Loudness is a measure of how we perceive a sound. Equally intense sounds of different frequencies may not sound equally loud. The reason is that the human ear has a low sensitivity to sounds near the upper and lower ends of the audible-frequency range.
People have been trying to reduce noise for centuries. In ancient Rome, there were laws restricting people from driving chariots on the stone streets at night because of the racket they created. Cars have been equipped with mufflers to stifle engine noise since the early 1900's. Many building features, such as carpeting and drapes, serve not only as decorations but also to absorb sound. People in schools and offices often use other materials to stifle sound. In some classrooms, for example, tennis balls cut in half and affixed to the bottoms of chair legs are used to eliminate the screeching of chairs against tile floors.
Some types of sounds are more difficult to block than others. For example, it is easier to block high-pitched noise than low-frequency rumbles. That is because the sound waves of low-frequency sounds are farther apart than those of high-frequency sounds, and they displace larger amounts of air. These larger amounts of air being pushed toward a sound-deadening structure make it difficult for the structure to absorb all of the sound. As a result, some of the sound waves pass through. About five times as much thickness of material is needed to block a sound with a frequency of 200 hertz, with a wavelength of about 1.7 meters (5.5 feet)—roughly the pitch of a church bell—than is needed to block the shrill 1,000-hertz sound of a police whistle, which has a wavelength of about 0.3 meters (1 foot).
There are two approaches to stopping noise: passive and active. In passive noise control, sound-absorbing materials and specially designed structures are used to muffle or block sound waves and structural vibrations. Active noise cancellation involves the generation of precisely tuned sound waves designed to cancel or greatly diminish bothersome sound waves.
(Source: How stuff works?)
