The lowest intensity of sound that can be perceived by the human ear is called threshold of hearing. For a sound of frequency 10 kHz, the threshold of hearing is, I0 = 10–12 W/m2. The ratio of intensities of the loudest to faintest sound is 1012. Humans can hear over a large range of intensities.
Instead of expressing the intensity I of a sound wave, it is more convenient to express of its sound level β, which is defined as
Here dB is the abbreviation for decibel, the unit of sound level. The unit bel was introduced in honour of Alexander Graham Bell.
(β2 – β1): Let β1 and β2 are the sound levels corresponding to sound intensities I1 and I2 respectively, then
