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Could a low frequencies from a sub break an overloaded glass shelf?

Discussion in 'Subwoofers' started by Jon Weaver, Nov 16, 2001.

  1. Jon Weaver

    Jon Weaver
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    I know that this might be an odd question, but its something which has bothered me for ages.

    I have an 'Optimum2000' stand, which is basically a 1m length of 10mm glass supported at either end (its not the one with the centre support)

    Its already supporting a Sony KV36FS70 which is almost 100kgs and the glass visibly bends (about 5-10mm in the middle)

    Optimum say that they have tested the glass upto 500kgs, but won't guarantee it. But they have assured me that 100kgs is fine.

    However, I asked 'Pilkington Glass' about their thoughts on toughened safty glass and with the measurements I have, they said that 50kgs would be safe!

    So, whilst the TV has been sat there for almost a year, I enter the living room every day expecting a scene of carnage.

    I have always wanted to upgrade my Yamaha SW80 sub to something more meaty.. But the sub is located right behind the shelf.

    This may sound silly.. But I wonder if it was possible that a really loud/low frequency could vibrate the shelf/TV enough to cause the shelf to fail.

    You hear people talk about their subs being able to move ornaments on neighbours shelfs and I wonder if there could be enough power to vibrate the stand.

    If the glass wasn't already overloaded, I would be worried, but I wonder what it would take to make the shelf break.

    I know that there isn't an easy answer this, but I would apprecate peoples thoughts/experinences.
     
  2. bob007

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    I was never that good at science at school, but this my understanding;

    First thing that came to mind was the opera singer who shatters the wine glass TRUE/FALSE . http://www.earthsky.com/1999/es990822.html


    RESONANCE WRECKAGE
    In the classroom, Professor Lewin demonstrated the impact of resonance on a smaller scale. He first rubbed a moistened finger around the rim of a wineglass to find the tone, or frequency, that is the resonance frequency of that glass. Then, using an electronic device to reproduce the tone at high volume, he shattered the vessel with sound waves.

    So from this i would say that if you found the tone, or frequency, that is the resonance frequency of that glass, your glass shelf would shatter, but i think highly unlikely in home cinema systems.

    You mention the shelf is bending in the middle and that you have heard reports of ornaments being moved in neighbours homes through vibrations caused by their sub bass, yes i can belive that to a certain degree, when i turn mine up to excess volumes i can rattle all my sons trophies on all three shelfs in his bedroom.

    I can see your worry as the human hearing goes down to roughly 20htz, then as home cinema folk know you feel the rest. But i don`t think that the subs we use for home cinema powerful enough to move your TV. Because the only way i can see the glass breaking is if the TV was to be physically moved up and down putting more stress on the glass.

    Low freqencies can do damage but you have to go right down to the bottom end sub 5htz and lower play that in your front room and the glass would be the last thing to worry about, more your foundations.

    hopefully someone may give a better explanation coz i only got a D in physics/science.

    Now i know why i hated science!



    :cool:
     
  3. Jon Weaver

    Jon Weaver
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    Its not actually the glass shattering from the sound of the sub as such.. Like you I have always thought that glass was more likely to break at high frequencies..

    I am actually worried about the 'thumping' being forceful enough to make the weight (TV) on top move and break the shelf.
     
  4. bob007

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    My opinion was;

    You would need something extraordinary to have an effect on a 100kgs of TV.
     
  5. paiger

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    If you are going to try and blow up your system can I come round and watch? A few years back I took the front off a home made speaker with Ace of Spades.


    S
     
  6. Nelly

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    Jon. I have no idea what your stand looks like, but can you not just make a centre support? Surely this would be the safest and surest answer?

    Matt
     
  7. Jon Weaver

    Jon Weaver
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    Its not possible I am afraid.

    Even if I could make the holes that I would need in toughened glass, there are 101 other reasons why it wouldn't work....

    But thanks for the idea...
     
  8. Reiner

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    I don't want to scare you and I am not a professional, but a resonating frequency can do a lot of harm (mainly the reasons why bridges collapse during an earthquake).
    However I would also assume it's quite unlikely in case of your subwoofer vs. stand dilemma as the sub's frequency would need to match the resonance frequency of the stand and that for quite some time, i.e. continously for several seconds or minutes perhaps.
    In fact you should be able to hear some rattling sound, sometimes happens in my setup where things in the room start vibrating and rattling occassionally, but it's quickly over as the soundtrack / movie goes on.

