Permitted Levels Reduced
OSHA's 29 CFR 1910.95 standard requires the employer to supply, but not mandate, hearing protection at 90 dBA when averaged over an 8 hour Time Weighted Average (TWA). An exposure of 85 dBA when averaged over an eight hour work period, the employer is mandated to implement a hearing conservation program, provide audiograms to employees, provide hearing protection, and training for the use of hearing protection. Other requirements are also required by the employer and are listed in the Occupational Safety and Health (OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard During the 1980s and 1990s many workers determined that the '90dBA for 8 hours' limit was far too high and an unacceptable number of workers would be damaged at these levels, so a level of 85 dB(A) for 8 hours was felt to be a better criterion. Even later the EU reduced the limits still further to the 80 dB(A) we have today, as given in the UK's "The Control of Noise at Work Regulations 2005.". These regulations follow closely the EU Directive 2003/10/EC, normally called the Physical Agents Directive.
A further complication for the sound level meter designer was that it was realised that a single very high noise peak could instantaneously damage hearing, so a limit was originally set by the then European Community so that no worker should ever be exposed to an rms acoustic pressure of more than 200 Pa - equating to 140 dB re 20 μPa - and that this should be measured using an instrument with no frequency weighting. This while a good idea, was patent nonsense as 200 Pa could be generated by trains going through tunnels, closing a door, in fact many everyday things could cause such a pressure wave below the frequencies that could be heard or could cause hearing damage. Accordingly C-frequency-weighting was specified to measure the peak level as this has a flat frequency response between 31 Hz and 8 kHz. However, this missed a significant amount of important energy and a new frequency weighting of 'Z' (zero) weighting was specified in IEC 61672 : 2003 that has a flat response from at least 20 Hz to10 kHz.
Read more about this topic: Noise Dosimeter
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