Basic notions

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Acoustic emission is generally due to the vibration of a body in contact with the fluid.

There are a multitude of sound sources in our daily environment: vocal cords, speakers, musical instruments, vehicles, etc.

The vibration produced by a sound source propagates through the fluid.


From a physical point of view, an acoustic wave is a mechanical vibration that propagates in a gas, a liquid or a solid.

The speed of sound is the speed of propagation of an acoustic wave. It depends on the nature of the medium in which the wave propagates. It also depends on the temperature.

Velocity of propagation of an acoustic wave at 20 ° C

in air: 344 m / s, about 1240 km / h.

in water: 1500 m / s, about 5400 km / h.

in steel: 5600 m / s or 20160 km / h.

And in vacuum? …

In a vacuum, there is no sound because there is no medium that can be used to support sound waves.

For proof, simply repeat the experience of Robert Boyle (1627-1691) – see the video

NB: Only vibrations or pressure variations associated propagate in the air, each molecule remaining in its original place. It’s the same for the circles in the water: the waves move, but the water remains in the same place. A cork floating on water goes up and down to the rhythm of the waves, but remains in place.


A sound can be received by the ear or a microphone.

The ear converts sound into sensory information transmitted to the brain via the auditory system.

The microphone converts sound into an electrical signal which can be recorded, stored, analyzed, reproduced,

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Objective / Subjective

The objective analysis of the sound received by a microphone consists in the description of the sound by mathematical tools. Main objective descriptors are the noise levels, duration, frequency and spectrum.

The subjective analysis of the perceived sound consists in the description of sounds by different subjects. Main subjective descriptors are the loudness, pitch and timbre.

Sound level/Loudness

The sound level is the sound intensity expressed in decibel (dB). This unit is used to best represent the feeling of a human being.

The perceived loudness depends on the sound level (dB) but not only.

Each time the sound intensity is multiplied by 10, which corresponds to an increase of 10 dB, the loudness is doubled.


The duration of a specific sound event determines, for a given volume level, the energy it contains, which can be likened to the “noise dose.”

Ear fatigue is directly related to the dose received. For example, the ear can withstand 85 dB for 8 hours, or 94 dB for 1 hour, or 100 dB for 1/4 hour.

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The frequency of a sound is defined as the number of times a periodic phenomenon occurs in 1 second.

The pitch is primarily related to the frequency of a sound.

The octave corresponds to a doubling of the frequency.

Spectrum / Timbre

Sound or vibration may be more or less complex.
The complexity of a sound can be understood using the concept of spectrum. The spectrum allows to know the distribution of energy versus frequency.

The timbre what differentiates two sounds of the same intensity, same duration and same pitch. The timbre is partly related to the spectrum of a sound. It also depends on the transients (attack, extinction of the sound).
Playing a sound backwards does not change its spectrum, but it changes the shape of the beginning (transient attack) and end (extinction transient).