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A noise is a setting vibration of the air. It is characterized by its frequency, expressed in hertz, ranging from severe to acute and by its intensity, expressed in decibels.

## The intensity of a sound

Zero decibel, which is absolute silence, can not be perceived to the extent that the human body itself emits a few decibels. The decibel scale starts at ten decibels (10 dB). This level corresponds to what is called silence.

### Scale of decibels

Very complicated to understand, the decibel scale is logarithmic. We must forget all his reflexes of arithmetic calculation. The calculation rules are not the same. For example, 60 dB + 60 dB is not 120 dB but 63 dB!
Two sounds of equal intensity will produce a sound of 3 dB more.
Another example is 70 dB + 100 dB = 100 dB. When a difference between two sounds is greater than 10 dB, the loudest sound covers the weakest.
Thus quantified from a well-identified noise, it is possible to define an isolation objective to obtain the desired sound level.

## The three types of noises

Depending on whether they are airborne, impact or equipment-related, noises are transmitted in different ways.

### Impact noise

Impact or shock noises (slamming doors, footsteps...) are transmitted by vibrating the walls that are the floors, walls, ceilings.

### Airborne noise

Airborne noise or pink noise (road traffic, conversation, television...) spread through the air, put the walls in vibration and pass from outside to inside, or from one room to another.

### Equipment noise

Equipment noises (lifts, valves, VMC...) are emitted by the technical installations and are propagated by air or in the form of impact noises.