Beneficial for engineering student

1. CE101
Environmental Science
Noise Pollution

2. Sound Characteristics and its Measurement
• Sound is produced in the environment by alternating pressure changes in the air,
caused by vibrations of solid objects or separation of fluids, as they pass over, around
or holes in solid objects
• Such vibrations cause the surrounding air to undergo compression, then rarefaction,
again compression, then rarefaction and so on
• Such phenomenon produces sound waves which propagate in the form of sinusoidal
path. The sine wave or sinusoid or sinusoidal signal is the most used graphic
representationof sound waves

3. Sine Waves
• Frequency, amplitude, wavelength
• Frequency - refers to the number of cycles of a wave per second. This is
measured in Hertz
• So if a sinusoid has a frequency of 100 hz then one period of that wave
repeats itself every 1/100th of a second. Humans can hear frequencies
between 20hz and 20,000 hz (20 Khz)

4. Amplitude
• Amplitude - variations in air pressure (measured in decibels)
• Amplitude is usually measured in decibels (abbreviated dB). People will hear amplitude
as loudness
• Frequency and amplitude are independent of each other. Two sine waves may have the
same frequency and different amplitudes, and vice versa

5. Wavelength & Period
• The wavelength of a wave is the physical distance between two
comparable points in neighboring cycles (e.g., the distance between to
pressure peaks or between two pressure troughs). For phoneticians, this
is the least interesting property
• Time between the successive peaks or troughs of oscillation is called the
period and its inverse represents the number of times a peak arrives in
one second is called frequency

6. Power of Sound
• Defined as rate of doing work by a travelling sound wave in the direction of
the propagation of the wave
• The energy transmitted by a sound wave in the direction of its propagation
is defined as its power, Watts (SI Unit)
• Sound intensity (I) – it is used to measure sound. Defined as the sound
power averaged over the time, per unit area normal to the direction of
propagation of the sound wave, intensity and power of sound wave are
related by the equation
• I = W/a
• I = Intensity of sound wave (W/m2)
• W = Power of sound wave in W
• a = a unit area

7. Levels of noise
• Sound heard by a normal healthy individual – 20 micro Pa
• Sound produced by a Saturn rocket at lift off stage 200 Pa
• This variation in sound pressure can be avoided by expressing sound pressure on a
scale based on log of the ratio of measure sound pressure and a reference standard
pressure, such measurement on this scale are referred as levels
• Sound level, L = Log10 (Q/Qo)
Where, Q = measured quantity of sound pressure or sound intensity
Qo = Reference standardquantity of sound pressure or sound intensity
L = sound level in Bels
• Unit of sound level is bels (B), since it is a large unit a smaller unit decibels (dB) is
generally used
• The decibel is defined as one tenth of a bel where one bel represents a
difference in level between two intensities I1, I0 where one is ten times
greater than the other

8. Levels of Noise
• For instance, the difference between intensities of 10-8 watts/m2 and 10-
4 watts/m2, an actual difference of 10,000 units, can be expressed as a
difference of 4 bels or 40 decibels
• Because of the very large range of sound intensity which the ear can
accommodate, from the loudest (1 watt/m2) to the quietest (10-
12 watts/m2), it is convenient to express these values as a function of
powers of 10. This entire range of intensities can be expressed on a scale
of 120 dB
• dB(A) - This is a measure of the overall noise level of sound across the
audible spectrum with a frequency weighting (i.e. `A' weighting) to
compensate for the varying sensitivity of the human ear to sound at
different frequencies

9. Levels of noise
• L (dB) = 10 * Log10 (Q/Qo)
• Reference standardin above equation is taken to be equal to 20 micro Pa
• Sound pressure level (Lp) in dB
• Lp = 10 * Log10 (Prms/20 µPa)2
• Since sound measuring instruments measure the rms pressure, the sound
pressure is computed by squaring their rms
• Similarly, the reference standard Qo is taken to be equal to 10-12 W/m2, when
sound intensity level is measured
• Sound intensity level (Li) in dB
• Li (dB) = 10*Log10 (I/ 10-12)

