Knowledge Bank

news category created 12 May 2006 written by Mick Glossop

What is Sound?

Sound is vibration transmitted through a solid, liquid, or gas; particularly, sound means those vibrations composed of frequencies capable of being detected by ears.1
Contents

* 1 Perception of sound
* 2 Physics of sound
o 2.1 Longitudinal and transverse waves
o 2.2 Sound wave properties and characteristics
o 2.3 Speed of sound
o 2.4 Acoustics and noise
* 3 Sound pressure level
o 3.1 Examples of sound pressure and sound pressure levels
* 4 Equipment for dealing with sound
* 5 References
* 6 Sound measurement
* 7 See also
* 8 External links

Perception of sound

For humans, hearing is limited to frequencies between about 20 Hz and 20,000 Hz (20 kHz), with the upper limit generally decreasing 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 (in humans) speech. Furthermore, humans have developed culture and technology (such as music, telephony and radio) that allows them to generate, record, transmit, and broadcast sound.

Physics of sound

The mechanical vibrations that can be interpreted as sound can travel through all forms of matter: gases, liquids, solids, and plasmas. However, sound cannot propagate through vacuum. The matter that supports the sound is called the medium.

Longitudinal and transverse waves

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. Through solids, however, it can be transmitted as both longitudinal and transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves in solids, are waves of alternating shear stress.

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 waves are characterized by the generic properties of waves, which are frequency, wavelength, period, amplitude, intensity, speed, and direction (sometimes speed and direction are combined as a velocity vector, or wavelength and direction are combined as a wave vector).

Transverse waves, also known as shear waves, have an additional property of polarization.

Sound characteristics can depend on the type of sound waves (longitudinal versus transverse) as well as on the physical properties of the transmission medium.

Whenever the pitch of the soundwave is affected by some kind of change, the distance between the sound wave maxima also changes, resulting in a change of frequency. When the loudness of a soundwave changes, so does the amount of compression in airwave that is travelling through it, which in turn can be defined as amplitude.

Speed of sound

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 air at sea level, the speed of sound is approximately 767.3 mph, in fresh water 3,315.1 mph (both at 20 °C, or 68 °F), and in steel about 13,332.1 mph.2 The speed of sound is also slightly sensitive (a second-order 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 measurements
Sound pressure p
Particle velocity v
Particle velocity level (SVL)
(Sound velocity level)
Particle displacement ξ
Sound intensity I
Sound intensity level (SIL)
Sound power Pac
Sound power level (SWL)
Sound energy density E
Sound energy flux q
Surface S
Acoustic impedance Z
Speed of sound c
v • d • e

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 in atmospheric air implies that the actual pressure in the sound wave oscillates between (1 atm -sqrt{2} Pa) and (1 atm +sqrt{2} 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

L_mathrm{p}=10, log_{10}left(frac{{p}^2}{{p_mathrm{ref}}^2}right) =20, log_{10}left(frac{p}{p_mathrm{ref}}right)mbox{ dB}
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
Nuclear Weapon explosion         approx 248
1883 Krakatoa eruption         approx 180 at 100 miles
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

Equipment for generating or using sound includes musical instruments, hearing aids, sonar systems and sound reproduction and broadcasting equipment. Many of these use electro-acoustic transducers such as microphones and loudspeakers.

References

1. ^ (2006) The American Heritage Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company.
2. ^ 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

Pitch Acoustics | Auditory imagery | Audio bit depth | Audio signal processing | Beats | Cycles | Diffraction | Doppler effect | Echo | Music | Note | Phonons | Physics of music | Pitch | Psychoacoustics | Resonance | Rijke tube | Reflection | Reverberation | Sonic weaponry | Sound localization | Soundproofing | Timbre |

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