Question

A spherical source of power $4\, W$ and frequency $800\, Hz$ is emitting sound waves. The intensity of waves at a distance $200\, m$ is

• A. $8 \times 10^{-6} W / m ^{2}$
• B. $2 \times 10^{-4} W / m ^{2}$
• C. $1 \times 10^{-4} W / m ^{2}$
• D. $4\, W / m ^{2}$

Answer 1

Answer: (A)

$I=\frac{I}{A}=\frac{I}{4 \pi r^{2}}=\frac{4}{4 \pi(200)^{2}}$
$=\frac{1}{4 \times 10^{4} \times \pi}=0.079 \times 10^{-4}$
$=8 \times 10^{-6} w / m ^{2}$