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NOISE POLUTION

Official complaints about noise pollution in the UK (adapted from Commission of the European Community, 1987)

Noise and SoundWhat is Noise ?

Noise is unwanted Sound.

Unwanted by whom?

What is sound?

Sound is air pressure waves which human ears can detect.

What is an air pressure wave?

Earth

Sound is defined as any pressure variation that the human year can detect

Amplitude, Wavelength, Period, speed of sound

Atmospheric Pressure = 1 Bar = 105 Pa

Atmospheric pressure / Sound pressure: Some physical media allow the propagation of disturbances.

Air is a physical media which allows the propagation of pressure disturbances.

The human ear is sensitive to some of these disturbances and this is what we call sound. Human ears can detect pressure fluctuations as low as 20µPa = 2*10-5 Pa.

An annoying sound (e.g. a loud horn) is about 2 Pa. This is still much smaller than 1 Bar.

Sound waves are extremely small pressure disturbances superimposed to a much larger atmospheric pressure.

Schematic representation of sound superimposed on top of the atmospheric pressure

Sound waves are characterised by their pressure amplitudes and their frequencies

There is another important difference between atmospheric pressure.

• The atmospheric pressure does not change very quickly; it varies with the weather so the time scale is, say, one day. By contrast, the human ear only detects pressure fluctuations which change at least 20 times per second, i.e. 20 Hz.

Propagation

Attenuation.

Geometrical Decay:

If a source of sound emits the same pressure fluctuation in all directions in free space (the so-called pulsating bubble), the surface with the same level of pressure will be concentric spheres. As the waves propagate outward, the spheres become larger and larger and the energy emitted by the source spread over an ever larger surface causing the amplitude to decay like 1/r2, where r is the distance from the source. This is called geometrical decay.

Absorption: sound wave would decay anyway due to the small but finite viscosity of the air and the absorbing capacity of most surfaces

Sources of noise pollution

• Traffic – main source• Industrial equipment• construction activities• Sporting and crowd activities• Low-flying aircraft

Most sounds are not purely sinusoidal variations. They vary both in frequency and Magnitude over time. To quantify their magnitude over time, the root mean square (r.m.s) pressure is defined

Sound Power and Intensity

Sound Power: The rate at which energy is transmitted by sound waves is called the sound power (W), measured in watts.

Intensity: The average sound power per unit area normal to the direction of propagation of a sound wave is termed in acoustic or sound intensity (I).

(W)

At sufficient distance from the sound source the intensity of the sound wave is proportional to the square of sound pressure

Intensity

Reference power level is 10-12 Watts

As mentioned before, the sound pressures perceived by human range from 20 µPa to 200 Pa.

Since sound power is proportional to the square of sound pressure, the sound pressure level in decibels is defined as

Sound pressure levels less than 25 decibels are not normally encountered except in broadcasting studies

Combining Sound Pressure LevelsExample 1:If a sound source has a pressure of 2000µPa at 10 m distance, compute:

(a)The sound pressure level in dB(b)The sound intensity in W/m2(c)The sound power in W

Example 2:If two sound sources have equal pressures of 2000µPa at 10 m distance compute

(d)The sound pressure level in dB(e)The sound intensity in W/m2(f) The sound power in W

Frequencies

Sound of a single frequency, called pure tones, rarely exist.

Audible sound ranges from : 0.015 to 15 kHz

Frequency lesser that 0.015 kHz is called infrasonic frequencies

Frequency greater than 15kHz is called Ultrasonic frequencies

Human voice contains frequencies 0.08 to 8 kHz (mainly concentrated in the band 0.5 to 2 kHz.

The sound of frequencies above 8kHz can usually be ignored in environmental noise monitoring

Octave Band:

An octave band is the frequency interval between a given frequency and twice that frequency. For example 0.05 – 0.1 kHz, 0.1 – 0.2 kHz, etc.

For noise analysis fixed octave bands are internationally recognized. These are centred on 0.0315kHz

For noise control purpose it is often necessary to identify the frequency components or spectra of sound.

Classification of Sound• ContinuousAn uninterrupted sound level that varies less than 5dB during the period of observation (ex. Household fan) • IntermittentIt is a continuous sound that lasts for more than one second but then is interrupted for more that one second (Ex. Dentist’s drill)

• ImpulsiveSound is for short duration, less than one second (ex. Hammering sound, typewriter sound). Change in pressure 40dB or more within 0.5 second with a duration of less than one second

General individual noise exposer patterns (Corbett, 1989)

NOISE CRITERIA

The loudness of sound is determined by its sound pressure level and its frequency.

