From Wikipedia, the free encyclopedia
- This page is about audible acoustic waves. For other uses
see Sound (disambiguation)
Sound is created when the membrane of this
drum vibrates
Perception
of sound
For humans, hearing is normally limited to frequencies between
about 12
Hz and 20,000 Hz (20
kHz)
[2],
although these limits are not definite. The upper limit generally
decreases with age. Other
species have a different range of hearing. For
example, dogs can perceive vibrations higher than 20 kHz. As a
signal perceived by one of the major
senses, sound is used by many species for
detecting
danger,
navigation,
predation, and
communication.
Earth's
atmosphere,
water, and virtually any
physical phenomenon, such as
fire,
rain,
wind,
surf, or
earthquake, produces (and is characterized
by) its unique sounds. Many species, such as
frogs,
birds,
marine and terrestrial
mammals, have also developed special
organs to
produce sound. In some species, these have evolved to produce
song and
speech. Furthermore,
humans have developed culture and technology
(such as
music,
telephone and
radio) that allows them to
generate, record, transmit, and broadcast sound.
Physics of
sound
Longitudinal and transverse
waves
Sinusoidal waves of
various frequencies; the bottom waves have higher frequencies than
those above. The horizontal axis represents time.
Matter in the medium is periodically displaced by a sound wave,
and thus oscillates. The energy carried by the sound wave converts
back and forth between the potential energy of the extra
compression (in case of
longitudinal waves) or lateral displacement
strain
(in case of transverse waves) of the matter and the kinetic energy
of the oscillations of the medium.
Sound wave
properties and characteristics
Sound characteristics can depend on the type of sound waves
(longitudinal versus transverse) as well as on the
physical properties of the transmission
medium.
Speed of
sound
U.S. Navy
F/A-18
breaking the sound barrier. The white halo is formed by condensed
water droplets which are thought to result from a drop in air
pressure around the aircraft (see
Prandtl-Glauert Singularity).
[3][4]
Main article:
Speed of
sound
The speed of sound depends on the medium through which the waves
are passing, and is often quoted as a fundamental property of the
material. In general, the speed of sound is proportional to the
square root of the
ratio of the
elastic modulus
(stiffness) of the medium to its
density. Those physical properties and the
speed of sound change with ambient conditions. For example, the
speed of sound in gases depends on
temperature. In 20
°C (68
°F) air at the
sea level, the speed of sound is
approximately 343
m/s (1,230
km/h;
767
mph)
using the formula "v = (331 + 0.6T) m/s". In fresh water, also at
20 °C, the speed of sound is approximately 1,482 m/s
(5,335 km/h; 3,315 mph). In
steel, the speed of sound is about 5,960 m/s
(21,460 km/h; 13,330 mph).
[5] The
speed of sound is also slightly sensitive (a second-order
anharmonic effect)
to the sound amplitude, which means that there are nonlinear
propagation effects, such as the production of harmonics and mixed
tones not present in the original sound (see
parametric
array).
Acoustics
and noise
The scientific study of the propagation, absorption, and
reflection of sound waves is called
acoustics.
Noise is a term often used to refer to an
unwanted sound. In science and engineering, noise is an undesirable
component that obscures a wanted signal.
Sound
pressure level
Main article:
Sound
pressure
Sound pressure is defined as the difference between the average
local pressure of the medium outside of the sound wave in which it
is traveling through (at a given point and a given time) and the
pressure found within the sound wave itself within that same
medium. A square of this difference (i.e. a square of the deviation
from the equilibrium pressure) is usually averaged over time and/or
space, and a square root of such average is taken to obtain a
root mean
square (RMS) value. For example, 1
Pa RMS sound pressure (94 dBSPL) in
atmospheric air implies that the actual pressure in the sound wave
oscillates between (1 atm

Pa) and (1 atm

Pa), that is between 101323.6 and 101326.4 Pa. Such a tiny
(relative to atmospheric) variation in air pressure at an
audio frequency
will be perceived as quite a
deafening sound, and can cause hearing
damage, according to the table below.
As the human ear can detect sounds with a very wide range of
amplitudes, sound pressure is often measured as a level on a
logarithmic
decibel scale.
The
sound pressure level (SPL) or
Lp is defined as

- where p is the root-mean-square
sound pressure and pref is a reference
sound pressure. Commonly used reference sound pressures, defined in
the standard ANSI S1.1-1994,
are 20 µPa in air and 1 µPa in water. Without a
specified reference sound pressure, a value expressed in decibels
cannot represent a sound pressure level.
Since the human
ear does not have
a flat
spectral
response, sound pressures are often
frequency weighted so that the measured level
will match perceived levels more closely. The
International
Electrotechnical Commission (IEC) has defined several weighting
schemes.
A-weighting
attempts to match the response of the human ear to noise and
A-weighted sound pressure levels are labeled dBA. C-weighting is
used to measure peak levels.
Examples
of sound pressure and sound pressure levels
| Source of sound |
RMS
sound pressure |
sound pressure level |
| Pa |
dB re 20
µPa |
Theoretical limit for undistorted sound at
1 atmosphere environmental pressure |
101,325 |
191 |
| 1883 Krakatoa
eruption |
|
approx 180 at 100 miles |
| Stun grenades |
|
170-180 |
| rocket launch equipment acoustic tests |
|
approx. 165 |
| threshold of pain |
100 |
134 |
| hearing damage during short-term effect |
20 |
approx. 120 |
| jet engine, 100 m
distant |
6–200 |
110–140 |
| jackhammer, 1 m distant / discotheque |
2 |
approx. 100 |
| hearing damage from long-term exposure |
0.6 |
approx. 85 |
| traffic noise on major road, 10 m distant |
0.2–0.6 |
80–90 |
| moving automobile,
10 m distant |
0.02–0.2 |
60–80 |
| TV set – typical home level, 1 m distant |
0.02 |
approx. 60 |
| normal talking, 1 m distant |
0.002–0.02 |
40–60 |
| very calm room |
0.0002–0.0006 |
20–30 |
| quiet rustling leaves, calm human breathing |
0.00006 |
10 |
| auditory threshold at 2 kHz – undamaged
human ears |
0.00002 |
0 |
Equipment for dealing with
sound
References
- ^
The American Heritage
Dictionary of the English Language, Fourth Edition.
Houghton Mifflin Company. 2006. http://www.bartleby.com/61/65/S0576500.html.
- ^
http://books.google.com/books?id=RUDTFBbb7jAC&pg=PA248
- ^
APOD: 19 August 2007- A Sonic
Boom
- ^
http://www.eng.vt.edu/fluids/msc/gallery/conden/mpegf14.htm
- ^
The Soundry: The Physics of
Sound
Sound
measurement
- Decibel, sone, mel, phon,
hertz
- Sound pressure level
- Particle
velocity, acoustic
velocity
- Particle displacement, particle amplitude, particle acceleration
- Sound power, acoustic power, sound power
level
- Sound
energy flux
- Sound
intensity, acoustic
intensity, sound intensity level
- Acoustic impedance, sound impedance, characteristic impedance
- Speed of
sound, amplitude
- See also Template:Sound measurements
See also
External
links