Use the relation between power, intensity and area to calculate the percentage of electric power converted by the speaker into sound power.
The power radiated from the source is spread evenly in all directions. The intensity is the ratio of power P radiated from the source to the area A.
Here, is power of the sound.
Since power radiated from the source is spread in circular wavelets so, the area of the circular opening is,
Substitute for in equation and rearrange equation for .
Here, is average sound intensity at the opening of the loud speaker and is radius of circular opening.