Concept in Definition ABC

The means of propagation of sound are: gases (for example, the air), liquids (such as water) or solids (metals, rocks, etc.). Sound propagates in gases and liquids as a longitudinal wave, where the direction of propagation through the medium is in the same direction as the wave propagation. On the other hand, in solids the waves can also be transverse, which means that the amplitude of the sound wave is perpendicular to the propagation of the sound.

What are its properties and characteristics?

Understanding sound as a wave, it presents its own phenomena, namely: reflection, interference, refraction and diffraction.

Reflection: occurs when a sound wave hits a hard surface, follows the law from "the angle of incidence is equal to the angle of reflection". This behavior is also observed with electromagnetic waves (light, radio, etc.). The

production of resonant standing waves on strings and air columns is due to the reflection of sound waves, which is a fundamental phenomenon in functioning of wind instruments. [1]

Interference: Occurs when wave patterns overlap each other. When reflected waves interfere with incident waves, constructive and destructive interference patterns are produced. Due to this, resonances called confined standing waves can be produced. This also implies that the intensity of sound near a hard surface is twice as high because the reflected wave and the incident wave add together in a narrow pressure zone. This sound behavior is exploited in pressure zone microphones allowing increased sensitivity. Another clear example of interference is "beating" [2], which consists of the superposition of two sound waves with frequencies of very close values.

Refraction: is the change in sound propagation when it enters a medium where its speed is different. Normally, sound propagates in all directions from a point source, with no 'privileged' direction, meaning the listener can only hear the sound wave that was initially directed at it, however, the refraction of the sound can bend the wave downward, in such a way that some sound is received additional. This occurs, for example, on the surface of cold lakes, which then turn out to be natural amplifiers. [4]

Diffraction: Occurs when sound waves 'bend' around small obstacles or propagate through small holes. A clear example is how sound is heard around corners. Diffraction is more important with longer wavelengths (low frequency) that is why it can be hear low frequencies around obstacles better than high frequencies (wavelengths short).

Finally, the parameters that characterize sound waves are speed, pressure and intensity.

Speed: the speed of sound in dry air is approximately equal to:

331.4+0.6T m/s (1)

A better approximation is given by expression for an ideal gas where the speed of sound is equal to:

vsound =√(γRT/M) (2)

Where γ is the adiabatic constant, R is the ideal gas constant whose value is 8.314 J/mol K, M is the molecular mass of the gas and T is the temperature absolute (measured in degrees kelvin °K).

Pressure: the variation of the pressure caused by the sound, in relation to the atmospheric pressure, allows you to quantize the sound. In the case of human hearing, the hearing threshold corresponds to a pressure variation of a little less than one billionth of atmospheric pressure. This hearing threshold can be formulated in terms of pressure, then the intensity of sound (measured in decibels) can be expressed in terms of acoustic pressure as:

I=10 log₁₀I/I₀=20 log₁₀P/P₀ (3)

Intensity: is the acoustic power per unit area measured by the 'observer' or better said listener. Units are watts/m2 or watts/cm2. Many measurements of sound intensity are made relative to the intensity of the standard threshold of hearing:

Yo₀ =10^-16 watts/cm² (4)

It is common to measure the intensity of sound in decibels:

I(dB)=10 log₁₀I/I₀ (5)

How did it start and in what situations can we apply the concept?

For some, it arises since time is time, however, the existence of sound began after creation of the universe, some fifteen billion years later according to the most accepted cosmological theories currently.

In its form as such, the evolution in the recording of sounds artificially created by human beings, it dates back to the year 1857, on March 25, when León Scott patented his autograph phone. We then go through the phonograph, the gramophone, the record player, the wire tape recorder, the reel tape recorder open, the compact cassette, the first optical system, the ProDigi, the DASH, the compact disc (CD), the MP3, the DVD, and the Blu-ray.