The intensity of Sound Wave
How strong the sound is designated by its loudness. Loudness of a sound is expressed by the intensity of sound. As the intensity increases i.e., as strongly as the sound wave transfers energy to our ears, the intensity of sound becomes more and more. Again, if the intensity decreases, loudness also decreases. So, intensity is the cause and loudness is its result.
This definition of loudness depends on individual. If the difference of frequency is more, sound of different frequencies but of the same intensity may appear to a listener more or less loud. So, loudness of a sound depends on frequency also. Clearly, loudness and intensity are not exactly same. Intensity of a sound is a physical property, but loudness is a feeling. Intensity is the characteristic of any sound. Noise has also intensity.
Intensity of sound depends on amplitude. Amplitude of tolerable sound in our ears is 10-5 m. On the other hand intensity of a feeble sound is about 10-11 m. A large range of intensity of sound is sensitive for human ears.
Thus, sound intensity is the power per square meter. The common unit of power is the watt (1 W = 1 Joules/s). The unit of SL is called the decibel (abbreviated dB). I is the intensity of the sound expressed in watts per meter and Io is the reference intensity defined to be 10-12 W/m2.
Sound intensity is defined as the amount of energy that passes through a unit area placed perpendicular to the propagation of sound.