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Sound can be defined as any pressure variation that the human ear can detect. The number of pressure variations per second is called the frequency of the sound, which was formerly measured in cycles per second but now is measured in hertz. Noise usually is sound that bears no information and whose intensity usually varies randomly in time. Noise is usually characterized as sound that is unwanted by the listener because it is unpleasant. Even thought the weakest sound pressure perceived as sound is a very small quantity, the range of sound pressure perceived as sound is extremely large. The weakest sound that can be heard by a person with very good hearing in an extremely quiet location is known as the threshold of hearing. For example the threshold of good hearing for an adult is right around 10 decibels. And the threshold of pain, the level of loudness at which a person experiences pain, in an adult is around 140 decibels. The loudness in a typical factory is approximately 90 decibels. If the ear is exposed to high levels of noise for a sufficient period of time, some loss of hearing may occur. There are many factors that affect the degree and extent of hearing loss: · The intensity of the noise · The type of noise