Measuring aircraft noise: monitoring stations, noise protection zones and noise quotas

If you regularly stand under an approach path, you know the gray masts with the spherical windscreen on top. These are aircraft noise monitoring stations – and they are only one part of a system that determines where building is allowed around an airport, who gets soundproof windows paid for and which aircraft are still allowed to take off late in the evening. This guide explains how aircraft noise is measured, how noise protection areas come about and what noise quotas are – and why all of this is more than just background knowledge for you as a spotter.

Measuring aircraft noise: the short answer

Aircraft noise is recorded at monitoring stations around the airport. A microphone continuously records the sound level, and software detects individual flyovers as events and assigns them to specific flights using radar data. From this come statistics, monthly reports and online portals.

The legally decisive noise protection areas, however, do not come from these measurements but from a calculation: for a forecast year, it is determined what levels the expected flight operations produce. And noise quotas, finally, cap the total amount of noise at some airports – often at night – instead of just counting flight movements.

Decibels in five minutes

Before we get to monitoring stations, it is worth a quick look at the unit. Noise is given in decibels (dB), a logarithmic scale. The “(A)” in dB(A) stands for A-weighting, which gives less weight to low and very high frequencies – roughly the way the human ear perceives them.

Two rules of thumb help you read any noise statistic:

• Plus 3 dB means double the sound energy. Two equally loud aircraft at the same time therefore do not give twice the number but about 3 dB more.
• Plus 10 dB is perceived by the ear as roughly twice as loud.

Two quantities are decisive for airport operations. The maximum level (L_Amax) is the loudest moment of a flyover – what you feel at the fence. The equivalent continuous sound level (L_Aeq) spreads the total sound energy of a period out evenly in calculation. Because energy is averaged, loud individual events carry disproportionate weight. A single very loud takeoff can raise the continuous sound level more than a dozen quiet landings.

How an aircraft noise monitoring station works

In Germany, monitoring is not voluntary image care. Under Section 19a of the Air Traffic Act (Luftverkehrsgesetz), the operator of a commercial airport with scheduled or package-holiday traffic – and of a commercial airfield with such traffic and more than 25,000 movements a year – must build and operate systems that continuously record the noise of arriving and departing aircraft, on the airport grounds and in the surrounding area. The results go to the licensing authority and to the aircraft noise commission (Fluglärmkommission) and must be published regularly. Where there is no need, the authority can grant exceptions.

The setup

A typical station consists of three building blocks. The heart is a weatherproof measuring microphone with a windscreen, often heated so that moisture and frost do not distort the measurement. Often there is also a small weather station, because wind and rain produce noise of their own and influence how sound propagates. A measuring computer stores the data and transmits it regularly to a central office. For measurement methods and assessment of aircraft noise there is a standard of its own, DIN 45643.

An example from the Rhine-Main region: the Environment and Community House (Umwelt- und Nachbarschaftshaus, UNH) in Kelsterbach has operated its own monitoring network there since 2011, independent of the airport operator, with nine fixed and three mobile stations. The stations work with class 1 microphones, a weather station and a measuring computer and are serviced and certified at regular intervals. Municipalities can have mobile stations set up in one place for about three months – that way areas with no fixed station can be studied too.

From noise to flight

A microphone hears everything: lawn mowers, leaf blowers, church bells, motorcycles. The real work is therefore in event detection. The software looks for level curves typical of a flyover – a rise above the background noise, a maximum and a decay over a few seconds. For each event, the maximum level and duration are stored, among other things.

The next step is matching with air traffic control radar data. In the case of the UNH, these are the flight tracks of Deutsche Flugsicherung (DFS, the German air navigation service) from its flight track recording. If a sound event matches in time and space an aircraft that was just nearby, it is assigned to that flight. Staff additionally check conspicuous events by ear. At the end there is a list: which flight, what time, what level at which station.

This very coupling makes the data interesting for spotters. Many airports and noise protection offices show measured values and flight tracks in online portals, usually with some delay. How to use such portals specifically to find spots is covered in detail in the guide Aircraft noise portals and WebTrak.

What measurements cannot do

Measured values are precise for exactly the point where the microphone stands. A few hundred meters away, terrain, buildings or a slightly different flight track can make things look completely different. In addition, the level depends heavily on the weather: a tailwind carries sound farther, a headwind weakens it, and in temperature inversion conditions aircraft often sound significantly louder. For area-wide statements about an entire surrounding region, calculations are therefore needed – and this is where the Aircraft Noise Protection Act comes into play.

