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SVS PCI ultra and room response

Discussion in 'Subwoofers' started by Imrahil, Aug 22, 2004.

  1. Imrahil

    Imrahil
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    I've recently made the jump and purchased the SVS PCI Ultra
    * oh boy does it rock * (and I think my neighbors agree :D )

    I bough the digital SPL meter from maplin's and measured the response using 5 Hz incremements from 10 to 100 Hz ...
    It was a bit of a palaver because initially I was using my pc and a test tone generator, but my pc was too loud and it seemed to be interfering with the measurements. So I ended up saving the test tones as WAV's to my iPod and using that I made the measurements.

    Now on most of the frequencies the SPL meter gave a very steady reading but at 2 particular frequencies (15Hz and 35 Hz) it just wobbled about with a slack of almost 40 dB (from 65 to about 100!)
    Ofcourse I can't make accurate measurements when this happens.
    Has anyone encoutered the same problem? is it normal?
    How do I fix it?

    Thanx in advance!
    Elco
     
  2. Ettepet

    Ettepet
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    I think your ipod doesn't do low frequency stuff all that well. If you have no "optical out" on your pc try burning a cd with these tones. A very slow 120Hz-15Hz sweep tone might also help.

    Furthermore you should have smaller gaps in between your test-tones, 5Hz doesn't cut it really. Many people use 1Hz, as do I. In fact I bought several items so I can let my pc do it automatically.
     
  3. Stellavision

    Stellavision
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    I'm asuming you mean you have a PC-Ultra? The PCi is a different model ;)
     
  4. Imrahil

    Imrahil
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    ehm yes I did ofcourse mean PC ultra :blush:
    I'm not using mp3's tho they are uncompressed WAV's
    and I get the same result from the tone generator using my PC (because of the weird response I decided to take the pc out of the equation cos it does make a fair ammount of noise - obviously this wasn't the guilty party tho)

    I spose I'll get a laptop and run the generator to rule it out completely
     
  5. Nimby

    Nimby
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    Bying a laptop just to test one subwoofer is a bit like overkill. (Even for an Ultra) Unless you intend to buy a laptop anyway. :)

    Does your meter have a C-slow setting? This is what you should be using. Do your testing with the meter in your normal listening position at seated head height. (the meter, not you) Preferably without holding the meter in your hand.

    You could move your computer temporarily to another room (or a cupbboard) and use a 10 metre phono cable. Though the computer may be making some noise, it would be continuous. Your meter will show how much noise if you test the computer noise alone (from the listening position) It shouldn't be a serious factor in the sub's response unless well above (say) 70dB. I would simply ignore it otherwise.

    I'd follow the advice to use tones in 1Hz steps. It shows the peaks and troughs far more clearly. 5Hz steps are perfectly okay to get a feeling for the response before going into a more lengthy 1Hz stepping test though.

    If you can save the stepping tones to an album in "My Music" in XP. You can have the tones played automatically as if they were music tracks. Then you can read your meter readings and record test tone frequency response in relaxed style. Instead of running backwards and forwards like a headless chicken between computer and meter.

    If you write down the frequency of your test tones in a column on the left on lined (or squared) paper. Then you only have to write down the dB figure from your meter beside the test tone frequency figure. An occasional glance at the computer screen will show that you haven't lost track of exactly which tone your are using. You can then carry out a number of tests by simply starting another column further to the right on your paper.

    Make sure you measure from the higher frequencies down to lower frequencies. If you start low you might turn the level way up too high. Just to be able to hear something that is really quite inaudible.

    The 40dB variations are a bit odd. Using 1Hz tones might pinpoint where the problem lies. Can you remind us of your room dimensions? Then we can see where the expected peaks and troughs would be most likely to occur.

    Remember to use the correction figures when you have finished response curve. Don't expect a +/-1dB response from your meter anyway. You are simply getting an idea of what your sub is doing in your own room and discovering how that can be optimised.

    Remember to shut doors and windows that are normally shut. These can affect the sub's in-room response. Apart from LF wind effects. Windows can badly affect results. Particularly if a brick comes sailing through after a morning of continuous testing! :blush:

    My apologies if you knew all of this already. :blush:
    But it might help someone else. :)

    Nimby
     
  6. Imrahil

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    Ah I wouldn't buy one just for this, I'll just borrow one from a friend ;)
    I've got the meter setup as required on slow, and C settings, it's mounted on a tripod in my seating position with me out of the way.

    It did say to use a minimum res of 5 HZ but since I had to make samples of each I thought saving out 20 samples was enough work as it was... if I use a laptop I can just use 1Hz increments without too much hassle so I'll definately go for that!
    - and no running around like a headless chicken ofcourse ! :D

    The rooms dimensions are approx 5m x 6m x 2.5m
    I did make a point of shutting all doors and windows when I made the initial readings and I was using the compensation chart outlined in the SVS manual.

    I'll try it again tonight with a laptop and 1HZ increments
    thanx for the useful sum-up!
     
  7. eviljohn2

    eviljohn2
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    If you're using a laptop you don't even need to use lined/squared paper as you can enter the values directly into a spreadsheet. You can also get a meaningful graph out immediately which is nice once you're halfway through and fed up of gathering results!

    Not sure if it's a good idea to use the correction values with the Maplin SPL meter, as far as I know they were only suitable for the old Radio shack model.
     
  8. Imrahil

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    Ah I wasn't 100% sure I needed to use the compensation chart, but since the meter is rated down to 300 Hz I thought it might be necesary.
    Also in the SVS manual it states "All meters are off by a similar ammount depending on the frequencies"
     

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