DIY low freq absorber

Discussion in 'Home Theater Projects' started by David_Rivshin, Apr 1, 2004.

  1. David_Rivshin

    David_Rivshin Second Unit

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    I've re-read alot of the old threads on DIY room treatments, but there seemed to be alot of conjecture and conflicting thoughts. So I thought I'd give some information on the problem I'm trying to solve at the moment, and hope that those more familiar with the theory can give me some advice.

    My basic goal is to reduce some background noise which is generated by my computers. The living room in my apartment is directly attached to a "dinning area", which I use instead as my office. My computers are on the floor the far corner, with no table above but a hard surface on 3 sides, and 2 feet in front. My guesstimate is that they generate around 50dB of noise at a distance of about 1 foot. I did some frequency analysis of the noise, and found it to be between 30 and 500Hz, with some spikes at a few specific frequencies, which depend on exactly what equipment I have on at the time.

    So, I what I would like to do is build some inexpensive panels to place in that corner with high absorbtion in the 30-500Hz range. Target size is about 1 foot tall, 3 feet along one wall, and 2 feet along the other. I'd like to keep depth below 4 inches if possible, as space is tight.

    Some thoughts I had from info in previous threads:
    1) rigid fiberglass insulation faced with pollyfill
    2) regular fiberglass insulation faced with pollyfill
    3) 1 or more layers of eggcrate foam with pollyfill
    4) a couple of these: http://www.partsexpress.com/pe/showd...number=300-895

    Any other ideas and/or evaluation of what materials would be best would be greatly appreciated.

    Thanks,
    -- Dave
     
  2. ThomasW

    ThomasW Cinematographer

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    Start by lining the metal insides of the PC chassis with some of the sound damping sheets that the car audio guys use. PE's generic brand is fine.

    Then use either the thick fiberglass insulation covered with poly or the thickest wedge foam you can find.

    The rigid fiberglass sheets are of limited use, but they're the quickest and easiest to use.

    If you can fabricate a temporary false wall between you and the machinery that will work best. Use the larger DIY sound absorbing panels
     
  3. TimForman

    TimForman Supporting Actor

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    What's actually making the noise in your PCs? Usually it's fan noise. You might be better off spending $20 or so for a quieter CPU fan. If it's the power supply fan damping the cabinet isn't going to help. In that case you'd have to swap out the power supply for a quieter one.
     
  4. David_Rivshin

    David_Rivshin Second Unit

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    I have two machines, and older one which is fairly loud by my new standards, and a new one which is pretty quiet. However, even with only the quiet machine on I can still hear it alittle. I think I just have abit of obsessive compulsiveness about me that wants to quiet it down further if I can. The louder machine will probably be getting replaced sometime this summer, although I am looking at spending a few dollars on quieter fans, but given the case I'll never get it as quiet as the other one.

    I do believe almost all the noise is fan noise. The quiet machine is in an Antec Sonata case which does a great job of isolating the vibration as it is. There is also a slight bit of high pitched HD whine on occation, but I don't think there's much I can do about that without replacing the responsible HD's.

    I'm not prepaired to go to extrordinary lengths in this regard, there are alot worse problems to take care of, but if there is something simple I can do I'm willing to experiment alittle in search of nirvana. As it is I'd estimate the level as around 25-30dB on the couch with only 1 machine running, not bad, but I'd love it to be lower.
    If I can just wraps some fiberglass in poly and duct-tape it to the wall and get a 5dB improvement in overall noise level I'll be quite pleased.

    Later on I'll be tackling the living room reflections and standing waves, but I figued this small project would get me started.

    Thanks for the responses guys, if there's any more advice to hand out I'm all ears.

    -- Dave
     
  5. TimForman

    TimForman Supporting Actor

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    Sounds like you're on the right track. Get rid of the reflections from the walls around the PCs.
     
  6. David_Rivshin

    David_Rivshin Second Unit

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    I think I'm going to pick up a small roll of regular fiberglass insulation, hold it together with either some wire or a cardboard backing, and wrap it up in something for a nicer appearance. I figure the total cost will be about $10-15 for everything.

    I have seen references to the idea that keeping a LF absorber off of the wall improves it's performance. Is this true for wider-band absorbers, or just for specific frequency resonators? If this would be appropriate for my application, how much airspace should I leave? Is there some relationship between the distance from the wall and the absorbtion coefficients?
    While I'm on the topic, what's the relationship between the thickness of the absorber and it's sonic characteristics? If I pack it in tight will that help/hurt/do-nothing? What differences would there be if I put two layers down instead of one? What if I put two layers in the space of one layer?
    Is there some book or online references for the theory of absorbers so I can stop asking so many questions?
    I'm guessing that the thicker the better, but I'm curious as to exactly how much better so I can gauge various tradeoffs.

