Acoustic Properties

[Stiganator]

I think we all understand the rudiments of case quieting. I'm more interested in how sound functions so I can think of a better way to dampen them.

Here's how I understand it. Sound travels in waves through a medium. One particle to the next. The waves are created by the pressure gradient the source creates.

Sound does not exist in a vacuum. Though air cooling would also not exist.

Sound travels through things and causes vibration in solids.

You can dampen sound with any material, but in effect you're really baffling???

What material types actually absorb sound?

Any info on sound and dissipation would be appreciated.

 

[CycloWizard]

I've never tried to dampen sound in a computer case or anything else, but here goes:

Sound is generated by an oscillating pressure source. The frequency of oscillation becomes the frequency of the sound (e.g. a violin string oscillates at a frequency dependent on your finger positions, which alter the string length. As the string vibrates, it displaces the air around it with a force related to the input energy of the bow).

Sound absorption, then, requires a medium that can absorb the energy of the sound. Many common materials found in your computer are nearly perfectly elastic; that is, they do not absorb energy - all the input energy that bends or deforms them is recovered when the pressure from the sound is removed. What you want is something that will dissipate this energy. The ideal solution is to use a purely viscous material (i.e. a fluid) that does not store energy. The input sound energy will be dissipated through viscous effects and transformed into thermal energy. However, this is probably not practical, for a variety of reasons, within a computer case. So, the best solution is probably to look for a viscoelastic material - one that has a solid-like structure but still dissipates energy. Probably the most common materials that would achieve this within hte constraints of your problem would be rubbers.

 

[Talcite]

I think you're confusing absorption and blocking stignator. If you have a case wall built of a vacuum, It will actually reflect the sound back into the case.

To remove soundwaves from air, you have to have acoutic foam or soft material, like cyclowizard said, to dissipate the energy. By the way, studies have shown that absorbing the sound won't have as much effect as expected. Sealing the sound is much more effective.

I don't agree with cyclowizard about rubber though. Rubber is very good for preventing sound transmission. However, it makes a very poor absorber of sound waves. One example is the hard drive enclosure build by leo from SPCR. Once a hole was opened through the wall of the case, the noise from the drive was much louder. That is also why you don't see rubber being applied in sound management applications such as recording studios and in homes.

 

[patentman]

Originally posted by: Stiganator

What material types actually absorb sound?

Viscoelastics. I.e. Dynamat

 

[CycloWizard]

Originally posted by: Talcite
I think you're confusing absorption and blocking stignator. If you have a case wall built of a vacuum, It will actually reflect the sound back into the case.

To remove soundwaves from air, you have to have acoutic foam or soft material, like cyclowizard said, to dissipate the energy. By the way, studies have shown that absorbing the sound won't have as much effect as expected. Sealing the sound is much more effective.

I don't agree with cyclowizard about rubber though. Rubber is very good for preventing sound transmission. However, it makes a very poor absorber of sound waves. One example is the hard drive enclosure build by leo from SPCR. Once a hole was opened through the wall of the case, the noise from the drive was much louder. That is also why you don't see rubber being applied in sound management applications such as recording studios and in homes.

You can't really say that 'rubber' is poor for this application, as there are literally infinitely many types of elastomers that can be called rubber. Some would be very effective at dampening sound, while others will not be. Which ones work better will depend on the formulation and the resulting mechanical properties. To move heat, you need a way for the air inside the case to get outside the case (in air cooling). Otherwise, you're relying on conduction alone which will never get the job done until you have a silver case sitting on a silver floor.

Like you mentioned, the solution that is probably found in homes and studios could be multi-layered glass/plastic with air between the panes or a foam (or possibly even a vacuum). If the glass/plastic is layered with air, it acts in the same way as the foam. The air is a viscous medium that will dissipate much of the energy, as well as cause the pressure waves to drop out of phase with the source. Once the waves are reflected off of a boundary (i.e. the next pane or the side of a pore in the foam), it will have a destructive interaction with the incoming sound. The reason I didn't necessarily suggest this solution right away is because these solutions are also very poor conductors. Rubbers generally are as well, but most will still conduct heat better than a foam or sealed air panes. Of course, if you have good enough ventilation and air flow, the conduction is a moot point.

 

[Shoal07]

Go to an AV website and look up the answer. There's millions of posts on this issue. AVS Forums and Soundandvisionmag.com are two good sources.

