Its (physics)

Propagation of the sound

In a compressible medium, generally in the Air, the sound is propagated in the form of a variation of Pression created by the sound source. A Loudspeaker, for example, uses this mechanism. Let us note that only compression moves and not the Molécule S of air, if it is not of some Micromètre S. When one observes rounds in water, the Vague S move but water remains at the same place, it nothing but does move vertically and not to follow the waves (a stopper placed on water remains with the same position without moving). the sound is also propagated in the solid in the form of Vibration S of the Atome S called Phonon S. Là still, only the vibration is propagated, and not the atoms which do nothing but very slightly vibrate around their position of balance.

The propagation velocity of the sound (one also speaks about celerity) depends on nature, the temperature and the pressure of the medium. As the air is close to a Perfect gas, the pressure has very little influence on the speed of sound. In a perfect gas celerity is given by the relation:

c= \ frac {1} {\ sqrt {\ rho \ chi}}

where

\, \ rho

is the Density gas and

\, \ chi

its Compressibilité.

It is thus seen that the speed of sound decreases when the density of gas increases (effect of inertia) and when its Compressibilité (its aptitude to change volume under the effect of the pressure) increases. When it is about the atmosphere, it is advisable to know in more the thermal structure of the mass of air crossed as well as the direction of the wind because:

the sound is propagated more badly with horizontal than under rising angles because of the density switching. (This property is taken into account in the design of the theaters in the open air since antiquity)
the attenuation is definitely less strong under the wind. (As long as its mode on the ground is not too turbulent)
the sound can be literally carried by a low inversion of the Gradient of Température. For example, following night cooling, it is possible to hear a train to 5 km of a railway under the wind in spite of the obstacles. The sound is then constrained to be propagated under the inversion indeed Guide of wave.

The sound waves move with approximately 344 meters a second in air with 20 °C, speed which one can round with approximately a kilometer every three seconds, which is useful to coarsely measure the distance from a flash at the time of a Orage (speed of light making its perception quasi instantaneous). In solid media (nongas) the sound can be propagated even more quickly. Thus in the Water, its speed is of 1482 m/s and in the steel of 5050 m/s. The sound is not propagated in the Vide, because there is no matter to support the produced waves (Soundproofing), the sound being propagated thanks to displacements of the molecules of air. It is a wave known as longitudinal, because the material points move in the same direction as the displacement of the wave (the other standard being transverse waves).

Frequency and height

The frequency of a sound is expressed in Hertz (Hz), it is directly related to the height of a perceived sound, but is only one of the components (see the article Psychoacoustique). To a low frequency corresponds a serious sound, at a high frequency an acute sound.

Any living being equipped with a Ouïe can perceive only part of the sound Specter:

the physiologists agree to say that the human Oreille average approximately perceives the sounds only in one certain beach of frequencies located (according to the age, the culture, etc), between 20 Hz (in lower part the sounds are described as Infrason S) and 20 Khz (beyond the sounds are described as Ultrason S);
the cat can perceive sounds up to 25 Khz;
the Chien perceives the sounds up to 35 Khz;
the Bat and the Dauphin can perceive the sounds of frequency 100 Khz.

Certain animals use their aptitude to cover a broad waveband at various ends:

the elephant S use infrasons them to communicate to several Kilomètre S of distance;
the dolphins communicate thanks to the ultrasounds;
the bat emit ultrasounds (~80 Khz) with their system of echolocation enabling them to move and drive out in the total black.

To have the frequencies corresponding to the notes of music of the moderate range (Western classical music), to see moderate Range > Comparison of 3 systems of division of the octave .

Amplitude and intensity

Measuring unit

Where usually the pressure is measured in Pascal S, in Acoustique the intensity is measured in Décibel S (dB). It is a unit which uses the Logarithme is report/ratio of the sound intensity on the intensity of reference expressed in Watt S per square meter (W₀ = 10^-12W.m^-2), is report/ratio of the pressure produced on the pressure of reference, expressed in pascals (P₀ = 2.10^-5 Pa). It was thus selected because that makes it possible to have figures easily easy to handle, which do not become extremely large or small (see the article Logarithmic scale), and because this approach corresponds better to what the human ear in term of sound feeling perceives.

