Question

The intensity of a sound wave gets reduced by 20% on passing through a slab. The reduction in intensity on passage through two such consecutive slabs is

• A. 40%
• B. 36%
• C. 30%
• D. 50%

Answer 1

The correct option is B 36%

Intensity after passing through one slab
$I^{\prime }=[I-\frac{20}{100}\times I]=[I-\frac{1}{5}]=\frac{41}{5}$
So, intensity after passing through two slabs
$I"=[I-\frac{20}{100}\times I^{\prime }]=\frac{4I^{\prime }}{5}=\frac{161}{25}\therefore \%decrease=\left[\frac{(I=-\frac{161}{5})}{I}\right]\times 100=36\%$