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 Post subject: Breaking up soundwaves?
PostPosted: Tue Nov 20, 2007 12:44 pm 
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I'm wondering, is it possible to break up a soundwave.... akin to the way you break up a microwave (metal mesh on the door of the microwave oven effectively stops microwaves because they cant pass through it)

I'm thinking, because i want to put a door on the front of my computer "rack".

the thing on the right of the desk in this pic

i'm wanting something that will allow air in relatively unimpeded, but stop some amount of noise from getting out.[/url]


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PostPosted: Tue Nov 20, 2007 1:27 pm 
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"Breaking up soundwaves" is not what you're looking to do, actually. You're looking to block the noise. The metal mesh in a MW oven door also blocks the waves, it does not selectively pass some of it. Anything that lets airflow through will also let sound through -- air is the medium for sound.

Breaking up soundwaves doesn't really make much sense anyway. The commonly used phrase is "breaking up standing waves" which refers in acoustics to stopping reflections between parallel walls or similarly symmetrical reflective surfaces which cause tonal resonances.

I'd suggest putting in a hinged door over the front, pushing back the components a couple of inches, and cut out a 2" slot across the front edge of the cabinet's floor so that it becomes like the front intake of a tower case. That assumes you have room to push the components back.

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PostPosted: Tue Nov 20, 2007 1:27 pm 
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Short, but incomplete answer: yes.

In fact my daughter and I were just discussing this at lunch. During my college years I studied, among other things, architecture, for two years. One of the classes covered this topic. For example, the little, apparently random sized (but not) holes in acoustic ceiling tile is designed to let sound waves of certain frequencies kind of get trapped inside and dissipate their energy. The back wall of one of our lecture halls was a black wall made of the stuff, but since there had been problems with other frequencies, it was covered with vertical strips of wood that stuck out from it, and were spaced about (memory is fuzzy here) 2 to 4 inches apart. These strips served the same function, trapping sound waves before they reflected back to the auditorium. I think they also helped direct some of the reflections away from just bouncing directly back at the audience and stage.

To do this right, you would have to run a spectral analysis of the sound frequencies that are coming out of your rack, and then design a door that would block those frequencies, or reflect them back into the box to dissipate. You would probably have to apply some kind of treatment to the sides, top, bottom, and rear of the rack too.


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 Post subject: Re: Breaking up soundwaves?
PostPosted: Tue Nov 20, 2007 2:52 pm 
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N7SC is talking about room acoustics which are obviously important in a lecture hall where you need to hear the lecturer.

But what you are asking for is sound attenuation so you don't hear your PCs.

Think of ripples in a pond. To stop the ripples you need a wall.

Aside from a door, you might need to damp the cabinet walls if there is any vibration coming from the PCs. You could stick some vinyl floor tiles to the inside walls of the cabinet and then maybe a layer of open cell foam to provide that last little bit.

But a door won't work if you don't have a rear opening for the PC exhaust.

You might also consider making your PCs quieter.


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PostPosted: Tue Nov 20, 2007 4:42 pm 
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i should probably mention that the most advanced scientific course i took in college was physics.... for business majors. (that's what it was actually called!)

so I am sorry for my noobishness on concepts of soundwaves. i kinda hoped they'd behave like electro-magnetic waves, which you can selectively block, depending on your material, depending on their wavelength.

my computers are outside of that big wooden case at the moment, because i'm preparing for something big. I have a 600 RPM, 25 CM fan that i'm drilling a hole in the back of the case for. but here's the thing. i want to put some other components in that case, on top of the computers. would an opening at the bottom, and one at the top, effectively move air through while cutting noise?

I'm going to be lining basically the whole inside of the wooden case with some sort of sound-absorbent foam, or other material. as well as adhering some to the wall behind the case, where the fan exhausts and where the power plugs go in.


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 Post subject:
PostPosted: Tue Nov 20, 2007 6:45 pm 
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Quote:
i kinda hoped they'd behave like electro-magnetic waves, which you can selectively block, depending on your material, depending on their wavelength.


they do. the same piece of acoustic foam will attenuate each individual frequency by a different amount, generally high frequencies more so than low frequencies. this is referred to using a sound transmission class/sound reduction index rating (which is a weighted average of the reduction across a selection of frequencies from 125Hz-4000Hz).


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 Post subject:
PostPosted: Wed Nov 28, 2007 1:24 am 
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to damp a frequency u need a dampening material atleast 1/4 of the soundwaves length. And the lower the frequency the longer the soundwave is...


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 Post subject:
PostPosted: Tue Dec 04, 2007 5:26 pm 
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MikeC wrote:
"Breaking up soundwaves" is not what you're looking to do, actually. You're looking to block the noise. The metal mesh in a MW oven door also blocks the waves, it does not selectively pass some of it. Anything that lets airflow through will also let sound through -- air is the medium for sound.

Breaking up soundwaves doesn't really make much sense anyway. The commonly used phrase is "breaking up standing waves" which refers in acoustics to stopping reflections between parallel walls or similarly symmetrical reflective surfaces which cause tonal resonances.

I'd suggest putting in a hinged door over the front, pushing back the components a couple of inches, and cut out a 2" slot across the front edge of the cabinet's floor so that it becomes like the front intake of a tower case. That assumes you have room to push the components back.