I know when designing acoustic panels many guys use fiberglass (like ceiling tiles), acoustic foam and cardboard in certain arrangements. Cheap, variable in size and (somewhat) easy to work with. Oh, and they put certain fabrics over them to finish the "look" off with the room and add a little to the dampining.

Design and placement is also critical. You need acoustic panels in certain places in a room relative to your listening position to have the desired effect. Improper placement and they're worthless. Now, studios usually encase the whole room which is what I magine you would try and do with a computer (encase the computer, not the room).

 

[Talcite]

Originally posted by: CycloWizard

Originally posted by: Talcite
I think you're confusing absorption and blocking stignator. If you have a case wall built of a vacuum, It will actually reflect the sound back into the case.

To remove soundwaves from air, you have to have acoutic foam or soft material, like cyclowizard said, to dissipate the energy. By the way, studies have shown that absorbing the sound won't have as much effect as expected. Sealing the sound is much more effective.

I don't agree with cyclowizard about rubber though. Rubber is very good for preventing sound transmission. However, it makes a very poor absorber of sound waves. One example is the hard drive enclosure build by leo from SPCR. Once a hole was opened through the wall of the case, the noise from the drive was much louder. That is also why you don't see rubber being applied in sound management applications such as recording studios and in homes.

You can't really say that 'rubber' is poor for this application, as there are literally infinitely many types of elastomers that can be called rubber. Some would be very effective at dampening sound, while others will not be. Which ones work better will depend on the formulation and the resulting mechanical properties. To move heat, you need a way for the air inside the case to get outside the case (in air cooling). Otherwise, you're relying on conduction alone which will never get the job done until you have a silver case sitting on a silver floor.

Like you mentioned, the solution that is probably found in homes and studios could be multi-layered glass/plastic with air between the panes or a foam (or possibly even a vacuum). If the glass/plastic is layered with air, it acts in the same way as the foam. The air is a viscous medium that will dissipate much of the energy, as well as cause the pressure waves to drop out of phase with the source. Once the waves are reflected off of a boundary (i.e. the next pane or the side of a pore in the foam), it will have a destructive interaction with the incoming sound. The reason I didn't necessarily suggest this solution right away is because these solutions are also very poor conductors. Rubbers generally are as well, but most will still conduct heat better than a foam or sealed air panes. Of course, if you have good enough ventilation and air flow, the conduction is a moot point.

?!?! I'm sorry... but I really can't agree with you there.

If some sound waves meet other soundwaves of equal amplitude and frequency, they double in amplitude

If sound waves meet meet other soundwaves of the inverse amplitude (not sure if i'm using the right words there. Physics kinda rusty... but you get the point.) Then they cancel out. That's the principal behind active noise cancellation.

Yes you're correct about certain types of rubber absorbing sound, but it is a very limited absorption. That's why when you walk into a sound studio/piano studio, or anechoic chambres, you see open celled foam, often in convoluted, pyramid, or similar patterns.

Furthermore, these patterns are designed to absorb certain frequencies of noise, high or low. Before soundproofing a room, you often have to consider what application it is being used for.

anyways, back to the original topic...

A flat panel filled with a vacuum will not absorb sound, even if it has a STC rating of 1000 (impossible). That is why when you enter a nearly empty room with plain walls and tiled flooring, there is often an echo.

If however the room was sealed, and there were no way for it sound to escape, it would eventually dissipate into heat. It would take MANY reverberations for this to happen though. Sound can lose energy slowly just becase the walls arn't perfect reflectors of sound (or any energy).

When you look for material to absorb sound, look for it's NRC rating. This is a number defined by a series of tests measuring sound absorption at several standard frequencies. The results are applied to a function, and the result is the NRC rating. This rating is often used by audio engineers when determining

The STC rating is actually how many dBs of sound an object is rated to stop. Often it has less to do with the object and more to do with the actual techniques while installing the object. Building contractors use this very often, and civil engineers too.

If you want more information on this, there's 2 guides I have written on this topic, it's probably somewhere in the old parts of this forum.

Wikipedia is also pretty good.

 

[CycloWizard]

Originally posted by: Talcite
?!?! I'm sorry... but I really can't agree with you there.

