noise pollution and its control

NOISE POLLUTION

INTRODUCTION

• Modern life has given rise to new form of pollution called noise pollution.

• Sound is pleasant or not depends upon its loudness , duration, rhythm and the mood of person.

• They are continuously disturbing human peace and tranquility

• Noise has become an important environmental pollutant and is a serious threat to the quality of our atmosphere that befoul air, water and soil.

Noise pollution

• Noise is a unwanted, unpleasant or disagreeable sound that causes discomfort.

• Noise pollution :-

Any unwanted sound that penetrates the environment is noise

pollution. In general noise pollution refers to any noise irritating one’s ear which come from an external source.

Sources of noise pollution

Types of noise :-

Noise can be studied under two heading :-

(1) Environmental noise- this type of noise is introduced into communication system and mainly includes cosmic noise and atmospheric absorption noise.

(2) System internal noise :-this type of noise is produced within the system.

Effects of noise pollution

Some of the important effects are:-• Affect human health, comfort and efficiency.• Impairment of hearing.• Psychological and pathological disorder.• Headache, nausea and fatigue.• Boiler-makers deafness syndrome.• Acoustic trauma.• Hyperglycaemia , hypokalaemia and hypoglycaemia.• Sonic boom.• Physical damage by cracks and broken windows.

Preventive and control of noise pollution

Noise pollution can be minimized by :-

(1) Reduction of noise at source.

(2) Reduction of population exposed.

(3) Duration of noise exposure.

Noise:-• Air-borne noise -less power, long duration and confined near

the origin.• Structure-borne noise-more power, short duration and

propagate over long distance.

Following measures may be taken for an effective control on noise pollution:-

• Using ear protective aids.• Proper designing of doors and windows.• Improving in working methods.• Tree planting.• Providing enclosure.• Use of silencers.• Town planning.• Treatment of walls, floors and ceilings.• Through law.

Noise pollution control in India

• Central pollution control board (CPCB).• Government of India have prescribed noise limit for

automobiles, domestic appliances and construction equipment. • Under the AIR ACT and the ENVIRONMENT ACT,1986.• Ban on the use of horn/siren and loudspeaker in the vicinity of

silence zone.

NOISE REDUCTION AND CONTROL

Basic Concept

• Fundamental elements in Noise Control

• Noise Can be Controlled by :– Passive Means– Planning and Forethought

SOURCE RECEIVERPATHS

General Noise Sources

• Energy contained in a Sound wave• Outdoor Sources • Indoor Sources

Noise Control at Source

• Proper Design• Proper Equipment Operation• Equipment Maintenance

Methods to control noise are:• Containment• Sound insulation and sound absorption

Proper Design

Good design may result from:• Using Shock Absorbing Techniques• Using Efficient Flow Techniques• Reducing Fluid Jet Velocities• Reducing Sound Radiating areas• Reducing Peak acceleration

Proper Equipment Operation

Relates to :• Sealed Acoustic guards • Work according to design• Proper installations• Proper Settings• Good support• Additional sound control devices

The potential maintenance activities are :• Maintain good dynamic balance• Consider noise specification while buying new

components• Use gradual transitions• Improve Lubrication• Correct Installation• Reduce mechanical run out

Noise Reduction Along Sound Paths

Common paths for noise are Airborne paths like Ducts and corridors.

Sound paths may be directly excited or indirectly excited through a sound field.

Reduced by considering the transmission of sound along ducts.

Usually by understanding the “reasonable noise requirement”

Noise Control Requirement Areas

• Noise from Construction and Civil engineering work• Fan noise• Noise from Industry• Road Traffic Noise• Aircraft and Airport noise

Legal Aspects of Noise Pollution

Various laws regarding control of noise pollution in India are:

• Constitution of India • Cr.P.C. Section 133• I.P.C. Public Nuisance 268-295

MACHINERY NOISE

INTRODUCTION

Mechanized industry creates serious noise problems, subjecting a significant fraction of the working population to potentially harmful sound pressure levels of noise.

The noise is due to machinery of all kinds and often increases with the power of the machines.

In industrialized countries it has been estimated that 15-20 % or more of the working population is affected by sound pressure levels of 75-85 dB.

The noise usually stems from a wide variety of sources or machines used in the industries.

SYMPTOMS OF MACHINERY NOISE

If the employees have to raise their voices to carry out a normal conversation when about 2 m apart .

