In physics, sound is a vibration that typically propagates as an
audible wave of pressure, through a transmission medium such as a gas,
liquid or solid.
In human physiology and psychology, sound is the reception of such
waves and their perception by the brain. Humans can only hear sound
waves as distinct pitches when the frequency lies between about
20 Hz and 20 kHz.
3.1 Longitudinal and transverse waves
4 Perception of sound
4.1 Pitch 4.2 Duration 4.3 Loudness 4.4 Timbre 4.5 Sonic texture 4.6 Spatial location
Experiment using two tuning forks oscillating usually at the same frequency. One of the forks is being hit with a rubberized mallet. Although the first tuning fork hasn't been hit, while the other fork is visibly excited due to the oscillation caused by the periodic change in the pressure and density of the air by hitting the other fork, creating an acoustic resonance between the forks. However, if we place a piece of metal on a prong, we see that the effect dampens, and the excitations become less and less pronounced as resonance isn't achieved as effectively.
A complex relationship between the density and pressure of the medium. This relationship, 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 does not have constant physical properties, it may be refracted (either dispersed or focused).
Spherical compression (longitudinal) waves
The mechanical vibrations that can be interpreted as sound can travel
through all forms of matter: gases, liquids, solids, and plasmas. The
matter that supports the sound is called the medium.
A 'pressure over time' graph of a 20 ms recording of a clarinet tone
demonstrates the two fundamental elements of sound:
Sounds can be represented as a mixture of their component Sinusoidal waves of different frequencies. The bottom waves have higher frequencies than those above. The horizontal axis represents time.
Although there are many complexities relating to the transmission of
sounds, at the point of reception (i.e. the ears), sound is readily
dividable into two simple elements: pressure and time. These
fundamental elements form the basis of all sound waves. They can be
used to describe, in absolute terms, every sound we hear.
In order to understand the sound more fully, a complex wave such as
the one shown in a blue background on the right of this text, is
usually separated into its component parts, which are a combination of
various sound wave frequencies (and noise).
Frequency, or its inverse, wavelength Amplitude, sound pressure or Intensity Speed of sound Direction
U.S. Navy F/A-18 approaching the speed of sound. 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. 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:
displaystyle c= sqrt p over rho ,
This was later proven wrong when found to incorrectly derive the speed. 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
displaystyle sqrt gamma ,
displaystyle sqrt p over rho ,
, thus coming up with the equation
displaystyle c= sqrt gamma cdot p over rho ,
K = γ ⋅ p
displaystyle K=gamma cdot p,
, the final equation came up to be
displaystyle c= sqrt frac K rho ,
, which is also known as the Newton-Laplace equation. In this equation, K = elastic bulk modulus, c = velocity of sound, and
= 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 20 °C (68 °F) air at sea level, the speed
of sound is approximately 343 m/s (1,230 km/h; 767 mph)
using the formula "v = (331 + 0.6 T) 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). 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 (see parametric
Perception of sound
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. Historically the word "sound" referred exclusively to an
effect in the mind. Webster's 1947 dictionary defined sound as: "that
which is heard; the effect which is produced by the vibration of a
body affecting the ear." This meant (at least in 1947) the correct
response to the question: "if a tree falls in the forest with no one
to hear it fall, does it make a sound?" was "no". However, owing to
contemporary usage, definitions of sound as a physical effect are
prevalent in most dictionaries. Consequently, the answer to the same
question is now "yes, a tree falling in the forest with no one to hear
it fall does make a sound".
The physical reception of sound in any hearing organism is limited to
a range of frequencies. Humans normally hear sound frequencies between
approximately 20 Hz and 20,000 Hz (20 kHz),:382 The
upper limit decreases with age.:249 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
(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.
Figure 1. Pitch perception
Pitch is perceived as how "low" or "high" a sound is and represents the cyclic, repetitive nature of the vibrations that make up sound. For simple sounds, pitch relates to the frequency of the slowest vibration in the sound (called the fundamental harmonic). In the case of complex sounds, pitch perception can vary. Sometimes individuals identify different pitches for the same sound, based on their personal experience of particular sound patterns. Selection of a particular pitch is determined by pre-conscious examination of vibrations, including their frequencies and the balance between them. Specific attention is given to recognising potential harmonics. Every sound is placed on a pitch continuum from low to high. For example: white noise (random noise spread evenly across all frequencies) sounds higher in pitch than pink noise (random noise spread evenly across octaves) as white noise has more high frequency content. Figure 1 shows an example of pitch recognition. During the listening process, each sound is analysed for a repeating pattern (See Figure 1: orange arrows) and the results forwarded to the auditory cortex as a single pitch of a certain height (octave) and chroma (note name). Duration
Figure 2. Duration perception
Duration is perceived as how "long" or "short" a sound is and relates
to onset and offset signals created by nerve responses to sounds. The
duration of a sound usually lasts from the time the sound is first
noticed until the sound is identified as having changed or ceased.
Sometimes this is not directly related to the physical duration of a
sound. For example; in a noisy environment, gapped sounds (sounds that
stop and start) can sound as if they are continuous because the offset
messages are missed owing to disruptions from noises in the same
general bandwidth. This can be of great benefit in understanding
distorted messages such as radio signals that suffer from
interference, as (owing to this effect) the message is heard as if it
was continuous. Figure 2 gives an example of duration identification.
When a new sound is noticed (see Figure 2, Green arrows), a sound
onset message is sent to the auditory cortex. When the repeating
pattern is missed, a sound offset messages is sent.
Sonic texture relates to the number of sound sources and the
interaction between them. The word 'texture', in this context,
relates to the cognitive separation of auditory objects. In music,
texture is often referred to as the difference between unison,
polyphony and homophony, but it can also relate (for example) to a
busy cafe; a sound which might be referred to as 'cacophony'. However
texture refers to more than this. The texture of an orchestral piece
is very different to the texture of a brass quintet because of the
different numbers of players. The texture of a market place is very
different to a school hall because of the differences in the various
Spatial location (see:
Particle velocity v, SVL
Particle displacement δ
Sound energy density w
Acoustic impedance Z
Speed of sound c
Audio frequency AF
Transmission loss TL
v t e
displaystyle - sqrt 2
Pa) and (1 atm
displaystyle + sqrt 2
Pa), 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 (SPL) or Lp is defined as
r e f
r e f
displaystyle 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
r e f
displaystyle p_ 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
Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications
Sound energy flux
^ Fundamentals of Telephone Communication Systems. Western Electrical
Company. 1969. p. 2.1.
^ ANSI S1.1-1994. American National Standard: Acoustic Terminology.
^ Acoustical Society of America. "PACS 2010 Regular
29. http://www.apkstub.com/2018/04/sounds-tool-apk-learn-and-download-free.html External links
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Library resources about Sound
Resources in your library
Sounds Amazing; a KS3/4 learning resource for sound and waves
v t e
Architectural acoustics Monochord Reverberation Soundproofing String vibration
Bark scale Combination tone Equal-loudness contour
Mel scale Missing fundamental
Mersenne's laws Overtone Resonance Standing wave
John Backus Jens Blauert Ernst Chladni Hermann von Helmholtz Franz Melde Marin Mersenne Werner Meyer-Eppler Lord Rayleigh Joseph Sauveur D. Van Holliday Thomas Young
Echo Infrasound Sound Ultrasound Musical acoustics