Loudness" is measured in decibels. The formula for the loudness of a sound is given by "dB = 10log[ I ÷ I0 ]" where I0 is the intensity of "threshold sound", or sound that can barely be perceived. Other sounds are defined in terms of how many times more intense they are than threshold sound. For instance, a cat's purr is about 316 times as intense as threshold sound, for a decibel rating of: