Air and Noise Pollution Studies

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Air and Noise Pollution Studies (CSE331) Dr. Tao
Wang CSE Office TU702 Phone [phone] FAX
2334-6389 E-mail cetwang_at_polyu.edu.hk Walk-in
Consultation Hours Thursday and Friday 5 - 6pm
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Contents to be covered

• Basics on noise
• Assessment and control
• Road traffic noise
• Railway noise
• Construction noise
• Industrial noise

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Introduction to noise pollution

• Noise is unwanted sound because it
• can cause hearing loss
• interferes speech communication
• disturbs moods, relaxation, and privacy
• Sound is a form of energy produced by a vibrating
  object or an aerodynamic disturbance

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Introduction to noise pollution

• Sound energy is propagated in the form of waves
  which represent a compression and decompression
  of molecules of air, liquids, and solids.
• The disturbance of air molecules by sound energy
  produces variations in normal atmospheric
  pressure. The pressure variations can be sensed
  by an instrument or animals sensory system (ear
  and brain).
• Sound can only be transmitted through a medium
  that contain molecules - it can not move through
  a vacuum.

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Sound energy When an object vibrates it radiates
acoustical energy. Sound power energy per unit
time (watts) Sound power level has unit of
decibels (dB), defined as SWL10 log 10 We/W0
dB Where We is measured sound energy W0 is
reference power, 10-12 w. Sound Intensity (LI)
energy per unit time per unit area SIL10 log 10
Ie/I0 dB
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• Sound pressure level
• SPL 10 log 10 Pe2/Pr2 dB
• 20 log 10 Pe/Pr dB
• where Pr is the threshold of human hearing, 2 x
  10-5 N/m2
• The reasons for the logarithmic nature of the
  decibels are
• The amplitude of audible pressure waves is a
  vast range. Taking logarithm of these extremes,
  we reduce the scale to 0-15 and by multiplying by
  10 to 0-150.
• In general, people judge that a sound which has
  increased in level by 10 dB is roughly doubled in
  subjective loudness

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Sound Pressure Level dB re 20µPa Description of sound source Typical subjective description
140 Moon launch at 100m artillery fire, gunners position Intolerable
120 Ships engine room rock concert, in front and close to speakers
100 Textile mill press room with presses running punch press and wood planers, at operators position Very noisy
80 Next to busy highway, shouting Noisy
60 Department store, restaurant, speech levels
40 Quiet residential neighborhood, ambient level Quiet
20 Recording studio, ambient level Very Quiet
0 Threshold of hearing for normal young people
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Example 1

• Determine the sound power level of a small siren
  that generates 0.1 W of sound power.
• Solution

Lw 10 log W dB
Lw 10 log Wre dB
10 log 0.1 dB
10 log 10-12 dB
110 dB
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Example 2

• Determine the sound power of a machine whose
  specified sound power level is 125 dB
• Solution
• W Wref x 10Lw/10 W
• 10-12 x 10125/10 W
• 3.2W

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Example 3

• Calculate the sound pressure level for a sound
  with an rms acoustic pressure of2.5 Pa
• Solution

Lp 10 log P2 dB
Lp 10 log Pref2 dB
Lp 10 log 2.52 dB
Lp 10 log (20 x 10-6)2 dB
Lp 101.9 dB
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Relationship of SIL and SPL

• Sound intensity (pressure2) x constant
• Sound intensity level Sound pressure level

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Wavelength, Frequency, and Amplitude of Sound
Wave -the shorter the wavelength, the more
frequent the waves per unit time -The higher the
wave, the more sound energy is has.
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Sound Propagation For a point source (Inverse
Square law) Intensity w/4?R2 (R is distance
from the source) SPLSWL 20 logR c 10
logQ (Q is directivity) (where c is a
constant) That is, the sound intensity decreases
by a factor of 4 (or by 6 dB) when distance from
the source is doubled. For a line source
(Inverse Law) Intensity w/2?RL (L is the
length of the source) SPLSWL 10 logR 10log
L c 10logQ That is, the sound pressure
level decrease by 3 dB when distance from the
source is doubled.
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Q values