Noise protection areas: what the law sets out

The Act for Protection against Aircraft Noise (FluLärmG) stipulates that a noise protection area is to be set for certain airfields. Affected are, among others, all commercial airports with scheduled or package-holiday traffic, commercial airfields with such traffic and more than 25,000 movements a year, and military airfields for jet aircraft. The area is set by a legal ordinance of the respective state government.

Two day zones, one night zone

The noise protection area consists of three zones. Different thresholds apply to existing civil airports and to new or substantially expanded civil airports:

Zone	Metric	Existing civil airport	New or substantially expanded civil airport
Day protection zone 1	daytime continuous sound level	from 65 dB(A)	from 60 dB(A)
Day protection zone 2	daytime continuous sound level	from 60 dB(A)	from 55 dB(A)
Night protection zone	nighttime continuous sound level	from 55 dB(A)	from 50 dB(A)
Night protection zone	maximum level criterion	on average 6 times 57 dB(A)	on average 6 times 53 dB(A)

The night protection zone thus applies in two ways: either through the continuous sound level or through the frequency of loud individual events. For military airfields, the day thresholds are each 3 dB(A) higher.

How the zones are calculated

The calculation follows the annex to the act and a calculation regulation of its own. You should know a few principles:

• Assessment periods: Day runs from 6 a.m. to 10 p.m., night from 10 p.m. to 6 a.m.
• Six busiest months: The calculation does not use the annual average but the six busiest months of a forecast year, that is, 180 days. The summer schedule therefore comes through in full.
• Runway direction with a safety margin: Because the wind blows differently from year to year, the number of movements per runway direction is increased by three times the spread of their usage shares over the last ten years. Both sides of an airport are thus protected somewhat more generously than the pure average would suggest. Why the direction fluctuates so much is explained in the guide Understanding runway direction in use.
• Forecast rather than hindsight: The basis is the foreseeable flight operations, not past traffic.

If operations change such that the continuous sound level at the edge of day protection zone 1 or the night protection zone rises or falls by at least 2 dB(A), the noise protection area must be set anew. Independently of that, no later than ten years after it was set, a check must be made as to whether the exposure has changed significantly.

What applies in the zones

A noise protection area is not a flight ban but a planning and compensation zone on the ground:

• Building bans: In the entire noise protection area, hospitals, nursing homes and similarly sensitive facilities may in principle not be newly built, and in the day protection zones additionally no schools or kindergartens. In day protection zone 1 and in the night protection zone, new housing is also generally not permitted – with exceptions, for example within existing built-up districts.
• Soundproofing: Owners in day protection zone 1 can apply for reimbursement of expenses for structural soundproofing, in the night protection zone for bedrooms including ventilation. When the claim arises depends on the level and the type of airport; the maximum amounts are regulated by a separate ordinance.
• Outdoor living area: At new or substantially expanded airports, there is in day protection zone 1 additional compensation for the impairment of terrace and garden.

For you as a spotter, the maps of noise protection areas are an underrated planning tool. The zones stretch like long tongues into the landscape along the approach and departure directions. Where day protection zone 1 reaches far out, aircraft fly particularly often and low – these are exactly the areas where spots under the approach path can be found. What you should keep in mind there for your own safety is in the article Spotting under the approach path.

Noise quotas: when noise, not flights, is counted

A pure movement cap treats every flight the same – no matter whether a modern short-haul aircraft or an old four-engine jet takes off. Noise quotas start differently: they limit the sum of the noise caused. If you use quieter aircraft, you get more flights into the same budget.

The Quota Count system in London

The best-known model is the Quota Count system (QC), which applies for the night hours at the London airports Heathrow, Gatwick and Stansted, among others. Each aircraft type receives a QC rating based on its certified noise values, separately for takeoff and landing. The classes are built logarithmically: with every roughly 3 dB louder, the QC value doubles, from fractions of a point for very quiet types to 16 points for the loudest.

Each airport gets a fixed points budget for the night per summer and winter season. Every movement uses up points according to its rating. Once the budget is used up, further night movements are possible only in narrow exceptions. Heathrow no longer allows the loudest ratings at night at all.

