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Sound is, essentially, variations of air pressure caused by vibrations. Sound has both frequency and intensity. Frequency, or pitch, refers to the number of sound vibrations per second. This is expressed as cycles per second, or hertz. Sounds can be low in pitch, like a bass drum, or higher in pitch, like a bird singing. Most people hear sounds between 20 and 20,000 hertz.

 
Sound intensity refers to the loudness of a sound. It is expressed and measured in decibels. The smallest unit of sound intensity a person can hear is 1 dB. A person’s voice during a normal conversation is about 65 dB, while a loud shout can be 90 dB or more.
 

Decibels measure both sound intensity and hearing ability. As the decibel level rises, the sound intensity increases more rapidly than people can notice. For example, a 90 dB sound is 10 times louder than a 80 dB sound. A 100 dB sound is 100 times louder than a sound that is 80 dB.

 
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