Transcript 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.
 “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 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.

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.
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.

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

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.

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.

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
amplitude and their frequency
their pressure
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.
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 mPa 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 mPa = 210-5 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.

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 = p2A + p2B.

This is illustrated in the diagram shown in Fig. 4.

(2) The second pitfall is that the logarithm
function is not linear:
Log(p2A + p2B)  Log(p2A ) + Log( p2B)
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 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.

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.
• 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).
Equal loudness contours

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
Transfer functions of the three main
weighting networks

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.
i=n

-
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

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.

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.
Figure 9 – Illustration of the definition of the Single Event Exposure
Level (SEL or LAX)
The main sources of Environmental Noise

(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.








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.

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/

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.