As an aircraft flies over a monitor, the noise level rises above the background noise level, peaks usually within 10 seconds and then slowly returns to the background level. This is a noise event and it lasts for around 20-40 seconds but varies depending on the height, type and loudness of the aircraft.
A noise monitor captures each individual noise event. An event begins when the noise level exceeds a certain threshold value set in the noise monitor (which will be above the background noise level) and ends when the noise level drops below it. The peak level and the noise level for each second of the event is stored. Noise monitors also measure average noise levels and track trends. This data is then matched with Airservices radar data. Data is usually averaged over a period of time, in order to reduce extreme results that may arise from unusual weather conditions or other noise sources (eg machinery, motorcycles).
Airservices uses a combination of permanent and portable noise monitors. Some permanent monitors have been in place for more than 10 years. Airservices regularly reviews the noise monitoring networks around major airports to ensure that they meet current needs. These reviews can cover a range of factors, such as:
- location in relation to flight paths and population
- monitor reliability
- security and access
- licensing issues
Airservices consults with community forums on the terms of reference for these reviews and publishes reports on them.