1. Air and Noise Pollution Studies (CSE331)
Dr. Tao Wang
CSE
Office: TU702
Phone:
FAX:
Walk-in Consultation Hours
Thursday and Friday : 5 - 6pm

2. Contents to be covered Basics on noise Assessment and control
Road traffic noise
Railway noise
Construction noise
Industrial noise

3. 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

4. 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.

5. Sound power: energy per unit time (watts);
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
SWL=10 log 10 We/W0 dB
Where We is measured sound energy; W0 is reference power, w.
Sound Intensity (LI): energy per unit time per unit area
SIL=10 log 10 Ie/I0 dB

6. 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
In general, people judge that a sound which has increased in level by 10 dB is roughly doubled in ‘subjective loudness’

7. Description of sound source
Sound Pressure Level
dB re 20µPa
Description of sound source
Typical subjective description
140
Moon launch at 100m; artillery fire, gunner’s position
Intolerable
120
Ship’s engine room; rock concert, in front and close to speakers
100
Textile mill; press room with presses running; punch press and wood planers, at operator’s 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
Threshold of hearing for normal young people

8. Lw = 10 log W dB Wre = 0.1 10-12 110 dB 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
Wre =
0.1
10-12
110 dB

9. Example 2
Determine the sound power of a machine whose specified sound power level is 125 dB
Solution:
W = Wref x 10Lw/10 W
= x 10125/10 W
= 3.2W

10. Lp = 10 log P2 dB Pref2 2.52 (20 x 10-6)2 101.9 Example 3
Calculate the sound pressure level for a sound with an rms acoustic pressure of 2.5 Pa
Solution:
Lp =
10 log P2 dB
Pref2
2.52
(20 x 10-6)2
101.9

11. Relationship of SIL and SPL
Sound intensity = (pressure2) x constant
Sound intensity level = Sound pressure level

12. 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.

13. Sound Propagation For a point source (Inverse Square law):
Intensity= w/4R2 (R is distance from the source)
SPL=SWL – 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)
SPL=SWL – 10 logR – 10log L – c’ + 10logQ
That is, the sound pressure level decrease by 3 dB when distance from the source is doubled.

14. 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

15. 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)

16. Human hearing range: 50-20000 hz
Best hearing range: hz

17. Sound Measurement
Sound pressure level meter
It measures sound pressure by converting it to electronic signals
-Microphone
-Attenuator
-Amplifier
-Indicating meter

18. Weighting Network or Scales
Different scales have different discriminations on lower frequency waves
-A scale: < 600 Hz
-B scale: < more moderate
-C scale: little discrimination
20 dB(A), 20 dB(B), 20 dB(C) etc.
It is common to use A scale.

19. 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

20. 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
500
1000
2000
4000
8000

21. 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.

23. 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.

24. 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

25. Example 4
Determine the total sound pressure level due to L p1 = 90 dB, L p2 = 95 dB and L p3 = 88 dB
Solution:
Lp = 10log(1090/ / /10) dB
= 10log( ) dB
= 96.8 dB

26. 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) dB
= 10log( ) dB
= dB

27. 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

28. Example 6
Determine the total sound pressure level due to L p1 = 90 dB, L p2 = 95 dB and L p3 = 88 dB by the Chart for Adding Decibels.
Solution:
88 dB
90 dB
95 dB
92.2 dB
96.9 dB

29. 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/ Lpb/10) dB
In this case, Lpt = 94 dB and Lpb = 85 dB
Lp = 10 log (1094/ /10) dB
Lp = 93.4 dB

30. Chart for Subtracting Decibels

31. Lp = Lpt – Lpb = 94 - 85 = 9 dB Lp = 94 – 0.6 dB Lp = 93.4 dB
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 = = 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

32. 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.

33. 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 s has a frequency of 500 Hz.

34. 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 (λ ) by:
c = f λ

35. 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.

36. 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.

37. 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.

38. 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

39. 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.

40. 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 S

41. 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 ρc
I = p2
= p2
(1.2)(344)

42. Decibel Scale (dB) Lq = 10 log q dB qref
The human ear responds to sounds over a very large range of sound intensities from (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
qref

43. 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
= watts/m2 (the threshold of hearing)
LI = 10 log I dB
Iref

44. 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
= watts
Lw = 10 log W dB
Wref

45. Sound Pressure Level (Lp)
Sound pressure level Lp = 10 log p2 dB
pref2
=20 log p
pref
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

46. Intensity Level, Sound Power Level & Sound Pressure Level
For a point source in a free field
I = W
was the relation ship of intensity, I, to power, W
4πr2
Changing to levels
10 log I
=10 log
-10 log(4πr2)
Iref
Wref
LI = LW – 10 log (r2) – 10 log (4 π)
= LW – 20 log r - 11

47. 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
2πrL

48. 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

49. 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

50. Frequency weighting networks
-A scale: < 600 Hz
-C scale: little discrimination

51. Exercise What is the wavelength in air at 21 蚓 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 Hz)?
(Tip: use equation 1.4A above).
If the temperature increases to 40 蚓, what is the frequency of a sound with a wavelength of 5m in air?
(Tip: use equation 1.5 first, then equation 1.4B).

52. 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.

53. Exercise
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?

54. Exercise Complete the following table
For combining two decibel levels of sounds
Difference between levels dB
Amount to be added to higher level dB 3 1
2.5 2 4 5 6 7 8 9 10

55. Exercise
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
i. What is the A-weighted level? (99 dB(A))
ii. What is the C-weighted level? (98 dB(C))
iii. Which is the higher? (The A-weighted level)

57. 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.

58. 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.)

59. Octave Bands Limits Centre frequency, Hz Limits of band, Hz 31.5 63
125
250
500
1000
2000
4000
8000
16000
Note that the centre frequency is the geometric mean and not the average of the band limit frequencies.

60. 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.