The effect can be read directly from the flight schedule: it pays for airlines to use the quietest types in the shoulder hours. When you photograph the first long-haul arrivals early in the morning in London, this calculation is also behind it. Spot tips for the airports are in the guides to Heathrow and Gatwick.

Frankfurt and the noise cap

Germany has no nationwide quota system on the British model. The discussion is an old one, though: at Frankfurt Airport, the mediation in 2000 already recommended allocating aircraft noise by quota and setting local caps. The planning approval decision of 2007 for the expansion contained no noise cap. In 2016, the Hessian state government presented a model of its own; since then, the parties have been working on a voluntary agreement with the aviation side. The topic accompanies the region to this day – in the Airport and Region Forum (Forum Flughafen und Region) it is considered an open point of the Alliance for Noise Abatement (Allianz für Lärmschutz).

Noise-related charges

The most widespread form of noise control is less visible: noise-related takeoff and landing charges. Many airports divide aircraft into classes according to their noise values and graduate the fees by class and time of day. A loud takeoff in the middle of the night then costs many times a quiet takeoff at noon. Zurich also works with such a class system. Such charges act like a soft quota: they prohibit nothing, but make loud aircraft expensive at sensitive times.

Instrument	What is limited	Effect on the fleet	What you notice at the spot
Movement cap	number of takeoffs and landings	low, every flight counts the same	less traffic, type mix unchanged
Noise quota (e.g. QC)	sum of noise in the period	strong, quiet types get more slots	shoulder hours dominated by modern jets
Noise-related charges	nothing directly, noise gets more expensive	medium, works through cost	older types disappear from the night hours
Night flight ban	operations at fixed times	indirect, bunches traffic before and after the ban	waves in the late evening and early morning

How the concrete night rules at German airports look is summarized in the article Night flight ban in Germany.

Why new aircraft are quieter

Behind all quotas stands the noise certification of the aircraft themselves. At certification, ICAO measures at three reference points: to the side of the runway, under the departure track a few kilometers away and under the approach shortly before the threshold. From this come noise chapters. Aircraft of the oldest Chapters 1 and 2 have not been allowed to take off and land in the EU since 2002; new types must meet significantly stricter standards.

At the fence you hear the difference. Modern short- and medium-haul jets such as the Airbus A320neo, the A321neo or the A220-300 rely on engines with a very high bypass ratio. In direct comparison with predecessors such as the Boeing 737-800, the takeoff in particular is quieter. How to tell neo and ceo apart visually is shown in the guide A320neo or ceo?.

Besides the aircraft, the flight procedure plays a part. Noise-abatement departure procedures regulate when thrust is reduced and the flaps retracted after takeoff. Continuous descents keep the engines at idle longer on approach. The position of the routes is noise policy too: departure routes are often laid out to avoid densely populated areas. More on this in the article about SIDs and STARs.

Using noise data for spot planning

Noise protection and spotting have more in common than you might think. Both are interested in where aircraft pass low and often. Here’s how to put the knowledge to use:

An example of a spot that lies directly under the traffic is the Zeppelinheim viewing platform in Frankfurt. In Hamburg, the Affenfelsen (“Monkey Rock”) shows how close residential areas and runway operations lie at Hamburg Airport – and why noise protection is a permanent topic at city airports.

What to keep in mind

• A measured value is not a limit value. A single flyover at 80 dB(A) says nothing about whether a protection zone is exceeded. The zones refer to averaged levels over many months.
• Portals show data with a delay. Noise portals are suited to planning and analysis, not to live hunting for an arrival.
• Respect for residents. If you stand under the approach path, you are a guest in an area that endures the noise every day. Park properly, be quiet and leave no litter behind.
• Your hearing counts too. At the fence and under the approach, maximum levels reach values at which hearing protection is worthwhile, especially in long sessions. Tips are in the article Aircraft noise and your hearing.
• Mind the legal situation. What is allowed around the airport is summarized in the article Is plane spotting allowed?.

Conclusion

Aircraft noise is captured in two ways: monitoring stations document what actually arrives at a point, and calculations determine which areas are legally considered affected. For this, the Aircraft Noise Protection Act defines two day protection zones and one night protection zone with clear thresholds, building bans and claims to soundproofing. Noise quotas and noise-related charges go a step further and count not flights but noise – with visible consequences for the type mix in the shoulder hours. If you understand this logic, you read noise maps and monitoring points as what they also are for spotters: a very precise map of flight paths.