Sound


Sound is a phenomenon in which pressure disturbances propagate through a transmission medium. In the context of physics, it is characterised as a mechanical wave of pressure or related quantities, whereas in physiological-psychological contexts it refers to the reception of such waves and their perception by the brain. Though sensitivity to sound varies among all organisms, the human ear is sensitive to frequencies ranging from 20 Hz to 20 kHz. Examples of the significance and application of sound include music, medical imaging techniques, oral language and parts of science.

Definition

According to the technical standard established by ANSI/ASA S1.1-2013, the American National Standard for Acoustical Terminology, sound is defined as:
This two-part definition of sound states that sound can be taken as a wave motion in an elastic medium, making it also a stimulus, or as an excitation of the hearing mechanism that results in the perception of sound, making it a sensation.

Acoustics

Acoustics is the interdisciplinary scientific study of mechanical waves, vibrations, sound, ultrasound, and infrasound in gaseous, liquid, or solid media. A scientist who works in the field of acoustics is called an acoustician, while an individual specialising in acoustical engineering may be referred to as an acoustical engineer. An audio engineer, by contrast, is concerned with the recording, manipulation, mixing, and reproduction of sound.
Applications of acoustics are found in many areas of modern society. Subdisciplines include aeroacoustics, audio signal processing, architectural acoustics, bioacoustics, electroacoustics, environmental noise, musical acoustics, noise control, psychoacoustics, speech, ultrasound, underwater acoustics, and vibration.

Physics

Sound travels as a mechanical wave through a medium. Sound waves are generated by a sound source, such as a vibrating diaphragm of a loudspeaker. As the sound source vibrates the surrounding medium, mechanical disturbances propagate away from the source at the local speed of sound, thus resulting in a sound wave. At a fixed distance from the source, the pressure, velocity, and displacement of the medium's particles vary in time. At an instant in time, the pressure, velocity, and displacement vary spatially. The particles of the medium do not travel with the sound wave; instead, the disturbance and its mechanical energy propagate through the medium. Though intuitively obvious for solids, this also applies for liquids and gases. During propagation, waves can be reflected, refracted, or attenuated by the medium.
The matter that supports the transmission of a sound is named the transmission medium. Media may be any form of matter, whether solids, liquids, gases or plasmas. However, sound cannot propagate through a vacuum because there is no medium to support mechanical disturbances.
The propagation of sound in a medium is influenced primarily by:
  • A complicated relationship between the density and pressure of the medium. This relationship, also affected by temperature, determines the speed of sound within the medium.
  • Motion of the medium itself. If the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement. For example, sound moving through wind will have its speed of propagation increased by the speed of the wind if the sound and wind are moving in the same direction. If the sound and wind are moving in opposite directions, the speed of the sound wave will be decreased by the speed of the wind.
  • The viscosity of the medium. Medium viscosity 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 isn't uniform in its physical properties, it may be refracted.
Some theoretical work suggests that sound waves may carry an extremely small effective mass and be associated with a weak gravitational field.

Waves

Sound is transmitted through fluids as longitudinal waves, also called compression waves. Through solids, however, sound 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 are waves of alternating shear stress perpendicular to the direction of propagation. Unlike longitudinal sound waves, transverse sound waves have the property of polarisation.
Sound waves may be viewed using parabolic mirrors and objects that produce sound.
The energy carried by a periodic sound wave alternates between the potential energy of the extra compression or lateral displacement strain of the matter, and the kinetic energy of the particles' displacement velocity in the medium.
Although sound transmission involves many physical processes, the signal received at a point can be fully described as a time‑varying pressure. This pressure‑versus‑time waveform provides a complete representation of any sound or audio signal detected at that location.
Sound waves are often simplified as sinusoidal plane waves, which are characterized by these generic properties:
Sometimes speed and direction are combined as a velocity vector; wave number and direction are combined as a wave vector.
To analyse audio, a complicated waveform—such as the one shown on the right—can be represented as a linear combination of sinusoidal components of different frequencies, amplitudes, and phases.

Speed

The speed of sound depends on the medium the waves pass through, and is a fundamental property of the material. The first significant effort towards measurement of the speed of sound was made by Isaac Newton. He believed the speed of sound in a particular substance was equal to the square root of the pressure acting on it divided by its density:
This was later proven wrong and the French mathematician Laplace corrected the formula by deducing that the phenomenon of sound travelling is not isothermal, as believed by Newton, but adiabatic. He added another factor to the equation—gamma—and multiplied
by
thus coming up with the equation
Since
the final equation came up to be
which is also known as the Newton–Laplace equation. In this equation, K is the elastic bulk modulus, c is the velocity of sound, and is the density. Thus, the speed of sound is proportional to the square root of the ratio of the bulk modulus 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 using the formula. The speed of sound is also slightly sensitive, being subject to a second-order anharmonic effect, to the sound amplitude, which means there are non-linear propagation effects, such as the production of harmonics and mixed tones not present in the original sound. If relativistic effects are important, the speed of sound is calculated from the relativistic Euler equations.
In fresh water the speed of sound is approximately. In steel, the speed of sound is about. Sound moves the fastest in solid atomic hydrogen at about.

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 is usually averaged over time and/or space, and a square root of this average provides a root mean square value. For example, 1 Pa RMS sound pressure in atmospheric air implies that the actual pressure in the sound wave oscillates between and, that is between 101323.6 and 101326.4 Pa.
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 or Lp is defined as
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 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.

Perception

A distinct use of the term sound from its use in physics is that in physiology and psychology, where the term refers to the subject of perception by the brain. The field of psychoacoustics is dedicated to such studies. Webster's dictionary defined sound as: "1. The sensation of hearing, that which is heard; specif.: a. Psychophysics. Sensation due to stimulation of the auditory nerves and auditory centers of the brain, usually by vibrations transmitted in a material medium, commonly air, affecting the organ of hearing. b. Physics. Vibrational energy which occasions such a sensation. Sound is propagated by progressive longitudinal vibratory disturbances." This means that the correct response to the question: "if a tree falls in a forest and no one is around to hear it, does it make a sound?" is "yes", and "no", dependent on whether being answered using the physical, or the psychophysical definition, respectively.
The physical reception of sound in any hearing organism is limited to a range of frequencies. Humans normally hear sound as pitch for frequencies between approximately 20 Hz and 20,000 Hz, The upper limit decreases with age. Below 20 Hz, sound waves are heard as discrete stuttering sounds or fast 'wow-wow-wow' sounds. Sometimes sound refers to only those vibrations with frequencies that are within the hearing range for humans or sometimes it relates to a particular animal. Other species have different ranges 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 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 that allows them to generate, record, transmit, and broadcast sound.
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. However, in sound perception it can often be used to identify the source of a sound and is an important component of timbre perception.
Soundscape is the component of the acoustic environment that can be perceived by humans. The acoustic environment is the combination of all sounds within a given area as modified by the environment and understood by people, in context of the surrounding environment.
There are, historically, six experimentally separable ways in which sound waves are analysed. They are: pitch, duration, loudness, timbre, sonic texture and spatial location. Some of these terms have a standardised definition. More recent approaches have also considered temporal envelope and temporal fine structure as perceptually relevant analyses.