2. Official complaints about noise pollution in the UK (adapted from
Commission of the European Community, 1987)
3. Noise and Sound
What 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
4. Sound is defined as any pressure variation that the human year
can detect
Amplitude, Wavelength, Period, speed of sound
5.
6. Atmospheric Pressure = 1 Bar = 105 Pa
Atmospheric pressure / Sound pressure: Some physical
media allow the propagation of 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. Human ears can detect
pressure fluctuations as low as 20µPa = 2*10-5 Pa.
An annoying sound (e.g. a loud horn) is about 2 Pa. This is
still much smaller than 1 Bar.
7. Sound waves are extremely small pressure disturbances
superimposed to a much larger atmospheric pressure.
Schematic representation of sound superimposed on top of the
atmospheric pressure
Sound waves are characterised by their pressure
amplitudes and their frequencies
8. There is another important difference between atmospheric
pressure.
• 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
9. Attenuation.
Geometrical Decay:
If a source of sound emits the same pressure fluctuation in all directions
in free space (the so-called pulsating bubble), 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.
Absorption: sound wave would decay anyway due to the small but
finite viscosity of the air and the absorbing capacity of most surfaces
10. Sources of noise pollution
• Traffic – main source
• Industrial equipment
• construction activities
• Sporting and crowd activities
• Low-flying aircraft
11. Most sounds are not purely sinusoidal variations. They vary
both in frequency and Magnitude over time. To quantify
their magnitude over time, the root mean square (r.m.s)
pressure is defined
12. Sound Power and Intensity
Sound Power:
The rate at which energy is transmitted by sound waves is called
the sound power (W), measured in watts.
Intensity:
The average sound power per unit area normal to the direction of
propagation of a sound wave is termed in acoustic or sound
intensity (I).
(W)
13. At sufficient distance from the sound source the intensity of the
sound wave is proportional to the square of sound pressure
14. Intensity
As mentioned before, the sound pressures perceived by human
range from 20 µPa to 200 Pa.
Reference power level is 10-12 Watts
15. Since sound power is proportional to the square of sound
pressure, the sound pressure level in decibels is defined
as
Sound pressure levels less than 25 decibels are not normally
encountered except in broadcasting studies
16. Combining Sound Pressure Levels
Example 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 W
Example 2:
If two sound sources have equal pressures 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 W
17.
18. Frequencies
Sound of a single frequency, called pure tones, rarely exist.
Audible sound ranges from : 0.015 to 15 kHz
Frequency lesser that 0.015 kHz is called infrasonic frequencies
Frequency greater than 15kHz is called Ultrasonic frequencies
Human voice contains frequencies 0.08 to 8 kHz (mainly
concentrated in the band 0.5 to 2 kHz.
The sound of frequencies above 8kHz can usually be ignored in
environmental noise monitoring
19. Octave Band:
An octave band is the frequency interval between a given
frequency and twice that frequency. For example 0.05 –
0.1 kHz, 0.1 – 0.2 kHz, etc.
For noise analysis fixed octave bands are internationally
recognized. These are centred on 0.0315kHz
For noise control purpose it is often necessary to identify
the frequency components or spectra of sound.
20.
21. Classification of Sound
• Continuous
An uninterrupted sound level that varies less than 5dB during
the period of observation (ex. Household fan)
• Intermittent
It is a continuous sound that lasts for more than one second but
then is interrupted for more that one second (Ex. Dentist’s drill)
• Impulsive
Sound is for short duration, less than one second (ex.
Hammering sound, typewriter sound).
Change in pressure 40dB or more within 0.5 second with a
duration of less than one second
23. NOISE CRITERIA
The loudness of sound is determined by its sound pressure level
and its frequency.
The environmental or community noise can be appropriately
measured in dBA units, which closely replicate the loudness
perceived by the ear.
24. There is a wide differences in people's responses to noise. A number
of different criteria have been proposed, the most commonly used
criteria are
• LAeq: The equivalent continuous level
• LAE : The sound exposer level
• LAN : Sound Level Exceeded for N% of the time in dBA
25. The equivalent continuous level
(LAeq)
It is applied to a fluctuating noise source. It is the constant noise
level over a given time period that produces the same amount of A
weighted energy as the fluctuating level over the same time frame.
27. Example:
An air conditioner generates a noise level of 75 dB for five
minutes every hour. If the background noise level is
55dB, compute the LAeq
28. Sound Exposure Level
The sound exposure level (SEL) is the constant level in dBA
lasting for one second which has same amount of A
weighted energy as a transient noise.
It can be used to express the energy of isolated noise
events such as aircraft flyovers.
29. 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 and
are often used in base line studies taken prior to the introduction of
new industrial or highway noise sources.
Commonly used percentile levels include the LA10, the sound level
exceeded 10% of the time (some time used to present the maximum
noise level.
They may be used as a complement to LAeq
31. Noise Pollution Level (LNP)
LNP=Laeq+Kσ
Where K is a constant taken as 2.56 and σ is the standard
deviation
Traffic Noise Index (TNI)
TNI=4(LA10 – LA50)+LA90 - 30
32. Noise Standards
Noise standards or thresholds are commonly specified as
part of the planning permission (consent) for proposed
development.
The specified values vary with
• existing land use
• background noise level of the area
• type of development
Typical noise level at nearby residences are
LAEQ – 40 – 70 dBA by day
LAEQ – 35 – 60 dBA by Night
33. EU Noise Directive: protection of workers from
the risks related to exposure to noise at work
• Daily personal noise exposure
38. When the dimensions of the source is small relative to the distance
to the receiving point the sound spreads spherically (no solid
surfaces and fluid boundaries)
The acoustic intensity (I) at a distance r meters is given by
Sound pressure level in (dB)
39. Since the acoustic intensity is proportional to the square of the
pressure, then we can write sound pressure level (LP) as
40. Directivity
Most sound sources do not radiate uniformly in all
directions. This is because of directional characteristics of
the sound source (which may be frequency dependence)
or because of external constraints by near by surfaces.
Directives of the surface constraints may be estimated by
examining the position of the source.
1. Close to the ground
2. remote from the ground close to a wall
Sound pressure level (LP) at a distance r is given by
43. • Attenuation due to meteorological conditions
• ground surface effect
• Sound attenuation by trees
• ground topography
• Reflecting surfaces and noise barriers
47. Noise Section of an EIA
• Baseline Noise Survey
• Probable noise emission level
• Statement of probable impact
• Proposed remedial measures
48. Noise Control
Source: The source could be modified by the acoustic treatment to
machine surfaces, design changes, etc. This is a specialized area which is
outside the scope of this chapter. However, an offending noise source
could be stopped or its operation limited to certain times of the day.
Transmission path: it could be modified by containing the source inside
a sound insulating enclosure, by constructing a noise barrier or by the
provision of absorbing materials along the path.
Receiver: the protection of the receiver by altering the work scheduled
or by the provision of ear protection
49. Source: The source could be modified by the acoustic
treatment to machine surfaces, design changes, etc. This is a
specialized area which is outside the scope of this chapter.
However, an offending noise source could be stopped or its
operation limited to certain times of the day.
Transmission path: it could be modified by containing the
source inside a sound insulating enclosure, by constructing a
noise barrier or by the provision of absorbing materials along
the path.
Receiver: the protection of the receiver by altering the work
scheduled or by the provision of ear protection
