Discerning noise levels from one mine with those from mines nearby
The Mount Pleasant Coal Mine in New South Wales, Australia, is a large-scale surface operating mine operating 24 hours a day, seven days a week, with an estimated 474 million tonnes of reserves to be mined. We were asked to carry out a noise and vibration assessment of a proposed major expansion at the site.
Noise levels from open cut coal mine operations are transmitted over distances exceeding one kilometer. The noise level at typical receivers varies significantly according to weather conditions. Air temperature above the ground is normally cooler than at the ground, and the denser air above tends to bend sound waves upward, whereas with “temperature inversions,” cold air trapped just above the surface bends the sound waves downward toward the earth increasing noise levels. Temperature inversions are common in the Mount Pleasant area at night and noise analysis is needed to take into account the increased propagation of noise under these weather conditions.
In New South Wales, mine noise is assessed according to the Noise Policy for Industry (NPfI) and this document is prescriptive in how to determine the relevant background noise level. However, despite the fact that the noise level emitted from a mine varies substantially from hour to hour, the document does not define how to assess such varying noise, other than to apply the LAeq measure.
We developed a procedure whereby the noise level (LAeq) exceeded for 10 percent of the time is predicted and assessed. In the vicinity of Mount Pleasant, information regarding temperature gradients is available, particularly from a nearby tower which measures temperatures at ground level and at elevated locations. This information has been used in the model to predict noise levels under a number of temperature gradient conditions. From this information, the 10 percent level was determined in order to assess the mine noise impact. Around Mount Pleasant, another challenge is there are several other open cut coal mines. Consequently, monitoring the mine noise using a standard omni-directional monitoring system can be influenced by noise from other mines. To overcome this difficulty, we have developed an innovative directional noise monitor, called the BarnOwl system, which measures noise in 5-degree segments over the full 360 degrees.
We predicted noise levels from the mine operation, along with air blast and vibration levels from proposed blasting. The report assessed the impact and recommended some controls.
On an ongoing basis the Mount Pleasant Mine uses BarnOwl units for monitoring their noise levels and dynamically adjusts operations as needed to meet regulatory requirements. This system is used to measure noise from each defined direction and segment, allowing for the monitoring of noise from the mine without contamination by noise from other mines nearby.