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Fluctuating Sub tone during avia test?

Discussion in 'Subwoofers' started by Dre, Oct 19, 2003.

  1. Dre

    Dre
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    This can't be normal can it? I decided to set up my system ( Velodyne CHT-10 and Gallo Nucleus) with Avia and a trusty SPL meter. All was fine until it came to the speaker and sub test. I found I could calibrate my Gallo at a constant tone aroung 85db but when it came to the sub this fluctuated by abouted 10db, i.e it started around 85db then quickly rose +10db and then slightly down and back up...so there is no way I could calibrate this accurately. Also when doing the sole LFE test on the sub nothing is heard but when doing the LFE crossover from my speakers to sub this is fine? Could you kind gents and ladies provide some advise to this novice. I;m sure that the fluctuating sub can't be right.

    Thanks in advance.

    Dre
     
  2. Stellavision

    Stellavision
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    I am afraid test tones on a sub do fluctuate.
    Spl meters are'nt really designed for handling the sort of frequency's that subwoofers produce.
    Best you can do is set the spl peak or take an average reading from -75db or whatever db you are using. I would use this as a guide and then probably tweak it by ear.
     
  3. dunkyboy

    dunkyboy
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    I've noticed this too - is there an accepted way of measuring it, then? I mean, should we use the peak, or the mean (or the median??), or what? I'm assuming it's the sub itself (or the room?) that adds the fluctuations, rather than the signal itself fluctuating... is that true? If so, what is the cause? And what can be done to minimise it? (Surely a sub's not very accurate if a stable signal is made to fluctuate wildly...)

    Cheers,

    Dunc
     
  4. Crustyloafer

    Crustyloafer
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    I've always tended to guestimate the average reading that the spl meter is displaying. So if it swings between 65 and 85dB then I'll recognise it as being 75dB. Seems to work pretty well so far.
     
  5. Smurfin

    Smurfin
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    I think you may be confused.

    If you play a constant test tone i.e to calibrate your speaker levels, then the most the SPL needle should jump around on the subwoofer is 3-4db, so it's easy to take a reading.

    It's never jumped around to the extent of 10db EVER, and it should measure pretty much the same every time.

    However, you may be using a frequency sweep rather than a constant test tone . If you are doing this, then the needle may easily jump around 10db, depending on the frequency response which is affected by both subwoofer positioning and room acoustics.

    If this is the case, then welcome to the lonely and frustrating world of subwoofer positioning, and the BFD:D
     
  6. Dre

    Dre
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    Definately fluctuates on a constant test tone, even the one on my Marantz SR4300 goes from -2 to +2 and freqeuntly peaks to +4...so is this to do with the positioning, if not what can cause this? HELP!!!!
     
  7. Dre

    Dre
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    Thought I would bump this up to the top as I had no response...cmon lads give a novice a helping hand ;-)
     
  8. Stellavision

    Stellavision
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    I too find it difficult to imagine how you would get as much as 10db fluctuations. All of my subs have wavered 4 or 5 db tops.
    If you are stuggling with the spl meter then setting by ear will be your best option. A great many people don't actually like the spl calibration and prefer the by ear method. Spl meters don't get on that well with lower frequency's so sometimes by ear is the best way. Choose a scene on a dvd like the Lobby from the Matrix which has a big bass riff the initial gun fight, and there's quite a lot of other lfe thrown in there. Mess about with your sub untill you are happy that you have enough bass/lfe without it being too much. There are of course other demp tracks to use as a guide.
     

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