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

C

Explanation

`L_(1) - L_(2) = 10 log_(10) ((I_(1))/(I_(2))) =10 log_(10) ((r_(2)^(2))/(r_(1)^(2)))`
`40 - 20 = 10 log_(10) ((r_(2))/(r_(1)^(2)))`
`20 = 20 log_(10) ((r_(2))/(r_(1)))`
`log_(10)((r_(2))/(r_(1))) = 1 rArr (r_(2))/(r_(1)) = 10`
` r_(2) = 10 r_(1) = 10 m`