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Architectural Acoustics: An Overview

By thouonesand Sep 30, 2013 973 Words
17.1 ARCHITECTURAL ACOUSTICS
Architectural acoustics may be defined as the
design of spaces, structures, and mechanical/electrical
systems to meet hearing needs. With proper
design efforts, wanted sounds can be heard properly
and unwanted sounds (noise) can be attenuated
or masked to the point where they do not cause
annoyance. Achieving good acoustics, however,
has become increasingly

frequency
is sometimes referred to using a term borrowed from
music—pitch. The higher a sound’s frequency, the
higher its pitch, and vice versa.

Hz. Most speech information is carried in
the upper frequencies, whereas most of the acoustic
energy exists in the lower frequencies.

A sound composed of only one frequency is
called a pure tone.

The frequencies in the scale of Fig. 17.4 all
stand in the ratio of 2:1 to each other—that is,
16:32:63:125:250, and so on. Borrowing again
from musical terminology, they are one octave
apart. These particular frequencies are also accepted
internationally as the center (reference) frequencies
of octave bands used for the purpose of sound
specification.

physical magnitude of sound is variously described
as sound power, sound power level (PWL), sound pressure,
sound pressure level (SPL), sound intensity, and
sound intensity level (IL).

Sound is therefore a longitudinal wave motion.

The upper limit decreases
with age as a result of a process called presbycusis
(Fig. 17.5). The loss is more pronounced in men
than women. Recognition of this phenomenon can
be of importance in schools,

The outer ear is funnel-shaped
and serves as a sound-gathering input device for
the auditory system. Sound energy travels through
the auditory canal (outer ear) and sets in motion
the components of the middle ear, comprising the
eardrum, hammer, anvil, and stirrup. The stirrup
acts as a piston to transmit vibrations into the fluid
of the inner ear. The motion of this fluid causes
movement of hair cells in the cochlea, which, in
turn, stimulates nerves at the bases of the hairs.
The nerves, in turn, transmit electrical impulses
along the eighth cranial nerve to the brain. These
impulses we understand as sound.
It is often assumed that the ear ignores

Architectural acoustics is defined as the technology of designing spaces to meet hearing needs. Wanted sounds are distinguished from noise (unwanted sounds). Providing for good acoustics in modern buildings is noted as an increasingly difficult task. The common elements of all acoustic situations are identified: source, transmission path, and receiver. Sound is thought to be best described for purposes of building design as “an audible signal.”

Sound generation and propagation are discussed. The fundamental properties of frequency, wavelength, and speed of propagation are explored. Means of characterizing frequency patterns (pure tones, harmonics, pitch, octaves) are considered. Human hearing and the functioning of the ear are reviewed. The response patterns of the ear are explored. The concept of equal loudness contours is presented and the implications of this concept considered. Masking and directivity effects are discussed.

The characteristics of sound sources—primarily speech—are outlined. The numerous ways in which the magnitude of sound is described are reviewed in detail. These include sound intensity, sound pressure, and sound power—as well as the derivative expressions of sound intensity level, sound pressure level, and sound power level. The relationship between sound intensity and free-field propagation is defined and illustrated by sample calculations. It is noted that the human ear responds to sound magnitude logarithmically, not arithmetically. The concept of the decibel is thus introduced and then illustrated by example and calculations. The relationships between sound intensity, pressure, and power are defined. The use of an integrating sound level meter to measure sound pressure levels is discussed. Weighting network use, to match sound level meter response to human response, is explained.

The effects of noise and of annoyance with sound are considered. Indicators of these effects, such as articulation index and speech interference level, are explained. Annoyance patterns (general patterns of human response) are given. Several design criteria for noise are presented and explained. These include noise criterion (NC) curves, room criterion (RC) curves, noise rating (NR) curves, and balanced noise criterion (NCB) curves. Special mechanical noise considerations are noted and hearing protection issues introduced. Vibration is identified as being distinct from sound (noise).

Chapter Outline

17.1 Architectural Acoustics
17.2 Sound
(a) Speed of sound
(b) Wavelength
(c) Frequency
(d) Octave bands
(e) The concept of sound magnitude
(f) Sound propagation
17.3 Hearing
(a) The ear
(b) Equal loudness contours
(c) Masking
(d) Directivity
(e) Discrimination
17.4 Sound Sources
(a) Speech
(b) Other sounds
17.5 Expressing Sound Magnitude
(a) Sound power
(b) Sound pressure
(c) Sound intensity
(d) The decibel
(e) Sound power level
(f) Sound pressure level
(g) Measuring sound
17.6 Noise
(a) Annoyance
(b) Noise criteria
(c) Noise criteria curves
(d) Room criteria curves
(e) High noise levels and hearing protection
17.7 Vibration
References

Key Concepts

architectural acoustics (as an important area of design) •difficulty of providing good acoustics (as a reminder to properly consider this aspect of design) •common elements of all acoustic systems (as a useful organizational structure) •frequency (as a fundamental property of sound)

speed of sound (as slower than light, critical to consider, and variable with material) •descriptions of sound magnitude (as involving numerous measures and units) •free-field sound propagation (as a design situation)

basics of human hearing (as the foundation for architectural acoustics) •equal loudness contours (as fundamental to understanding human response to sound) •masking (as a design tool or as a problem)

logarithmic response of human ear (explaining qualitative evaluations of sound) •decibel scale (used universally to quantify sound magnitude) •meaning of the term “level” (to signify a ratio, expressed as a decibel value) •weighting (of sound measurement devices; available options) •noise and annoyance (as undesirable acoustic situations) •noise or room criterion curves (as a typical means of expressing design criteria) •hearing protection versus comfort (as a design objective in some spaces) •vibration (felt by occupants versus being heard)

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