• Free space 1
• Centered in a flat surface 2
• Centered at junction of 2 surfaces 4
• At a corner by three flat surfaces 8

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Interaction of sound with an obstacle

• Reflection- sound turning back from solid surface
  ( 3dB for one surface)
• Diffraction sound turning around barriers (-10
  dB for typical barriers)
• Refraction sound changing direction due to
  change of speed in different medium (e.g., change
  in temperature, wind)
• Transmission sound passing through panels by
  solid vibration (-10 dB for glass window, -40db
  for walls)
• Air absorption - sound energy absorbed by humid
  air (-5 dB/km)

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Human hearing range 50-20000 hz Best hearing
range 500-4000 hz
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Sound Measurement Sound pressure level meter It
measures sound pressure by converting it to
electronic signals -Microphone -Attenuator -Amplif
ier -Indicating meter
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Weighting Network or Scales Different scales
have different discriminations on lower frequency
waves -A scale lt 600 Hz -B scale lt more
moderate -C scale little discrimination 20
dB(A), 20 dB(B), 20 dB(C) etc. It is common to
use A scale.
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Meter response and instrument accuracy Fast/slow
options. Accuracy Class I 1dB for research
use Class II 2 dB for general purpose Class II
3 dB not for regulatory and technical uses
Spectrum analysis -Sound pressure levels at
different frequency band The most commonly used
instrument is octave band analyzers
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Center frequencies and frequency ranges of octave
bands
Center frequency, Hz Frequency range, Hz
31.5 18-45
63 45-90
125 90-180
250 180-355
500 155-710
1000 710-1400
2000 1400-2800
4000 2800-5600
8000 5600-11200
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Commonly used L for different averaging periods

• Lmax is the maximum noise level during a
  designated time interval or a noise event.
• Leq (24 hours) is the equivalent continuous
  source level and is a measure of energy level of
  a time-varying noise.
• L10 (1 hour) is the noise level exceeded for 10
  of the one-hour period and is generally used for
  road noise at peak traffic flow.
• L90 (1 hour) is the noise level exceeded for 90
  of the one-hour period and is generally used as a
  measure of the background noise level.

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Averaging Sound Measurements Because decibels
are geometric values they can not be added
directly. - First convert readings to arithmetic
values which are then added and converted back to
a decibel. The equivalent sound level is Leq
10 log ? 10Li/10/N Where Li corresponds to each
measured value in dB.
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• Addition
• Similarly, when individual values are added
• L 10 log ? 10Li/10
• Subtraction
• L 10 log (10Ls/10-10Lb/10)
• Where Ls is the noise source and Lb is the
  background

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Example 4

• Determine the total sound pressure level due to L
  p1 90 dB, L p2 95 dB andL p3 88 dB
• Solution

Lp 10log(1090/10 1095/10 1088/10) dB
10log(109 109.5 108.8) dB 96.8 dB
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Example 5

• Determine the total sound power level due to the
  contributions from three octave bands, designated
  as L w1 100 dB, L w2 103 dB, and L w3 106
  dB.
• Solution

Lp 10log(10100/10 10103/10 10106/10) dB
10log(1010 1010.3 1010.6) dB 108.4
dB
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Chart for Adding Decibels(1) Determine the
difference between the two(2) use the following
table to add the corresponding increment to the
HIGH level
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Example 6

• Determine the total sound pressure level due to L
  p1 90 dB, L p2 95 dB andL p3 88 dB by the
  Chart for Adding Decibels.
• Solution

88 dB
90 dB
95 dB
92.2 dB
96.9 dB
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Example 7

