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Sub setup and room gain

Discussion in 'Subwoofers' started by slingshot, Mar 5, 2003.

  1. slingshot

    slingshot
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    Just been testing my new sub, and I get a reasonably flat graph when plotting the frequency, it drops off to around -5db at 20Hz. However I get a lovely +12 db around 50Hz, this is just were I want the sub to start coming in to cover the mains.

    It appears to be because of the room, I get the same peak, with just the mains (+9db) on their own or just the sub on it's own, and both together is just ridiculous.

    I've tried moving the sub, but this doesn't help, tried drawing the curtains and it's pretty constant +12db wherever I move in the room.

    Any ideas of a cheapish, easy fix to this problem, and moving house is definatley out of the question.

    Cheers

    Slingshot

    If it makes any difference the room is approx 13' X 24' and it's stud and plasterboard.
     
  2. paulfd

    paulfd
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    Hi,
    I have a similar problem in that i get around a 15db increase between 38 and 44Hz. I haven't found anything cheap yet and certainly low frequency baffles in the corners are not an acceptable solution in my lounge. Not wife friendly.

    I'm currently using the AVIA low frequency sweep to reposition the sub (when I'm on my own) to try and increase the gain across a larger frequency range. Currently managed to get the range increased from 38 to 52 Hz just by careful repositoning the sub in to the corner of the room.

    The gain I have is due to the room resonance for which I understand that baffles are one solution the other being the Behringer Feedback Destroyer Pro. See thread http://www.avforums.com/forums/showthread.php?s=&threadid=65439&highlight=feedback. If I can't get the improvement via repositioning then this is probably the route I'll try next.

    Hope this helps
     
  3. deckard

    deckard
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    I have to second Paul's reply, sounds as though you've got a troublesome room mode at this frequency. I get the same problem at 39 and 78Hz (1st and 2nd harmonics). No end of repositioning on my part could get around it.

    The feedback destroyer sounds like the best solution currently, Apocalypse has in fact made a new post since the link given above detailing how he set it up and opinions etc - sorry but I don't know how to link it into this post, have a search.
     
  4. Apocalypse

    Apocalypse
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    slingshot - The BFD will easily sort out that peak you get, a peak around the 50Hz range could cause room boom, I had a huge peak from 50 -63hz but once it was reduced the overall sound was vastly improved. If you have any specific questions fire away by all means.

    deckard - To post the thread in your post you have a number of ways of doing this but I use this one :

    Find the thread in question, place your cursor over the thread and click the right mouse button, from the options that appear select "Copy Shortcut". This saves the link to your clipboard, now when you are posting your reply simply press "Ctrl+V" and the link will appear where ever the cursor is flashing. During the actual typing of the post it will apear in normal text but when you click "Submit Reply" it will appear as a link.

    P.S - If I insulted your intelligence I apologize right now

    Phil
     
  5. deckard

    deckard
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    Wot inteligens? Apoc, cheers m8!
     
  6. slingshot

    slingshot
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    Cheers, so where do I put the BFD (in my system I mean ?) does it mess with the signal on output from the preamp and before power amps, or does it work at speaker level ?

    Basically I'd need something capable of adjusting the volume at a certain frequency on both outputs to the main speakers, and a line output to the sub. Is this possible ?

    Cheers

    Slingshot

    P.S. tried to look at the site for info, but it's currently down, so maybe the answers are all on there.
     
  7. EvilMudge

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    The BFD is a line level device, so put it in the chain between pre-amp and power or processor and sub if using it to control a subwoofer. For gods sake don't feed it at speaker level or you will most likely kill it.

    Trouble with using it in stereo mode is that you can adjust two channels and two channels only with it, so to get it working with two satellites and a sub, you need to split the signal to drive both your sub and the power amp, then adjust the subs crossover until it blends with the satellites, then start playing with the filters on the BFD.
    EDIT: You can't then adjust the filters seperately for the sub and the mains - around the crossover frequency things could get interesting.
     
  8. Apocalypse

    Apocalypse
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    slingshot - To hook up the BFD requires 2 x phono cables and 2 x phono-->6.3mm jack adaptors (these are only £2 each). The other hook-up method is 2 x phono-->XLR cables but this option is more expensive as you can't use your existing phono cables.

    You can see the actual connection in this article here

    There is a very slight delay to the signal once the BFD is added to the system so you have to change the distance you've set the sub at and bring it in 1 foot. IOW my sub setting used to be 7 feet in my processor but now it's set to 6 feet to compensate for the delay of the BFD. I've had no indication at all of any signal problems, dance music is handled with ease and accuracy.
     
  9. Apocalypse

    Apocalypse
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    You've hurt my brain now Evil, the BFD in this instance is only used on the sub so it has no effect on the mains or the power amp. I think that's what you were saying but I'm really hungry and I can't think very well atm :laugh:
     
  10. EvilMudge

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    Sorry Phil! Get some food, and when you get back there should be an explanation waiting for you.

    1. Pre-amp outputs go into BFD inputs
    2. BFD Outputs use a Y splitter to feed into the line ins on the back of the Subwoofer and the Inputs on the Power Amp
    3. You then set the crossover on the sub so that it comes in below the stereo pair.
    4. Now you can play about with the filters to tame any room modes above and below the crossover between sub and satellite.

    So now if the crossover is at 50Hz, you can adjust the frequency response for both channels above fifty, and for the sub below fifty. That way if you've got a room mode at 65Hz, but only the left speaker excites it, you can tame it. If you've got one at 35Hz which the sub excites you have to set a filter at 35Hz for both channels.
     
  11. Maukka

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    This sounds something you should be able to cure by adjusting the phase control on the subwoofer. This might also be done with your receiver. At least the Sony STR-DB1080 alters the sub's phase when you change the distance setting for the sub. It has come handy for my HGS-18 only features a 0°/180° phase switch.

    I've managed to flatten a nasty 50Hz bump (+10dB) in my room with this method. My sub is crossed over at 40Hz and when slightly out of phase with the mains, the freq. response is quite nice with only about +3dB bump at 50Hz. This is of course with music only. When watching movies, the Sony handles the bass management, but I've never been able to achieve the same results (re. frequency response) that way.
     
  12. Godfather

    Godfather
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    I thought you were supposed to increase the distance you put into the processor/amp:

    "Something that you should remember to do when you are setting up your BFD is add a foot to the value you enter for sub distance in your pre/pro or receiver set up. The 1 msec DSP processing delay in the BFD would account for approximately a foot in distance. If you add a foot to the distance you tell your pre/pro or receiver that your sub is from your ears, then it will advance the sound the 1 msec required. This will compensate for the BFD's delay."

    :confused:
     
  13. Apocalypse

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    Quite right, I got it the wrong way round, blame the momentary hunger :suicide:

    EvilMudge - I see what you mean (now that I've eaten) and it's an interesting angle, not sure if I'll be trying it though. Any articles on it by any chance as the BFD articles I've read all use the method presently used by me and others?
     

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