If some sound waves meet other soundwaves of equal amplitude and frequency, they double in amplitude

If sound waves meet meet other soundwaves of the inverse amplitude (not sure if i'm using the right words there. Physics kinda rusty... but you get the point.) Then they cancel out. That's the principal behind active noise cancellation.

My physics is not rusty. As I said, sound is nothing but oscillating waves of pressure. If one pressure wave is coming from the left and another is coming from the right and they have the same amplitude, they will interact in a complex fashion, but they will not produce an additive effect. They would produce an additive effect if they were in-phase and travelling in the same direction, but that is not the case in the situation I was describing.

Yes you're correct about certain types of rubber absorbing sound, but it is a very limited absorption. That's why when you walk into a sound studio/piano studio, or anechoic chambres, you see open celled foam, often in convoluted, pyramid, or similar patterns.

The amount of absorption will depend on the viscoelastic behavior of the material. If I have a rubber with a phase angle (that is, the inverse tangent of the loss modulus over the storage modulus) near 90° at all frequencies, it will be extremely effective at absorbing and dissipating sound energy. It is true that it's difficult to achieve this material because such a material would probably want to creep, among other things, but it's certainly theoretically possible. And I did acknowledge that rubber is not the material of choice. I suggested it as a cheap alternative. Instead of discussing my premises, you've simply told the OP what he (and everyone else here) probably already knew. Since I assume he's not a super-rich guy trying to completely sound-proof his computer case, I thought my suggestion was a reasonable alternative.

Furthermore, these patterns are designed to absorb certain frequencies of noise, high or low. Before soundproofing a room, you often have to consider what application it is being used for.

Not really true. Owing to the nature of sound, one can have a single design that will work at all audible frequencies. I've been in such a room where wood 2x4 boards were cut at angles and various lengths, then fixed to the walls. It was a small, completely enclosed room. I could clap, whistle, yell, stomp my feet, and there was no echo at all. This was because of two things. First, the wood absorbed a good deal of the energy from the sound, though how much depends on the frequency (as it does with virtually all viscoelastic materials). Second, the angles and lengths of the boards reflected the sound waves at angles such that they would destructively interfere with one another. By having a variety of angles and lengths, it covers a very broad frequency range.

A flat panel filled with a vacuum will not absorb sound, even if it has a STC rating of 1000 (impossible). That is why when you enter a nearly empty room with plain walls and tiled flooring, there is often an echo.

But we're talking about a computer case here. The sound can stay inside the computer all it wants. The goal is to minimize the sound outside.

If however the room was sealed, and there were no way for it sound to escape, it would eventually dissipate into heat. It would take MANY reverberations for this to happen though. Sound can lose energy slowly just becase the walls arn't perfect reflectors of sound (or any energy).

The walls not being 'perfect reflectors of sound' is exactly the viscoelastic effect I mentioned (and you dismissed) above. If the walls were 'perfectly elastic', then the sound would still dissipate through viscous dissipation and destructive wave interference.

 

[Talcite]

Well after reading your post cyclowizard, I realize this is never going to end, so.... I submit to your absolute intellectual superiority over this matter and all future matters. Have fun sound proofing your system/directing other how to sound proof their systems.

I just would like to point out, no one has ever used rubber to absorb sound in industrial or residential applications.

And i'd also like to point out that you don't need to be rich to use acoustic foam. I got mine from a shipping crate carrying computer equipment. You have to be lucky enough to find open celled foam, not closed cell foam. Many sites resell smaller sized cutouts of sonex foam. 20 or 30 dollars is more than enough to cover the inside of the computer case. Acoustic tiles are also very cheap, and can be purchased from home depot or specialty locations.

Anyways, stiganator, just make sure you get quiet components first, then you can work on silencing the case, then work on silencing the room. The wall behind the computer and the floor under the computer will make a pretty big difference in the sound you hear from the computer. Oh and don't neglect sounds made by vibrations from the case, that one's a biggie.

 

[CycloWizard]

Originally posted by: Talcite
Well after reading your post cyclowizard, I realize this is never going to end, so.... I submit to your absolute intellectual superiority over this matter and all future matters. Have fun sound proofing your system/directing other how to sound proof their systems.

Thanks. But really, all you had to do was say 'Oh yeah, I guess the OP is asking for an alternative approach rather than the same old...', which he did, indeed, say:

I think we all understand the rudiments of case quieting. I'm more interested in how sound functions so I can think of a better way to dampen them.