Sound

A drum produces sound via a vibrating membrane
Spherical compression (longitudinal) waves
A 'pressure over time' graph of a 20 ms recording of a clarinet tone demonstrates the two fundamental elements of sound: Pressure and Time.
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.
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).
Figure 1. Pitch perception
Figure 2. Duration perception
Figure 3. Loudness perception
Figure 4. Timbre perception
Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications

Vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.

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Frequency

Number of occurrences of a repeating event per unit of time.

A pendulum making 25 complete oscillations in 60 s, a frequency of 0.41 Hertz
A pendulum with a period of 2.8 s and a frequency of 0.36 Hz
Diagram of the relationship between the different types of frequency and other wave properties.
Modern frequency counter
Complete spectrum of electromagnetic radiation with the visible portion highlighted
The sound wave spectrum, with rough guide of some applications

Frequency is an important parameter used in science and engineering to specify the temporal rate of change observed in oscillatory and periodic phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light.

Wavelength

Spatial period of a periodic wave—the distance over which the wave's shape repeats.

The wavelength of a sine wave, λ, can be measured between any two points with the same phase, such as between crests (on top), or troughs (on bottom), or corresponding zero crossings as shown.
Sinusoidal standing waves in a box that constrains the end points to be nodes will have an integer number of half wavelengths fitting in the box.
A standing wave (black) depicted as the sum of two propagating waves traveling in opposite directions (red and blue)
Wavelength is decreased in a medium with slower propagation.
Refraction: upon entering a medium where its speed is lower, the wave changes direction.
Separation of colors by a prism (click for animation)
Various local wavelengths on a crest-to-crest basis in an ocean wave approaching shore
A sinusoidal wave travelling in a nonuniform medium, with loss
A wave on a line of atoms can be interpreted according to a variety of wavelengths.
Near-periodic waves over shallow water
Wavelength of a periodic but non-sinusoidal waveform.
A propagating wave packet
Pattern of light intensity on a screen for light passing through two slits. The labels on the right refer to the difference of the path lengths from the two slits, which are idealized here as point sources.
Diffraction pattern of a double slit has a single-slit envelope.
Relationship between wavelength, angular wavelength, and other wave properties.

Examples of waves are sound waves, light, water waves and periodic electrical signals in a conductor.

Audio signal processing

Subfield of signal processing that is concerned with the electronic manipulation of audio signals.

Signal transmission using electronic signal processing. Transducers convert signals from other physical waveforms to electric current or voltage waveforms, which then are processed, transmitted as electromagnetic waves, received and converted by another transducer to final form.

Audio signals are electronic representations of sound waves—longitudinal waves which travel through air, consisting of compressions and rarefactions.

Audio engineer

An audio engineer (also known as a sound engineer or recording engineer) helps to produce a recording or a live performance, balancing and adjusting sound sources using equalization, dynamics processing and audio effects, mixing, reproduction, and reinforcement of sound.

An audio engineer with audio console, at a recording session at the Danish Broadcasting Corporation.
Noted audio engineer Roger Nichols at a vintage Neve recording console.
Acoustic diffusing mushrooms hanging from the roof of the Royal Albert Hall.
The Pyramid Stage
Live sound mixing
At the front of house position, mixing sound for a band
Correcting a room's frequency response.

Sound engineering is increasingly seen as a creative profession where musical instruments and technology are used to produce sound for film, radio, television, music and video games.

Psychoacoustics

An equal-loudness contour. Note peak sensitivity around 2–4 kHz, in the middle of the voice frequency band.
Audio masking graph
Perceptual audio coding uses psychoacoustics-based algorithms.
Psychoacoustic model

Psychoacoustics is the branch of psychophysics involving the scientific study of sound perception and audiology—how humans perceive various sounds.

Acoustical engineering

The transparent baffles inside this auditorium were installed to optimise sound projection and reproduction, key factors in acoustical engineering.
Disney's Concert Hall was meticulously designed for superior acoustical qualities.
Ceiling of Culture Palace (Tel Aviv) concert hall is covered with perforated metal panels
At outdoor concerts like Woodstock, acoustic analysis is critical to creating the best experience for the audience and the performers.
Ultrasound image of a fetus in the womb, viewed at 12 weeks of pregnancy (bidimensional-scan)

Acoustical engineering (also known as acoustic engineering) is the branch of engineering dealing with sound and vibration.

Acoustic wave

Acoustic waves are a type of energy propagation through a medium by means of adiabatic compression and decompression.

For a simple substance, during an adiabatic process in which the volume increases, the internal energy of the working substance must decrease

Some examples of acoustic waves are audible sound from a speaker (waves traveling through air at the speed of sound), ground movement from an earthquake (waves traveling through the earth), or ultrasound used for medical imaging (waves traveling through the body).

Speed of sound

An F/A-18 Hornet displaying rare localized condensation in humid air while traveling near the speed of sound
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Density and pressure decrease smoothly with altitude, but temperature (red) does not. The speed of sound (blue) depends only on the complicated temperature variation at altitude and can be calculated from it since isolated density and pressure effects on the speed of sound cancel each other. The speed of sound increases with height in two regions of the stratosphere and thermosphere, due to heating effects in these regions.
Approximation of the speed of sound in dry air based on the heat capacity ratio (in green) against the truncated Taylor expansion (in red).
Speed of sound in water vs temperature.
Speed of sound as a function of depth at a position north of Hawaii in the Pacific Ocean derived from the 2005 World Ocean Atlas. The SOFAR channel spans the minimum in the speed of sound at about 750-m depth.

The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium.

Reflection (physics)

Change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated.

The reflection of Mount Hood in Mirror Lake.
Diagram of specular reflection
Refraction of light at the interface between two media.
An example of the law of reflection
General scattering mechanism which gives diffuse reflection by a solid surface
Working principle of a corner reflector
Multiple reflections in two plane mirrors at a 60° angle.
Sound diffusion panel for high frequencies

Common examples include the reflection of light, sound and water waves.

Refraction

Redirection of a wave as it passes from one medium to another.

A ray of light being refracted in a plastic block.
Refraction of light at the interface between two media of different refractive indices, with n2 > n1. Since the phase velocity is lower in the second medium (v2 < v1), the angle of refraction θ2 is less than the angle of incidence θ1; that is, the ray in the higher-index medium is closer to the normal.
A pen partially submerged in a bowl of water appears bent due to refraction at the water surface.
When a wave moves into a slower medium the wavefronts get compressed. For the wavefronts to stay connected at the boundary the wave must change direction.
A pencil part immersed in water looks bent due to refraction: the light waves from X change direction and so seem to originate at Y.
An image of the Golden Gate Bridge is refracted and bent by many differing three-dimensional drops of water.
The sun appears slightly flattened when close to the horizon due to refraction in the atmosphere.
Heat haze in the engine exhaust above a diesel locomotive.
Mirage over a hot road.
Water waves are almost parallel to the beach when they hit it because they gradually refract towards land as the water gets shallower.

Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction.