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When a plastic bag is crushed in air it makes noise. But when immersed in water and then crushed we hear very little noise. Why is it so?

Even if I go inside water, I do not hear the noise the same as I hear in the air.

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Do you mean we hear less noise in the air? Or listening in water? Did you try the experiment underwater e.g. in a pool? –  paisanco Jul 26 at 17:12
    
@paisanco under water also. When i observed it makes less sound in air, i tried under water. –  Signal strength Jul 26 at 17:19

2 Answers 2

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.

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Very interesting. Now, the situation is similar the other way around: you do not hear much of people outside the water when you are submerged. Possibly this is because of the air in your ears. –  Enredanrestos Jul 26 at 20:14

Another reason is that due to the inertia of the water the plastic can't accelerate as quickly and it needs to accelerate very quickly to make those noises.

http://www.quora.com/Physics/Why-do-some-types-of-plastic-wrappers-make-so-much-noise-when-crumpled

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