The environmental or community noise can be appropriately measured in dBA units, which closely replicate the loudness perceived by the ear.

There is a wide differences in people's responses to noise. A number of different criteria have been proposed, the most commonly used criteria are

• LAeq: The equivalent continuous level

• LAE : The sound exposer level

• LAN : Sound Level Exceeded for N% of the time in dBA

The equivalent continuous level(LAeq)

It is applied to a fluctuating noise source. It is the constant noise level over a given time period that produces the same amount of A weighted energy as the fluctuating level over the same time frame.

Sampling methodology is discrete

Example:

An air conditioner generates a noise level of 75 dB for five minutes every hour. If the background noise level is 55dB, compute the LAeq

Sound Exposure Level

The sound exposure level (SEL) is the constant level in dBA lasting for one second which has same amount of A weighted energy as a transient noise.

It can be used to express the energy of isolated noise events such as aircraft flyovers.

The Sound Level Exceeded for N% of the time in dBA (LAN)

It indicates how frequently a particular sound level is exceeded.

The percentile levels revel maximum and minimum noise levels and are often used in base line studies taken prior to the introduction of new industrial or highway noise sources.

Commonly used percentile levels include the LA10, the sound level exceeded 10% of the time (some time used to present the maximum noise level.

They may be used as a complement to LAeq

The Sound Level Exceeded for N% of the time in dBA (LAN)

Noise Pollution Level (LNP)

LNP=Laeq+KσWhere K is a constant taken as 2.56 and σ is the standard deviation

Traffic Noise Index (TNI)

TNI=4(LA10 – LA50)+LA90 - 30

Noise Standards

Noise standards or thresholds are commonly specified as part of the planning permission (consent) for proposed development.

The specified values vary with• existing land use• background noise level of the area• type of development

Typical noise level at nearby residences areLAEQ – 40 – 70 dBA by dayLAEQ – 35 – 60 dBA by Night

EU Noise Directive: protection of workers from the risks related to exposure to noise at work• Daily personal noise exposure

Weekly exposure

Outdoor Propagation of Sound

• Geometrical Spreading• Directivity• Non-point sound sources• Acoustic Near Field• Attenuation• Outdoor noise level prediction

Geometrical Spreading

When the dimensions of the source is small relative to the distance to the receiving point the sound spreads spherically (no solid surfaces and fluid boundaries)

The acoustic intensity (I) at a distance r meters is given by

Sound pressure level in (dB)

Since the acoustic intensity is proportional to the square of the pressure, then we can write sound pressure level (LP) as

Directivity

Most sound sources do not radiate uniformly in all directions. This is because of directional characteristics of the sound source (which may be frequency dependence) or because of external constraints by near by surfaces.

Directives of the surface constraints may be estimated by examining the position of the source.

1. Close to the ground2. remote from the ground close to a wallSound pressure level (LP) at a distance r is given by

Non-point sound source

Attenuation• Attenuation due to distance• Atmospheric attenuation

• Attenuation due to meteorological conditions• ground surface effect• Sound attenuation by trees• ground topography• Reflecting surfaces and noise barriers

Outdoor noise level prediction

Noise Contours

Noise contour map of an airport runway

Noise Section of an EIA

• Baseline Noise Survey• Probable noise emission level• Statement of probable impact• Proposed remedial measures

Noise Control

Source: The source could be modified by the acoustic treatment to machine surfaces, design changes, etc. This is a specialized area which is outside the scope of this chapter. However, an offending noise source could be stopped or its operation limited to certain times of the day.

Transmission path: it could be modified by containing the source inside a sound insulating enclosure, by constructing a noise barrier or by the provision of absorbing materials along the path.

Receiver: the protection of the receiver by altering the work scheduled or by the provision of ear protection

Source: The source could be modified by the acoustic treatment to machine surfaces, design changes, etc. This is a specialized area which is outside the scope of this chapter. However, an offending noise source could be stopped or its operation limited to certain times of the day.

Transmission path: it could be modified by containing the source inside a sound insulating enclosure, by constructing a noise barrier or by the provision of absorbing materials along the path.

Receiver: the protection of the receiver by altering the work scheduled or by the provision of ear protection