Environmental Engineering

Lecture # 6

Chemical Engineering Department UET Lahore

Synopsis

Noise Pollution Noise & Our Hearing Level of Noise Measurement of Noise Classification of Noise Sources of noise Impacts of noise Control of noise pollution Conclusion

What is Noise?

Noise is unwanted sound.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. Or we can say when the effects of sound are undesirable it termed as NOISE

Noise Pollution and Our Hearing

The inner ear of humans contains a snail-shaped structure called a cochlea that is lined with thousands of microscopic hairs. When sound vibrations enter the cochlea, they cause the tiny hairs to move back and forth. If strong vibrations blast into the cochlea, the hairs can be flattened and damaged.

Sound is the quickly varying pressure wave travelling through a medium. When sound travels through air, the atmospheric pressure varies periodically. The number of pressure variations per second is called the frequency of sound.

The response of the human ear to sound is dependent on the frequency of the sound. The human ear has peak response around 2,500 to 3,000 Hz and has a relatively low response at low frequencies.

To express sound or noise in terms of Pa is quite inconvenient because we have to deal with numbers from as small as 20 to as big as 2,000,000,000. A simpler way is to use a logarithmic scale. As such, the loudness of sound is commonly expressed in decibel (dB).

Levels of Noise in the Environment

Sound Levels and Human ResponseCommon sounds Noise Level [dB] Effect

Rocket launching pad, (no ear protection) 180 Irreversible hearing lossCarrier deck jet operation, Air raid siren 140 Painfully loud

Thunderclap 130 Jet takeoff (200 ft), Auto horn (3 ft) 120 Maximum vocal effort

Pile driver, Rock concert 110 Extremely loudGarbage truck, Firecrackers 100 Very loud

Heavy truck (50 ft), City traffic 90 Very annoyingHearing damage (8 Hrs)

Alarm clock (2 ft), Hair dryer 80 AnnoyingNoisy restaurant, Freeway traffic, Business

office 70 Telephone use difficultAir conditioning unit

Conversational speech 60 IntrusiveLight auto traffic (100 ft) 50 Quiet

Living room, Bedroom, Quiet office 40 Library, Soft whisper (15 ft) 30 Very quiet

Broadcasting studio 20 10 Just audible 0 Hearing begins

Measurement of Noise

Sound Power

Sound Intensity

Sound Pressure

IrW 24

c

pI

2

010log20p

pLp

Example

If a sound source has a pressure of 2000µPa at 10m distance, compute:

a) The sound pressure level in dB,

b) The sound intensity in W/m2,c) The sound power in W.

Answers

a) Lp = 40dBb) I = 9.9x10-9 W/m2

c) W = 12.5x10-6 W

Classification of Noise

1) Community Noise/Environmental Noise

2) Occupational Noisea) Continuous Noiseb) Intermittent Noisec) Impulsive Noise

Community Noise

It is also known as, Environmental Noise Domestic Noise Residential Noise

Sources: Road, Rail, Air traffic Construction and Public works Restaurant, cafeteria etc. Sporting events like motor

sports Play grounds Parks

Occupational Noise

Sources: Industrial Machinery

includes, Rotors, gears, electrical

machines. Combustion engines Drilling, crushing Pneumatic equipment

Occupational Noise

Continuous Noise

Impulsive Noise

Intermittent Noise

Continuous Noise

It is produced by the machinery that operates without interruption e.g.

Pumps, Compressors and Processing equipments

Intermittent Noise

Impulsive Noise