Sound

Drum cadences performed by the United States Navy Band.

For other uses, see Sound (disambiguation).

Sound is a phenomenon in which pressure disturbances travel through an elastic material medium. In physics, it is described as a mechanical wave of pressure or related quantities, such as displacement. In physiological-psychological contexts, it refers to a hearing process.

Sensitivity to sound varies among living things. The human ear is sensitive to audio frequencies ranging from 20 Hz to 20 kHz. More broadly, acoustic waves include mechanical longitudinal waves in general.

Sound has many important uses. Examples include music, medical imaging techniques, oral language and parts of science.

Definition

According to the technical standard ANSI/ASA S1.1‑2013, the American National Standard for Acoustical Terminology, sound has two parts. First, it is an oscillation in pressure, stress, or particle movement that spreads through a material with internal forces, such as an elastic or thick, sticky material. Second, it is the hearing sensation caused by that oscillation. This means sound can be seen as a wave moving through a material, which makes it a stimulus, or as the hearing process that lets us perceive sound, which makes it a sensation.

ANSI/ASA S1.1‑2013 is one of the few major standards organisations to define sound clearly, and it is the standard used in U.S. acoustics. Physics and engineering books often describe sound differently, as a moving disturbance that travels as a wave through a material, rather than as a vibration. These books describe sound as changes in pressure or particle movement that travel through a material, made of areas that are squeezed together and areas that are spread apart. They use this description because it is clear and precise.

Acoustics

Main article: Acoustics

Acoustics is the science that studies mechanical waves, vibrations, sound, ultrasound, and infrasound in gases, liquids, or solids. A scientist in this field is called an acoustician, and a person who specialises in acoustical engineering may be called an acoustical engineer. An audio engineer works with recording, changing, mixing, and reproducing sound.

Acoustics is used in many parts of modern life. Its subfields 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 such as water, crystals, or air. A sound source, like the vibrating diaphragm of a loudspeaker, pushes the medium around it, and these disturbances spread out at the local speed of sound. The particles of the medium do not travel with the wave. Instead, the disturbance and its mechanical energy move through the medium, and this is true for liquids and gases as well as solids. The material that carries a sound is called the transmission medium. It can be any form of matter, including solids, liquids, gases, or plasmas, but sound cannot pass through a vacuum because there is nothing to carry the disturbances. Sound is mainly affected by the density and pressure of the medium, by temperature, by the movement of the medium (such as wind), and by viscosity, which decides how quickly sound fades. Theoretical work suggests that sound waves carry an extremely small effective gravitational mass, which comes from nonlinear corrections to the stress–energy of the wave.

Sound waves can be refracted, diffracted, absorbed, and weakened by attenuation, and in a mixed medium they may be dispersed. In fluids, sound travels as longitudinal waves, also called compression waves, which push the pressure above and below the equilibrium level, creating compression and rarefaction. In solids, sound can also travel as transverse waves, which move through shear stress across the direction of travel. Sound waves are often described as sinusoidal plane waves with a frequency, wavelength, amplitude, and intensity. The first serious attempt to measure the speed of sound was made by Isaac Newton. Later, the French mathematician Laplace found that sound is adiabatic rather than isothermal, and added the gamma factor to the formula. The speed of sound is proportional to the square root of the ratio of the bulk modulus of the medium to its density. In 20 °C air the speed is about 343 m/s, and it is about 1,482 m/s in fresh water, 5,960 m/s in steel, and about 36,000 m/s in solid atomic hydrogen, the fastest of these. Sound pressure is the difference between average local pressure and the pressure in the sound wave, and it is often measured on a logarithmic decibel scale. Commonly used reference sound pressure values, defined in ANSI S1.1-1994, are 20 μPa in air and 1 μPa in water. Because the human ear does not respond equally to all frequencies, the International Electrotechnical Commission has defined weighting schemes, and A-weighting is used to match human hearing.

Main article: Speed of sound

Perception

Main article: Psychoacoustics

In physiology and psychology, the word sound refers to the experience produced when a living thing is stimulated by sound. This differs from the physical meaning used in acoustics. The study of how living things detect and understand these signals is called psychoacoustics, and the wider field is psychophysics. Webster's dictionary gives both meanings, as "the sensation of hearing" and "vibrational energy which occasions such a sensation." This is why the question "if a tree falls in a forest and no one is around to hear it, does it make a sound?" can have different answers. The physiological reception of sound in organisms with auditory systems works only within a range of frequencies. In humans, pitch is heard from about 20 Hz to 20 kHz, and the upper limit gets lower with age. Domestic dogs can detect frequencies above 20 kHz. Sound is one of the major senses and helps many species with detecting danger, navigation, predation, and communication. Atmosphere, water, and many natural events such as fire, rain, wind, surf, or earthquakes each produce their own sounds. Many animals, including frogs, birds, marine and land mammals, have special organs to make sound, and in some species these produce song and speech. Humans use music, telephones, and radio to make, record, send, and broadcast sound. Noise is often used to mean an unwanted sound, and a soundscape is the part of the acoustic environment that people can perceive.

Historically, sound waves are studied in six separate ways: pitch, duration, loudness, timbre, sonic texture, and spatial location. Some of these terms have standard definitions, such as in the ANSI/ASA S1.1-2013 terminology. Newer approaches also treat temporal envelope and temporal fine structure as important. Pitch is how "low" or "high" a sound seems, and simple sounds are heard by their slowest vibration. Sounds are placed on a range from low to high, so white noise seems higher than pink noise because it has more high-frequency sound.

Duration is how "long" or "short" a sound seems. It often lasts from when a sound is first noticed until it is heard to change or stop. In noisy places, sounds that start and stop can seem continuous, which helps people understand radio messages with interference.

Loudness is how "loud" or "soft" a sound seems. Louder sounds move the basilar membrane more, which activates more auditory‑nerve fibres. Very brief sounds seem softer than longer ones at the same intensity, and this effect levels off after about 200 ms. Complex tones often seem louder than simple tones.

Timbre is the quality of a sound, such as a rock's thud, a drill's whir, or an instrument's tone, and it helps people recognise what is making it. It depends on how the sound changes over time. A clarinet and a piano can be told apart by their changes in loudness and harmonics.

Sonic texture relates to how many sound sources there are and how they interact. In music, texture can be unison, polyphony, or homophony, while a busy café may be called a cacophony.

Spatial location is where a sound seems to come from, including its side-to-side and up-and-down placement and its distance. In a busy sound environment, people can often pick out several sources by combining spatial location and timbre.

Frequency

See also: Audio frequency

Ultrasound is sound waves with frequencies higher than 20,000 Hz. It is not different from sound people can hear in its physical properties, but humans cannot hear it. Ultrasound devices work with frequencies from 20 kHz up to several gigahertz. Medical ultrasound is commonly used for diagnosis and treatment.

See also: Perception of infrasound

Infrasound is sound waves with frequencies lower than 20 Hz. These sounds are too low for humans to hear as a pitch, but people can hear them as separate pulses, like the 'popping' sound of an idling motorcycle. Whales, elephants and other animals can detect infrasound and use it to communicate. It can be used to detect volcanic eruptions and is used in some types of music.