A person speaking normally produces a sound intensity of `40 dB` at a distance of `1 m`. If the threshold intensity for reasonable audibility is `20 dB`, the maximum distance at which he can be heard cleary is.

Options

A. `10 m`
B. `5 m`
C. `4 m`
D. `20 m`

Answer

A

Explanation

`dB=10log_(10)[(I)/(I_0)]`
where `I_0=10^(-12)wm^-2`
Since `40=10log_(10)[(I_1)/(I_0)]implies(I_1)/(I_0)=10^4`
Also, `20log_(10)[(I_2)/(I_0)]implies(I_2)/(I_1)=10^2`
`implies(I_2)/(I_1)=10^-2=(r_1^2)/(r_2^2)`
`impliesr_2^2=100r_1^2impliesr_2=10m`