Sound

sound is produced when its membrane vibrates]] Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.

Propagation of sound

Sound is a sequence of waves of pressure that propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes of propagation). During propagation, waves can be reflected, refracted, or attenuated by the medium.

The behavior of sound propagation is generally affected by three things:

A relationship between density and pressure. This relationship, affected by temperature, determines the speed of sound within the medium.

The propagation is also affected by the motion of the medium itself. For example, sound moving through wind. Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported.

The viscosity of the medium also affects the motion of sound waves. It determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.

When sound is moving through a medium that does not have constant physical properties, it may be refracted (either dispersed or focused)., 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, but are deaf to anything below 40 Hz. 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 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. The scientific study of human sound perception is known as psychoacoustics.

Physics of sound

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

Longitudinal and transverse waves

s of various frequencies; the bottom waves have higher frequencies than those above. The horizontal axis represents time.]] 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 waves 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 at right angle to the direction of propagation.

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 often simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties:

Frequency, or its inverse, the period

Wavelength

Wavenumber

Amplitude

Sound pressure

Sound intensity

Speed of sound

Direction

Sometimes speed and direction are combined as a velocity vector; wavenumber and direction are combined as a wave vector.

Transverse waves, also known as shear waves, have the additional property, polarization, and are not a characteristic of sound waves.

Speed of sound

breaking the sound barrier. The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft (see Prandtl-Glauert Singularity).]] The speed of sound depends on the medium the waves pass through, and is 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 the sea level, the speed of sound is approximately using the formula "v = (331 + 0.6 T) m/s". In fresh water, also at 20 °C, the speed of sound is approximately . In steel, the speed of sound is about . 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

Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical or audio engineer. The application of acoustics can be seen in almost all aspects of modern society with the most obvious being the audio and noise control industries.

Noise

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

Sound pressure is the difference, in a given medium, between average local pressure and the pressure in the sound wave. 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 this average provides 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 $-\backslash sqrt\{2\}$ Pa) and (1 atm $+\backslash sqrt\{2\}$ Pa), that is between 101323.6 and 101326.4 Pa. Such a tiny (relative to atmospheric) variation in air pressure at an audio frequency is perceived as a deafening sound, and can cause hearing damage, according to the table below.

As the human ear can detect sounds with a wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale. The sound pressure level (SPL) or L_p is defined as :$L\_\backslash mathrm\{p\}=10\backslash ,\; \backslash log\_\{10\}\backslash left(\backslash frac^2\}\backslash right)\; =20\backslash ,\; \backslash log\_\{10\}\backslash left(\backslash frac\{p\}\{p\_\backslash mathrm\{ref\}\}\backslash right)\backslash mbox\{\; dB\}\backslash ,$

:where p is the root-mean-square sound pressure and $p\_\backslash mathrm\{ref\}$ 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 matches 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.

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.

Sound measurement

Decibel, Sone, mel, Phon, Hertz

Sound pressure level, Sound pressure

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

Acoustics

Academic Programs in Acoustics

Auditory imagery

Audio bit depth

Audio signal processing

Beat

Diffraction

Doppler effect

Echo

Music

Musical tone

Note

Phonons

Physics of music

Pitch

Psychoacoustics

Resonance

Refraction

Reflection

Reverberation

Signal tone

Sonic weaponry

Sound localization

Soundproofing

Stereo imaging

Structural acoustics

Timbre

Ultrasound

List of unexplained sounds

References

External links

Sounds Amazing; a KS3/4 learning resource for sound and waves

HyperPhysics: Sound and Hearing

Introduction to the Physics of Sound

Hearing curves and on-line hearing test

Audio for the 21st Century

Conversion of sound units and levels

Sound calculations

Audio Check: a free collection of audio tests and test tones playable on-line

More Sounds Amazing; a sixth-form learning resource about sound waves

Category:Acoustics Category:Hearing Category:Waves