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

Answer

B

Explanation

`40=10log(10)((I_1)/(I_0))`
`(I_1)/(I_0)=10^4`
Also, `20=10log_(10)((I_2)/(I_0))`
`implies(I_2)/(I_0)=10^2` .(ii)
`(I_2)/(I_1)=10^-2=(r_1^2)/(r_2^2)`
`r_2=100r_1^2impliesr_2=10m`