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Sub in an alcove - bad idea in principle?

Discussion in 'Subwoofers' started by theo cupier, Oct 6, 2005.

  1. theo cupier

    theo cupier
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    I currently have my sub just in front of a chimney breast which is in the centre of the longer wall in my AV room/living room. It is on (carpeted) suspended wooden floorboards, but is on a marble slab and I have 6 halved squash balls providing isolation to a reasonable degree.

    The hearth in the chimney breast is being converted to an empty, plastered alcove which will be about 3ft wide, 18in deep and arched to a max of 3ft high. The hearth's base is brick over concrete and will be finished off with a decorative marble or stone slab.

    There will be a wood/board shelf near the top of the alcove for my centre speaker which will be directly below my plasma screen. The listening/viewing position is 8-9ft away, along the opposite wall.

    In terms of maximising the space in the room, it would be great if I could put my sub (XLS200DF) into the alcove and, on the face of it, I would expect a better result from the stone-brick-concrete base (and the fact that the sub is centrally positioned - I know bass is less directional but I have found I prefer the sub positioned between the fronts rather than to the sides).

    Are all of the above plus-points for the alcove location valid? If so, are they all going to be outweighed by any unforseen (by me) downsides to this location? Would I be better leaving it where it is/was? Or should I aim for 'anywhere out of the alcove'? Would the sides of the alcove be better than in the middle?

    I know the ultimate answer is 'try it and see' but a bit of guidance and pointers before I try to get too clever and move everything else around to accommodate this would be most appreciated.

    Supplimentary questions for bonus points:
    1. The chimney breast/alcove are on the party wall with my neighbours. Is putting it in the alcove likely to transmit more or less bass to them? The house is 1930s brick built.
    2. Should I be using the rubber feet or spikes if it is standing on either the current slab or the larger slab in the alcove?

    Many thanks in advance for your advice.
     
  2. binbag

    binbag
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    From what I've read of room acoustics you'll be booking a ticket to Mode City. Putting a speaker next to a wall introduces a reflection from that wall at 1/4, 1/2 and the whole wavelength of a note it produces.

    As it crosses a room this reflection interferes with the original note it either adds to it or cancels it out. These are known as standing waves and are fixed by the frequency used and the size of the room and the only real variable - the position of the speaker. If you are at a point of cancellation then you'll get a diminished response at that frequency.

    If you put your speaker in a corner equidistant from the walls then you get reflections from one wall interacting with reflections from the other wall and if you have the roof at twice or four times the distance then you'll get reflection interaction from that aspect as well. So what you could end up with is lattice of places within the room that you have no bass at some point your frequency response and possibly large peaks at other frequencies as the 'adds to' reflections gang up too.

    You should still try it as your seating position may be unaffected though.

    Supplementary: 1) More bass unless there is a clear cavity in the wall. 2) Whatever gives the most stability.
     
  3. theo cupier

    theo cupier
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    binbag - thanks for that. Whatever location around the room the sub sits, it's going to be near a wall (if only to keep the children away from the exciting knobs and switches on the rear panel). It's question really of whether the alcove worsens this or not.
     
  4. Nimby

    Nimby
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    I would think that the frequencies, at which your sub is operating, would be too low to be reinforced by the chimney/alcove.

    I'm not keen on party walls for speakers and subs as it increases the chances of the noise carrying through the wall and disturbing your neighbours. If you can find an alternative then I would turn your back to the party wall. You can always bung your equipment rack in the alcove if it helps. I know it's not always possible to rotate your listening triangle 180 degrees but do consider any alternatives.

    If you know your neighbour you could try playing a track with a repetitive bass beat and then going next door to see how much can be heard. Never assume your neighbour will compain to your face. He may simply be polishing his arsenal of weapons and sticking pins in your effigy as he waits for the right moment to strike with deadly force. :suicide:

    Can you hear his TV through the party wall when it's quiet in your room? If you can, then don't put your speakers or sub on that wall. It's just asking for trouble. At least with brick you have a chance you are well insulated sound-wise. But don't assume anything without checking.

    Nimby
     
  5. theo cupier

    theo cupier
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    The neighbours are louder than us (teenage kids win hands-down over toddlers, although we have the slight edge between 6-10am on weekends) and I think the room adjoining my AV room is not used much by them so this was only a side issue (to make sure it didn't seem like we were retaliating for another late night rappers battle).

    The present listening / viewing triangle is the only practicable one really, due to the position of doors, windows and chimney breasts etc. The speakers are either side of the chimney, about 5ft apart, it's really a question of whether the sub goes in the alcove on the solid floor or in front/beside it on the suspended boards.

    The only other options (beside the viewing sofa or against a short wall) would involve new high and low level cables to the sub which I can't afford right now.

    By the sounds of it, Nimby, the alcove shouldn't do anything actively detrimental to the sub so I will give it a go and let you know how it gets on (when it ever gets finished!)
     
  6. binbag

    binbag
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    Certainly not at the lower end; a 20hz wave is something like 56' long. But they shorten dramatically as you raise the frequency - 30hz is nearer 30'. So its possible that 1/4 wavelengths of e.g. 125 hz will interact.

    The best thing to do is to stick it in there and run a sweeping test tone.
     
  7. AngelEyes

    AngelEyes
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    From my own experience of placing a sub in an alcove I would suggest you are best off buying the BFD now and accepting a lower output than you would get in an 'open' environment. That isn't a problem if you sub has plenty of power (just turn up the gain) but if you already run it near it's limit you may find the alcove takes too much away.

    I am not suggesting you will have the same issues but I had a lot of trouble getting things to work in an alcove. I have now given in and taken it out.

    Try it and see but use an SPL meter so you can see what the affect is before and after moving it to the alcove.
     
  8. Nimby

    Nimby
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    I told you to raise your sub in its alcove to let it breathe better.
    But if you ignore my advice then let the BFD be upon you! :devil:

    Nimby
     
  9. AngelEyes

    AngelEyes
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    Well I did setup an oxygen mask but it hasn't made any difference, not even the Sinex :(
     

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