• Determine the sound pressure level at a point due
  to a particular machine if, at the point, L p
  85 dB with the machine off and 94 dB with the
  machine operating.
• Solution

Lp 10 log (10Lpt/10 - 10Lpb/10) dB In this
case, Lpt 94 dB and Lpb 85 dB Lp 10 log
(1094/10 - 1085/10) dB Lp 93.4 dB
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Chart for Subtracting Decibels
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Example 8

• Determine the sound pressure level at a point due
  to a particular machine if, at the point, L p
  85 dB with the machine off and 94 dB with the
  machine operating, using Chart for Subtracting
  Decibels
• Solution

Lp Lpt Lpb 94 - 85 9 dB That is, the
difference between the total level and the
background level, Lp 9 dB. Entering the chart
at 0 dB on the abscissa, we find that 0.6 dB is
to be subtracted from Lp. Therefore, Lp 94
0.6 dB Lp 93.4 dB
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Sound Waves

• Sound is a disturbance, or wave, which moves
  through a physical medium (such as air, water or
  metal) from a source to cause the sensation of
  hearing in animals. The source may be a vibrating
  solid or turbulence in the air.

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Period Frequency

• Period is the time taken for one vibration cycle.
  Its symbol is T and its unit is seconds (s).
• Frequency is the number of vibration cycles per
  second. Its symbol is f and it is measured in
  units called hertz (Hz).
• Frequency and period are related by f 1/T
• For example, a sound with a period of 0.002 s has
  a frequency of 500 Hz.

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Speed, Frequency and Wavelength of Sound

• Wave velocity is the speed with which sound
  travels through the medium. Its symbol is c and
  its unit meters per second (m/s).
• It is related to the frequency (f) and wavelength
  (? ) by
• c f ?

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Reflection

• Reflection occurs when an obstacle's dimensions
  are larger than the wavelength of the sound. In
  this case the sound ray behaves like a light ray
  and, for an obstacle with a flat surface, the
  reflected ray will leave the surface at the same
  angle as the incident ray approached it, so that
  the angle of incidence is equal to the angle of
  reflection.

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Diffraction

• Diffraction occurs when an obstacle's dimensions
  are of the same order or less than the wavelength
  of the sound. In this case the edge of the
  obstacle acts like a source of sound itself and
  the sound ray appears to bend around the edge.
  This limits the effectiveness of barriers.

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Refraction

• Refraction occurs when a sound ray enters a
  different medium at an angle. Because of the
  differing speed of travel of the sound wave in
  the two media, the sound ray will bend. This can
  be an important consideration in outdoor sound
  propagation over long distances. When weather
  conditions produce a temperature inversion, sound
  rays originally propagating upwards can be bent
  back to the ground.

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Transmission Absorption

• When a sound wave strikes an obstacle, part of it
  is reflected, part is absorbed within the
  obstacle and part is transmitted through to
  become a sound wave in air again on the other
  side

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Sound Pressure

• Sound Pressure is the force per unit area and
  gives the magnitude of the wave. Its symbol is p
  and its unit is pascal (Pa).
• A quantity known as the root-mean-square
  pressure, prms, is often used in acoustic
  measurements, to overcome the problem of the
  average pressure being zero.

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Sound Intensity

• Sound intensity, at a point in the surrounding
  medium, is the power passing through a unit area.
  Its symbol is I and its unit, watts/m2.
• where W is the sound power in watts
• S is the surface area in m2

I W
I S
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Sound Intensity and Sound Pressure

• As most measurements of sound are in terms of
  sound pressure (p), it is useful to know the
  relationship between sound intensity and sound
  pressure
• where I is the sound intensity in watts/m2
• p is the sound pressure in Pa
• r is the density of medium in kg/m3
• c is the speed of sound in m/s
• For air at 21oC , r 1.2 kg/m3 and c 344 m/s
• Strictly speaking, this equation is for plane
  waves (ie waves propagating with parallel
  wavefronts). However, away from a point source,
  the spherical waves approximate plane waves.

