Noise-related hearing loss is the most common occupational disease in Europe and North America, accounting for almost one-third of all work-related illnesses. These disorders can lead to long-lasting effects causing stress, fatigue or isolation that significantly increases the risk of work-related accidents caused by other factors. Hearing loss is irreversible and often detected late, so PPE covering this risk is classified as Category III.
The ear and sound
Noise, as a vibratory phenomenon is characterized by:
- Its intensity (in decibels [dB]) corresponds to the amplitude of the vibrations emitted by the sound source. 0 dB corresponds to the minimum sound level audible by the human ear. The pain threshold is 120 dB and the ear can be damaged from 85 dB.
- Its frequency (in Hertz [Hz]) which will define the perceived height. The higher the frequency and the higher pitched the sound, the lower it is and the lower pitched it will be. The human ear is able to perceive sounds at frequencies between 20 Hz (very low pitch) and 20,000 Hz (very high pitch).
- Its duration and its variation, which makes it possible to differentiate very brief sounds, of a duration in the order of a second, such as impulse noises (a shot, impacts) of sounds that have longer durations (hours, or a day) for which it is important to consider the resulting dose of noise received.
The ear can be broken down into three distinct parts:
- The outer ear consisting of the horn and the ear canal
- The middle ear between the eardrum and the inner ear. It is filled with air and allows, using the ossicles, to transform the aerial vibrations into structure borne vibrations that can be analyzed by the inner ear.
- The inner ear, the heart of the auditory system, consisting of a cavity filled with liquid containing the cochlea where the body of Corti is found. Within it, the vibrations of the liquid transmitted to the ossicles are picked up by the hair cells which select them by frequency. The information is then conducted by the auditory nerve to the cerebral cortex which can interpret it.
The human ear has a particular sensitivity to each frequency range. At moderate sound levels, it is less sensitive to low pitched sounds. To represent this particular sensitivity,
the noise measurements and standards use a weighting of the measured sound levels called weighting A. The thus weighted decibels are denoted dB (A).
How to protect yourself?
To choose the correct product for hearing protection.
- Identify the nature of the noise: stable, fluctuating, intermittent, pulse.
- Measure the noise at the working station: intensity (dB) and volume (Hz).
- Determine the exposure time.
- Calculate the reduction necessary to return to an acceptable ambient level (cf Directive 2003/10/CE).
The performance of the hearing protector (its attenuation level) must be adapted to the risk assessment of the workplace. It should bring the noise level to a level that is not harmful to health, while avoiding overprotection would cut the operator from his environment (warnings, communication...).
Regulatory daily dose authorized according to the sound level
Attenuation values for noise
How do you use the attenuation values?
3 indicators, from the simplest to the most precise are made available to the user:
- SNR (Single Number Rating): Single average value of attenuation.
- HML : Attenuation values expressed in terms of average levels of frequency:
- H : Attenuation of PPE at high frequencies (pitched noises)
- M : Attenuation of PPE at medium frequencies
- L : Attenuation of PPE at low frequencies (bass sounds)
- APV (Assumed Protection Value) : Attenuation values expressed on 8 specific frequency levels (see the data sheet of the hearing protector).
When hearing protection equipment can be worn in several ways (on the head and under the chin for example), it must be tested for each method of wearing.
When wearing a single hearing protector is not enough, it is possible to combine them. The attenuation resulting from the simultaneous wearing of earplugs with a B SNR and a noise-cancelling headset with an ST SNR is calculated by the following formula: 33 × log ((0.4 × B) + (0.1 × ST))
Level of noise