Sound

In physics, sound is a vibration that propagates as an acoustic wave 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.

In air at atmospheric pressure, these represent sound waves with wavelengths of 17 meters (56 ft) to 1.7 centimeters (0.67 in).

Acoustics is the interdisciplinary science that deals with the study of mechanical waves in gasses, liquids, and solids including vibration, sound, ultrasound, and infrasound.

[3] An audio engineer, on the other hand, is concerned with the recording, manipulation, mixing, and reproduction of sound.

At a fixed distance from the source, the pressure, velocity, and displacement of the medium vary in time.

[5] The mechanical vibrations that can be interpreted as sound can travel through all forms of matter: gases, liquids, solids, and plasmas.

[6][7] Studies has shown that sound waves are able to carry a tiny amount of mass and is surrounded by a weak gravitational field.

In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m (56 ft) to 17 mm (0.67 in).

[13] 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.

Thus, 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 (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 [m/s] = 331 + 0.6 T [°C].

"[17] 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?"

The physical reception of sound in any hearing organism is limited to a range of frequencies.

[18]: 249  Sometimes sound refers to only those vibrations with frequencies that are within the hearing range for humans[19] or sometimes it relates to a particular animal.

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.

Furthermore, humans have developed culture and technology (such as music, telephone and radio) that allows them to generate, record, transmit, and broadcast sound.

In science and engineering, noise is an undesirable component that obscures a wanted signal.

Selection of a particular pitch is determined by pre-conscious examination of vibrations, including their frequencies and the balance between them.

Timbre is perceived as the quality of different sounds (e.g. the thud of a fallen rock, the whir of a drill, the tone of a musical instrument or the quality of a voice) and represents the pre-conscious allocation of a sonic identity to a sound (e.g. "it's an oboe!").

This identity is based on information gained from frequency transients, noisiness, unsteadiness, perceived pitch and the spread and intensity of overtones in the sound over an extended time frame.

Even though a small section of the wave form from each instrument looks very similar, differences in changes over time between the clarinet and the piano are evident in both loudness and harmonic content.

Less noticeable are the different noises heard, such as air hisses for the clarinet and hammer strikes for the piano.

[31][32] The word texture, in this context, relates to the cognitive separation of auditory objects.

[33][34] In a thick texture, it is possible to identify multiple sound sources using a combination of spatial location and timbre identification.

Ultrasound is not different from audible sound in its physical properties, but cannot be heard by humans.

A drum produces sound via a vibrating membrane .
Experiment using two tuning forks oscillating usually at the same frequency . One fork is hit with a rubberized mallet, causing the second fork to become visibly excited due to the oscillation caused by the periodic change in the pressure and density of the air. This is an acoustic resonance . When an additional piece of metal is attached to a prong, the effect becomes less pronounced as resonance is not achieved as effectively.
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 ). [ 14 ]
Pitch perception. During the listening process, each sound is analysed for a repeating pattern (orange arrows) and the results forwarded to the auditory cortex as a single pitch of a certain height (octave) and chroma (note name).
Duration perception. When a new sound is noticed (Green arrows), a sound onset message is sent to the auditory cortex. When the repeating pattern is missed, a sound offset messages is sent.
Loudness information is summed over a period of about 200 ms before being sent to the auditory cortex. Louder signals create a greater 'push' on the Basilar membrane and thus stimulate more nerves, creating a stronger loudness signal. A more complex signal also creates more nerve firings and so sounds louder (for the same wave amplitude) than a simpler sound, such as a sine wave.
Timbre perception, showing how a sound changes over time. Despite a similar waveform, differences over time are evident.
Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications