Noise Pollution

Dr. Wesam Al Madhoun

Noise is unwanted sound

Unwanted by whom? “Whom” is people (human – no one cares whether the rabbits living near Heathrow are bothered by aircraft noise).

People’s tolerance thresholds to noise vary from individual to individual, with time, location (country to country etc…).

Not easy to find a universal definition.

What is noise?

Reflecting on noise “Noise" derived from "nausea," meaning seasickness

Noise is among the most pervasive pollutants today

Noise is unavoidable for many machines

Noise negatively affects human health and well-being

The air into which second-hand noise is emitted and on which it

travels is a "commons“, a public good

Sound is air pressure waves which human ears can detect.

What is sound?

What is an air pressure wave? The air in the atmosphere is a mixture of gases.

A gas kept in an enclosed space applies a force on the wall of the container (think of a blown-up balloon).

The pressure is the force per unit surface applied by the gas on the container walls.

Earth isn’t a container but because of the Earth gravity, the atmosphere is kept pushing on and around Earth’s surface.

This attraction results in a pressure which is around 1 Bar = 105 Pa at the surface.

1 Pascal (Pa) is the IS unit for pressures. 1 Pa =1 Newton/m2.

This is very small: 1 Newton is roughly the weight of an apple.

1 Pa is the weight of an apple spread out over a 1-meter side square.

Some physical media allow the propagation of disturbances.

This surface allows the propagation of disruptions: throwing a

stone in a pound causes ripples to radiate away from the

source of disturbance.

Throwing a stone in the middle of a sandpit causes no such

ripples: the surface interfacing air and sand does not

propagate disturbances.

Atmospheric pressure / Sound pressure.

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.

A wave is a disturbance travelling in a propagating medium.

Human ears can detect pressure fluctuations as low as 20 m Pa = 2*10-5 Pa.

This is 10 orders of magnitude below the atmospheric pressure.

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.

The atmospheric pressure does not change very quickly; it

varies with the weather.

The human ear only detects pressure fluctuations which

change at least 20 times per second, i.e. 20 Hz.

Human hearing and Frequency

0 20 Hz 20 kHz 5 MHz

These pressure disturbances can be quite complex (think of the pressure fluctuations produced by a jet engine) but

however complex they are, they propagate outwards, or away from the source as long as the medium is homogeneous.

Medium non-homogeneities can be anything like a glass wall, the ground…

They partially redirect and transmit the incident sound wave. This is represented diagrammatically on Fig. 2.

Propagation

Sound waves travel at a specific speed – the speed of sound – which is roughly c=340m/s in air.

This speed depends very little on the frequency of the wave: high frequencies, i.e 12 kHz travel as fast as a 50 Hz wave.

However these two parameters define another important one: the wavelength .

If T is the period of the pressure fluctuation, then its frequency is f=1/T and the wavelength is defined by:

=cT=c/f

Wave speed, wavelength.

It is the distance between two successive wave fronts (like

the distance between two wave crests with sea waves).

For sound waves, at 50 Hz, the wavelength is about 5m; at

5kHz, it is about 7cm so there is a significant different.

wavelength

If a source of sound emits the same pressure fluctuation in

all directions in free space, 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.

Attenuation.

This is called geometrical decay. Even if this did not

happen, sound wave would decay anyway due to the small

but finite viscosity of the air and the absorbing capacity of

most surfaces.

Sound waves are characterized by their pressure

amplitude and their frequency

Source power

Sources of sound (a loudspeaker, a hammer drill) have a

characteristic acoustic power measured in Watts.

This is the acoustics energy emitted by a source regardless

of the subsequent propagation of the sounds.

Measures of Sounds – Measures of Noise

Acoustic intensity I.

The energy produced by the source spreads out in space.

The acoustic intensity is the amount of acoustic energy that

flows per unit surface.

The acoustic intensity indicate the amount of energy a given

surface receives.

It is proportional to the square of the sound pressure.

Noise levels.

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

This range is enormous. As the intensity is proportional to the square of the pressure, its range of variation is even greater.

When a quantity varies over several orders of magnitudes, it is usually more helpful to look at its Logarithm and this is what people working with noise do.

A number of these logarithmic levels are used:

Intensity Level: LI=Log10(I/I0) (in Bell),

where I0=10-12 W/m2 is a reference level which roughly corresponds to the lower threshold of hearing.

These levels are actually non-dimensional numbers but they are commonly assigned a fictitious unit, the ‘Bell’.

Most intensity levels are fairly small numbers in Bells, one usually counts them in decibels (dB) i.e. a tenth of a Bell. In decibel, the intensity level is therefore:

LI=10XLog10(I/I0) (dB)

Intensity is physically the meaningful quantity (as an indicator of the ‘strength’ of a sound),

pressures are much easier to measure experimentally using a simple microphone.

The intensity is proportional to the square of the pressure.

So an alternative to LI is the Sound Pressure Level LP :

Lp=10XLog10(p2/p0 2) = 20X Log10(p/p0) in dB,

Where p0= 20 Pa = 210-5 Pa is the reference pressure so that Lp=0 at the standard threshold of hearing.

The pressure p used here is the root-mean square pressure, which is more representative than the maximum amplitude for complex non-harmonic sounds.

Due to the different reference chosen for both levels, the numerical values of Lp and LI are different but this difference is very small (0.5 dB) and usually ignored.

Effectively, they both represent the same thing – the strength of the sound at a given instant in time and space.

Sound pressure for known sounds

Both intensity and pressure define what is occurring at a point in space.

The more fundamental quantity is the Sound Power Level of the source, Lw defined by:

Lw= 10 Log (W/W0)

Note! Noise levels in dB are not additive. There are two separate issues which make the addition of SPL delicate.

(1) Standard acoustics is a linear science which means that if a

noise source A working alone produces an instantaneous pressure pA

at some point M in a room and if a source B produces a pressure

pB at M simultaneously (when the source A is not on) then the

resultant pressure at M when both sources are working is pA + pB.

However if the two sources are uncorrelated (which is usually the

case), this instantaneous pressure fluctuate widely.

To get a measure of the magnitude of the noise, we need its root mean square value over a couple of periods (say 1s).

It turns out that for uncorrelated sources, the resultant rms pressure is such that is such that p2 = p2

A + p2B.

This is illustrated in the diagram shown in Fig. 4.

(2) The second pitfall is that the logarithm function is not linear:

Log(p2

A + p2B) Log(p2A ) + Log( p2

B)

dB noise levels cannot be simply added

The human ear is the organ that allows us to perceive sound

waves (among other things).

When a sound wave enters the external auditory canal, it

impinges on the eardrum which activates a small mechanism

of bones effectively transmitting the air pressure to the fluid

contained in the cochlea.

The Ear and human hearing perception

The cochlea is long coiled canal whose wall is covered with small nerve ended hairs – the cilia – which detect the motion of the fluid.

Section diagram of the human ear

The strongest the sound, the furthest down the spiral the cilia

will be disturbed causing an appropriate nerve response to

feed the brain.

The semi-circular canals are connected to our perception and

our keeping in balance.

The Eustachian tube is a simple passage connecting the throat

to the internal ear allowing internal and external (static) air

pressures to balance out.

Physiological damage to the ear is often manifest by an

increase in the threshold of hearing which is monitored by

audiometric assessments.

Sleep disorders, loss of concentration, stress and other

psychological factors are also common and well known

consequences of noise exposure.

The ear is not simple linear perceiving sensor.

The subjective impression of the intensity or magnitude of a

sound depends on the frequency content, the waveform and

the duration of the noise.

The loudness level of a given sound is measured by making

a(statistical) subjective comparison between the perceived

loudness of that sound and that of a pure tone of specified

amplitude and frequency that seems equally loud.

Loudness Level

The sound pressure level of the pure tone in Phons is then

called the loudness level of the sound.

Equal Loudness Contours monitor how the same

impression of loudness changes with frequency. An

example of such contours is shown in Fig. 6.

Equal loudness contours

• These contours show that human is more sensitive to frequencies in the 1-10 kHz range than below 1 kHz (it takes a lot more actual pressure to reach the same impression of loudness at 100 Hz than at 1 kHz).