    Thanks again,
    -- Dave
     
  7. ThomasW

    ThomasW Cinematographer

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    Google will get you the answers to most of your questions. Just do searches for bass traps, sound absorbing panels, etc, etc.
     
  8. David_Rivshin

    David_Rivshin Second Unit

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    Eek, I was afraid of that type of answer [​IMG]
    Last time I tried searching for such things I either ended up looking at:
    A) marketing propaganda
    B) "I did X and it just sounds so much better", with no scientific testing or textbook knowledge to back it up, or discussion of why that way was used instead of the 300 other techniques that are mentioned once elsewhere.
    C) talk about Hemholtz resonators which just went straight over my head, and I don't think is applicable to this project
    anyways

    I'd really love some educated statements of type
    "Doubling the thickness of any absorber will increase the absorbtion by XdB at xHz, and YdB at yHz."
    or
    "putting an are gap of X inches behind an absorber will shift it's absorption curve down YHz."

    That said, I will do some more searching, 'cus the more info the better. I just have more faith in the members of this forum than most of what I find floating around the web...

    -- Dave
     
  9. Pete Mazz

    Pete Mazz Supporting Actor

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    Way better to address the issue of a noisy PC from inside the case. Quieter fans, isolating HDs and anything mechanical, and case damping is the way to go.

    Unless you know the freqs involved, specific sound absoption info is useless. Basic rules of freq absorption are to put the absorber in a high particle velocity zone for best performance, which is never directly on the surface of a wall. Used at 1/4 wavelength of the offending freq from the wall is best.

    Pete
     
  10. ThomasW

    ThomasW Cinematographer

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  11. Clay Autery

    Clay Autery Stunt Coordinator

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    One word... Sorbothane. If your noise probs are truly in the low freqs like you say in the first post, I'd use sorbothane to isolate your fans and your hard drives from the metal case...

    You can even do it to the power supply fan if you are VERY careful... Note: do NOT open the power supply if you don't know what you are doing!!!

    The idea is to use the sorbathane to remove any metal to metal contact between the vibrating device and the surrounding case. They actually make isolation dampers specifically for mounting hard disk drives... Although, you need to mount the HDD in a 5-1/4" space to get the room to put the dampers in...

    On the fans, you can use the same type damper or sheet Sorbathane cut to size with through screws... the fan mounting holes need to be machined for screw clearance, and you need to use a sorbathane piece under the screw head with a washer too to make sure that the fan frame ONLY touches sorbathane and not the case and/or mounting hardware.

    Now, the next level is to line the inside of your case with Sorbathane sheets that come with PSA on them for just such a purpose... This will help to further reduce any low freq vibrations that make it into the case panels...

    I used this method to quiet down an overclocking project I did a few years back where I HAD to use high speed 4+"fans to ventilate the case so the coling solutions inside the case had a bigger temp delta to work with... IAW, I had to keep the case vents open.

    Another place you might check is to isolate the cooling fan(s) on your CPU heatsink from the heatsink themsleves...

    Additionally, make sure NO wiring from any device touches the metal case panels. Where they much touch the frame pieces for routing, make sure they are rigidly attached with wire ties... OR wrap the the 1" area where the wire tie will go with sorbothane to isolate the bundle from the case...

    Doing this INSIDE the case, witll drastically reduce the noise...

    The harder noise to get rid of is air flow noise... Not a whole lot you can do (easily) with this... You can however cut the fan grills out to remove some of the turbulence... if you can mount the fans inside the case with tubular standoffs (iso-damped of course), the 'tunnel effect will tend to lower the external heard...
    This should get rid of about 80+% of the noise using BIG fans... Normal fans should just about be dead quiet at this point...
     
  12. ThomasW

    ThomasW Cinematographer

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    One should be careful using sorbothane, it likes to melt when it gets too hot.

