Noise Pollution

Noise is defined as unwanted sound.

Characteristics of sound and its measurement :

Sound is produced by alternating pressure changes in the air, caused by various reasons. Sound waves propagate in the form of sinusoidal path as shown in the fig.

The amplitude of the sound wave is measured in units of pressure such is N/m2.

The equivalent pressure of the above sine wave is represented by root mean square pressure,Prms

When the sound waves are periodic, regular and of long duration, they produce a pleasing effect and such a sound is known as musical sound. When  the sound waves are non - periodic, irregular and of short duration, they produce a displeasing effect and such a sound is known as noise

The noises that pose the greatest threat to the human body are those with high pitch high amplitude and longest duration.

MEASUREMENT OF SOUND: The sound is measured in units called decibels(db). One db unit is approximately the smallest change of sound intensity which the human ear is able to appreciate.

The pressure waves which our ears detect, and thus classified as audible sound, range from 0.00002N/m2 to 200N/m2. The sound becomes louder as the pressure increases. The pressure at which the sound first becomes audible is known as threshold of hearing or beginning of feeling

of sound, which for a normal healthy ear is about 0.00002 N/m2(20 u - Pa).

The ratio of two pressures is known as sound pressure Ievel (SPL) which is defined as

SPL =20 log 10 (P/Pref )

Where, SPL = sound pressure level in decibel(dB);

P = pressure of sound wave in N/m2

Pref = reference pressure in N/m2 (0.00002 N/m2) 

The threshold of hearing, 0.00002 N/m2  is usually taken as the standard reference pressure.

SOME TYPICAL SOUND LEVELS:

SNo.	Source of location	Sound level in db
1	City  traffic in busy street	70
2	Loud noise accompanying lighting	120
3	Aero plane noise at a distance of 3 meters (painful)	130

The combined effect of different sound , pressures lasting for different period is worked out by using some statistical measures such as LN and Leq systems.

The LN concept: The parameter LN represents the sound pressure level (or noise level) that

will be equaled or exceeded for N% of the measuring time. e.g  if a sound pressure level (or noise level) of 70 dB is equaled or exceeded for 60% of the measuring time, the parameter L, is represented as L60 and its value is 70 dB. i.e L60 = 70 dB.

The Leq concept : the parameter Leq represents the sound pressure level (or noise level) which is equivalent to a number of different sound pressure levels produced at a place for different time intervals.Thus L is defined as that uniform value of sound pressure level which over a given time,

will produce the same effect as is produced by the fluctuating sound pressure levels over the same time.

Where

Leq = Equivalent sound pressure level in dB

n = total number of sound pressure levels recorded

Li = values of sound pressure levels recorded in dB with 1,2,3....n

ti = time duration of the different sound pressure

levels expressed as a fraction of the total measuring or recording time.

EFFECTS OF NOISE:

• Noise has an influence on blood pressure, muscular strain.
• It leads to fatigue and consequently the efficiency of persons decreases considerably.
• Prolonged exposure to the noise may result into temporary deafness or nervous breakdowns.
• Cardiac disturbances in human beings.

 EFFECT OF NOISE IN HUMAN BEINGS:

No	Noise  level in dB	Effect observed
1	0	Threshold of audibility
2	120	Pain threshold
3	190	Major Permanent damage, if prolonged

Acceptable Noise Levels  for different Structures:

No	Type of building	Acceptable noise level In dB
1	Auditoriums and music rooms	35 to 40
2	Small offices, court rooms, libraries	40 to 45
3	Schools	45 to 50
4	Large public offices, banks and stores	50 to 60
5	Factories	60 to 65

Depending upon the duration, the noises can also be grouped in the following three types:

• Continuous noise: It is an uninterrupted sound level that varies less than 5 db during the entire period of observation. A running fan is an example of such a noise.
• Intermittent noise: It is a noise which continues for more than one second and is then interrupted for more than one second. A drilling machine used by a dentist produces such type of noise.
• Impulse noise: It characterized by a change of sound pressure of at least 40 db within 0.50 second with a duration of less than one second. Noise produced from firing of a w capon would indicate such type of noise.

Following measures may be taken to have an effective control on the noise pollution:

• Design of doors and windows: The excellent sound insulation is obtained by constructing glazed windows with double or triple panes of glass
• Enclosures: Provide enclosures, shields and barriers so that some of the sound waves arty cut off from propagating.
• Planting of trees: The presence of trees act as noise barriers