Airflow...cooling,can be pretty efficient with a somewhat indirect path if not bottlenecked. Sound loses some punch anytime it's not a direct path. This especially applies to high frequency trebles. Further...absorbant materials and geometry that makes sound bounce in a managed pattern so little "echo" escapes,is a plus.

Totally blocking sound/noise takes density-mass-weight. Unless a 120 pound Adobe c omputer case fits your space,the best bet is to deflect noise and disperse it.....and to simply not have much noise in the first place.

I'm an advocate of using eSATA to get the HDDs farther away and in a seperate box. I'm also big on sticking a Ninja on a CPU that's a sensible amount of power for your needs,and then using a very big fan at a very low RPM for ventilation.

If your goal-need-desire involves an overclocked $400 CPU Quad and a Vid card that could fry chicken-then you live with some trade-off


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 Post subject:
PostPosted: Wed Dec 05, 2007 1:39 am 
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eXa wrote:
to damp a frequency u need a dampening material atleast 1/4 of the soundwaves length. And the lower the frequency the longer the soundwave is...

This is the key. Human hearing is typically rated at 20 Hz to 20 kHz. The speed of sound is ~ 340 m/s. So, a sound wave at 20 Hz would have a wavelength of ~ 17 m! Clearly, you are not going to do much about filtering out those bass noises unless your damping material is meters think. What about the other end? Well, 20 kHz should have a wavelength of ~ 17 mm, so almost any non-resonant obstruction between you and the noise source will dampen these high pitched sounds. So the ends of the audible spectrum are kind of moot, either trivial or impossible to dampen. What you are left with are sounds that can be dampened, but it will take several cm of material and will almost certainly have to significantly impede airflow to be effective at lower frequencies.


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 Post subject:
PostPosted: Thu Dec 06, 2007 4:41 am 
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like someone said before, sound waves needs a medium to travel through. Take away that medium, and you take away the noise. So what you could do, if you were really serious, is have the walls of the case made of a material that has hollow interior that consists of a vaccum

ofcourse, this would create heat issues (heat also travels via some medium, and it would have a tough time escaping that pc)


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 Post subject:
PostPosted: Thu Dec 06, 2007 5:32 am 
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hi fyad,

sound absorbing material helps reduce reflections, it cant stop the sound from passing thru it (after all its just foam). for this you will need mass like a glass or wooden door.

1. your idea of venting the back is good and a MUST in my opinion, just make sure the fan is quiet,
2. like mikeC said - build a door for the front and intake either from a gap at the bottom of the door or from the bottom of the wood box
3. the door itself could be made of triplex glass (2 sheets of glass with rubber/slicon/air layer between them) most glass people know what this is... seal the door from the sides and top/bottom using rubber door seal that can be purchaced at any hardware (not computer hardware :wink: ) store.
4. this will stop the direct path of sound from the pcs to your ears, but I have come to realize using quiet components and slow fans work much better by lowering the amount of sound emmited from the machine in the firs place. it is usually cheaper too...

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 Post subject:
PostPosted: Tue Dec 18, 2007 1:47 pm 
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Fayd wrote:
i should probably mention that the most advanced scientific course i took in college was physics.... for business majors. (that's what it was actually called!)

so I am sorry for my noobishness on concepts of soundwaves. i kinda hoped they'd behave like electro-magnetic waves, which you can selectively block, depending on your material, depending on their wavelength.

my computers are outside of that big wooden case at the moment, because i'm preparing for something big. I have a 600 RPM, 25 CM fan that i'm drilling a hole in the back of the case for. but here's the thing. i want to put some other components in that case, on top of the computers. would an opening at the bottom, and one at the top, effectively move air through while cutting noise?

I'm going to be lining basically the whole inside of the wooden case with some sort of sound-absorbent foam, or other material. as well as adhering some to the wall behind the case, where the fan exhausts and where the power plugs go in.


A lot of soundproofing is about managing the bounce of soundwaves. If you take a room,and have the entrance a bit of a maze,paint the walls black,you get a darkroom,as the light has no direct line and the black walls won't bounce/reflect light. If instead of black...you lined the walls with thick textured carpet,soundwaves would be absorbed and deflected as they bounce,and while in the room,you will still have some sound getting in and out-it will be much reduced. Look at the entry to some theatres,this layout keeps the noise from the lobby from getting into the seating area. The Acoustic wall in an auditoreum is mostly to absorb echo. In that,you want the direct sound from the front. Echo muddys the sound and clarity. However a few offset walls like that between you and the sound..so sound has to zig zag between the source and you will substantially reduce the sound level you hear. Of course....if you were in a sealed room made of 12" thick adobe blocks...you would hear almost total silence unless you are next to a rock band with a loud bass or an F-15 jet.

If you have a 1000 rpm fan on the back of the computer and the computer is under a desk with the fan facing a carpeted wall....you will percieve MUCH less sound then if the same fan is in front and on the desk at ear level.

A great way to quiet HDDs is to use eSATA and then you can have the HDD 6' from the puter---perhaps behind a dresser full of socks and sweaters.

Distance helps. Any bend in the path helps--especially if the barriers absorb or break up the waves (similar to the concept of stealth aircraft).

Airflow,for cooling,can bend and isn't much effected if the pathway is lined in foam,carpet or whatever.


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