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The main point can be understood just be reading the question. But there's more to it. I want to buy around 300 liters of water to stack them up against the wall through which I get (and produce) noise.

I know there are better ways for reducing noise in a room, but please don't consider them.

There are many unknowns like: the height of the wall, the volume of the room etc. I hope they aren't crucial for answering the question and that having in mind "all other things being equal" will be enough. I would love to hear the answer to the question if I add some variations to it:

  • What is better? Big bottles of water (i.e. 5l) or smaller ones (i.e. 1.5l or even 0.5l).

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  • How much is the form of the bottles (for example, circular or square base.) important for the solution.

  • is there any difference in the efficiency of the solution if I use regular mineral water (in this case the bottle is tender and easier to deform) over carbonated, sparkling water (of which bottles are stiff and can bounce if dropped them on a concrete surface)

I have a pretty good understanding of physics but no experience regarding acoustics. Even though it is a real-life problem, I would love to read some deeper insights about the solution/strategy. Hopefully for me, It's gonna be a good introduction to water acoustics.

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    $\begingroup$ I'd go for smaller bottles and maybe two layers? Once the sound wave is in water it travels quite nice; some damping is supposed to happen at the surface and probably very "strong" due to the plastic bottle itself. I guess the empty bottle is even better, and I definitively would not use glass bottles...On the other hand, smaller bottles are probably stiffer, so less damping. The thinner the plastic the better, I guess, so "low quality" should be better. $\endgroup$ – mikuszefski May 22 '17 at 9:39
  • $\begingroup$ Shouldn't the answer of that question not also depend on the frequency range? You could get some resonances. $\endgroup$ – domj33 May 22 '17 at 10:10
  • $\begingroup$ Water leads sound very well, actually much better as air. You would experience some dispersion on phase borders, but it surely isn't the best sound insulator. In your place I would use dense textilies and not water. $\endgroup$ – peterh says reinstate Monica May 24 '17 at 0:50
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    $\begingroup$ Did you end up doing this? Can you give an update? I'm curious about efficiency of this approach to sound insulation. $\endgroup$ – JBeurer Jun 11 at 15:39
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Shielding sound is surprisingly hard. The reason can be gleaned by thinking about the dB scale - the decibel is a logarithmic scale, and it is generally accepted that "twice as loud" requires a 10dB change in sound. But 3 dB is a doubling of sound intensity (energy). So if sound is coming uniformly through your wall, and you block half of the wall perfectly, you expect a 3 dB drop in sound intensity. You would need to cover 90% of the wall perfectly to get the sound to be half as loud.

What this means is that you are asking the wrong question. You will do much better making sure that you have "complete" coverage of those parts of the wall that are moving in response to sound, and that therefore transmit the sound. The key concept here is acoustic impedance mismatch. When a sound wave hits a surface with different density / sound velocity, you get a partial reflection. If you have multiple layers, you get better damping. It also helps to have uneven surfaces - so that reflections from different parts are out of phase and don't result in "echo". This is why layers of heavy curtains, floor-to-ceiling, might be a better solution. Certainly you will gain very little by having a thick layer of water, versus multiple thin layers. Just make sure there are no gaps - that's what will be the key to good insulation.

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I was thinking very soft bottles like those of milk cartons. I remember that in normal noise dampening they use soft materials that literally absorb the sound instead of bouncing them back. I'm not really sure how the water part will help, but if this is a bet with your friends or you just want to technically have water, maybe some fuzzy soft water bottles should work best.

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