The speed of a certain compressional wave in air at standard temperature and pressure is `330 m//s` . A point source of frequency `300 H_(Z)` radiates energy uniformly in all directions at the rate of `5` Watt. (a) What is the intensity of the wave at a distance of `20 m` from the source? (b) What is the amplitude of the wave there? [ Density of air at `STP = 1.29 kg //m^(3)`]

Options

A. `1.15 xx10^(6) m`
B. `1.15 xx10^(-6) m`
C. `1.15 xx10^(-6)cm`
D. none

Answer

B

Explanation

(a) `I = (P)/(4 pi r^(2)) = (5)/(4 pi(20)^(2))`
`= 9.95 xx 10^(-4) W//m^(2)` (b) `I= (1)/(2) rho omega^(2) A^(2) nu`
`:. A = sqrt((2l)/(rho (2pi f^(2))nu))`
`= sqrt((2 xx 9.95 xx10^(-4))/(1.29 (2pi xx300)^(2)(300)))`
`= 1.15 xx10^(-6) m`