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- The study of sound, including its production, transmission, and
effects.
- Those qualities of an enclosure that together determine its
character with respect to distinct hearing.
- The branch of science and technology that is devoted to the production,
transmission, control, processing, transformation, reception, and effects of
sound, longitudinal waves, particularly as vibration, pressure, or elastic
waves and shock phenomena in material media. [After Weik '96]
Source:
http://www.its.bldrdoc.gov/projects/devglossary/_acoustics.html
-
The
science of sound. It can also refer to the effect a given environment has on
sound.
Source:
http://www.owenscorning.com/around/sound/glossary.asp
What Were They Thinking?
From the 1882 edition of The American Universal Cyclopædia,
published by S. W. Green's Son, New York.
Acoustics (Greek akouo, I hear)
is the science of sound. This part of physics is often treated in connection
with the atmosphere – an arrangement that seems inappropriate; for the
atmosphere is only the most common conductor of sound; and every substance,
whether solid or fluid, is capable, as well as air of sounding itself, or of
conveying the sound of other bodies. Acoustics is rather a part of the
science of motion. All motion is either rectilineal circular, or vibratory;
and when a vibratory motion is quick enough to affect the sense of hearing –
for which at least thirty vibrations in a second are required – it
constitutes a sound. A definable, uniform sound is a note or tone, and the
rapidity of the vibrations is its pitch; a confused indeterminate sound is a
noise. The chief subjects treated of in acoustics are:
1. Musical sounds, or
notes. Here the question is concerning the absolute and relative velocities
of the vibrations and those modifications, called temperament, to which
their original proportions are subjected for the practical purposes of
music.
2. The origin of sound, and
the laws which guide the vibrations of sounding bodies, and which give rise
to different phenomena in different substances. In all sounding bodies, it
is elasticity that is to be looked upon as the moving power. The elasticity
of a sounding body may arise from stretching, as in the strings of a violin
or the head of a drum; or from its own stiffness, as in rods, bells, etc.
3. The propagation of
sound, as well through the air and other gases as through solids and
liquids; and the reflection of sounds or echoes. All elastic bodies conduct
sound, many much more powerfully than air. In water the conducting power is
four times stronger than it is in air; in tin, seven times; in silver, nine
times; in iron, ten times; in glass, seventeen times.
4. Perception of sound, or
the structure and functions of the ear.
The ancients had made attempts to cultivate
acoustics. Pythagoras and Aristotle were aware of the way that sound is
propagated through the air, but as a science independent of its application to
music it belongs almost entirely to modern times. Bacon and Galileo laid the
foundation of this new mathematical science; Newton showed by calculation how
the propagation of sound depends upon the elasticity of the atmosphere or other
conducting medium. He observed that a sounding body acts by condensing the
portions of air that lie next to it, and in the direction of the impulse. These
condensed portions then spring back by their elasticity, and at the same time
impel forwards the portions lying next them. Each separate portion of air is
thus driven forwards and backwards; and thus all round the sounding body there
is an alternate condensation and rarefaction of air, constituting, as it were,
waves of sound. In determining the velocity of sound, Newton, Lagrange, and
Euler erred in their calculations; the best researches on this subject are those
of Laplace. Chladni first raised acoustics to an independent science. In recent
times, comparatively little has been done in this branch of physics. Savart has
determined more exactly the number of vibrations in a second necessary to
produce an audible sound; and Cagniard de Latour invented the sirene, and
discovered many of the conditions under which both solids and fluids sound. The
soundings of heated metals, when laid on cold metallic supports, has occasioned
much discussion. Faraday and Marx have examined the figures of sound;
Wheatstone, the phenomena of sympathetic sounds; and Willis, the formation of
vowel-sounds by the human voice.
While the principles of acoustics are well
known in theory, they are seldom carried out to a satisfactory result in
practice. We allude more particularly to the many instances in which costly
assembly halls and churches are defective as regards public speaking; it being
seemingly a mere chance that new edifices of this kind exhibit proper acoustic
qualities. In some cases, the sounds uttered cause echoes and reverberations,
perplexing alike to a speaker and his auditory, and in others the sounds are
dispersed at a high elevation and are lost. This subject urgently demands
consideration in connection with architecture. As a general rule, the ceilings
of halls should be at a moderate elevation; the lowering of a ceiling and the
removal of the chandeliers have been known to improve the speaking and hearing
properties; and the hanging up of flags and draperies has, in a variety of
instances, had a similarly good effect. The whispering gallery of St. Paul’s,
London, offers an interesting example of one of the phenomena in acoustics
(echo).
