Underground » Health and Safety
Noise induced hearing loss and its consequences with regard to Occupational Health and Safety remain a major problem in the Coal Industry, especially underground. Meanwhile, National Standards for exposure to noise in the occupational environment are being lowered from an 8-hour equivalent continuous A-weighted sound pressure level of 90 dBA to 85 dBA. The most desirable solution is of course to treat the noise problem at source. Where noise control strategies are not feasible, the use of hearing protection devices remains the most widely used strategy for limiting the exposure to noise in the work place.
In the underground coal mining industry however, it is widely recognised that the use of certain forms of hearing protection is far from satisfactory. Some of the reasons given by miners for their reluctance to wear hearing protection are that hearing protection devices such as ear muffs are uncomfortable, they interfere with speech communication, and they . impair the miners ability to hear the audible signals of potential roof fall, the roof talk.
Active Noise Control as applied to hearing protection is a technique which uses an electronically generated sound field to cancel unwanted noise, based on the principle that superposition of a signal on an identical signal which is 180° out-of-phase results in mutual cancellation of the two signals.
This project aims to demonstrate the applicability of and to establish design specifications for ANC (Active Noise Control) Headsets for use in the coal mining industry, especially underground.
Noise measurements on production equipment, development equipment, and personnel transport vehicles were carried out at several underground coal mines. These measurements were supplemented with data from previous underground noise surveys and analysed to identify equipment with excessively high noise levels and to determine the sound pressure levels and frequency characteristics of the offending noise. The spectral analysis of the recorded data showed considerable variation in the noise levels experienced by miners due to the wide range of equipment used. Overall however, the noise is broadband having most energy in the 400 to 2000 Hz band.
A market survey of existing ANC Headsets was carried out and six units were acquired for evaluation. The evaluation of the headsets involved a series of noise attenuation performance tests carried out on an artificial head at National Acoustics Laboratories (NAL), a series of environmental tests carried out at Vipac's Victorian Technology Centre in Melbourne and a series of subjective evaluation tests carried out at several mines via user interviews.
The NAL tests showed some devices to benefit from a significantly improved noise attenuation performance at low frequencies thanks to the ANC system. Using the measured noise from a continuous miner and the measured noise attenuation performance of one of the ANC headsets under evaluation as an example, it was demonstrated that with Active Noise Control, the overall Leq noise level was reduced from 90 dBA (with passive hearing protection only) to 77 dBA.
The subjective tests, of the headsets involved playing back tape recorded underground coal mining machinery noise to mine personnel through an amplifier speaker system and inviting the participants to listen to the noise with and without the headset operating. Although only a limited number of people took part in the trials, the user impression of the ANC headsets collected via a questionnaire at three different mines revealed a generally positive response of the miners to the idea of using ANC headsets for hearing protection. In particular, the improvement in speech intelligibility with the Active Noise Control system was the highlight of the ANC devices. However, the general operator acceptability for these devices in their current form was very low, especially with regards to power supply and ruggedness.
The Vipac Laboratory environmental tests revealed that, in their current form, the ANC headsets tested are also unsuitable for use in underground coal mines and would need to be re-engineered if they are to be used successfully underground.
The final part of the project has been to establish a set of specifications for the manufacturers to use in developing ANC headsets for use underground.