Acoustics refers to the study of sound, namely, its production, transmission through
solid and fluid media, and any other phenomenon engendered by its propagation
through media. Sound may be described as the passage of pressure fluctuations
through an elastic medium as the result of a vibrational impetus imparted to that
medium. An acoustic signal can arise from a number of sources, e.g., turbulence
of air or any other gas, the passage of a body through a fluid, and the impact of a
solid against another solid.
Because it is a phenomenon incarnating the nature of waves, sound may contain
only one frequency, as in the case of a pure steady-state sine wave, or many
frequency components, as in the case of noise generated by construction machinery
or a rocket engine. The purest type of sound wave can be represented by a sine
function (Figure 2.1) where the abscissa represents elapsed time and the ordinate
represents the displacement of the molecules of the propagation medium or the
deviation of pressure, density, or the aggregate speed of the disturbed molecules
from the quiescent (undisturbed) state of the propagation medium.
When the ordinate represents the pressure difference from the quiescent pressure,
the upper portions of the sine wave would then represent the compressive
states and the lower portions the rarefaction phases of the propagation.Asinewave
is generated in Figure 2.2 by the projection of the trace of a particle A traveling in
a circular orbit. This projection assumes the pattern of an oscillation, in which the
particle A’s projection or “shadow” A onto an abscissa moves back and forth at a
specified frequency. Frequency f is the number of times the sound pressure varies
from its equilibrium value through a complete cycle per unit time. Frequency is
also denoted by the angular (or radian) frequency
ω = 2π f =
2π
T
(2.1)
expressed in radians per second. The period T is the amount of time for a single
cycle to occur, i.e., the length of the time it takes for a tracer point on the
sine curve to reach a corresponding point on the next cycle. The reciprocal of
period T is simply the frequency f . The most common unit of frequency used
in acoustics (and electromagnetic theory) is the hertz (abbreviated Hz in the SI
system), which is equal to one cycle per second. An acoustic signal may or may
not be audible to the human ear, depending on its frequency content and intensity.
If the frequencies are sufficiently high (>20 kilohertz, which can be expressed
more briefly as 20 kHz), ultrasound will result, and the sound is inaudible to the
human ear. This sound is said to be ultrasonic. Below 20 Hz, the sound becomes
too low (frequency-wise) to be heard by a human. It is then considered to be
infrasonic.
Sound in the audio frequency range of approximately 20 Hz–20 kHz can be heard
by humans. While a degree of subjectivity is certainly entailed here, noise conveys
the definition of unwanted sound. Excessive levels of sound can cause permanent
hearing loss, and continued exposure can be deleterious, both physiologically and
psychologically, to one’s well-being.
With the advent of modern technology, our aural senses are being increasingly
assailed and benumbed by noise from high-speed road traffic, passing ambulances
and fire engine sirens, industrial and agricultural machinery, excessively loud radio
and television receivers, recreational vehicles such as snowmobiles and unmuffled
motorcycles, elevated and underground trains, jet aircraft flying at low altitudes,
domestic quarrels heard through flimsy walls, and so on.
Young men and women are prematurely losing their hearing acuity as the result
of sustained exposure to loud rock concerts, discotheques, use of personal
cassette and compact disk players and mega-powered automobile stereo systems.
In the early 1980s, during the waning days of the Cold War, the Swedish navy
reported considerable difficulty in recruiting young people with hearing sufficiently
keen to qualify for operating surveillance sonar equipment for tracking
Soviet submarines traveling beneath Sweden’s coastal waters. Oral communication
can be rendered difficult or made impossible by background noise; and
life-threatening situations may arise when sound that conveys information becomes
masked by noise. Thus, the adverse effects of noise fall into one or
more of the following categories: (1) hearing loss, (2) annoyance, and (3) speech
interference.
Modern acoustical technology also brings benefits: it is quite probable that the
availability (and judicious use) of audiophile equipment has enabled many of us,
if we are so inclined, to hear more musical performances than Beethoven, Mozart
or even the long-lived Haydn could have heard during their respective lifetimes.
Ultrasonic devices are being used to: dislodge dental plaque; overcome the effects
of arteriosclerosis by freeing up clogged blood vessels; provide noninvasive medical
diagnoses; aid in surgical procedures; supply a means of nondestructive testing
of materials; and clean nearly everything from precious stones to silted conduits.
