Engineering Stack Exchange is a question and answer site for professionals and students of engineering. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Are there any materials known to be sound-insulating as well as thermally-conductive?

Cost is not an overriding concern, but preferably costing "reasonable amounts" and much less than national defense spending levels.


This is a thought experiment that I'm pondering regarding the idea of a computer case which conducts heat while isolating or dampening all of the sound inside.

There's multiple holes in the idea when fully formed regarding financially reasonable, allowing air-flow, etc, but to keep this question narrow I want to just dismiss all of those concerns for the thought experiment.

I recognize conducting one type of energy while insulating against another is not a straight-forward thing. The closest material that comes to mind is water, but that's not completely accurate because it does conduct kinetic energy (i.e. hydraulics). It is better described as acting as a sort of low-pass filter.

So if no materials come to mind, I'd be interested in hearing of something that could filter sound very effectively while conducting heat as well.

I'd like to put the qualifier that the material is a solid, but I'd be interested really in any material with these properties just because I believe (perhaps wrongly?) that there are likely a very small number of them. Again, don't think in terms of the goal concept I mentioned regarding a computer case, this is just one step towards that thought experiment (which is likely not reasonable, or else products would be on the market right now that provide such).

share|improve this question
    
Forced convection cooling would not depend on the material of the case. – Nick Alexeev Feb 19 at 18:58
    
Water is an excellent 'sonic conductor'. The attenuation is more than an order of magnitude less than in air at most frequencies and temperatures (reference). – Chris Mueller Feb 19 at 19:19
1  
Food for thought - solid wood conducts sound much better than air, yet a wooden door blocks sound when it's closed; AFAIK the exact mechanism is the reflection on the border between air and wood, and and internal reflection within the door. Perhaps layers of two materials that both conduct heat but have very different speeds of sound will do what you want? – Peteris Feb 19 at 22:51

I can think of a few possible ways to approach this.

Probably the first thing to look at is the design of the case itself. They generally tend to be made up of thin, flat panels riveted to a frame. In this sort of situation you can get a lot of benefit from the use of a relatively small amount of sound deadening material. This approach is widely used on automotive body panels and has the advantage that a fairly small strip can reduce the ability of the panel to transmit sound significantly and can be as simple as bitumen impregnated felt.

Also relatively thin films or coatings can have a significant effect on acoustic response without providing much thermal insulation.

Similarly there is probably a lot of potential in increasing the damping of the whole structure of the case, for example by using rubber gaskets or spacers at metal to metal joints.

In more general terms it is often useful to think not so much in terms of sound insulation as a general material property but to look at the way that the structure as a whole responds to the vibration frequencies that it is subject to and 'tune' it to respond in the way that you want by adjusting its mass, stiffness and damping so that its resonant frequency is as far away as possible from the excitation frequency.

So overall I would advise looking at the dynamic behaviour of the case as a whole rather than just a specific material.

Having said that there are a few materials which are worth considering. Aluminium and copper (especially in high purity) have excellent thermal conductivity and are fairly 'soggy' in their mechanical properties, also cast iron has particularly good vibration damping characteristics as well as reasonable thermal conductivity (and I would love to see a cast iron PC case).

It may also be worth considering composites. Carbon fibre has the potential for good thermal conductivity and it may be worth investigating aluminium filled resins and these may give a bit more flexibility in tuning the structure of teh case to get the best acoustic response.

Another possible approach might be to sandwich a gel or powder between two sheets of aluminium or copper. There have also been experiments in completely submerging a computer system in oil which would provide both acoustic damping and convective cooling.

share|improve this answer
    
The gel-between-metal approach is one that I had pondered over.. what would be a gel material to use? Also Cast Iron came to mind as well. Initially I had thought of using water between 2 sheets of metal but the risk of leaking-fries-everything give pause to the safety of such an approach, gel on the other hand... I don't know anything about available materials. I wonder if the stuff they use in ice-packs would be a good thought? Hmm.. – Jimmy Hoffa Feb 19 at 21:41
    
One possibility is something like the thermal paste used for interfacing heatsinks and coolers to processors. There are various different types available and the self adhesive tape version, if used to laminate two sheets of aluminium together might work. An alternative might be to make up a paste from aluminium powder with some sort of liquid binder. – Chris Johns Feb 21 at 15:04

The first couple of things that come to mind are metallic 'wools' (ex. steel wool/stainless steel 'scrub pads'), or carbon foams.

  • Metal wools will provide minimal thermal insulation (metal strands conduct, air gaps insulate, so it balances out mostly), but a large amount of vibration dampening 'limp mass.'