10. Noise Measurement
• Noise measuring instruments most widely used in the practice of occupational hygiene
• Many types of measuring systemscan be used for the measurement of sound
depending on the purpose of the study, the characteristics of sound and the extent of
information that is desired about the sound
• The various elements in a measuring system are:
• the transducer; that is, the microphone
• the electronic amplifier and calibrated attenuator for gain control
• the frequency weighting or analyzing possibilities
• the data storage facilities
• the display
• Not all elements are used in every measuring system. The microphone can, for
instance, be connected to a sound level meter or directly to a magnetic tape recorder
for data storage and future measurement or reference

11. The two main characteristics are:
1. The frequency response: that is, the deviation between the measured value and
the true value as a function of the frequency. As the ear is capable of hearing
sounds between 20 Hz and 20 kHz, the frequency response of the sound level
meter should be good, with variations smaller than 1 dB, over that range
2. The dynamic range: that is, the range in dB over which the measured value is
proportional to the true value, at a given frequency (usually 1000 Hz). This range
is limited at low levels by the electrical background noise of the instrumentand
at high levels by the signal distortion caused by overloading the microphone or
amplifiers

12. Microphone
• The microphone is the interface between the acoustic field and the measuring system.
It responds to sound pressure and transforms it into an electric signal which can be
interpreted by the measuring instrument (e.g. the sound level meter)
• Microphone characteristics - frequency response, dynamic range, directivity, stability
• The microphone can be of the following types: piezoelectric, condenser, electret or
dynamic
• In a piezoelectric microphone, the membrane is attached to a piezoelectric crystal which generates
an electric current when submitted to mechanical tension
• In a condenser microphone, the microphone membrane is built parallel to a fixed plate and forms
with it a condenser. A potential difference is applied between the two plates using a DC voltage
supply (the polarization voltage). The movements, which the sound waves provoke in the
membrane, give origin to variations in the electrical capacitance and results small electric current
• A variation on the condenser microphone which is currently very popular is the electret. In this case
the potential difference is provided by a permanent electrostatic charge on the condenser plates
and no external polarizing voltage

13. Sound Level Meters
• Sound Level Meters - Sound level meters provide instantaneous noise
measurements for screening purposes
• The specifications of sound level meters are given in IEC 60651 for four
types 0, 1, 2, 3 differing by the measurement precision
• The measurement precision is reduced as the type number increases,
affecting manufacturing costs significantly
• The IEC 60651 standard specifies the following characteristics:
• directional characteristics
• frequency weighting characteristics
• time weighting, detectorand indicator characteristics
• sensitivity to various environments

14. • The type 0 sound level meter is intended as a laboratory reference
standard
• Type 1 is intended especially for laboratory use, and for field use where
the acoustical environment has to be closely specified and controlled
• The type 2 sound level meter is suitable for general field applications
• Type 3 is intended primarily for field noise survey applications
• The frequency response for all types is defined from 10 Hz to 20000 Hz
with a higher accuracy at frequencies from 100 Hz to 8000 Hz

15. Noise Control
• In general, noise control techniques may be classified in three categories
1. Noise reduction at the source
2. Noise control of the transmission path
3. The use of noise protective measures at the receiver
• Which method or which combination of methods, is employed depends
on the amount of noise reduction that is required and on economic and
operational considerations

16. Noise Reduction at the Source
• The reduction of exciting forces e.g., reduction of impacts or impulsive
forces, balancing of moving masses, reduction of frictional forces by
proper alignment and lubrication etc.
• The reduction of the response of various components of the system to
these exciting forces, e.g., by application of vibration-damping materials
to the radiating forces
• Changes in operating procedures e.g., a factory, adjacent to the
residential areas, suspend or reduce noise generating operations at night

17. Noise Control of the Transmission Path
• Siting, e.g., by increasing distance between the source and the receiver
• Path deflection e.g., use of barrier
• Properly designed enclosures
• Absorption e.g., by providing sound absorbing material in a room where
both the source and the receiver are present, most of the reflected sound
can be avoided

18. Protective measures at the receiver
• Use of personal protective equipment e.g., earplugs, earmuffs, noise
helmets etc.
• Education and public relations
• Exposure control e.g., rotation of personnel so that work assignments in
the intense noise area are for a limited period of time only