The velocity of sound has been determined by
ascertaining the time intervening between the flash and report of a gun, as
observed at a given distance, and dividing the distance by the time. After many
experiments in various countries, Van der Kolk assigned
1091 ft.
8 in. per second, with a
probable error of
3.7 ft. as the velocity of
sound in dry air at
32° Fahr. More recent
experiments by the astronomer royal at the cape of Good Hope, give
1096 ft. To this velocity
may be added
1.11 ft. for each degree
Fahr. But air is not a perfect gas, and the variations of elastic force caused
by a wave of sound passing through it are not uniform; so these measures, though
approximately, amy not be absolutely, correct. Furthermore, the rapidity of
transmission depends upon the loudness of the sound; and capt. Parry found, in
the polar regions, that the discharge of a cannon at a distance of
2 ½ miles was heard perceptibly
sooner than the word ordering to fire, which, of course, preceded the discharge.
There is also a gradual falling off in the speed of sound; and Regnault
determined that a sound decreased in speed by
2.2 ft. per second in passing
from a distance of
4000 ft. to one of
7500 ft. He also found that
the velocity depended upon the pitch, the lower notes traveling faster than the
higher ones; thus, the fundamental note of a trumpet travels faster than its
harmonies. Sound travels faster in liquids than in air, and faster in solids
than in liquids. In the river Seine, at
59° Fahr. the speed was
4714 ft. per second. Through
iron, sound travels ten and a half times faster than through air. Experiments on
telegraph wire produce almost identical results. Different metals transmit sound
in widely different degrees. Wertheim assigned
16,822
for iron and
4030 for lead, at a
temperature of
68° Fahr. Except in a few
cases, the loudness of a sound is less as the distance increases between the
source of the sound and the ear. In an unlimited and uniform medium, the
loudness of the sound proceeding from a very small sounding body varies
inversely as the square of the distance. But to verify this fact it would be
necessary to make a test at a considerable elevation above the earth’s surface,
the ear and source of sound being separated by air of constant density. As the
density of the air diminishes, it would be found that the loudness of a sound at
a given distance would decrease. The decay of sound due to this cause is
observable in the rarefied air of high mountain regions. De Saussure found that
the report of a pistol at a great elevation appeared no louder than would a
small cracker at a lower level. But it must be stated that when air-strata of
different densities are interposed between the sound and and the ear placed at a
given distance, the intensity depends only on the density of the air at the
source itself; whence it follows that sounds proceeding from the surface of the
earth may be heard at equal distances as distinctly by a person in a floating
balloon as by one situated on the surface itself; whereas any noise originating
in the balloon would be heard at the surface as faintly as if the ear were
placed in the rarefied air on a level with the balloon. This was exemplified by
Glashier, the aeronaut, who, at an elevation of
20,000
ft. heard with great distinctness the whistle of a locomotive passing beneath
him. The prolonged roll of thunder, with its manifold varieties, is partly to be
ascribed to the reflection of the sound by mountains, clouds, etc., but is
mainly due to the comparatively low rate of transmission through air. The
explanation will be more easily understood by noting the case of a volley fired
by a long line of troops. A person at a given point in the line would hear the
sound of the nearest musket first, and of the others in the order of distance,
and the effect would be a prolonged roll, concluded by the musket most remote
from the hearer though all were fired at the same instant; and the roll would
gradually decrease in loudness. If he stood exactly opposite the centre of the
line, the reports from either end would reach him simultaneously and the effect
would be more nearly a loud crash. If the soldiers formed a circle, the listener
in the centre would hear a single explosion, since the report of every gun would
reach his ear at the same instant, and the whole explosion would be equal to
that of the sum of all the separate discharges. By varying the form of arranging
the troops, corresponding variations in the sound would be produced. Keep in
view, then, the fact that flashes of lightning may be regarded as representing
lines of troops, at the points and along the ranks of which explosions are
generated at the same instant of time; then consider the variety of distance and
position relative to the electric discharge of the listener, and we find no
difficulty in accounting for the rolling peals of thunder. In a mountainous
region this rolling is greatly augmented by reverberations or echoes from the
steep declivities.
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