Les chiffres du bruit

Abstract

Parmi les organes des sens qui nous permettent d’entrer et de rester en contact avec notre environnement, l’audition a une place privilégiée. Elle s’est adaptée aux besoins de l’homme au fil de son évolution, complétant la fonction d’alerte à distance (bruits des prédateurs ou des proies potentielles) par une fonction de communication de plus en plus évoluée. Le niveau du monde sonore qui nous entoure s’est beaucoup élevé, suivant les progrès et l’intensification de la mécanisation (moteurs, machines de plus en plus rapides) et même ceux de l’amplification volontaire des émissions sonores (sirènes, musique). Les phénomènes bruyants, dont les caractéristiques physiques sont à connaître, peuvent être gênants, voire dangereux pour l’homme. Si l’énergie qu’ils véhiculent est suffisante, ils peuvent en effet provoquer la destruction de certaines structures internes de l’oreille. Pour prévenir ce risque, il est utile de connaître les caractéristiques physiques du phénomène, celles de sa mesure, les niveaux rencontrés et certaines valeurs limites d’exposition.

High levels of noise encountered both in leisure activities and at workplaces can be somewhat annoying, but they can also cause hearing damage. In order to lessen these risks, some physical characteristics of the sound phenomenon need to be understood. The level of a sound is given in dB, a logarithmic unit in which simple addition is not available: 100 dB + 100 dB = 103 dB. The highest level of noise which can be tolerated by the human ear is considered to be 120 dB. Another component of sound characteristics is the frequency, which describes the height of a sound. The frequency is given in Hz, the human hearing field is comprised in the range of 20 to 20 000 Hz. Regarding the sensitivity of the ear, depending on the frequency, acusticians use a weighed dB, called dB(A), which takes into account a lower risk to hearing below 500 Hz and above 6 kHz. They also integrate the energy measured during a period of time to take the fluctuation of usual noise levels into account. So that currently, the levels of noise are often given in LAeq (equivalent to the level of continuous noise given in dBA). For moderate levels of noise, another weighted filter is used in sound level meters: the C curve, because low frequencies, although they are less dangerous for the ear, are more disturbing. In every day life, we sometimes have noise levels reaching 100 dB, and even 120 dB (fire alarms). Amplified music can reach 110 dBA, but a French regulation limits the output of PCPs (Walkmans) to 100 dB and the levels in concerts and discotheques to 105 dBA. At the workplace, the maximum level of noise allowed by French Law is 90 dBA for an 8 hour exposure, and 140 dB for peaks. In order to improve the protection of all workers in the EC, a recent European Directive will decrease the maximum level to 87 dBA before March 2006.