ACOUSTIC SIGNATURE OF OPEN CUT COAL MINES

The NSW Department of Planning and Environment (DPE) has a regulatory role in managing the noise from coal mines. Currently there are 56 coal mines in NSW which generally have Leq(15 minute) noise criteria in the order of 35 to 40 dB(A) at the nearest private residences.
As a consequence of its responsibilities, DPE undertakes a range of high quality audits and noise monitoring studies. This paper presents a brief selection of ancillary data that has been extracted from those studies. The object of this exercise was: to document the noise signature of coal mines; to better understand how to measure mine noise in a noise environment dominated by insect, frog and bat noise; and to identify if there are any shortcomings or areas of potential improvement in the way low frequency noise is identified.
The exercise found that under normal meteorological conditions, differential attenuation over large distances effectively reduces the spectral energy > 630 Hz to very low levels, (if not below the threshold of hearing) with both dB(A) and dB(C) of mine noise usually being controlled by sub 250 Hz energy. The use of low pass filtering as an effective tool to eliminate extraneous high frequency noise such as that from frogs and insects was validated by the exercise. The exercise also found that the characteristic peaks in the 16 and 25 Hz range are unlikely to be audible unless the overall level of mine noise is in excess of at least 45 dB(A). Consequently, it is recommended that these frequencies should not be included in an assessment of low frequency annoyance (with reference to a C – weighted level).