3.11 Construction, commissioning and operations phase

The third phase in the management of noise is to implement a comprehensive monitoring and audit program during the construction, commissioning and operations phase, and even the closure and rehabilitation phases. The monitoring program provides the mining company with a means to maintain a continuous record of environmental noise emissions. Technology also allows the mine manager to have real time access to data, from monitoring locations at residences around the mine, on which operational decisions can be made. The audit program also addresses the company’s procedures for dealing with complaints and ensuring quality objectives are met.

3.11.1 Compliance monitoring on site

During the commissioning or early operational phase of the mine, the mine owner will often want to confirm that the equipment supplied meets the sound power level values that were assumed in the environmental assessment process and incorporated into equipment specifications in the detailed design phase.

This form of monitoring is normally undertaken by experienced acoustic consultants in attendance at the mine site, using either a conventional sound level meter or more sophisticated measurement techniques such as noise intensity measures or even acoustic cameras. Generally, sound pressure level measurements are made at a known distance from a source then converted to a sound power level for comparison with a specification.

In addition to an overall sound level measurement (dBA), these sorts of measurements can also be done in octave bands or third octave bands in order to determine the frequency content of noise. Detailed procedures for such measurements are generally found in Australian or overseas standards. To enable comparison with contract specifications these procedures need to be followed accurately.

Intensity measurement techniques can be useful to isolate particular sources or breakouts of noise from buildings. The acoustic camera gives a clear visual indication of hot spots, when used by a skilled operator who understands the technical limitations of the particular device.

3.11.2 Compliance monitoring off site

On-site monitoring for compliance is probably the most controversial area of noise monitoring, not because the measurements are complex, but because there is much room for interpretation regarding the quality and quantity of data required to get the ‘right’ answer. Much of this interpretation relates to the question of whether consent conditions require that noise criteria never be exceeded under any circumstances, or whether they would allow noise criteria to be exceeded over a small proportion of the time (typically less than 10 per cent).

Compliance monitoring has traditionally required attended visits to the site once a quarter to monitor at a number of representative receivers surrounding the mine site, for possibly only one or two hours each. The monitoring is generally conducted at night. Clearly, the noise levels at are heavily dependent on the actual activities conducted and, in particular, the weather conditions on the night. On a particular night, a residence upwind could have noise levels that are virtually inaudible and not measurable, while a residence downwind has high levels of noise that are not experienced very often. There are clear shortcomings in compliance for a whole year being determined by measurements taken over such a short amount of time.

As an alternative to attended compliance monitoring, equipment similar to that used for background noise monitoring can be deployed at the site. However, in rural and semi-rural environments there are often many other sources of ambient noise which mask the noise from mine activities, meaning that it is almost impossible to determine whether noise can be definitely attributed to the mine. This technique can only be used with confidence in cases where the noise impact from the mine site is constant (for example, the noise impact on a residence very close to a ventilation fan).

One advantage of attended over unattended measurements is the ability of the engineer who is collecting the measurements to estimate what proportion of the noise is attributable to the mine site. However, depending on the relative contribution of mine noise and ambient noise, this could be up to 2–3 dBA in error. While this is not a large difference, in some cases it is enough to change the result from assumed compliance to non-compliance.

As a result of significant technological improvements in noise monitoring and communications equipment, approval conditions are increasingly being updated to include requirements for real time monitoring. As these sorts of conditions become prevalent, both unattended monitoring with ‘unintelligent’ devices and attended monitoring are likely to be used much less frequently.

A remote monitoring station including noise and weather instruments and wireless communications. Source: Wilkinson Murray.

The changes in measuring technology evolve as computing power increases. It is now possible to store large amounts of data, which can be downloaded either real time over a network or physically onto an external hard drive or ‘memory stick’. As a minimum, unattended monitoring devices should now be able to:

  • measure overall dBA levels
  • include a low pass filter (so low-frequency noise normally associated with a mine can be separated from higher frequency noise from birds and insects)
  • record audio data (continuously or on trigger as required) in a format dependant on quality required
  • update a central database on a real time basis—typically, every 5 minutes.

These minimum equipment features, together with post processing of data (including listening to recorded fi les to eliminate non-mine noise), combined with some knowledge of the site and its operations, allow for a much better evaluation of compliance.

Enhancements to the technology are also available. One example is directional noise monitoring. This technology is capable of determining the level of noise which comes from the direction of the mine (including options such as low pass filtering), and can compare this value directly to the criteria for the mine, without the need for post processing.

Directonal noise monitor. Source: Wilkinson Murray.

Monitoring equipment can be set up remotely (using solar panels and batteries) and, with either wireless or mobile phone communications, can feed data back to the mine site in real time. The information can not only be used for compliance reporting (on a daily, weekly, monthly or quarterly basis), but can also give real time information to the production manager such that mine operational activities can be altered on an hourly basis (if need be) to ensure that noise limits are achieved despite changing weather conditions.

In the context of maximising production and remaining within noise limits at all times this technology is a viable approach.

3.11.3 Noise profiling

As discussed in Section 3.10.2, often the ambient noise environment includes many noise sources, and noise from mining operations is only one component of overall noise. Therefore, if the ambient noise exceeds a project noise goal, this does not necessarily mean that the noise criteria have been exceeded because of the mine.

By using a site-specific noise model, the noise contribution from the mine can be predicted for the prevailing weather conditions and consequently compared to both measured levels and project noise goals to more accurately understand the likelihood of exceedances from the mine. Once a mine has been using this sort of approach to noise management over several years, with regular validation against measurement results, it becomes a very useful tool for managing noise and understanding likely implications for ‘what if’ scenarios when planning or changing operations.

Some leading practice businesses are already attempting to link their real time noise monitoring data with real time weather data in a noise modelling tool to deliver the most sophisticated information possible to assist mine managers to ensure operations can always comply with noise limits.

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