Sound is the form of energy that causes sensation of hearing. Vibration of an object is the necessary condition for the production of sound. In humans, sound is produced by the voice box or larynx. Sound needs a material medium for its propagation. Sound travel through solids, liquids and gases but it cannot travel through vacuum. Hearing is the process in which eardrum senses the vibrations of sound and sends the signals to the brain. Properties of sound enable us to distinguish between two sounds produced by different source. The two important properties of sound are amplitude and frequency. Amplitude is the maximum displacement of a body from its mean position. Amplitude decides the loudness of sound. Frequency is the number of vibrations produced by an object in one second. Frequency determines the shrillness or pitch of a sound. The frequency is expressed in hertz (Hz). The sounds that cause pleasant sensations on ears are known as music. The sounds that cause unpleasant sensation on our ears are known as noise. Noise pollution is the disturbance produced in the environment by undesirable, loud and harsh sound that comes from various sources.

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