Encyclopedia
Sound is a disturbance of mechanical energy that propagates through matter as a
wave. Sound is characterized by the
properties of sound waves which are
frequency,
wavelength, period,
amplitude and velocity or speed.
Explanation
Noise and sound often mean the same thing, when they differ a noise is an unwanted sound. In science and engineering noise is an undesirable component that obscures a signal.
Humans perceive sound by the sense of hearing. By sound, we commonly mean the vibrations that travel through air and can be heard by humans. However, scientists and engineers use a wider definition of sound that includes low and high
frequency vibrations in air that cannot be heard by humans, and vibrations that travel through all forms of matter, gases,
liquids and solids. The matter that supports the sound is called the
medium. Sound propagates as
waves of alternating
pressure, causing local regions of compression and rarefaction. Particles in the medium are displaced by the wave and oscillate. The scientific study of sound is called acoustics.
Perception of sound
Sound is perceived through the sense of hearing. Humans and many animals use their
ears to hear sound, but loud sounds and low frequency sounds can be perceived by other parts of the body through the sense of touch. Sounds are used in several ways, most notably for communication through
speech or, for example,
music. Sound can also be used to acquire information about properties of the surrounding environment such as spatial characteristics and presence of other animals or objects. For example,
bats use
echolocation, ships and submarines use
sonar, and humans can determine spatial information by the way in which they perceive sounds.
The official human hearing range is 20 Hz to 20,000 Hz; however, the majority of people can no longer hear 20,000 Hz by the time they are teenagers and the upper edge only continues to decrease as age increases. The hearing range varies by individual and, mostly in the upper part of the range, hearing damage accumulates with age. The ear is most sensitive to frequencies around 1000-3,500 Hz. Sound above the hearing range is known as
ultrasound, and that below the hearing range as infrasound.
The amplitude of a sound wave is specified in terms of its pressure. The human ear can detect sounds with a very wide range of amplitudes and so a
logarithmic decibel amplitude scale is used. The quietest sounds that humans can hear have an amplitude of approximately 20 µPa or a sound pressure level of 0 dB re 20 µPa . Prolonged exposure to a sound pressure level exceeding 85 dB can permanently damage the ear, sometimes resulting in tinnitus and hearing impairment. Sound levels in the hearing range, in excess of 130 dB are considered more than the human ear can withstand and may result in serious pain and permanent damage.
At very high amplitudes, sound waves exhibit non-linear effects including
shock.
Speed of sound
The speed at which sound travels depends on the medium through which the sound waves pass, 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 stiffness of the medium and its density. Those physical properties and the speed of sound change with ambient conditions. For example, the speed of sound in air and other gases depends on temperature. In air, the speed of sound is approximately 345 ms
-1, in water 1500 ms
-1 and in a bar of steel 5000 ms
-1.
Sound pressure
Sound pressure is the
pressure deviation from the local ambient pressure caused by a sound
wave. Sound pressure can be measured using a
microphone in air and a hydrophone in water. The SI unit for sound pressure is the pascal . The instantaneous sound pressure is the deviation from the local ambient pressure caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. In a sound wave, the complementary variable to sound pressure is the acoustic particle velocity. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance. The acoustic impedance depends on both the characteristics of the wave and the medium. The local instantaneous sound intensity is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity.
Sound pressure level
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 or
Lp is defined as
- where p is the root-mean-square sound pressure and p0 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.
Since the human
ear does not have a flat spectral response, sound pressure levels are often
frequency weighted so that the measured level will match perceived sound level. The International Electrotechnical Commission 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 sound levels.
Examples of sound pressure and sound pressure levels
| Source of sound | sound pressure | sound pressure level |
|---|
| | pascal | dB re 20 µPa |
|---|
| threshold of pain | 100 | 134 |
| hearing damage during short term effect | 20 | approx. 120 |
| jet, 100 m distant | 6 - 200 | 110 - 140 |
| jack hammer, 1 m distant / discotheque | 2 | approx. 100 |
| hearing damage during long-term effect | 6×10-1 | approx. 90 |
| major road, 10 m distant | 2×10-1 - 6×10-1 | 80 - 90 |
| passenger car, 10 m distant | 2×10-2 - 2×10-1 | 60 - 80 |
| TV set at home level, 1 m distant | 2×10-2 | ca. 60 |
| normal talking, 1 m distant | 2×10-3 - 2×10-2 | 40 - 60 |
| very calm room | 2×10-4 - 6×10-4 | 20 - 30 |
| leaves noise, calm breathing | 6×10-5 | 10 |
| auditory threshold at 2 kHz | 2×10-5 | 0 |
References
- Beranek, Leo L, "Acoustics" Acoustical Society of America. ISBN 0-88318-494-X
Measurement of sound
- Decibel, sone, mel, phon
- Sound pressure, acoustic pressure, sound pressure level
- Particle velocity, acoustic velocity, sound velocity
- Particle displacement, particle amplitude, particle acceleration
- Sound power, acoustic power, sound power level
- Sound intensity, acoustic intensity, sound intensity level
- Acoustic impedance, sound impedance, characteristic impedance
- Speed of sound, amplitude
- Sound energy flux
- See also
See also
Acoustics | Auditory imagery | Audio signal processing | Beats | Cycles | Decibel |
Doppler Effect | Echo | Infrasound |
Loudspeaker |
Microphone |
Music | Noise |
Phonons |
Physics of music | Pitch | Radiation of sound | Resonance |
Rijke tube | Reflection | Reverberation | Sonic weaponry | Sound localization | Soundproofing |
Sound reproduction |
Steam whistle | Timbre | Tinnitus |
Ultrasound |
Voyager Golden Record |
Wave |
External links