If the employees use noisy powered tools or machinery for more than half an hour each day.

If any one works in a noisy industry→engineering, textile, forging, pressing, bottling and foundries.

If there noises due to impacts (such as hammering, forging, pneumatic impact tools etc), explosive sources such as detonators or guns.

SOURCE

A considerable number of industrial machines and processes generate high levels noise that can cause physical and psychological stresses.

Industrial machines and processes consists of various elements such as: rotors, stators, gears, bearings, fans, vibrating panels etc.

The source of noise may be: – Air bome noise (propagation through air).– Structure bome noise (propagation through solids) .– Reflection from floor, wall, ceilings and machine surfaces.

High noise producing machines consists of :

Fans and Blowers Generators Compressors and Pumps Die casting machines Lathes Tumblers and Grinders Punch presses Power saws Stamping machines Conveyers

IMPACT OF MACHINERY NOISE

Cost of noise hazard to workers: Hearing loss: – Temporary– Permanent

Tinnitus Stress, annoyance and fatigue Communication problems Task distraction Clinical health effects

Cost of noise hazard to the organization:

Compensation claims

Decrease in productivity

Hearing protectors

Medical surveillance

Risk of accidents

MACHINE NOISE EXPOSURE LIMITS

Noise exposure limits depends upon: The levels of exposure to noise of the employees averaged

over a working day or week. The maximum noise (peak sound pressure) to which

employees are exposed in a working day.

Noise exposure limits are:– Daily or weekly exposure of 80 dB– Peak sound pressure of 135 dB

CONTROL OF MACHINERY NOISE

Noise control in Industry

Need for noise control

• Noise produced by industry causes annoyances to neighbors, particularly in mixed residential/industrial areas and It causes actual damage to hearing of workers due to over-exposure at the workplace.

• A noise control program should provide an effective yet economical technique.

Types of Noise

Noise can be divided into two types:• Continous• Variable or intermittent

Noise control criteria

Two types of criteria, are generally followed for industrial noise control.

– Hearing conservation

– Speech communication

Personal protective equipment

• Earplugs• Earmuffs or Helmets

• Control at Source • Control of transmission path • Control at the receiver

Noise Control by Planning

Noise control by planning can be divided into three sections:

1. Selection of quiet processes2. Layout3. Making provision in advance for noise control procedures

Noise control by Maintenance

• Correct assembly of machine parts or replacements.• Replacement of worn parts.• Tightening of loose parts.• Regular lubrication to reduce friction.

Noise control by modification

Some noise sources can be made quieter by altering the equipment operation.

• Reduction of equipment operation speed.• Reduction of peak forces (impact) by extending the

force application time.• Reduction of flow velocities of gases and liquids.• Reduction of impact noise by the use of softer or

more resilient materials.

Noise control by enclosing the noise source

Engineering acoustic enclosure design consists of structurally controlling all noise paths at the source.

• Wise selection of materials.• Good enclosure design.• Continuous supervision of enclosure.

DAMPING

• Typical applications: Chutes, hoppers, machine guards, panels, conveyors, tanks etc.

• Techniques:1. Unconstrained layer damping2. Constrained layer damping• Limitation:The efficiency falls off for thicker sheets

Vibration isolation

Vibration isolators prevent noise from being transmitted through the base of the equipment, and through conduit, piping, ductwork and other devices which may be rigidly connected to the noise source.

Acoustic design criteria

• Acoustic design goal.• Attenuation characteristics.• Use of sound absorptive materials.

• Location of noise source relative to listener.• Flexible noise control materials.• Isolating the employee from the source.

Administrative controls

These are the measures that management can institute to reduce occupational noise exposure by changing work procedure, policy and routine.

• Duration of exposure• Employing the deaf• Purchase specifications• Developing the quieter products

PROPERTIES OF SOUND

SOUND

Sound, and therefore all noise, is physically a rapid alteration of air pressure above and below atmospheric pressure.

All sounds travel as sound pressure waves from a vibrating body such as a human larynx, radio, TV, record player speaker, or vibrating machine.

Sound is transfer of energy without transfer of mass. Sound is a form of energy which is emitted by a vibrating

body and on reaching the ear causes the sensation of hearing through nerves.