The relatively new technique of active noise cancellation utilizes computerized
sensing to duplicate the histograms of offending sounds but at 180 degrees out
of phase, which effectively counteracts the noise. This technique can be applied
to aircraft to lessen environmental impact and to automobiles to provide quieter
interiors.
solid and fluid media, and any other phenomenon engendered by its propagation
through media. Sound may be described as the passage of pressure fluctuations
through an elastic medium as the result of a vibrational impetus imparted to that
medium. An acoustic signal can arise from a number of sources, e.g., turbulence
of air or any other gas, the passage of a body through a fluid, and the impact of a
solid against another solid.
Because it is a phenomenon incarnating the nature of waves, sound may contain
only one frequency, as in the case of a pure steady-state sine wave, or many
frequency components, as in the case of noise generated by construction machinery
or a rocket engine. The purest type of sound wave can be represented by a sine
function (Figure 2.1) where the abscissa represents elapsed time and the ordinate
represents the displacement of the molecules of the propagation medium or the
deviation of pressure, density, or the aggregate speed of the disturbed molecules
from the quiescent (undisturbed) state of the propagation medium.
When the ordinate represents the pressure difference from the quiescent pressure,
the upper portions of the sine wave would then represent the compressive
states and the lower portions the rarefaction phases of the propagation.Asinewave
is generated in Figure 2.2 by the projection of the trace of a particle A traveling in
a circular orbit. This projection assumes the pattern of an oscillation, in which the
particle A’s projection or “shadow” A onto an abscissa moves back and forth at a
specified frequency. Frequency f is the number of times the sound pressure varies
from its equilibrium value through a complete cycle per unit time. Frequency is
also denoted by the angular (or radian) frequency
ω = 2π f =
2π
T
(2.1)
expressed in radians per second. The period T is the amount of time for a single
cycle to occur, i.e., the length of the time it takes for a tracer point on the
sine curve to reach a corresponding point on the next cycle. The reciprocal of
period T is simply the frequency f . The most common unit of frequency used
in acoustics (and electromagnetic theory) is the hertz (abbreviated Hz in the SI
system), which is equal to one cycle per second. An acoustic signal may or may
not be audible to the human ear, depending on its frequency content and intensity.
If the frequencies are sufficiently high (>20 kilohertz, which can be expressed
more briefly as 20 kHz), ultrasound will result, and the sound is inaudible to the
human ear. This sound is said to be ultrasonic. Below 20 Hz, the sound becomes
too low (frequency-wise) to be heard by a human. It is then considered to be
infrasonic.
Sound in the audio frequency range of approximately 20 Hz–20 kHz can be heard
by humans. While a degree of subjectivity is certainly entailed here, noise conveys
the definition of unwanted sound. Excessive levels of sound can cause permanent
hearing loss, and continued exposure can be deleterious, both physiologically and
psychologically, to one’s well-being.
With the advent of modern technology, our aural senses are being increasingly
assailed and benumbed by noise from high-speed road traffic, passing ambulances
and fire engine sirens, industrial and agricultural machinery, excessively loud radio
and television receivers, recreational vehicles such as snowmobiles and unmuffled
motorcycles, elevated and underground trains, jet aircraft flying at low altitudes,
domestic quarrels heard through flimsy walls, and so on.
Young men and women are prematurely losing their hearing acuity as the result
of sustained exposure to loud rock concerts, discotheques, use of personal
cassette and compact disk players and mega-powered automobile stereo systems.
In the early 1980s, during the waning days of the Cold War, the Swedish navy
reported considerable difficulty in recruiting young people with hearing sufficiently
keen to qualify for operating surveillance sonar equipment for tracking
Soviet submarines traveling beneath Sweden’s coastal waters. Oral communication
can be rendered difficult or made impossible by background noise; and
life-threatening situations may arise when sound that conveys information becomes
masked by noise. Thus, the adverse effects of noise fall into one or
more of the following categories: (1) hearing loss, (2) annoyance, and (3) speech
interference.
Modern acoustical technology also brings benefits: it is quite probable that the
availability (and judicious use) of audiophile equipment has enabled many of us,
if we are so inclined, to hear more musical performances than Beethoven, Mozart
or even the long-lived Haydn could have heard during their respective lifetimes.
Ultrasonic devices are being used to: dislodge dental plaque; overcome the effects
of arteriosclerosis by freeing up clogged blood vessels; provide noninvasive medical
diagnoses; aid in surgical procedures; supply a means of nondestructive testing
of materials; and clean nearly everything from precious stones to silted conduits.
The relatively new technique of active noise cancellation utilizes computerized
sensing to duplicate the histograms of offending sounds but at 180 degrees out
of phase, which effectively counteracts the noise. This technique can be applied
to aircraft to lessen environmental impact and to automobiles to provide quieter
interiors.
EmoticonEmoticon