What is Noise?

Noise is defined as "unwanted" sound. Humans are very sensitive to sound and hear sounds that vary over a range of more than 1,000,000,000,000 to 1 in acoustic energy — from the whining of a mosquito’s wings to the racket of a jackhammer. Sounds levels vary so greatly in magnitude that it is impractical to describe their sound pressures in familiar linear units. Sound pressures are therefore measured on a logarithmic "decibel" (dB) scale. "A weighting" of the sound level is widely used in measurements of environmental sound to ensure electronic instruments match the sensitivity of the human ear across the audible spectrum. Hence the notation "dBA" or dB(A) etc.

Basic Principles

A sound wave has wavelength, frequency and amplitude. Each vibration of the source produces one pressure wave in an elastic transporting medium, usually air in the case of sound. The wavelength is the distance between successive pressure waves, that is, during one cycle from low - high - low pressure as shown below in Figure 1 .

Figure 1: Propagation of a sound wave[i].

Frequency

Frequency is the rate at which the sound source vibrates, and subsequently produces a pressure wave. It is measured in cycles per second referred to as Hertz (denoted Hz). One Hertz is one cycle per second.

Frequency determines the pitch of the sound. A doubling of frequency produces an approximate increase of one octave.

The frequencies that the normal "healthy" human ear can detect range from 20 Hz to 20 kHz (20,000Hz) although individuals vary greatly in terms of their sensitivity. Generally below 16 Hz is the range of infrasound and above 20 kHz is the ultrasound range.

Amplitude

The amplitude is the maximum excursion of the pressure difference of a sound wave as shown in Figure 2.

The Speed of Sound

The speed with which sound travels depends on the medium through which it travels, particularly its elasticity and density. The speed of sound in air at 20°C is approximately 344ms-1 this equates to approximately 0.34km/s. Sometimes the speed is referenced at 340ms-1

Relationship between Frequency and Wavelength by Velocity

As the velocity (speed with direction) of sound is constant for any given medium in a fixed state, if the frequency increases then the wavelength decreases.

The decibel Scale

The human ear is a very sensitive system with a considerable hearing range. To accommodate this very large range, noise levels are measured using the decibel (dB) scale. The following graph gives examples of common sound levels with an idea of their subjective effect.

The sound pressure in micropascals (µPa) is shown to illustrate the need for the use of the decibel scale. The decibel scale is logarithmic and not an arithmetic scale meaning decibels cannot be added in the traditional way.

The noise generated at sound is known as their "sound power level". The sound power level will always be higher than the sound pressure levels. The sound power output must be known before the sound pressure can be calculated or predicted. Sound pressure levels received at the ear should therefore not be confused with sound power levels that are used in prediction models to quantify sound sources. Sound Power and Sound Pressure are described as follows:

Noise Source	Sound Pressure Level (dB)	Sound Pressure (µPa)	Subjective Description
30m from a military jet aircraft during take off	140	200,000,000	Painful, intolerable
Concert	105	3,500,000
Inside night club	100	2,000,000
Concert at mix desk	98	1,600,000
Heavy vehicle at 7m	90	630,000	Very Noisy
Alarm clock at 1m	80	200,000
Vacuum cleaner at 3m	70	63,000	Noisy
Busy Office	60	20,000
Conversation at 1m	55	11,000
Reading room inside museum	35	1,100
Inside bedroom in residential area with windows closed	30	630	Quiet
Remote location without any identifiable sounds	20	200	Very Quiet
Theoretical threshold of hearing	0	20	Uncanny silence

Figure XX: Theoretical Sound Pressure (µPa) and Sound Pressure Level (dB) of common noise sources.

Using a decibel scale (sound pressure level) can complicates the mathematics, however the scale makes the numbers more manageable and, with familiarity, more meaningful.

The following schematic illustrates some typical sound pressure levels:

Subjective (Personal) Human Response to Sound

It is commonly accepted that for the "average healthy" young person a change of 1dB is just perceptible under controlled conditions. A change of 3dB is noticeable, 6dB obvious and a change of 10dB is significant. A 10dB change corresponds approximately to subjectively halving or doubling the loudness of a sound for a pure tone.

A general summary as to how the human ear responds to sounds measured is described as follows:

• Except under laboratory conditions, a change in sound level of 1dB cannot be perceived;
• A doubling of the energy of a sound source corresponds to a 3dB increase;
• Outside of the laboratory, a 3dB change in sound level is considered a barely discernible difference;
• A change in sound level of 5dB may typically result in a noticeable community response;
• A 6dB increase is equivalent to moving half the distance towards a sound source;
• A 10dB increase is subjectively heard as an approximate doubling in loudness
• The threshold of pain is an sound pressure levels any where between 120dB to 140dB

Causes of Airport Noise

Aircraft

Aircraft are an obvious cause of airport related noise. Noise is generated by aircraft in two main ways:

• Airframe noise: occurs when air passes over the plane’s body (the fuselage) and its wings. This causes friction and turbulence, which make a noise. Even gliders make a noise when in flight, and they have no engines at all.
• Engine noise: is created by the sound from the moving parts of the engine, just like in a car, but also by the sound of the air being expelled at high speed once it has passed through the engine. Most of the engine noise comes from the exhaust or jet behind the engine as it mixes with the air around it.

Modern Noise levels

Planes today are much quieter than they were 40, 30 or even 20 years ago. And these will be replaced by even quieter aircraft in the future. However even though each individual aircraft is becoming quieter, there are more planes flying now, which means that while the average level of noise from each plane is lower than before, you hear individual planes more often

Engine testing

Aircraft engines sometimes need testing for extended periods while the aircraft is stationary, and this can generate noise

Other airport related noise

This can include such things as construction noise and noise from operating machinery.