ELEMENTS OF A SOUND WAVE

Frequency The frequency of a sound indicates the number of cycles

performed in 1 s:

f =1/T Hz

where T is the period of one full cycle. The unit for frequency is the hertz (Hz):

1 Hz = 1 cycle/sec 1000 Hz = 1 kilohertz = 1 kHz

Wavelength The wavelength λ is equal to the distance the oscillations

have propagated in the time period T:

λ = cT = c/f

Speed In a free field, sound propagates with the velocity c defined

by:

c = 20.05√T (m/s)

where T is the temperature in Kelvin. A simpler formula for the velocity of sound in air

sufficiently accurate at normal temperatures, 0–30ºC, is:

c = 331+0.6 T˛ (m/s)

where T˛ is the temperature in centigrade.

Pure Tone A sound that contains only one frequency is a pure tone.

Pitch Pitch represents the perceived fundamental frequency of a

sound. Timbre

Timbre is the quality of a sound or tone that distinguishes different types of sound production.

Loudness Loudness is the quality of a sound that is the primary

psychological correlate of physical strength. Sound Pressure

It is the pressure generated by a sound wave.

SOUND PRESSURE

Root Mean Square Sound Pressure (rms) The rms value is obtained by squaring the sound pressures

at any instant of time and then integrating over the sample time and averaging the results. The rms value is then the square root of this time average.

Physically, the rms value is indicative of the energy density of the disturbance.

Sound Pressure Level The sound pressures that are normally measured with a

sound-level meter cover an extremely large range. In order to handle in a simple manner such a large

measurement range, a logarithmic measurement scale is used.

The sound pressure level then is a logarithmic ratio L defined as:

where is sound pressure in interest (pa), and is reference sound pressure usually chosen as the limit of hearing of 20 µPa. The unit for SPL is Decibel (dB).

Decibel The human ear is capable of identifying a wide range of

sounds. If we used the microbar to describe sounds, the numbers

would be very cumbersome. A sound that is perceived to be twice as loud as another is

said to be one Bel greater in sound level. A Bell is a unit of comparison. One decibel (1 dB) is one-tenth of a Bel.

ADDITIONS OF SOUND LEVELS

To add sound-level values, it is first necessary to convert each decibel reading to sound intensity.

All sound intensities are added and then the sum is converted to a resultant decibel reading. For example,

Table 1. Approximate increase when adding two sound levels.

SOUND POWER LEVEL & LOUDNESS

Like SPL, Sound Power Level also exists over a wide range. It is expressed using a logarithmic scale which is based on an

internationally selected sound power as a reference:

Lw = 10 log(W/W0) dB

where W0 is 10-12 Watts.

Loudness Loudness is the human impression of the strength of a

sound. The unit of Loudness is phons. It is equal to the sound

pressure level (in dB) reference to 20 μPa of an equally loud 1000-Hz tone.

MECHANISM OF HEARING

Figure 3. Cut-away drawing of the human ear

The human ear is an incredible instrument. Yet only recently have we become aware of the devastating

psychological effects of noise Sound pressure waves caused by vibrations set the eardrum

(tympanic membrane) in motion. This activates the three bones in middle ear, which physically

amplify the motion received by the ear drum and transfer it to the inner ear.

This fluid-filled cavity contains the cochlea, in which are present hair cells which deflect and send bioelectric signals to brain.

HEALTH EFFECTS OF NOISE

Temporary Hearing Loss – Temporary Threshold Shift. Permanent Hearing Loss – Permanent Threshold Shift. Presbycusis. Environmental Noise. Effect on other body parts.

Cardiovascular. Eyes. Headache.

Noise Measurement Instruments

Microphone

• A microphones convert a sound wave into an electrical signal in which the voltage and current are proportional to the original sound.

• Most microphones today use electromagnetic induction , capacitance change, piezoelectric generation to produce the signal from mechanical vibration.

• To perform this task microphones use a thin membrane, known as a diaphragm, which mimics the function of the human ear.

Cross Section of a Microphone Diaphragm

Types of Microphones on basis of internal configuration

Dynamic Microphones

Ribbon Microphones

Condenser Microphones

Types of Microphones - Microphone pick up patterns

Unidirectional

Bidirectional

Omnidirectional

Sound Level Meters

Noise Dosimeter and Sound Analyzer

• The noise dosimeter will measure the amount of potentially injurious noise to which an individual is exposed over a period of time.

• The noise dosimeter does not, however,identify noise sources.

• A frequency analyzer may be necessary to measure complex sound and sound pressure according to frequency distribution.

• It will supplement readings obtained with a sound-level meter. Noise analyzers cover different frequency bands.