Official complaints about noise pollution in the UK (adapted fromCommission 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 yearcan detectAmplitude, Wavelength, Period, speed of sound
Atmospheric Pressure = 1 Bar = 105 PaAtmospheric pressure / Sound pressure: Some physicalmedia allow the propagation of disturbances.Air is a physical media which allows the propagation ofpressure disturbances.The human ear is sensitive to some of these disturbancesand this is what we call sound. Human ears can detectpressure fluctuations as low as 20µPa = 2*10-5 Pa.An annoying sound (e.g. a loud horn) is about 2 Pa. This isstill much smaller than 1 Bar.
Sound waves are extremely small pressure disturbancessuperimposed to a much larger atmospheric pressure.Schematic representation of sound superimposed on top of theatmospheric pressure Sound waves are characterised by their pressure amplitudes and their frequencies
There is another important difference between atmosphericpressure.• 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 directionsin free space (the so-called pulsating bubble), the surface with the samelevel of pressure will be concentric spheres. As the waves propagateoutward, the spheres become larger and larger and the energy emittedby the source spread over an ever larger surface causing the amplitudeto decay like 1/r2, where r is the distance from the source. This is calledgeometrical decay.Absorption: sound wave would decay anyway due to the small butfinite 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 varyboth in frequency and Magnitude over time. To quantifytheir magnitude over time, the root mean square (r.m.s)pressure is defined
Sound Power and IntensitySound Power:The rate at which energy is transmitted by sound waves is calledthe sound power (W), measured in watts.Intensity:The average sound power per unit area normal to the direction ofpropagation of a sound wave is termed in acoustic or soundintensity (I). (W)
At sufficient distance from the sound source the intensity of thesound wave is proportional to the square of sound pressure
IntensityAs mentioned before, the sound pressures perceived by humanrange from 20 µPa to 200 Pa. Reference power level is 10-12 Watts
Since sound power is proportional to the square of soundpressure, the sound pressure level in decibels is definedas 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 WExample 2:If two sound sources have equal pressures of 2000µPa at 10 mdistance compute(a)The sound pressure level in dB(b)The sound intensity in W/m2(c)The sound power in W
FrequenciesSound of a single frequency, called pure tones, rarely exist.Audible sound ranges from : 0.015 to 15 kHzFrequency lesser that 0.015 kHz is called infrasonic frequenciesFrequency greater than 15kHz is called Ultrasonic frequenciesHuman voice contains frequencies 0.08 to 8 kHz (mainlyconcentrated in the band 0.5 to 2 kHz.The sound of frequencies above 8kHz can usually be ignored inenvironmental noise monitoring
Octave Band:An octave band is the frequency interval between a givenfrequency and twice that frequency. For example 0.05 –0.1 kHz, 0.1 – 0.2 kHz, etc.For noise analysis fixed octave bands are internationallyrecognized. These are centred on 0.0315kHzFor noise control purpose it is often necessary to identifythe frequency components or spectra of sound.
Classification of Sound• ContinuousAn uninterrupted sound level that varies less than 5dB duringthe period of observation (ex. Household fan)• IntermittentIt is a continuous sound that lasts for more than one second butthen 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 aduration of less than one second
NOISE CRITERIAThe loudness of sound is determined by its sound pressure leveland its frequency.The environmental or community noise can be appropriatelymeasured in dBA units, which closely replicate the loudnessperceived by the ear.
There is a wide differences in peoples responses to noise. A numberof different criteria have been proposed, the most commonly usedcriteria 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 noiselevel over a given time period that produces the same amount of Aweighted energy as the fluctuating level over the same time frame.
Example:An air conditioner generates a noise level of 75 dB for fiveminutes every hour. If the background noise level is55dB, compute the LAeq
Sound Exposure LevelThe sound exposure level (SEL) is the constant level in dBAlasting for one second which has same amount of Aweighted energy as a transient noise.It can be used to express the energy of isolated noiseevents 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 andare often used in base line studies taken prior to the introduction ofnew industrial or highway noise sources.Commonly used percentile levels include the LA10, the sound levelexceeded 10% of the time (some time used to present the maximumnoise 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 standarddeviationTraffic Noise Index (TNI)TNI=4(LA10 – LA50)+LA90 - 30
Noise StandardsNoise standards or thresholds are commonly specified aspart of the planning permission (consent) for proposeddevelopment.The specified values vary with• existing land use• background noise level of the area• type of developmentTypical noise level at nearby residences areLAEQ – 40 – 70 dBA by dayLAEQ – 35 – 60 dBA by Night
EU Noise Directive: protection of workers fromthe risks related to exposure to noise at work• Daily personal noise exposure
When the dimensions of the source is small relative to the distanceto the receiving point the sound spreads spherically (no solidsurfaces 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 thepressure, then we can write sound pressure level (LP) as
DirectivityMost sound sources do not radiate uniformly in alldirections. This is because of directional characteristics ofthe sound source (which may be frequency dependence)or because of external constraints by near by surfaces.Directives of the surface constraints may be estimated byexamining 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
Noise Section of an EIA• Baseline Noise Survey• Probable noise emission level• Statement of probable impact• Proposed remedial measures
Noise ControlSource: The source could be modified by the acoustic treatment tomachine surfaces, design changes, etc. This is a specialized area which isoutside the scope of this chapter. However, an offending noise sourcecould be stopped or its operation limited to certain times of the day.Transmission path: it could be modified by containing the source insidea sound insulating enclosure, by constructing a noise barrier or by theprovision of absorbing materials along the path.Receiver: the protection of the receiver by altering the work scheduledor by the provision of ear protection
Source: The source could be modified by the acoustictreatment to machine surfaces, design changes, etc. This is aspecialized area which is outside the scope of this chapter.However, an offending noise source could be stopped or itsoperation limited to certain times of the day.Transmission path: it could be modified by containing thesource inside a sound insulating enclosure, by constructing anoise barrier or by the provision of absorbing materials alongthe path.Receiver: the protection of the receiver by altering the workscheduled or by the provision of ear protection