EQing stereo subs

Discussion in 'Archived Threads 2001-2004' started by Stephen Dodds, Mar 21, 2001.

  1. Stephen Dodds

    Stephen Dodds Second Unit

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    When EQing stereo subs should you do each one individually, or have the same settings for both?
    Thanks
    Steve
     
  2. Wayne A. Pflughaupt

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    If they are identical subs and in the same location (i.e., corner), it is fine to adjust them the same. However, if you are really “picky” there might be some miniscule differences between the two. For instance, if they are stacked, the sub on top might not have quite the boundary reinforcement as the one on the floor. However, the difference is probably not enough to worry about.
    If the subs are in two different locations, they need to be EQd separately, because they will not have the same room response, even if they are identical.
    However, Stephen, take it from one who knows, it is a really bad idea to have the subs in different locations.
    To begin with, EQing separated subs is very difficult and time consuming, at least ten times more work than one alone. First of all, I wasted many hours trying to EQ them independently with an RTA, only to find that with test tones they were miles off the mark. I had the best luck just using an SPL meter with test tones. You have to get dB readings for each one independently and EQ each not only for room response, but to compensate for shortcomings of the other. Then when you take dB readings running both simultaneously you will find the results are inconsistent with the EQ settings you set for them separately. In other words, if you were able to EQ each for flat response independently, when they are run together response is no longer flat.
    At the end of the (several) days, the results will never be optimal. This was my experience with a pair of identical subs. I wouldn’t even want to think about precisely EQing non-identical subs.
    In my situation, I wanted symmetrical placement, left and right. The right sub was in a corner, the left was on a long, flat wall. The right sub had a sizable hump at 40Hz, but after that was tamed it would get down to 20Hz easily. The left sub, however, was rolling off below 40Hz and needed to have the lowest bands boosted to get output down to 20Hz.
    The results were fine for music, but during demanding passages with DVDs (like the explosions and depth charges in U-571) the left sub would regularly bottom out. I had to reduce the low end boost, but then, combined, the subs obviously suffered significant dB reduction below 40Hz, because only one sub was “putting out” that low. To compensate I tried boosting the lowest bands on the sub in the corner, which restored extension, but then that sub would bottom out.
    A couple of weeks ago I finally bit the bullet and moved both subs to the same corner, and the improvement was immediate and amazing. I knew I was losing something with symmetrical placement, but I had no idea it was so much! Gain was significantly increased. I was able to turn the sub level up at least 4dB, and run the whole system at a higher volume higher than I have had the nerve before! Now I have so much energy at 20Hz the couch vibrates (and so do the viewers!). I measured an ear-bleeding 107dB peak when I re-played U-571 the other night! The sub amp (325 watts per channel) was clipping, but the drivers never bottomed out. I have them stacked, and initially I have simply duplicated the right sub’s EQ on the (former) left sub. I’ll eventually get around to testing and EQing the other sub separately, if need be, but if I did not have the capability to do that I’d be happy with the way it is right now.
    Hope this helps,
    Wayne
     
  3. Stephen Dodds

    Stephen Dodds Second Unit

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    I have a slightly different situation. I have a pair of the Infinity Prelude MTS speakers. They have a satellite portion which attaches to sub sections. Each sub section is an 850 watt amp with a 12 inch woofer.
    Extension and volume are not a problem, I have a Velodyne sub I use for LFE and sides and rears and the Infinities go deeper and louder. My room helps here as I have a big peak starting at 30Hz.
    I'm more interested in getting flat response for music. The Ininity subs come with a built in single channel EQ (RABOS) but I'd prefer to do it properly with a Behringer FD or similar. I also like and notice the difference in stereo bass.
    I realise it will be tricky, but a FD is cheap, and it's only time. I have Spectraplus to play with too.
    Thanks for the tips, I'll try and do each sub separately and see what happens.
    Steve
     
  4. Jones_Rush

    Jones_Rush Stunt Coordinator

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    According to what Wayne say, I think that if you want to place your subs at different locations (one left, one right) it will be best to send tone signals to both, simultaneously, and measure the output at your listening position. When you'll finish measuring, you'll have a graph which will show both subs working together. After this, you should apply the EQ to flatten this graph. (you'll have to use an EQ device which can work with two channels simultaneously)
    Shortcomings:
    1) You won't be able to manually compensate with one sub if the other sub has shortcomings (ie - dips). On the other hand, since the chances that both subs will have dips at the same frequency at your listening position are small, you'll get a better response than with only one sub. Your dips won't be so sharp (although you'll have more dips, but they will have less impact since you're using two subs).
    Gains:
    1) You'll get a flat frequency response from both subs when playing together at your listening postion (as flat as your EQ device enables)
    2) You'll get approx +3db at most frequencies in comparison to using only one sub.
    3) If you run the subs with a stereo signal, you won't have any localization problems.
    It seems to me, like Wayne said, that it will be a very wrong idea to try to independently flatten each sub for flat frequency response (when using them at different sides), since this way, you can't take into account the changes that one sub will have on the other while both running at the same time (boosting, cancellation...)
     
  5. Wayne A. Pflughaupt

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