Noise – an overview for students

Planning for the new Martha Mine had to take into account that the mine would be operating very near to people’s houses and properties as well as a primary school. The planners had to look at ways that noise from the mine and its activities could affect the neighbouring areas.

Noise levels were monitored around Waihi for seven years before mining started so that information could be collated about pre-mining noise levels. This information was used to set the standards for noise levels once mining began.

Noise monitoring continues throughout the operation. Technicians take regular weekly and monthly readings and other readings are taken when they are needed. The location and number of sites are reviewed from time to time to make sure the monitoring clearly reflects the activities happening on site.

How is sound monitored?

Noise monitoring is carried out with a sound level meter connected to a chart recorder that graphs the sounds as they happen.

While noise monitoring the technician has to note down any significant sounds such as traffic, birds, cicadas or barking dogs.

The microphones are very sensitive and a lot of care must be taken to see that the sound samples are recorded in the right way.

A fifteen minute sample is recorded at three separate times over one day for each monitoring site. The technician must also use an anemometer to check the wind speed and wind direction for each sample. This is because wind speed and direction can affect noise levels.

After samples are recorded the information is loaded into a computer and later downloaded and the results sent to the Hauraki District Council.

How is sound measured? … and what are the permitted levels?

The basic unit of sound is the decibel (dB).

The A weighted sound level (dBA) is a good measure of sound that is detectable by the human ear.

L10 denotes the sound level which is equalled or exceeded for 10% of the measurement time. This level may be considered as the average maximum sound level.

L95 denotes the sound level which is equalled or exceeded for 95% of the measurement time. This level may be considered as the average minimum sound level.

… a sound reading of L10 75dBA means that the average human ear is hearing up to 75 decibels for at least one tenth of the measurement time.

Noise monitoring is carried out at houses close to mining activities. The permitted noise levels vary, depending on the activity and the time of day. At night time the limits for noise are lower (L10 40dBA) than for the day time. The day time noise limit is either L10 55dBA or L10 50dBA, depending on when and where the activity is taking place.

How is noise managed?

The mining company and mining contractor take all practical steps to keep down noise levels by:

  • choosing and maintaining good equipment so the engines are running quietly and efficiently
  • building noise bunds (earth barriers) and planting them with trees
  • building close-boarded fences and using acoustic materials where needed around noisy machinery or on perimeter fencing
  • keeping stockpiles low so that any machinery working on the stockpiles stays below the level of the noise bunds
  • reviewing excavation methods and using different methods when mining machinery is near the crest of the open pit.