorptance and reverberation)
· sound absorption (and related characteristics and indicators)
· acoustic materials (types, properties, and performance)
· effectiveness of material installation details (as a design consideration)
· reverberation (as a key acoustic property and design criterion)
· articulation (as the converse of reverberation)
· sound fields in enclosed spaces (as a design concern)
· relationship between SPL and PWL (as controlled by design)
· law of diminishing returns (with respect to adding absorption)
· NRC (noise reduction coefficient, as a misnamed acoustic property)
· optimum or maximum reverberation time (as a design criterion)
· acoustic characteristics (numerous characterizations of sound quality)
· ray diagramming (as a design tool)
· auditorium design (as a particular concern of room acoustics)
· remedial treatment (for existing spaces)
sound reinforcement system (as often required in larger spaces)
Terminology and Metrics
Important Terminology
· ASTM (American Society for Testing and Materials)
· intelligibility
· room acoustics
· noise control
· sound (as being)
reflected
transmitted
absorbed
· absorption
coefficient of absorption
noise reduction coefficient (NRC)
· total absorption
· reverberation
liveness
deadness
anechoic chamber
· reverberation time
optimum reverberation time
· articulation
· sound field
near field
free field
reverberant field
far field
diffuse
· sound source
· inverse square law
· acoustic materials
fibrous materials
panel resonators
volume resonators
Helmholtz resonators
“sound blocks”
sound-reflecting
sound-absorbing
· sound power level
· sound pressure level
· directivity constant (factor)
· room factor
· acoustic characteristics
directivity
echo (and clear echo)
fullness
body
clarity
definition
brilliance
sound envelopment
flutter
diffusion
focusing
creep
standing waves
resonance
· early decay time
· specular reflection
· reflecting panel
· midfrequency
· ray diagram
· sound (reinforcement) system
input devices
amplifiers and controls
loudspeakers
woofers
horns
central
distributed
· equalization controls
· delay mechanism
· wavelength
Important Metrics
· α (coefficient of absorption, a ratio or percentage)
· A (total absorption, Sabins—square feet or square meters)
· λ (wavelength, inches)
· Tr (reverberation time, seconds)
· PWL (sound power level, dB)
· SPL (sound pressure level, dB)
· Q (directivity factor or constant, integer number)
· R (room factor, square feet or square meters)
NRC (noise reduction coefficient, a ratio or percentage)
Chapter 18: Sound in Enclosed Spaces
Sweet’s System Online (Division 9, including acoustical treatments):
http://www.sweets.com/sdff09.htm
Reverberation time demonstration (audio files):
http://www.bkla.com/reverb.htm
BKL Consultants
Reverberation Time Demonstration:
http://www.bkla.com/reverb.htm
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