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When object vibrate in air, they produce sound waves.  These waves of pressure travel through the air and hit our eardrums, causing them to vibrate in specific ways.  The result is something we sense as sound, as our brains recognize certain patterns and tell us what the sounds are.  In this way we can tell the difference between a barking dog and a car’s screeching tires.
The level of sounds is measured by the amount of power the sound waves carry as they push through the air.  Normally, this is measured in decibels.  Humans can hear very quiet sounds that produce the tiniest sound waves.  The level of 0 decibels (0 dB) is the quietest sound that any human can hear – this is called the threshold of hearing.

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Above that, every increase in 10 dB represents ten times more power in the sound waves.  For example, a whisper is about 20 dB.  A normal car engine makes a sound at about 70 dB.  That’s a 50 dB difference, but it’s 105 or 10x10x10x10x10 = 100,000 times more power!  At 110 dB, a loud rock band’s sound produces 90 dB more than a whisper, or 109 = 1 billion times more power.  The loudest sound that humans can listen to (but causing intense pain), 130 dB, is about 100 billion times more powerful than the threshold of hearing.  And a sound of 140 dB would instantly break your eardrums if you were exposed to it!

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