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# Vacuum Sound Barrier

Posted on 2008-10-20
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I have learnt that sound doesn't travel through a vacuum.

If you surrounded an object with a vacuum layer, what is the minimum width of the vacuum layer required such that it acts as a sound barrier - no sound gets in and no sound gets out.

Another way of thinking of this question is: what is the maximum distance between atoms allowed such that sound can still be transferred?
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Question by:Lolly-Ink
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Your thin layer of vacuum would have to be contained.  The containment would be traveling with and be part of the vehicle is what I would say.
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Therefore, not having the desired effect.
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In practical terms I would imagine that if you had a vacuum enclosed by two diaphragm, the act of hitting one of those diaphragms with sound would transmit to the far diaphragm by virtue of the fact that the size of the enclosed vacuum remains constant.
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Don't agree the size of the enclosed vacuum has to remain constant - if one side is moved without contacting the other, there is effectively nothing to pass on to the movement to the other side.

imagine two parallel steel plates with vacuum between them, plate A exposed to noise, silence wanted beyond side B.  Now when noise hits plate A, it causes longitudinal waves, and if the amplitude of these is large enough to cause the atoms on the inner surface of A to interact with those of B, noise has been transmitted.  But if you have gap greater than amplitude of sound in metal, then there is no interaction and no sound.

Not sure how to know what this amplitude is other than for loud sounds it appears to become visible to the naked eye, so I think you need a visible constant gap is my speculation, the gap needs to be greater for loud sounds, where amplitude becomes greater.

The construction of such a thing is another issue, air pressure wants to push the plates together, so they need to be thick enough to resist collapse or bending into each other.  Also need to be large enough not to let sound around the edges, or enclose you in a box, but if a box then there has to be something to keep the boxes apart (assuming there is gravity).
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An old fashioned high school physics experiment is to put an electric bell in a Bell Jar.  No pun intended.

Turn on the bell and start the pump.  As the pressure goes down, the bell gets fainter and fainter and disappears.

You have to make sure that there isn't an acoustical path through the support structure.  Sound (vibration) travels through solids quite well.
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Notes on the demo I mentioned above from a UCLA website:

http://www.physics.ucla.edu/demoweb/demomanual/acoustics/effects_of_sound/bell_in_vacuum.html
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in theory, for absolute zero sound, you would need an absolute vacuum.  An absolute vacuum would be pretty near impossible to achieve because the container would colapse.  The very small amount of air in the vacated area that is to be the vacuum will allow a small bit of sound to pass (molecules vibrate, which is how sound moves), even if you left an infinately small amount of air (or any gas, e.g. argon, hydrogen...whatever) in what is meant to me a vacuum, you would then allow an infinately small amount of sound to be genereated by their vibration

The earth has an atmosphere, so we can hear things, as we go up, it gets thinner and thinner, even out of orbit where we call it a vacuum, there is still very thin air, go out another thousand miles and there will be stiill some thin air...Lets say for the sake of it we went to a location is space a 10 billion miles from any other object, then, since there will be the odd molecules flying around (space dust), you would then ask yourself, the air is very thin here, about 1 molecule in a foot square of space, so if I made a vacuum wall an inch wide, there will be a 1 in 144 (12 *12) chance of this being a complete vacuum.
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1 in 1728 (12 * 12 * 12)
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All good discussion. Thanks.
Assuming we have the technology to create a perfect vacuum layer, would anyone like to suggest what the minimum width of the vacuum layer is to be a sound barrier?
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If the barrier between the vaccum and the medium is also a barrier to momentum, then no width is necessary.
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>> An absolute vacuum would be pretty near impossible to achieve because the container would collapse.

not true in my opinion, removing every atom or molecule from a space leads to practical problems, but the strength of the vessel to hold a pure vacuum need be no stronger than that to hold a near vacuum, since such low pressures can be achieved, the pressure differential is near identical in both cases.
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robrandon earned 500 total points
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Not sure on the exact minimum width the vacuum would need to be, but I would say it would be a function of the distance that the materials used to create the vacuum vibrate.

So if you are using material X on both sides of the vacuum, and material X can vibrate a maximum distance of Y, then you would want the minimum distance to be at least 2Y.

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I don't think you are asking the right question.
You can't really put a layer of vacuum around something.

You do need to look at how sound waves are produced in detail.

So you put a speaker and a microphone in a bell jar at 1 atm.

Start by not driving the speaker.
Air molecules bounce off the front and back of the speaker cone.
The energy of the molecules before and after each bounce doesn't change.
No sound/pressure waves are generated.
The microphone does not detect a signal.

Next drive a sunusoidal tone at 1 atm.
Air molecules bounce off the front and back of the speaker cone.
When the cone is moving forward, molecules bouncing off the front pick up energy.
Molecules bouncing off the back lose energy.
When the speaker is moving backward, the process is reversed.
Sound/pressure waves are generated in this case.
The microphone will detect a signal.

Finally drive a sinusoidal tone while evacuating the bell jar.
Fewer molecules hit the speaker as the pressure goes down.
The impedance of the transmission medium goes down.
The energy content/intensity of the sound waves decrease.
The signal detected at the microphone gets weaker and weaker.

If you reduce the pressure to the point where the amplitude of the
pressure waves is smaller than the Brownian motion noise, then the
signal becomes undetectable.

The value would depend on the size of the speaker, the distance to the microphone, and the input power signal.
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ISO 9613 Part 1 has formulae for doing this calculation (Atmospheric attenuation).  Since it's an ISO document, you probably won't be able to google an original.  It is excerpted in a forum here:
http://forum.studiotips.com/viewtopic.php?t=158

And here is an online calculator which I think implements the formula:
http://www.csgnetwork.com/atmossndabsorbcalc.html

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Thanks. I can understand this.
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"Another way of thinking of this question is: what is the maximum distance between atoms allowed such that sound can still be transferred?"

The closer the atoms the louder the sound transfered since the energy of the atom weakens as it travels, just like somebody shouting from the distance compared to right in your ear, also, depends on what you mean by hearing the sound??  are you refering to some super scientific computer that can detect the lightest evedence of enery on distant llanet, or are you refering to the hearing ability of joe bloggs.

Also, if you put it your way, how loud was the initial sound that is to try and propogate through, and how far away from the sound were you?  Also, what temperature as the vibration will increase with temperature.

Interesting discussion!
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