Question

The intensity of sound after passing through a slab decreases by $20\text{\%}$. On passing through two such slabs, the intensity will decrease by

• A. $30\text{\%}$
• B. $36\text{\%}$
• C. $40\text{\%}$
• D. $50\text{\%}$

Answer 1

Answer: (B)

$36\text{\%}$

The sound wave on passing through a slab, loses intensity of $20\%$, in other words it retains $80\%$.
Assume a wave passing through two blocks has an intensity of $I$.
After passing through block one, the new intensity becomes $\frac{80}{100}I=0.8I\left(I{}_2\right)$
After passing through block two, the final intensity is $\frac{80}{100}$$I{}_2=0.64I$
Hence the intensity decreased by $0.36I$ which is $36\%$ of the initial