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Removing Room Resonance

Discussion in 'Home Cinema Building DIY' started by jpwhaley, Jan 26, 2004.

  1. jpwhaley

    jpwhaley
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    Having modified my sub with a LinkWitz transform circuit to make go a little lower I am left with a big problem with room resonance around 40Hz. Has anyone had any experience with using Feedback Destroyer or graphic equalisers to remove these kind of problems?

    I have searched around and see that some people use Behringer Feedback Destroyer Pro DSP1124P (around 100GBP) Is it any good?

    Thanks
     
  2. deckard

    deckard
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    Hi there, I too have a problem with a peak at 40 Hz and have just been playing about with a BFD to try and ameliorate it.

    The BFD certainly can be very effective at cutting peaks, but my problem (and possibly yours too) is not just the peaks, but the dips too.

    These are caused by reflections from the sub coming back out of phase and cancelling it's own output. Hence I have peaks at 40 and 80 Hz plus a dip at 60 Hz.

    The BFD cannot sort my problem out, only room treatments can help my situation.

    However, a lot of people have done wonders with their BFDs. Could you post a frequency sweep?
     
  3. MuFu

    MuFu
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    jpwhaley, you're not using an HTPC as a source by any chance, are you? There's a pretty good software solution (Foober2000 Convolver) that can sort this kind of thing out quite easily.

    MuFuy.
     
  4. stegalv

    stegalv
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    hellooo,just reading your thread and the reply, been into home cinema for a few years but only just getting more into the technical bits,can you explain what you are asking about,how do you know the room resonance are at the frequencies you and the reply are talking about,how do you know there is even a problem,i am about to order parts to build my own sub so would be grateful if you could explain in simple terms
     
  5. deckard

    deckard
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    Ok, just a little bit of physics required here...

    You'll be aware that if you blow across a bottle you can create a particular frequency. This is the basis of standing waves (kind of), you excite the air in the bottle at a particular frequency, one with a wavelength the length of the bottle. In fact you excite multiples of that frequency too, ie 40, 80, 120 etc, but the first resonant frequency is the strongest.

    The same happens in rooms. Each dimension of your listening room has has a primary, secondary, tertiary etc resonant frequency that are frequently also called modes.

    This first frequency can be predicted if you divide the speed of sound by the relevant dimension: eg for a 4m dimension 340 m/s / 4.0m =85 Hz, the second frequency is 2x85=170 Hz etc.

    Do this for each dimension and you'll get an idea of your 'problem' frequencies.

    I'd also recommend excel users to get hold of this program to make life a bit easier instead of doing the above :look here. I'd also recommend reading the white papers on that site.

    HTH
     
  6. stegalv

    stegalv
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    thanks for that, but are you actually hearing a problem or theoretically there should be,
     
  7. Gary Lightfoot

    Gary Lightfoot
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    A friend of mine used his BFD to make a notch filter at 42hz so that his radiator would stop rattling.

    You can use it to increase and reduce certain frequencies too. Using a test CD, create a graph of all the readings, then use the BFD to increase/reduce the necessary frequencies until the graph is flatter.

    Gary.
     
  8. deckard

    deckard
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    Not necessarily Gary. You can't use a BFD to raise all dips, some are caused by self-cancellation - reflections coming back out of phase with the main signal - so boosting the main signal only boosts the reflections, ie no effect on the level but increased strain on your subwoofer.
     
  9. jpwhaley

    jpwhaley
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    After annoying my neighbours by listening to test tones and frequency sweeps last night I was able to minimze some of the resonance by changing my funiture & sub postitions. As well as some resonance peaks I am also getting a dip like deckard at ~60Hz. Removing any reflection will be difficult as I don't want to carpet the walls.

    Has anyone got a spreadsheet for designing notch filters or know where to get a Feedback destroyer cheap?

    I am not using a HCPC so I can't use software.
     
  10. deckard

    deckard
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    jpwhaley, I got mine from soundslive.co.uk for £97.23 inc delivery.

    You might want to look here for a filter calculator/simulator.
     
  11. chrisgeary

    chrisgeary
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    resonance is also a problem. you can reduce the peaks with the BFD, but you cant remove the resonance. you need an anti-sub. a sub at the other end of the room running out of phase with the main sub and at about half the volume of the main sub. BFD that too and use it to only generate sound at the frequencies that get cancelled or boosted - use the BFD to cut those desirable frequencies (ie those around the peaks and dips that you still want and don't have resonance problems with). you end up with a kind of push-pull in the room - works really nicely. i have a near flat response in my room where i once had a massive 33hz peak, and i dont have the reverberation associated with it either!

    thus you have solved your peaks and also got resonance control too!. for the record i use a REL Strata III at the front, and a REL Quake II at the back. (the anti-sub needn't be massive as it only needs to generate about half the volume of the main sub)
     

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