Sound is often measured in decibels and the sound level is given by the formula ? ? = 10 log ( ? ? 0 ) dB=10log( I 0 ? I ? ) where ? 0 I 0 ? is the threshold of human hearing and is equal to 1 0 ? 12 10 ?12 ? ? ? ? ? ( ? ) ? ? ? ? ? ? ( ? ) 2 meters(m) 2 Watts(W) ? . and ? I is in ? ? 2 m 2 W ? . Intensity, in decibels, ? ? dB of a spherical sound wave is dependent on the distance from the source of the sound and is given by the formula ? = ? 4 ? ? 2 I= 4\pi r 2 P ? in ? ? 2 m 2 W ? where ? P is the output power, measured in Watts ( ? W) and ? r is the distance, in meters( ? m), from the center of the sphere.