How aviation noise is measured

What the aviation industry classifies as 'quiet' planes, and what you experience when a plane flies overhead, are very different things. Understanding this relationship is key to understanding the multitude of applications and claims made by Leeds Bradford Airport.

How we experience noise

Before we go into the details of how noise from planes is classified, we need to understand how we experience noise as an observer. People experience noise in different ways. It changes as we get older; children can hear higher pitched noise than adults; something that annoys one person will not be annoying to another; a constant noise is less disturbing than an irregular noise because our brain can filter it out.

Our auditory system is a very sensitive instrument. There are two parts: the ear, a membrane and sensors which detect sound pressure, and the auditory cortex in the brain which processes the information.

With one hand you could drop a penny on the floor and hear it spinning as it comes to rest, then, a few moments later, you could fire a cannon with the other hand and experience an explosion of such high sound pressure that it could damage your ear drum, but your brain will normalise the two events and make sense of the information so you can hear one and not be overwhelmed by the other. No electronic measuring device can provide that range and sensitivity.

What would you prefer flying over your house: one Concord per hour or one Boeing 737-MAX per minute? They are equivalent according to the aviation industry

What is a Decibel?

Noise is measured in Decibels (dB). This is a measure of the sound pressure as it hits your ear drum or sound meter. Sound is basically a pressure wave travelling through air. Think of a wave in water: when it hits an object it moves it, or, if it’s a solid object like a sea wall, reflects off it. Similarly, a sound wave will move your ear drum and will reflect off a hard surface.

Decibels are not a measure of the loudness you experience when your brain tries to make sense of the information sent by your ear. Decibels are measured on a logarithmic scale, so an increase of 10 dB causes a doubling of perceived loudness and represents a ten-fold increase in sound pressure.

The significance of the 3 dB band

Planes are classified into 3 dB bands. You will hear people in the aviation industry claiming that a plane that is 3 dB quieter than another is half as loud, but that isn’t true: it is half the sound pressure as detected at the sound meter or your ear drum. Your brain translates that sound pressure into loudness and 3 dB actually represents the smallest increment in loudness that the human ear can distinguish.

So, if one plane is 3 dB quieter than another, it is barely distinguishable to the human observer. If one plane is in the lower half of one 3 dB band, and the next ‘quieter’ plane is in the upper half of the 3 dB band below, you won’t be able to tell the difference.

Your environment when you hear a plane makes more of a difference to the loudness you experience than the 3 dB band in which it is classified. Hearing a plane in a city scape with hard reflective surfaces, or next to a glass window on your balcony, will add 3 dB from the sound reflections around you. If you are in a grassy field where there are no reflections, that plane will seem 3 dB quieter. Similarly in a woodland area, the canopy will make the plane seem quieter. There are many other factors: cloud cover, fog, rain, air pressure, wind strength, proximity to the plane, altitude, etc.

The reason the industry uses the 3 dB band is because it can be measured objectively and is a convenient size to classify the variety of planes. But measurements are made in controlled conditions, it is not a realistic measure of what you hear. The claim that each 3 dB band is half the loudness of the previous one is likely to be the result of pressure from the aviation industry who gain by allowing double the number of flights in each subsequent band.

How aircraft noise is classified

Each make and model of plane is certified with a Quota Count (QC) value depending on the amount of noise it makes on takeoff and landing, accounting for weight during takeoff and landing, engine type and any special design features such as wing tips. This is carried out in accordance with the International Civil Aviation Organisation (ICAO) certification process.

The QC System

The certified QC value means that the plane fits into a ‘noise’ band that is 3 dB wide. The QC can be different for takeoff and landing, it’s even possible for a plane to be quieter on takeoff than landing. The QC value for any civilian aircraft can be looked up in tables issued by the UK’s National Air Traffic Services (NATS). Exempt planes are small propeller driven planes used for specific purposes such airport inspection.

Noise Classification 2023 Quota Count (QC)
Exempt
Below 81 EPNdB 0
81 – 83.9 EPNdB 0.125
84 – 86.9 EPNdB 0.25
87 – 89.9 EPNdB 0.5
90 – 92.9 EPNdB 1
93 – 95.9 EPNdB 2
96 – 98.9 EPNdB 4
99 – 101.9 EPNdB 8
Greater than 101.9 EPNdB 16

 

In the QC system, an airport is allocated a quota, say 1000 night-flights per year. The QC value is a divisor, so, a quota of 1000 would allow 1000 planes of QC 1, or 2000 planes of QC 0.5, or 4000 planes of QC 0.25, or any combination of these that adds up to the quota of 1000.

The system is flawed because 2000 planes at QC 0.5 is not equivalent to 1000 planes at QC 1. But there is an obvious incentive for the industry to use quieter planes. The latest generation of planes are QC 0.5, QC 0.25 and QC 0.125 so there is a tendency to have many more slightly quieter planes as we move forward.

So, what would you prefer flying over your house, one noisy QC 16 plane (e.g. Concord) per hour, or 64 QC 0.25 planes (e.g. Boeing 737-max) per hour – more than one every minute? They are equivalent according to the QC system.

However, it is commonly accepted by the UK and other national aviation regulators, that the number of events is as at least as important as the average perceived loudness, and this is shaping aviation noise policy. New metrics have been developed which count the number of events above a particular threshold. For example, N60, indicates the number of events over 60 dB for a fixed time period and is a more realistic and understandable measure of disturbance.

The system at Leeds Bradford Airport

The QC system is not fully implemented at Leeds Bradford Airport. Only the London airports fully implement the QC system and are regulated in law. Many regional airports select parts of the QC system and implement a hybrid system in agreement with the local planning authority who are the regulators.

This is case at Leeds Bradford Airport, where the QC values are used to define what can and can’t fly at night, but an absolute cap is placed on the number of flights that are permitted. There is no mixing and matching of QC values to mathematically add up to a quota.

Leeds Bradford Airport is big enough already