Units of sound are measurements used to quantify the intensity of a sound. The two most common units of sound are bels and sones. Bel is a unit of measurement for sound intensity, named after Alexander Graham Bell. It is equal to 10 decibels and is used to measure the loudness of a sound. Sone is a unit of measurement for perceived loudness, which is based on subjective human perception. It is equal to 40 decibels and is used to measure the loudness of a sound as perceived by humans.