Recent developments in firearms noise and hearing conservation: hearing protection fit testing,noise measurement and hearing surveillance

摘要

Firstly, a bench-mark exposure standard is needed to indicate excess exposure.? The impulse noise exposure standards are different from the continuous standards both in terms of how they have been set and what needs to be measured. ?The continuous noise standards are a Sound Pressure Level (SPL) of 85 dBA over a period of 8 hours, with an equal energy relationship so that 88 dBA for 4 hours (double the SPL) is also 100% of the dose. 1?This standard was set by looking at the permanent hearing loss (Permanent Threshold Shift, PTS) found in workers exposed to these levels of noise over long periods. ?It does not prevent, but limits the prevalence of, noise- induced hearing loss.? The duration of the impulse noise from firearms is in the order of a few milliseconds and there is a limit to which the equal energy hypothesis can be extrapolated for ?such very short time durations. A ‘peak’ exposure standard of 140 dB has been adopted by many countries including New Zealand,2 primarily because, in excess of this level, the behaviour of the sound field is different and requires special measurement techniques. ?Impulse energy is also measured by level, the peak level, ?and an ‘energy like’ measure of duration, of which there are a number.? The “B” duration, where the impulse has fallen 20dB from the peak, is an example : it is identifying this parameter which requires? special equipment.? Because impulse noise is more unpredictable in terms of long term exposure, the effects were looked at in terms of the temporary hearing loss (Temporary Threshold Shift, TTS) that occurs directly after noise exposure, the TTS or risk of hearing loss being different for each impulse noise ?parameter. A number of impulse noise exposure criteria have been developed.? Amongst the first in the 1960’s, and illustrative of the concept, is that of Coles et al3 with a limit of 159 dB Lpeak for 5 ms, the exposure being limited to 100 rounds fired at rates of between 6 and 30 per minute.? This should limit the TTS to be not more than 10 dB at or below 1000 Hz, 15 dB at 2000 Hz, and 20 dB at or above 3000 Hz in 75% of the normal hearing ?persons exposed. The continuous and impulse standards are not entirely congruous in terms of both “equivalent energy” and the likelihood of harm, but more recently the Sound Exposure Level, the SPL normalised to 1 second, has been proposed as a unifying metric.4 The limit is a critical level of 116 dBA SEL per impulse, with 50 exposures allowed. This should not result in a TTS2 (the TTS measured 2 minutes after exposure) of greater than 25 dB HL at 4 and 6 kHz in 95% of those exposed. This limit has the advantage that it can be measured with a standard Sound Level Meter.