Noise monitors

Heathrow has 39 noise monitors currently located at sites around the airport and has added over 30 new monitors within the last ten years. 

Twelve monitors are specifically located in fixed locations to comply with the DfT’s requirement to monitor noise limits for departing aircraft. These have been sited on the advice of the CAA. The specific requirements are set out in the United Kingdom Aeronautical Information Publication (UK AIP) and are shown on maps in the CAA publication CAP1149 (Noise monitor positions at Heathrow, Gatwick and Stansted Airports). CAP1149 also sets out the positions of the remaining mobile monitors which were correct at the time of publication. The locations for community requests (up to 4 currently) for mobile monitors are selected with input from NACF members and reviewed annually, seeking to identify suitable areas with characteristics that add to the understanding of aviation noise and to enhance noise modelling. The remaining mobile monitors are in more permanent locations that were agreed through the NACF and reflect a range of aircraft noise exposure areas (e.g. arrivals, departures, arrivals and departures, close in/far out etc).

The mobile monitors are placed according to a range of criteria that will vary depending on each monitor’s intended purpose. For example, Heathrow is required to produce annual noise exposure contours and monitors are deployed in specific locations to produce these contours. In addition, mobile monitors may be deployed at more distant locations to measure the noise of specific operations, for example to study departure noise along a specified route or to measure the impact of operational trials. We also position monitors to assist noise track keeping, so these tend to be located under the main arrival and departure routes for the airport. We also respond to requests from residents for monitors to be placed in their communities, to gain a better understanding of local noise impacts. Site selection criteria considers issues such as access, available power sources, feasibility (e.g. presence of livestock), security, landowner permission, other noise sources in the area, relevance (e.g. if the area is not regularly overflown by aircraft). In addition, we want to ensure that community monitors add value to long-term noise assessment. Our aim is that all study sites add to stakeholders’ understanding of aircraft noise and are free from political bias or influence.

Our noise monitors are supplied by Brüel & Kjaer. The noise detection technology for each monitor is identical, but power supply to the equipment varies between mains supply, solar panels and hydrogen fuel cells (which provide backup when solar levels are low). Each monitor has a 4G router which transmits noise data back to our central server. All monitors are the same model.

Noise monitor calibration is carried out regularly following external audit requirements set by Bureau Veritas and in accordance with the manufacturer’s specifications. Heathrow employs an acoustics consultancy which undertakes this work, and it follows a strict regime. Daily ‘ping’ tests are carried out to communicate with the monitors to check they are streaming live data and if not, a service ticket is raised, and an engineer attends the monitor site to investigate and resolve any issues. 

The monitors are Class 1* and are regularly calibrated for accuracy. For the departure limit monitoring a maximum 0.7db is permitted to allow for potential monitor tolerance.

* A Class 1 Sound Level Meter is a noise measurement instrument that meets the requirements of IEC 61672-1:2002 (or an equivalent such as BS EN 61672-1:2003) to Class 1 performance.

The monitor thresholds are set differently based on the acoustic environment in which they are set. The noise event will need to exceed this threshold level to be stored as a single noise event. This enables distinct events to be captured and then correlated with aircraft overflight where appropriate.

Heathrow does not keep data on noise monitor failure rates, as the rate is very low. With automatic monitoring and daily ping tests, any failed monitors are detected and replaced using spare units within days. For the departure noise monitors the array has been set up to ensure that at least two monitors should capture the noise event.

All Heathrow monitors have a dB threshold which triggers noise events. The thresholds are set in the 60 to 65dB range. Noise levels are continuously recorded by the monitors throughout the 24-hour period and in real-time, although there is a small delay in this data being available in ANOMS/WebTrak while it is processed (correlated to tracks). The monitors will only create a record if the noise event can be correlated with an aircraft track. This is carried out by comparing the time of the event with an aircraft flight plan.

The following data is provided:

Event based metrics:

• LASmax (Maximum noise level)
• SEL (Sound Exposure Level)
• EPNL (Effective Perceived Noise Level)
• LAeq (Equivalent Continuous Sound Level)
• Duration (in seconds)
• One-second noise values for each event (the event time history)
• One-third octave band noise levels for each event

Hourly Noise Climate values:

• Total LAeq (Equivalent Continuous Sound Level for each hour, all noise sources)
• Aircraft LAeq (Equivalent Continuous Sound Level for each hour, aircraft noise only)
• LA90 (Background noise level during each hour)

Sound levels are measured by the device’s microphone and converted into a sound file (WAV files, mp3 files) and (x type) decibel readings. The decibel readings are recorded digitally as (y type) file formats and all data is sent over a 4G mobile telecoms connections to our central server. 

The data collected from Heathrow’s noise monitors is stored on a central server and we currently have data for each monitor relevant to its location. For some monitors this can be over a decade depending on when the monitor was deployed.

In addition to the calibration checks on the monitor, the noise data is validated by automated processes which flag unusual or unmatched events. Individual events can be assessed by conducting playback of the monitor’s audio recordings. This is done for all potential departure noise infringements. Any noise event which exceeds the departure noise limits is thoroughly investigated through the use of the playback function to ensure that we are not imposing fines on an airline for events which are not aircraft related. 

Heathrow provides tools on its website for members of the public to locate and query data from noise monitors. The primary application is WebTrak and can be accessed by clicking here. Heathrow’s internal noise track keeping (NTK) system is called ANOMS and uses the same data to report departure noise limits at the fixed monitor sites set out in the AIP. The CAA is able to freely access ANOMS to check compliance. Remaining mobile monitors are used for a number of purposes to aid the understanding of noise impacts, such as new operational procedures that might improve operational efficiency and/or mitigate noise. The findings of these investigations are used in reports and presentations that are published in the public domain for stakeholder engagement. Examples include Heathrow’s recent Steeper Departures trial and the Slightly Steeper Approaches trial.

The annual noise contours produced on behalf of Heathrow by the CAA utilise the noise data to validate and update their modelling assumptions. The CAA have access to all our data without need for a manual transfer. Any adjustments to their modelling assumptions are typically noted in the reports. The data can also be exported to other third-party suppliers for use in studies. 

Yes, we have carried out several noise studies tracking historic trends. We are also exploring how we can develop our NTK systems to undertake more automated assessments.