    I would also agree that the sub won't move the TV, however the bending would give me reason to worry.
    Had the same problem with my amp on the top of my shelf so I ordered a custom made pane of glass at double the thickness (10mm instead of 5mm) - and my amp weights around 22kg only.
    At least I can sleep better now .... ;)
     
  9. Nic Rhodes

    Nic Rhodes
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    Resonance frequency is not what we should worry about here. Glass is an immensely strong engineering material. That is why we build buildings out of it. Toughend glass is particularly strong and that is why these shelves are made from it. I have no doubt that it will hold 500Kg, the reason no one wants to guarantee it is all that can happen is that it fails!

    Glass is however a brittle eng material and one tiny weakness (crack / scratch) will make a weak point. The glass will always fail at this point. This is how you cut glass after all (even Tough glass). If it has worked to date you should be fine, if you drag you Tv and it scratches the glass, remove it! If some asked me to quantify weight limits on 10mm shelves then I would guarantee 75kg. There is a good FOS in there as well (essential for glass).

    I don't think you need to worry too much but don't come to me if it fails!


    Dr Nic R
    Engineer
     
  10. Nic Rhodes

    Nic Rhodes
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    I have asked my glass research team to give me some figures. I will post when I have them.
     
  11. MarkB

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    Glass is strong in tension or compression along its length, but a lateral load is different and I don't think it would take much to break it under this load. Certainly if you tapped the glass with something hard it could shatter (in a similar way to how a bus window will shatter when hit in the corner with a tiny metal hammer).

    However, I don't think that the power of the sub woofer would have any effect because of it's low frequency – it would need to move the mass of the TV at a high frequency to break the glass, I believe that glass may be slightly elastic at lower frequencies. I might be wrong though, sounds can cause resonance at different frequencies in "nodes" (magnified areas), and this might be enough to break it.

    You could ask the manufacturing company for a copy of the calibration method that they used to test the load, they should also have UKAS traceable certificates of the test instruments, this may cover you with the house insurance if things do go wrong. Just don't be near it if it does go!

    Mark
     
  12. bob007

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    Sorry nic, but toughend glass can`t be cut, toughend glass is made to measure only.

    Toughened glass is generally two opposing surfaces under tension and any work carried out on the glass after it has been toughened will cause the panel to 'explode'.

    Toughened glass is essentially made in the same way as normal, annealed glass. The only difference is that there is an extra stage at the end of the manufacturing process which adds the greater strength to the glass.

    Whilst the glass is still hot from production (at about 650 degrees centigrade), the outside surfaces of the glass are rapidly cooled to room temperature by jets of water or cold air jets. This cools the surface of the glass quicker than the glass in the centre.

    use to be a glass cutter;



    ;)
     
  13. Jon Weaver

    Jon Weaver
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    Nic (and everyone)

    Thanks for your help.

    As you are going to get your research team to get some figures, I wondered if you wanted to know the exact details.

    The glass in question is 300x950mm.

    Its 10mm thick and has 1 hole in each corner.

    Its supported by a round pole about 2" in diameter in each corner.

    The TV is 90kgs.

    The base of the TV is around 800mm wide.

    I currently have the TV on some rubber sheet, as without it, the plastic against glass gave almost zero friction, and the TV could be moved without any trouble.

    I have been assured countless times that 90kgs is well within spec, but any confirmation of that would be appreciated
     
  14. micb3rd

    micb3rd
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    Generaly to my understanding low frequency bass would not normally effect the glass by resenating because the frequencies are to low for glass vibration BUT you have to remember that shelf is connected to a wall and that wall can transfer vibrations straight to the glass.

    I have seen this happen in my house, I have a small window placed in a door way into a part of the house, I have placeds speakers and subwoofers in this alcove to see the effect of bass and midbass in the room even from 35 hz right up to 150hz + the sound vibrated the window quite badly, so I would be carfull with that overloaded glass, esspessialy if you are going for a quite a big SPL bass system.
     
  15. Nic Rhodes

    Nic Rhodes
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    Right, I heard back from my glass team, there only real concern was whether there were any cracks or scratches, however 100kg is a big weight. Never heard of one going yet and Sony have sold a fair few heavy 36 inchers.
     
  16. Jon Weaver

    Jon Weaver
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    Thanks for the feedback.

    I could of responces:

    To Micb3rd,

    The shelf isn't connected to the wall.. Its actually a free standing unit made up of glass sheets supported by an array of poles.

    The only contact that it has with the house is where it touches the floor.


    To Nic,

    Whilst Sony might have sold a fair few 36" TVs, one thing I might not have explained is that my shelf is a 3rd party product. I would therefor assume that very few Sony 36" TV owners have this kind of shelf... But there must be some..

    Again, thanks for all your help
     

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