Unit of Intensity:
According to definition intensity the rate of flow of energy through unit area. So, its S.I. unit is Joule s-1m-2 or, Wm-2 (Watt per square meter). But, since this unit is very large, so microwatt par square metre i.e., μWm-2 unit is used.
We can hear a wide range of sound by our ears. Intensity of minimum sound of 100 Hz frequency that we can hear is about 10-12 Wm-2. It is called the threshold of hearing or audibility. The maximum loud sound that our ears can withstand comfortably is about 1 Wm-2. Above this limit of intensity, ears feel pain. The upper limit of this sound is called threshold of pain or feeling. The range of intensity of hearing of sound is so large that in order to compare the intensities of different sounds we need to use logarithm of intensity. Intensity I is a physical quantity, hence it has unit, so we cannot take its logarithm. But ratio of two intensities I/I0 is dimensionless quantity, hence we can take its logarithm.