    Some of the serious overclocker supply places sell silicon gaskets for damping. It handles heat much better than sorbothane
     
  13. Clay Autery

    Clay Autery Stunt Coordinator

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    Yep, Sorbothane will melt... especially the softer stuff... But I have never had any problems with it... I don't let anything in my systems get that hot. [​IMG]
     
  14. Hans H

    Hans H Auditioning

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    I've done some audio work over the years. A couple of quick, cheap things to try are stapleing some egg crate type foam to the wall behind the offending machines and then putting some drapes over the foam to hide it. Also set the machines on some type of isolation. I use carpet samples. They can usually be had for pocket change at carpet stores. I also like the quieter fan idea.I've also had good luck using egg crate or moving blankets covered by nice heavy drapes in room corners to help break up standing waves. You can also hang some drapes from the ceiling in front of the machines using a temporary type of install from the ceiling.
     
  15. Clay Autery

    Clay Autery Stunt Coordinator

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    Let me add something here.... Although insulating the device from the surrounding area rather than eliminating the source of the noise MAY eliminate the noise, you may ALSO be creating a problem for yourself...

    Insulation, draping and otherwise isolating the machine from the rest of the room, will likely reduce airflow to/through/around the device... Restricting the air supply to the device will cause the available air supply to heat up... The heated air is repeatedly reused and repeatedly heated to higher and higher temps... At some point, your device will likely not be cooled suficiently, and you will have heat related performance problems...
     
  16. David_Rivshin

    David_Rivshin Second Unit

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    First off my apologies for leaving this thread hanging. I've run into some car issues which needed my attention, so this project has gone on the back burner for the time being. I won't be able to get to it this weekend either, but I'm hoping perhaps next weekend.

    Thanks again for all the input from everyone, I'm taking it all under consideration. I did want to address a few of the comments, in no particular order.

    I don't really think I can quiet the main computer down too much at this point. It's in an Antec Sonata case, which comes with a single 80mm PSU fan and a 120mm exhaust fan, both temperature controller by the PSU. In addition I installed a 120mm Vantec Stealth intake fan to cool the 4 HD's installed in the lower bays. CPU fan is an 80mm Top Motor model, sitting ontop of a Thermalright SLK-947U heatsink. Both intake and CPU fan are MB (Asus A7N8X) controlled according to CPU socket temperature. The video card is a Matrox P650, which is passively cooled, and there are no other fans in the system.
    Current readings are:
    CPU Fan: 2280 RPM
    Front Fan:1454 RPM
    PSU Fan: 1722 RPM
    CPU Temp: +35°C
    M/B Temp: +28.5°C
    Don't know the speed of the exhaust fan, but I presume it's in the same range as the PSU fan.
    The two case fans are mounted via Antec supplied rubber "screws" to dampen vibration, and the CPU fan has some rubber grommets shoved in between it and the retaining clips. Although the HD's (with the exeption of one mounted in a 5.25" bay) are mounted via rubber grommets, I do detect some minor floor vibration and alittle HD whine.
    Overall I'd estimate the total volume around 33-35dB at 1 meter. The noise level actually seems to be below the self-noise of my EMC8000/UB602 mic/pre-amp at seating distance, but I can still hear it if I pay attention.
    I don't think I could get any quieter without either spending alot more money than I intend, or pairing down some of the hardware (replacing 3 smaller HD's with a single larger one is an option I'm considering). As far as I can tell almost all the noise is just air noise from the PSU and exhaust fans, which seems wide-band low-freq in nature.

    The other computer, on the other hand, is a comparitive jet engine. At this point I can hardly believe I used to sleep with it about 5 feet from my head. But as I said before I'll probably replace it with a much quieter one sometime this summer, and in the meantime I leave it off when I care about noise level.

    I have recently though about putting something heavy under the computers, such as a slab of marble, but maybe something soft like carpet remnants would work better? I actually have some pieces of old carpet laying around I could try that with. Heck, it's free [​IMG]

    I don't think I'm going to go through the trouble of doing something so intrusive as putting drapes infront of the equipment, it's just not *that* big of a problem right now.
    There is a surface somewhat infront of the cases from a desk leg where I'll probably put a small absorber as well, just for good measure.

    Right now I'm really limiting my options to things I can do outside of the cases, specificially wall treatments. Current plan is to pick up a small roll of R13 from Lowes and put it down double thick (7" normally) compressed into the depth of a 1x4 frame. Put some pollyfill infront of that, and some burlap covering, and I think I've got just about what I want. I assume it's possible to take the backing off that fiberglass, otherwise I don't think doubling it up will work, and I'll have to just get a roll of R21 instead.

    I have picked up The Master Handbook of Acoustics (as well as some other related books) at my library, and have started reading it. I will most likely wait to actually build anything until I've finished reading the interesting chapters. I only have a few hours a week to even think about this, so it may take me some time.

    -- Dave
     

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