I p2
I ?c
I p2 0.0024 p2
I (1.2)(344) 0.0024 p2
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Decibel Scale (dB)

• The human ear responds to sounds over a very
  large range of sound intensities from 10-12
  (quietest) watts/m2 to 10 (painful) watts/m2
• To handle the range we make use of a logarithmic
  ratio scale called the decibel scale.
• In general, a decibel scale for any quantity, q,
  is defined as
• the decibel is not an absolute measure but is
  referenced to a selected quantity, qref.
• the ear itself 'hears' logarithmically and humans
  judge the relative loudness of two sounds by the
  ratio of their intensities, a logarithmic
  behavior.

Lq 10 log q dB
Lq 10 log qref dB
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Sound Intensity Level (LI)

• Sound intensity is expressed as a decibel it is
  referred to as sound intensity level and is given
  the symbol LI.
• To convert sound intensity, I, to sound intensity
  level LI the following relationship is used
• I sound intensity whose level is being
  specified, in watts/m2
• Iref reference intensity
• 10-12 watts/m2 (the threshold of hearing)

LI 10 log I dB
LI 10 log Iref dB
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Sound Power Level (Lw)

• Sound power, W, can also be expressed in decibels
  and is then referred to as the sound power level,
  Lw.
• where W sound power of the source in watts
• Wref reference sound power
• 10-12 watts

Lw 10 log W dB
Lw 10 log Wref dB
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Sound Pressure Level (Lp)
Sound pressure level Lp 10 log p2 dB
Sound pressure level Lp 10 log pref2 dB
20 log p dB
20 log pref dB

• where p rms sound pressure in Pa
• pref 2 x 10-5 Pa
• (sometimes written as 20 m Pa 20 x 10-6 Pa,
  which is the sound pressure at the threshold of
  hearing at 1000 Hz)
• The sound pressure level at the threshold of
  hearing is thus

Lp 20 log 2 x 10-5 20 log 1 0 dB
Lp 20 log 2 x 10-5 20 log 1 0 dB
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Intensity Level, Sound Power Level Sound
Pressure Level

• For a point source in a free field

I W W was the relation ship of intensity, I, to power, W was the relation ship of intensity, I, to power, W was the relation ship of intensity, I, to power, W was the relation ship of intensity, I, to power, W
I 4pr2 4pr2 was the relation ship of intensity, I, to power, W was the relation ship of intensity, I, to power, W was the relation ship of intensity, I, to power, W was the relation ship of intensity, I, to power, W
Changing to levels Changing to levels Changing to levels Changing to levels Changing to levels Changing to levels Changing to levels
10 log 10 log I I 10 log W -10 log(4pr2)
10 log 10 log Iref Iref 10 log Wref -10 log(4pr2)
LI LW 10 log (r2) 10 log (4 p) LI LW 10 log (r2) 10 log (4 p) LI LW 10 log (r2) 10 log (4 p) LI LW 10 log (r2) 10 log (4 p) LI LW 10 log (r2) 10 log (4 p) LI LW 10 log (r2) 10 log (4 p) LI LW 10 log (r2) 10 log (4 p)
LW 20 log r - 11 LW 20 log r - 11 LW 20 log r - 11 LW 20 log r - 11 LW 20 log r - 11 LW 20 log r - 11 LW 20 log r - 11
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Intensity Level, Sound Power Level Sound
Pressure Level

• For a line source in a free field
• where L is the length of the source

I W was the relation ship of intensity, I, to power, W
I 2prL was the relation ship of intensity, I, to power, W
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Distance

• If the distance from the point source (r) is
  doubled, the sound pressure level is decreased by
  (20 log 2), which is 6 dB

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Frequency weighting networks

