Sound is caused by waves of pressure that spread out though the air, making the air molecules squeeze together and then push apart. These waves are started by anything that is vibrating, from a car engines to the strings of violin. Unlike light, sounds always has to travel through something, so if you were floating in empty space, you would hear nothing but total silence.
How loud do sounds get?
Scientists normally measure the loudness of a sound by using units called decibels (dB). The softest sound that the human ear can hear has a decibel rating of about 4, while the loudest ones that it can cope with have a rating of over 150. The decibel scale works logarithmically. This means that if two sounds differ by 10 decibels, one of them is 10 times as loud as the other.
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