But attention, the concept of level noise gives only one vague idea of the perceived feeling, because it is necessary to take into account the sensitivity of the ear, which varies mainly according to the frequency of the sound (the ear is less sensitive to the low frequencies). A better approximation of perceived volume is given in balanced decibel has (dBa), it can be measured electronically after filtering of the signal by a weighting filter has (there exist also weightings B and C adapted to measurements of sounds of larger intensities).

0 dB corresponds at least that the human ear can perceive called Seuil of audibility, and not with the Silence Absolu. This value was chosen by experimentation for a sound of frequency 1000 Hz, it is worth 10^-12 W.m^-2, but the majority of the people have a threshold of audibility higher than 0 dB (approximately 4 dB). The threshold of pain is of 130 dB, but the ear can suffer damage starting from 85 dB (see the article Psychoacoustics).

It is enough to change the reference of power or pressure (P₀ or W₀ in the formulas below) so that the scale of volumes is completely changed. This is why the decibels graduated on the button of volume of a Hi-fi system do not correspond at all to acoustic levels but to electric outputs of exit of the amplifying , which does not have almost anything to see: the value 0 dB very often represents the maximum power which the amplifier is able to deliver.

Various measurements of the amplitude

There exist several ways of measuring the amplitude of a sound, and by extension, of an unspecified signal of undulatory nature:

the average amplitude (the arithmetic value Mean of the positive signal)
the effective amplitude (equivalent continuous amplitude in power)
the amplitude peak (maximum positive)
the amplitude peak with peak (the maximum variation of positive and negative amplitude)

In practice, the average amplitude presents little interest and is not used. On the other hand, the effective value or RMS, for Root Mean Public garden in English, is the average quadratic Valeur signal is universally adopted to measure the value of the alternating voltages, within the general framework as much as in acoustics. An amplifier which is given for 10 Watts RMS will make 14 Watts in peak and 28 Watts in peak with peak (also noted DC). Measurements of power peak to peak are rather often called “musical Watts” by the salesmen of audio-visual material, because the figures are more flattering.

Stamp

Space time

Like all the perceived phenomena, the Temps plays a fundamental role for acoustics (and even more in Musique). There exist even very close relations between the space and time, considering the sound is a wave which is propagated in space during time.

One distinguishes three big classes from acoustic signals:

Periodic S, whose form is repeated with identical in time;
Aléatoire S, which does not have periodic characteristics. In what follows, and generally, one is interested only in one restricted unit of these signals; those which have stable characteristics Statistique S in time. They are called random signals ergodic. Concretely, it is the case of the Bruit S “white or pink” used by the scientists and certain artists;
Impulse S: who do not repeat themselves in time and have a given form.

All the signals can be defined and analyzed indifferently in the temporal Espace or the frequential Espace. In this last, one will often have recourse to the use of the spectrum of the signal, calculated since his frequential definition (known as of the field of Fourier). The spectrum of a signal represents the various “notes” or pure sounds that a sound contains, called partial S. Danslecasde a signal Périodique Stable like a siren, the spectrum does not evolve/move during time and presents to only one value called “line”. It is indeed possible to consider any sound as the combination of a whole of “pure sounds” which are sinusoids (see on this subject the article on the Transformée of Fourier).

