1)Soundisproducedwhen something vibrates. The vibrating body causes the medium (water, air, etc.) around it to vibrate. Vibrations in air are called traveling longitudinal waves, which we can hear.Soundwaves consist of areas of high and low pressure called compressions and rarefactions, respectively.

2)Audible sound:The human ear can easily detect frequencies between 20 Hz and 20 KHz. Hencesound waveswith frequency ranging from 20 Hz to 20 KHz is known are audible sound.

3)Sources of noise pollution:Noise can come from many places. Let us see a few good sources:

Household sources:Gadgets like food mixer, grinder, vacuum cleaner, washing machine and dryer, cooler, air conditioners, can be very noisy and injurious to health. Others include loud speakers of sound systems and TVs, ipods and ear phones. Another example may be your neighbor’s dog barking all night everyday at every shadow it sees, disturbing everyone else in the apartment.

Social events:Places of worship, discos and gigs, parties and other social events also create a lot of noise for the people living in that area. In many market areas, people sell with loud speakers, others shout out offers and try to get customers to buy their goods. It is important to note that whey these events are not often, they can be called'Nuisance'rather than noise pollution.

Commercial and industrial activities:Printing presses, manufacturing industries, construction sites, contribute to noise pollutions in large cities. In many industries, it is a requirement that people always wear earplugs to minimize their exposure to heavy noise. People who work with lawn mowers, tractors and noisy equipment are also required to wear noise-proof gadgets.

Transportation:Think of aero planes flying over houses close to busy airports like Heathrow (London) or Ohare (Chicago), over ground and underground trains, vehicles on road—these are constantly making a lot of noise and people always struggle to cope with them.