• Low frequencies is less sensitive to the ear.
• Several different weighting networks have been
  developed.
• A-weighting network
• the best describing the damaging effect to ear.
• reduces the low frequency response and some of
  the high frequencies.
• C-weighting network
• useful in estimating the attenuated noise when
  personal hearing protectors are used

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Frequency weighting networks
-A scale lt 600 Hz -C scale little discrimination
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Exercise

• What is the wavelength in air at 21 ? of a sound
  with
• (i) frequency at the lowest end of the range of
  audibility (i.e. 20 Hz)?
• (ii) frequency at the highest end (i.e. 20 000
  Hz)?
• (Tip use equation 1.4A above).
• If the temperature increases to 40 ?, what is the
  frequency of a sound with a wavelength of 5m in
  air?
• (Tip use equation 1.5 first, then equation
  1.4B).

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Exercise

• Calculate the sound pressure level of a sound
  with a sound pressure of 2 Pa. (100dB)
• If the sound power level of a machine is 102 dB
  what are the sound pressure levels
• at 2m. (65 dB)
• at 4m (79 dB)
• assuming the source is in a free field.
• Add together two sounds of 94 dB and 99 dB.
• Add together 3 sounds of 96, 89 and 92 dB.

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Exercise

1. If the sound pressure level at a worker's
   location is 93 dB with one machine and the
   ventilation system operating and it is 87 dB with
   the machine switched off and the ventilation
   still on, what is the sound pressure level due to
   the machine by itself?

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Exercise
Complete the following table Complete the following table
For combining two decibel levels of sounds For combining two decibel levels of sounds
Difference between levelsdB Amount to be added to higher leveldB
0 3
1 2.5
2
3
4
5
6
7
8
9
10
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Exercise
Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw Try the following addition of octave band levels of noise from a circular saw
Octave Band Centre Frequencies, Hz 31.5 63 125 250 500 1000 2000 4000 8000 16000
Circular saw octave band levels, dB 73 75 77 80 87 85 88 96 92 87
i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A)) i. What is the A-weighted level? (99 dB(A))
ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C)) ii. What is the C-weighted level? (98 dB(C))
iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level) iii. Which is the higher? (The A-weighted level)
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Frequency Analysis

• to obtain information about the frequency
  spectrum of a sound to design effective noise
  control
• to select appropriate personal hearing
  protectors.
• Octave Band
• is sufficient to measure the sound pressure level
  in bands of frequencies, rather than at
  individual frequencies.
• is the width of the band usually chosen.
• Is a band where the upper frequency is twice that
  of the lower. Each band is denoted by its centre
  frequency.

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Internationally Preferred Frequencies
31.5 Hz 63 Hz 125 Hz 250 Hz 500 Hz 1k Hz 2k Hz 4k Hz 8k Hz 16k Hz
Example of a typical spectrum of a circular saw
cutting aluminum
(Note that the line is simply a guide to move
your eye from one band to another and does not
imply the magnitude at the frequencies between
the centre frequencies.)
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Octave Bands Limits
Centre frequency, Hz Limits of band, Hz
31.5 22 - 45
63 45 - 89
125 89 - 177
250 177 - 353
500 353 - 707
1000 707 - 1414
2000 1414 - 2828
4000 2828 - 5657
8000 5657 - 11314
16000 11314 - 22627
Note that the centre frequency is the geometric
mean and not the average of the band limit
frequencies.
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Reference

• Beranek, L. L, Noise and Vibration Control,
  Revised Edition, Institute of Noise Control
  Engineering, Washington, 1988.
• Bies, D.A. and Hansen, C.H., Engineering Noise
  Control 2nd Edition, E FN Spon, London, 1996.
• Norton, M. P., Fundamentals of noise and
  vibration analysis for engineers, Cambridge
  University Press, Cambridge, 1989.
• 01dB, Mediacoustic - Teaching Acoustics by
  Computer, 01dB, France 1996.
• Engineering Noise Control Theory and Practice,
  David A. Bies and Colin H. Hansen, 2000.