Recording

Music

The sound and data processing

Acquisition

For the digital processing of the sound (treatment by a computer), it is necessary to carry out an analog-to-digital conversion, what is called its acquisition. This operation consists in transforming the variations of Pression sound, in a succession of numbers which average data processing will be able to treat. One calls this transformation the sampling signal. A Microphone converts the variations of pressures of the air into electrical signals which, connected has an analog-to-digital converter (EDGE or ADC in English, for Analog to DIGITAL Converter ) which will digitize this signal with regular step, to transform it into a succession of numbers. This work is now completed by the charts its of the personal computers.
Figure 9: Practical filter and théorique.
I: Filter ideal
P: Filter practices
R: Undulation
Effective b: Band-width

A filter behaves like a manipulator of signal and this has effects at the same time on the form Temporel of the wave and on its spectrum. Thus a square signal with 100 Hz which one filter 200 Hz will become a sinusoid with 100 Hz because one will have removed the upper part of his spectrum to him (see figure 6c). Same manner, a note of piano with 1000 Hz (C 6) will sound as a vulgar whistle if it is filtered to 1200 or 1500 Hz. The base frequency of the signal is called fundamental. The others are multiples of this fundamental and are called Harmonique S. Dimensioned temporal, a filter introduces also modifications called Distorsion S. That comes mainly from the delay which the harmonics take the ones compared to the others.

In order to illustrate the influence of a filter on a signal, let us consider a simple square impulse (figure 10a), the amplitude of its spectrum (figure 10b) and the phase of its spectrum (figure 10c). It is that this rectangular impulse is anything else only one filter which lets pass the sound to t=0 and cuts it after T seconds. The spectrum of the impulse represents frequency response of the filter. It is seen that the more the frequency of the signal is raised, the more there is shift between the components whereas their amplitude decrease.

Appear 10a: Temporal signal. rectangular Impulse with t=0.

Appear 10b: Spectrum (Amplitude).

Appear 10c: Spectrum (Phase).

Figure 11 represents the influence of our rectangular filter on a simple signal like a sinusoid.

Appear 11a: Impulse rectangulaire.
Impulse with t=0.

Appear 11b: Impulse sonore.

The fact of cutting in a brutal way the sound at time T introduced of new frequencies into the spectrum of the sinusoid. If the filtered signal is more complex like the square signal of the figure 6c, the various frequencies which compose it will be more out of phase ones compared to the others (introduction of a delay). -->

Figure 7a: Integral transform of Fourier.

Infinite and continues in time and frequency

Figure 7b: Fourier series.

Periodic in time and discrete in frequency

Figure 7c: Sampled functions.

Discrete in time and periodical in frequency

Figure 7d: Discrete transform of Fourier.

Discrete and periodical in time and frequency

--> Conclusion

This review of the physical aspects, human and techniques of acoustics gives an idea of the extent of the parameters which can define the sound. However, the appreciation of your ear nevertheless remains the ultimate criterion of detection of quality, without being a measuring device. The figures given by mathematics or of the sophisticated measuring instruments can be useful to include/understand why a recording appears odd, but they will never say to you if Beatles made better music than Rolling Stones in the Sixties. -->

See too

progressive mechanical Wave;
Speed of sound;
Wall of the sound;
Effect Doppler;
Sound engineer;
Sonothèque ;
Wiring for sound;
Formation with the trades of the sound.

External bonds

the nature of the sound;
Audiomaniac - universe of the sounds and the sound effects: Physique of the sound.
Natural and perception of the sound

figure 1a watch how a Stylet attached to a source of vibration, like a Loudspeaker for example, oscillates and produces a wave, when one makes ravel a paper band in front of his point.

Z: Vibration of the stylet of an amplitude ±A0
λ (lambda): Wavelength
X: Displacement of the band at the speed C
W: Resulting wave

Figure 1a: Vibration of a stylet on a tape in displacement

P: Vibrating piston
T: Tube
T: time

Figure 1b: vibration of a piston in a fluid

-->

S. Danslecasde a signal Périodique Stable like a siren, the spectrum does not evolve/move during time and presents to only one value called “line” as on figure 1. It is indeed possible to consider any sound as the combination of a whole of “pure sounds” which are sinusoids. See on this subject the article on the Transformed of Fourier .

Figure 1: Pure sinusoid (simple and periodic)

Figure 2: Combination of two sinusoids

Figure 3: Square signal (complex but periodic)

Figure 4: Random signal (complex and not periodical)

Figure 6: Aural signals and their spectra -->

Fiu-vro: Helü Simple: Sound Zh-yue: 聲

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