Questions about using REW

Discussion in 'Subwoofers' started by Dave964, Jan 11, 2008.

  1. Dave964

    Dave964
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    I used REW last night, and have added 4 filters to my BFD - and according to the new plots, it does look a little better. However - there were a few things I didn't do and I plan to do it again over the weekend "properly". But I have a few of questions :

    1) I didn't load the correction values for the SPL. HTS seemed to suggest wild varations between RS SPLs and that the correction values might not make much sense anyway. But do people usually load them?

    2) I didn't calibrate the soundcard - because I couldn't. The plot was clearly a feedback loop. The help file mentions my card (Creative Labs Audigy 2ZS) and says I need to disable some advanced monitor recording setting. But I can't find it! Has anybody done this?

    3) I've seen some comments about using main speakers as well as the sub. Should I?

    4) I can use the filter to get rid of the peaks - but I have quite a big dip at about 78Hz. How do I get rid of that?
     
  2. mattkhan

    mattkhan
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    without calibrating your meter yourself you can't know if it's correct or not. It is likely to be in the right ballpark however and more accurate then if you didn't use it. IME the settings one comes up with when you use the wrong cal file sound light at the low end which does tally with what the calibration file is doing AFAIK.

    whether it makes a difference will depend on the card itself and what its output is like. Typically they rolloff at the low and high end but are flat inbetween, if so it won't make much difference. I don't know whether the presence of the monitor makes a difference to actual readings though.

    I only bother turning off my mains on the rare occasions equipment changes otherwise I just run full range each time. My filters tend to be <60Hz anyway so it makes no real difference and saves time.

    if it's pretty narrow then don't bother as you're unlikely to hear it, if it's wide then play around with phase to see what is best. If you still can't do anything then you're probably stuck with it, whether it is audible or not is another question.

    Cheers
    Matt
     
  3. Andori

    Andori
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    I found a spiked dip in my response around 75 Hz, but it seemed to go away when I had the mains plumbed in.
     
  4. Dave964

    Dave964
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    OK - I've done some more runs this evening, and added more filters. Then run again / add more filters etc.

    I've given up trying to cal the sound card and still haven't used calibration values for the meter. I haven't used my mains just because it's easier not to - amp has multi-channel in so I can connect direct to sub in.

    Phase seems to make no difference to the dip but I guess it won't without my mains running.

    Below is my final result. Top graph is without EQ, bottom one is with my final filters. Now I just need to watch some movies and see if it actually sounds any different!

    By the way - I've only applied filters below 80Hz. I assume this makes sense, since the crossover in the amp is set to that?
     

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  5. mattkhan

    mattkhan
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    if it were me, before I listened, I'd REW it with the mains and extend the sweep beyond 200Hz to check the interaction (as you say phase will do nada without the mains active) & the relative levels. OTOH at some point you just have to start listening and enjoying what you have before getting back into tweaking :)

    I don't really understand why you haven't used the meter cal file though, I don't really see how you can have much faith in the low end results without it.

    Also have you thought about applying a house curve? I now use what has been described as a hard knee house curve (odd name, not sure I see why but I mention it for google referencing) as I find the logic makes sense. It is basically applying a logarithmic interpolation instead of linear when applying the house curve in REW, i.e. it looks like a straight line instead of the curved line you get with a simple house curve in REW right now. Practically I suspect it makes no real difference to the sound but the logic is good so I use it :D

    Assuming the cal file ends up generally boosting the low end though then you looks like you've got a nice raw response to work with (just 1 big peak which is often easily tamed). It also looks like you know how to use the BFD to get the response you want so basically you're all set :thumbsup:

    what filters did you use out of interest?

    Cheers
    Matt
     
  6. Dave964

    Dave964
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    I'm actually not sure how to use my mains because I'm not sure what to connect the line out of the sound card to on my amp. When I connect it to Sub in, it works very well - but if I take a regular phono input then it sounded absolutely awful and like it wasn't doing my main speakers any favours at all.

    I'll try it again over the weekend with the meter cal file - and I'll look into house curves too, so the filters may change.

    I've ended up using 11 filters - I couldn't tell you what they are without checking them on the BFD though because I didn't save them - just kept adding filters and re-running until I got the final result.

    Once I've loaded the cal, I'll probably start from scratch again. Doesn't take long - it's the moving the PC downstairs which is the biggest pain.
     
  7. mattkhan

    mattkhan
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    you should just be able to connect from the soundcard line out to any spare input on the receiver/processor. When you say it sounded awful, in what way? You've mentioned some feedback issues with the loopback connection so it might be related to that. It would be worth resolving that, I'm pretty sure I've read threads along those lines on the appropriate forum but never paid much attention to them. A bit of searching should reveal answers though if someone doesn't pop up here.

    that's a lot of filters. IME you can get typically get much the same effect with a lot less, there is always the temptation to keep tweaking to hit the line exactly but it's not really necessary IMO. A few dB here or there don't make much difference (except on a graph) in practice. It's also somewhat futile as you don't *always* sit in precisely the same position (even over the course of 1 film) so the actual response will differ. On that note it can be worth taking readings from 2-3 positions across the listening area and using the average graph to see a composite response then EQ for that. OTOH you may get a big dip turn into a peak in 2 different positions on the same sofa, then you have to choose who gets the good bit :)
     
  8. Nimby

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    If you don't load the matching calibration file for your SPL meter you won't get an accurate picture of your subwoofer's response. The difference can be at least 10dB! This is much more important than soundcard calibration which usually work well into the deep infrasonics.

    Here is the same subwoofer and the same microphone/SPL meter position with various calibration files:

    [​IMG]

    Are you feeling lucky? :)
     
  9. Dave964

    Dave964
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    I've now re-run the tests with the cal file loaded, and adjusted the filters. These settings will stay for a while since the novelty of playing with REW has worn off a bit!

    I raised the gain on the bass a couple of days ago (by 3dB) after using the SPL meter to adjust the levels of all the speakers. In bass heavy moments, this did cause a few rumbles I didn't want. The BFD filter settings now seem to cure that - I wouldn't say the differences are huge, but it does sound much tighter. How much of this is placebo is questionable - but I'm happy enough with it.
     

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