Sound Level Meters are the instruments commonly used to

measure environmental noise.

They have 3 main components: a microphone, some

filtering electronics (the weighting networks) and some

display.

Sound Level Meter and Weighting Networks

Transfer functions of the three mainweighting networks

Sound Level Meter

A filter is an electronic circuit which cuts out part of the frequency content of an input signal.

Weighting networks are filters that are applied on the raw noise signal (measured by the microphone).

They are meant to take into account the distortion introduced by the human perception.

"A" weighting network weights a signal in a way that approximates an inverted equal loudness contour at low Sound Pressure Levels,

"B" network corresponds to a contour at medium pressure levels.

"C" network to an equal loudness contour at high pressure levels.

A specialized filter, the "D" weighting, has also been introduced

for aircraft noise measurements.

In addition to one or more of these weighting networks, sound

level meters (noise measuring instruments) usually also have a

Linear or "Lin." network.

This does not weight the signal but enables the signal to pass

through unmodified.

Equivalent sound level (Leq) can be applied to any fluctuation noise level.

It’s the constant noise level that, over a given time, expand the same amount of energy as the fluctuating level over the same period.

Leq = 10 log Σ 10Li/10(ti)

n: the number of samples taken Li

: the noise level in dBA of the ith sample ti : fraction of total sample time

Equivalent Noise Levels

i=n

For highly fluctuating noises, LAEq is not enough.

It is sometimes complemented by slightly more refined statistical analysis of the noise based on percentiles.

Thus LAX = Y dB(A) where X is a percentage and Y a dB level means that for the noise considered, the level Y dB(A) is exceeded X% of the time.

For example: LA10 =60 dB(A) means that the level of 60 dB(A) is exceeded 10 percent of the time.

This is a good indication of noise events which are extreme but sporadic.

Percentiles

In many cases, community reaction to noise is governed by a single noisy event or by a series of identifiable noisy events (like blasts).

A parameter is needed to quantify the effect of such events on the overall noise climate.

The parameter used is the single event noise exposure level, noted LAX or SEL

The SEL or LAX of a single discrete noise event is the level which if maintained constant for a period of 1s would have as much A-weighted energy as is contained in the actual noise event.

The SEL can be thought of as a standardized impulsive strength of a noise event. This definition is illustrated in Fig. 9.

LAX or SEL

Figure 9 – Illustration of the definition of the Single Event Exposure Level (SEL or LAX)

The main sources of Environmental Noise

Environmental Noise Legislation

(a) You become deaf because you’ve been working for thirty years on a noisy press machine.

You can sue your employer for damage and negligence.

This is a private law suit and will take place in a civil court.

(b) A Health and Safety executive comes to a factory and suspects workers are dangerously exposed. S/he orders a noise assessment and finds that worker’s noise exposure exceeds the limit.

As a consequence, the employer will be prosecuted in a criminal court for not following the specifications of the Noise at Work Regulation 1989.

Noise at work

1. Control of Pollution Act 1974 (almost completely superseded by EPA 1990)

2. Environmental Protection Act 1990 3. Noise and Statutory Nuisance Act 1993 4. Health and Safety at Work Act 1974 [Noise at Work

Regulations 1989] 5. Planning Policy Guidance PPG 24 6. Land Compensation Act 19773 7. Building Act 1984 [Building Regulations 1991, Part E] 8. Road Traffic Act 1972/1988 + other transport Acts.

Parliamentary Acts relating to Noise

If a new road is being planned, it is not possible to assess experimentally the impact of the new road on existing neighboring areas.

In this case, noise calculations taking into account the most common effects have been standardized to predict pressure levels at some distance.

See for example: http://www.npl.co.uk/acoustics/techguides/crtn/

Noise Calculations

Such calculations exist for Road, Rail and Air traffic.

They are published by the Department of Transport in the form of booklets.

They are used for planning purposes,

1- When noise contour maps are necessary (the actual measurements to get so many data would take too long),

2- When measurements are difficult (because of access, or background noise…)

3- or when various alternative control solutions need to be tested.