Intensity of sound:
The intensity of sound at a point is defined as the amount of sound energy crossing the point per unit area per second. Then the unit of intensity of the sound is given by j/m2s or wm-2.
Threshold of hearing:
The threshold of hearing is the lowest intensity of the sound that can be detected by our ear within the range of audibility. The sensitivity of ears caries with the frequency i.e. the sensitivity of ears is different range of frequencies. Also the threshold of hearing at a frequency may very form ear to ear. Hence, the threshold of hearing has been defined for a normal ear at a frequency of 1000 Hz. The threshold of hearing is taken as 10-12 watt m-2 for frequency of 1000Hz.
Relation between I and L
Since, we know,