What’s that noise?

There has been some recent debate in the Netherlands about the aircraft noise exposure calculations used to set government regulatory policy. Community groups and experts alike have expressed concerns about the trustworthiness of aircraft noise calculations in general. As noise modelling is a big part of what we do, we’d like to take a moment to explain what noise exposure levels are – and what they aren’t.

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Noise and nuisance

Growth in air traffic combined with large populations around busy airports naturally creates friction. Aircraft make noise. Noise regulation policies, such as noise abatement procedures and land-use planning, are intended to help reduce this friction by balancing economic benefits and the adverse effects of aircraft noise.

Imagining the noise level of a single aircraft as it flies overhead – quantifiable as audible sound expressed in decibels – is easy. We can also reasonably estimate what certain decibel levels will sound like, especially if we can compare them to other familiar noise sources such as a passing car or train, or even the different ambient noise levels between a city, a residential neighbourhood and the countryside.

Setting noise level policies is, however, not quite so straight-forward as that. The noise levels used to regulate noise exposure are more complicated than just the individual aircraft noise levels. Every flight is different, with different aircraft types, flight altitudes, etc. Noise modelling must take all these multiple events, with all their multiple variables, into account. To the communities, the mathematical complexities of these calculations can make it seem less than transparent and therefore more easily manipulated.

The reason for complexity

Different types of aircraft and aircraft movements have different noise footprints and different weather conditions have different effects on how noise spreads. A helicopter does not sound like a jet. What we hear and how we experience noise on the sunny day today will be different from tomorrow when it’s windy and raining. Noise is also experienced differently when it happens often or seldom.

These are only a few of the factors; there are many more that affect how aircraft noise is experienced and at what level it is considered a nuisance. That includes ambient noise levels and even non-acoustic factors such as time of day, the general prosperity level of an area or a community’s economic ties to aviation or the airport.

Noise exposure calculations take a variety of such factors into account. The fact that they contain so many variables does make them difficult to understand in terms of what our personal experience will be. However, factoring them in remains important because most aircraft noise regulations are intended to limit total aircraft noise to a level that is tolerable to the community and considered acceptable when balanced against the benefits.

How aircraft noise exposure is calculated

The numerous variables that affect noise exposure and noise nuisance are weighted and combined into an annual average exposure level. These average exposure levels are still expressed in terms of decibels, but they do not represent the same thing as the sound we imagine when we think of a single aircraft flying once overhead. To put it into very simple terms: they represent the average of all flights at all times of day in all weather conditions during a period of one year.
For simplicity, let’s consider the most obvious and directly-related elements:

·        Noise exposure level of an aircraft passing

·        Number of aircraft passages

·        Time of day of an aircraft passage

For example, 100 flights per day might be experienced as a greater nuisance than 50 flights. Aircraft noise in the evening and at night is more likely to be annoying than during the day. Quieter aircraft create less noise exposure than loud ones. Annual average aircraft noise exposure is the result of calculations that incorporate these factors. Often, they are also weighted for day, evening and night nuisance over the course of a year (the metric known as Lden). A single night flight, for instance, is equal to 10 daytime flights in Lden calculations.

In addition, using averages makes it possible to include factors that may be extremely complex. The art of creating a very good model is the ability to simplify complex variables enough to make them useful. The use of averages does, however, also mean that most actual instances of an event will be either higher or lower than the average.

Is there another approach?

European regulations prescribe annual average noise exposure (like Lden) as the standard calculation measure. It helps to ensure policies are consistent and it is more representative of noise over a longer period of time. The weighted averages of the Lden calculations represent noise exposure in the same way that we experience it, so one would imagine that Lden is trustworthy as a measure of noise nuisance. Numerous international studies confirm this.

Yet, there is increasing debate and lack of trust amongst the general public in the calculated Lden values. They are often misunderstood, especially if they are (incorrectly) compared to individual aircraft noise levels. Trusting complex calculated averages of myriad variables is naturally more uncomfortable than the single noise event that is easy to imagine.

Given that trust is the ultimate currency, could there be another way to present aircraft noise nuisance in a way that is easier to understand and might help rebuild trust? That is the topic of our next blog.


About To70. To70 is one of the world’s leading aviation consultancies, founded in the Netherlands with offices in Europe, Australia, Asia, and Latin America. To70 believes that society’s growing demand for transport and mobility can be met in a safe, efficient, environmentally friendly and economically viable manner. To achieve this, policy and business decisions have to be based on objective information. With our diverse team of specialists and generalists to70 provides pragmatic solutions and expert advice, based on high-quality data-driven analyses. For more information, please refer to www.to70.com.

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