Luton and District Association for the Control of Aircraft Noise

Alternative noise indicators

Leq, the equivalent average noise exposure over a significant time period, is not an intuitively obvious indicator of disturbance from noise which actually occurs as short bursts of intense noise separated by long periods of quiet. It is more suited to noise from motorways where, for long periods of the day, the noise level can be heard as a wavering level of noise.

Its proponents argue that, because of the logarithmic dB scale, it weights the noisiest events heavily so that it puts pressure on operators to eliminate the noisiest aircraft from the fleet. This is true: campaigners against Heathrow calculated that one flight of Concorde generated noise equivalent to 35 flights by Boeing 747-400s. But the effect of this has been that airports claim, on the basis of average noise indicators, that their operations are getting quieter at the same time as the number of flights is increasing hugely. A simple example shows this to be nonsense: if all aircraft were to be replaced by types which were 3 dB(A) quieter, most people on the ground would be hard pressed to notice the change. Yet Leq noise indicators would not return to the same levels until the number of flights was doubled, a change which would be very difficult for those living below to ignore.

Most people perceive noise as increasing with increasing frequency of flights whether the aircraft are quieter or not so average noise does not represent what they experience. In addition, average noise is not easy to measure. This has led to interest in alternative ways of characterising noise around airports.

One such indicator is N70, a simple count of the number of overflights at a location for which the peak noise on the ground exceeds 70 dB(A). It is usually calculated for the “average day” in a time period. N70 has been used extensively in Australia to supplement average indicators. N60 is sometimes used for the night period.

Before Leq was adopted in the UK, the standard indicator was the Noise and Number Index (NNI) which attempted to reflect both the frequency of noise events and their loudness. It may, therefore, be better suited to indicate disturbance from aircraft noise than either Leq or N70. A recently completed study for the UK Government known as ANASE has provided some support for this view. It concluded that either flight numbers are under-represented by Leq indicators or that, if Leq is a valid indicator of annoyance, people are now more sensitive to aircraft noise than they were in the early 1980s when a similar study was completed.

The first of these conclusions seems to us to be much more realistic as, in the 1980s, there were far fewer flights by aircraft which were perceptibly noisier than today’s. The data set which led to the adoption of the Leq indicator of disturbance was, therefore, unrepresentative of the environment around airports today.

Individual events

In addition to the indicators which attempt to aggregate the impacts of multiple flights, there are others which address the effects of individual flights. These are most often quoted in relation to sleep disturbance where one noise event is enough.

LAmax is the most easily understood of these as it simply indicates the peak A-weighted noise generated on the ground as the noise rises and falls with the passing of the aircraft. A more complex indicator, and one which is not intuitively useful but which is favoured by the industry because it can be related back to Leq, is the Sound Equivalent Level (SEL), sometimes known as the Single Event Level. This is the equivalent noise level in dB(A) which would have been reached if all of the sound energy generated by the event had been concentrated in a single pulse of one second’s duration.

When used as a predictors, these indicators are usually calculated for the “typical” aircraft expected to operate at night as it is difficult to predict the absolute noisiest ever likely to visit. As an example, the SERAS study which preceded the December 2003 White Paper, the Future of Air Transport,  produced noise “footprints” based on an SEL of 90 dBA (“SEL90”) for the noisiest aircraft in the Quota Count 2 (QC/2) category.

The Quota Count (QC) is a noise scale operating at the three airports at which night noise is regulated by the Secretary of State for Transport. Each aircraft’s Quota Count is based on its noise classification and, for departures, destination and the airports, Heathrow, Gatwick and Stansted, each have a nightly noise quota as well as a cap on total movements at night (defined as 23:30 to 06:00).