  • Carbon foams/aerogels/what not: aerogels are very effective sound dampening materials, but are also quite insulative. Carbon-based foams (i.e. make an organic gel/foam, then carburize in a reducing-atmosphere oven) could be easily made with a larger pore size using freeze-drying processes to help lower the thermal insulation, without reducing the acoustic properties too much.

An afterthought: You could also use a vinyl/carbon-fiber type composite (maybe filled with alumina for enhanced thermal conductivity) to make a somewhat flexible (albeit a bit pricey) panel for constructing your computer box. The panel flexibility should work well for damping many of the vibrations & absorbing the energy (esp. by not 'buzzing' as much where panels meet, or where noisy fans/HDDs are mounted to panels).

share|improve this answer
    
Steel wool is an interesting idea I hadn't thought of! That has the added benefit of being naturally porous so I wouldn't actually need independent air vents too... I could use a frame and make a sealed steel-wool-bag effectively around the entire frame with many thick layers of steel wool... This would both filter the air, and require zero individual air-vents so the sound of the air intake and exhaust would be diffused across the entire thing which would likely further dampen the sound, and cool the air. This is a very interesting thought! Thanks! – Jimmy Hoffa Feb 21 at 18:04
    
and if I think this through, I could take that whole thing, place it in an aluminum or other case with a 3" buffer space all the way around it (including beneath so it's floating in the center) and have intake/exhaust holes (not screens) in that, and that entire thing would further act like a muffler to the air intake and exhaust... – Jimmy Hoffa Feb 21 at 18:08
    
With that approach, you would run into the problem of air re-circulation through the 'thick layers' of steel wool. If you wanted to make such an approach practical, you would need to have separate section in your "outer box" for air intake/exhaust, then have either intake or exhaust from the inner (wool bagged) box communicate dirextly to the outer box, protected from the opposing-flow air through the rest of the inner box. Once sealed away from the opposing-flow, however, you could then baffle that flow with wool for sound absorption & filtration as well. – Robherc KV5ROB Feb 22 at 0:44

Since nearly all the sound (aside from speakers themselves) originates in cooling fans, it's going to be really tough to block sound while allowing exhaust airflow. You'll need a different, expensive, cooling system.

Probably the next noisiest is the hard drive, which if well heat-sunk could be covered locally with sound-absorbing material (or switch to all-SSD at some expense).

In sum: you are far better off eliminating sources of sound than trying after the fact to ameliorate the problem.

share|improve this answer
    
as I said to start with though, this is more of a thought experiment. I find it interesting to try and think through novel approaches to common problems. Today's common problem: computer's are noisy. I'm well aware of the myriad of typical solutions, but this question is based on attacking the problem from a stance of: What haven't I read of or thought about before? The above mention of metal sheets with a gel between them speaks to an interesting thought! Imagine your case is loaded with fans, but your case is 2 cast-iron cubes nested with gel between them? – Jimmy Hoffa Feb 19 at 21:39
1  
"Imagine your case is loaded with fans, but your case is 2 cast-iron cubes nested with gel between them?" then as long as you have appropriate mufflers on the airflow in and out you should be fine regardless of the thermal conductivity of the gel. On the other hand if you block the airflow your computer will likely overheat even if your gel was perfectly thermally conductive unobtainium. Computers (at least desktops and regular laptops) are cooled through air exchange, not through case condution. – Peter Green Feb 20 at 1:42
    
"Imagine a spherical case with a uniform distribution of fans..." – Carl Witthoft Feb 20 at 13:37
    
@PeterGreen so you think a muffler could be a valuable asset? I realize the air-flow is one of the biggest problems, but I was intending to push questions about handling that off to a separate question to keep this one narrow and the next one as well. – Jimmy Hoffa Feb 21 at 18:01

If the hard disk drive was replaced by with a solid state drive and the heat sink for the CPU chip was made bigger and externalized it would be possible to have a silent computer which cooled directly to the atmosphere or could be cooled by other means.

Currently, a heat sink is attached to a CPU chip and all this is contained within the computer case, whether it be a laptop or a desktop. To cool the heat sink fans are placed in the computer case to provide airflow over the heat sink.

If one on the larger panels of the computer case acted as a heat sink it could be connected to the CPU chip via thermal conductors and the heat produced by the CPU chip drawn away and radiated to the atmosphere or cooled by other means, such as a water jacket. This would eliminate the need for noisy cooling fans.

Additionally, replacing noise producing hard disk drive with silent solid state drives would eliminate the other main source of noise produced by computers.

share|improve this answer

While not a singular material, and a very crude approximation of the concept, would an approach like this not allow more heat then sound to pass through?

enter image description here

share|improve this answer
    
I think this is akin to the idea others mentioned which I should have realized before: A muffler exists for precisely this reason. Ever heard a car without one? – Jimmy Hoffa Mar 25 at 12:44

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.