Measuring noise

At NATS we are very aware of the impact that aircraft noise has on those who live under flight paths. That’s why we work with airports, airlines and communities to help shape and inform options to better manage the effect of noise and minimise the impacts wherever possible.

Noise is defined as unwanted sound that may result in disturbance and annoyance. Aircraft noise is caused by airflow around the aircraft fuselage and wings as well as noise from the engines, with different aircraft producing different noise levels and different noise frequencies and tones.

Aircraft are individually less noisy than in previous generations with a reduction of noise by more than 90% since jet aircraft entered service in the 1960s. However as traffic continues to grow as demand for air travel increases, this improvement is often counteracted by the number of aircraft overflying an area.

The way that people experience noise from all types of sources can significantly differ. But noise is not always just about decibels; the pitch, vibration, variation in intensity and the length of exposure time can have impacts too.

The level of annoyance also varies owing to factors such as the length of time a person lives in an area affected by aircraft noise, personal sensitivity, the impact of outside influences and the activity the individual is engaged in at the time e.g. sleeping, working, watching TV.

The noise level of aircraft can vary immensely depending on a number of factors;

• How high aircraft are above the ground.
• Whether aircraft are directly overhead or how far they are laterally displaced from the receiver (in any direction).
• Whether aircraft are arriving or departing which can affect the amount of engine thrust they are using (and therefore the noise level) and the amount of air resistance around the fuselage, wings and undercarriage.
• The weather which can increase or decrease the experience of noise depending on conditions. Weather can also affect where aircraft are in the sky since aircraft take-off and land into the wind, affecting which runways are used.

The human ear can handle an enormous range of sound levels. To measure this the decibel scale (dB) is used, which encapsulates the energy of sound with reference to the threshold of hearing using a logarithmic scale. This relates sound intensity to the smallest audible sound of 0dB, so a sound 10 times more powerful is 10dB, whilst a sound 100 times more powerful than the threshold of hearing is 20dB.

Noise measurement also needs to take account of the varying response of the human ear to different frequencies of sound with most sensitivity occurring at the 2-4 kHz range. Therefore the decibel unit used to express human response to loudness or annoyance includes a weighting that varies with both intensity and frequency. The most common measure of this is the A-weighted sound level known as dBA.
Knowing the scale of noise is only one element of capturing its impact, it is also important to consider how we measure the impact of an individual event. There are a number of decibel metrics by which aircraft noise is often described:

• Lmax which is a measure of the loudest part of a flight.
• Leq16h which describes the cumulative noise exposure from aircraft noise events over a 16 hour period. This measure is used to create noise contours connecting areas with the same noise exposure from 0700 to 2300 (the UK official description of daytime – a sixteen hour period). Research globally has found that annoyance due to aircraft noise is correlated with this cumulative metric.
• SEL is the sound exposure level of an aircraft event, measured in dBA of a one second burst of steady noise that contains the same total A-weighted sound energy as the whole event. SEL is often used to characterise the likelihood of sleep disturbance relating to aircraft noise as research has found that single event metrics are a better predictor of sleep disturbance than long term average noise metrics such as Leq16h
• DNL is a variant of Leq which includes a 10dB weighting for noise events at night and a 5dB weighting for events during evening periods, reflecting the potential for increased sensitivity to noisy events during those time periods.
• View Lmax data on a range of aircraft

The reality of aircraft flying overhead can be difficult to describe. Typically aircraft noise is given in the context of certain types of aircraft flying at certain levels. However, this can be difficult to understand in practice. To help with this the table below provides links to video clips of aircraft of different types at different heights.

The aim of these clips is to be illustrative rather than scientific, as the noise experienced from an overflight will depend on a range of factors including how directly overhead the flight is, the weather, background noise and local environment. The absolute level of noise from these clips will depend on your volume setting; most clips do however have some background noise such as birdsong, wind in trees and road/rail noise which should allow you to set the noise from the aircraft in a recognisable context.

Changing flightpaths can mean a change in noise impacts.  The airspace change process requires wide consultation if those changes are below 7,000ft.  All airspace changes undertaken by NATS are summarised here.  The airspace change process is described in the CAA’s publication CAP1616.