When plastic bags/wrappers are crumpled in air they make noise. But when crumpled inside water we hear very little noise. Why is it so? Would I hear more noise if I go inside water and crumple the wrapper? I have tried this and the noise was less inside water but I was uncomfortable as the water went inside my ears. Why is it so?
[Physics] Why do we hear lesser noise when plastic bags/wrappers are crumpled inside water than when they are crumpled in air
acousticseveryday-lifeexperimental-physics
Best Answer
If you mean listening in the air while crushing the bag under water - the main reason is due to the different acoustic impedances of air and water. Transmission of the sound of the bag popping through the water probably plays a secondary role.
Acoustic impedance is defined as
$I =\rho c$
where $\rho $ is the density of the medium, and $c$ the sound speed in the medium.
The acoustic impedance of water is larger primarily because it is so much denser than air.
The acoustic impedance of water is much larger (about 3400 times) that of air. The transmission coefficient of sound from water to air (and vice versa) is roughly the ratio of the acoustic impedance of air to water, i.e 1/3400.
The actual value of the intensity transmission coefficient from medium 1 to 2 with acoustic impedances $I_1$ and $I_2$ is
$T =4\dfrac{\dfrac{I_1}{I_2}}{(1 + \dfrac{I_1}{I_2})^2}$
Which works out to 0.0012 for medium 1 = water, medium 2 = air
So hardly any underwater sound transmits to the air above.