There are some noises in life that are really annoying and others that we just learn to live with; aircraft and airport noise are often subject to a lot of debate as different people would put them into different categories. But with it being such a hot topic, how do the airports keep track of their noise emissions?
First of all, we must understand what causes aircraft noise.  There are 2 sources of noise from aircraft: air friction (as it passes over the wings or fuselage) and engine noise. These both occur in the direction of the planes travel (i.e. most sound travels in the direction of approach/descent) rather than travelling sideways from the runway.
There are 2 main methods that are used to keep track of airport noise, both with their advantages/disadvantages. The first method is to measure the Sound Level created by one aeroplane taking off, then to add levels accordingly depending on the number of flights that will occur. An average Sound level for the whole period of time can then be calculated, depending on the number of flights. This is often used in prediction of airport noise, as it is quick and easy. However it is not always accurate because each plane makes a slightly different level.
The second method is to measure what is known as an LAeq. This is where a fixed microphone is set up so the sound level can be measured over a prolonged period of time for many departures/arrivals. Because the sound level varies with time (i.e. increasing when planes take off) it is then averaged. From this, the constant equivalent level (hence the eq part of LAeq) that would have to be made to create the same overall noise level can be calculated. At Norwich International Airport (NWI), my local airport, 3 fixed microphones are placed on the airport perimeter, so that the noise levels are constantly monitored and do not exceed regulations.
If an LAeq measured over a time period of 1hour is taken and reads 75dB,* this does not mean that it is constantly below 75dB. This could however mean that it is 90dB for 2 minutes and 45dB for the rest of the hour, which would give an average equivalent noise level of 75dB for the hour. The graph below shows this example:
One problem with airport noise is that some planes take off at night, which is a time where the noise is generally considered more of a nuisance. For this reason another measurement can be made using the LAeq, which takes this into account. The night day and evening measurements are done separately and then the evening one has 5dB added to it and the night one has 10dB added to it, because they are perceived as louder at night. These new values are then averaged again over the whole time period of a 24-hour day.
With these measurements, noise maps can be made, where ‘contour’ lines mark different dB levels. In 2005 NWI commissioned an external body to produce a noise map for its surrounding areas.  The values used for these maps depend on a day-to-day basis, but are an average over a period of time. Factors such as wind direction/speed and flight path can affect these, but measures are taken by airports to take this into account (e.g. flight paths over lesser populated areas).
The map below shows the LAeq noise contours of the airport over a 16-hour period :
This next map is similar to the one above, but this time is based on measurements over a 24-hour period with the evening and night penalties added, so it is more representative of the general perception of the level :
You will see, as stated before, that the noise mainly travels in the direction of flight and the levels are greater towards the ends of the runways; this is because of the noise created by the engines and the friction on the wings in takeoff/landing. In order to get an idea of comparison, 65-70dB is roughly the level of a busy restaurant or a busy A-road, which is 20m away. The background level for a housing area would generally be expected to be up to around 50dB without noise from the airport.
A general rule of thumb for sound is that it will decrease at -6dB for every distance doubling (e.g. if you are at 5m and it is 85dB then at 10m it will be 79dB). This rule is true if there are no reflections, so in reality it is often not quite this much. This rule can be seen on the maps where the distance between the contours increases the further away from the runway.
For many, the airport can be seen as a nuisance and should be made to reduce noise; in ways this has happened as planes have become quieter, but despite this there has been an increase in the number of planes. For local authorities, noise complaints need to be assessed as whether the source (i.e. the airport) has many pros (e.g. local jobs). Also, the type of nuisance needs to be addressed as to whether it is interfering with the individual’s enjoyment of their land or whether it has affected many.
What are your views on your local airport; do you feel like the disadvantages of the noise outweigh the benefits of the airport?
*dB stands for decibels which is the units we use to measure sound.
Thank you to Norwich International Airport for kindly supplying me with the noise survey data in order to write this blog.
 What causes aircraft noise? – http://www.gatwickairport.com/business-community/aircraft-noise/air-traffic-noise-explained/causes-of-aircraft-noise/
 Norwich Airport Noise Policy – http://www.norwichairport.co.uk/content.asp?pid=52
 Department for Environment, Food and Rural Affairs Airport Noise Action Planning for NWI (Noise Contour Mapping) – http://www.norwichairport.co.uk/downloads/airport_datapack_2013_norwich_airport_egsh.pdf