Please help me cross over the subwoofers

Discussion in 'Monitoring' started by CherylJosie, Mar 12, 2017.

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  1. CherylJosie

    CherylJosie Active Member

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    Yes, thanks for the final notes. I expect I will be back with questions on EQ, compression, and effects after the next gig.
     
  2. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    Hello, I mixed my first show in 1993. Since then, up to about 2012, I've mixed hundreds of shows, thousands of bands (thanks partly to open mic nites) in venues ranging from coffeehouses to 500 seat theaters to outdoor events with thousands. Most of my gigs have been club level shows. Your biggest problem is the drummer. I see two options, drum some sense into him or walk away.

    That said, I'll add some tech opinions.

    First, having aux-fed subs doesn't relieve you of needing a crossover. Bass instruments have a lot of information above the bass range so they need to be in the mains as well as the subs. Having higher frequencies coming from two different kinds of speakers at the same time is not a good thing due to their physical separation and drastically different phase response. Don't use eq for a crossover, use a crossover.

    Second, ditch aux-fed subs. It's a brilliant idea but for a beginner running a system like that it's a distraction. Distractions can ruin a show. Instead, aggressively high pass inputs. I assume the DL mixer has variable slope, variable frequency HPF. Set it to 12 or 18dB/octave and sweep up until the tone degrades, then back it down a touch. Also consider running low shelf eq on vocal mics, cutting several dB up to 300-1000Hz, to offset proximity effect. A high pass filter on the whole system can be a good thing, though perhaps those powered subs are doing that internally.

    Third, learn how to eq bass and kick. Kick drums often have too much energy around 200Hz. Set up a 1 octave filter with some boost and sweep it around between 100 and 300Hz. The mud should jump out somewhere in that range. Cut it as you see fit. Find the "click" frequency and boost if needed for clarity. I'll usually add a low shelf below the mud to fine tune the bottom of the kick. That could be a boost of cut depending on the situation. With bass I often find too much from 50-100Hz and down, perhaps because it's hard to hear that range when standing next to the amp. Then there's generally a "definition" frequency that you can boost, which could be anywhere between 300Hz and 2kHz. Getting clarity from bass instruments is often enough to get people thinking it's louder than it really is.

    Fourth, be aware of the room's contribution. For example, with the subs on the floor they are in "half space", where you get a low frequency boost from boundary effect. Move them to a wall and you've in quarter space for an additional boost. In the corner of the room they are in eighth space for yet another boost. Aim your mains at the audience and away from walls. The back row is a pretty good target for the tweeters.

    Fifth, control the stage volume. Do what you can to get people to set their amps to reasonable volume. There are various amp positioning options that can help. The drum/bass/guitar mix should be balanced with the PA off, at least it should be balanced on stage. This will not only make it better for you out front, it will make the monitors work better.

    Sixth, mix to the stage volume. Whether or not the band controls their volume you'll still have some kind of stage volume to deal with. When setting up the mix (after line checks, tweaking kick eq etc.) start with the vocals and direct instruments. Make sure they get over the stage noise and then fill in other stuff as needed. If an amp is loud out front don't put it in the mains.
     
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  3. CherylJosie

    CherylJosie Active Member

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    If anything, I think involving a technically inclined noob to run the sound represents more of a risk than the drummer himself being involved. ;) He got more than he bargained for. We are both doing our best to come up to speed and he seems open-minded, if cautious about making major changes at this point.

    Thanks for your other, more technical input. I was vaguely aware of most of it, but having never done it or studied it in detail, I can use all the help I can get.

    The last club we performed at hires their own pro sound guy for the gigs, and he re-did all the input EQ (amazingly fast) during the sound check. So we did actually get some pro help, and it cleaned up lots of the 'mud'. I think I learned more about pro sound in 20 minutes of watching him pull curves around with his fingertips than from any reading I did so far. Thanks to you, now I have a written explanation for some of what I observed.

    The lack of crossover may be technically wrong, but the equipment is good and the configuration is working OK compared to most of the acts performing at these venues. I guess that is more a condemnation of the competition than praise for us, but everything is relative.

    The subs have undefeatable 120Hz 12dB/decade lowpass internally, probably cascaded (Linkwitz-Riley) Butterworth with 6dB of attenuation and 90 degrees of phase at 120Hz. They probably are expecting the other half of that crossover to be a cascaded second-order Butterworth highpass and a phase inversion between mains and subs to keep the amplitude smooth at crossover.

    I am not sure if I can exactly implement that cascaded highpass for the mains on Master Fader. I may be limited to a single-stage 12dB/octave Butterworth highpass with 3dB of attenuation instead of 6dB at 120Hz. Maybe I can simulate that integration with SPICE and pick a slightly modified (higher) cutoff frequency to compensate/optimize. This seems an unorthodox approach, but it does avoid me having to make major changes to an already working setup, or invest in an external crossover highpass for the mains.

    Another potential consideration is that, lacking any strong reason to 2nd order highpass every signal going to the imminently full-range-capable mains at 120Hz or thereabouts through a crossover, I am reluctant to put the highpass on the output channel feeding the mains. I think I prefer to keep the mains full range for inputs that do not use the subwoofer, and highpass all extraneous low frequencies with the individually tuneable inputs EQ instead.

    If the inputs will not let me split the EQ between main and aux, maybe I can run the bass, kick and keys through a submix and use that EQ to implement the highpass for them on the mains. I would need to check that the submix highpass on the mains path does not perturb the aux-fed subwoofer output.

    However I do the crossover, actually implementing it and measuring the result is going to be a challenge. The band is on the other side of the SF Bay from me, and I am not, so far, part of their practices.

    I am still a warm body to run the faders, and making these structural changes to the system is more than the drummer bargained for. In a way, my taking ownership of this is intruding.

    I think he can probably comprehend the issues just fine because he is an intelligent tech professional, but everything is ultimately up to him, including how much effort we put into conforming to the original design intent of crossover.
     
  4. CherylJosie

    CherylJosie Active Member

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  5. dvdhawk

    dvdhawk Well-Known Member

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    Hello again, I'm glad to see you've been sticking with it.

    The pseudo-crossover looks reasonable. The Frequency Response of the KW181 is spec'ed at 40Hz to 112Hz (within -6dB), so the only thing I'd question is the efficacy of boosting the frequencies below 40Hz so much on the graphic EQ. The sub cabinet cannot produce 20Hz, but may die (and waste a lot of its very limited electricity reserve) trying. Class D amplifiers, like the one built into the QSC, are not usually great at riding out big transients. They are trying to convert wall-current into audio as efficiently as possible to eliminate the need for big heavy transformers. Trying so hard to amplify the octave below the range of the speaker seems like it would waste a lot of energy with no benefit.

    That's my opinion anyway. I'll be curious to see what some of the other guys have to say.

    My only complaint with the video is the editing, or lack of editing to be more exact. The band sounds quite good through my computer monitors. Outdoors you have to fight the wind and weather, there's nothing you can do about that, but you don't have to fight any acoustical boundaries (walls & ceiling) so it's a good gauge for how good the band can truly sound. The flip-side of that being, not being confined within a box means you need a good bit more PA to fill the same size area.
     
  6. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    I agree the pseudo crossover seems ok.
    That graphic EQ on the sub aux is nonsense. You are sending signal with frequencies below 100hz. Why would you apply EQ on upper frequencies ??
    And that boost up to 20hz is also not a good idea.. most of what the audience feel in the low end happend between 40-70hz. This is were the goodness is. So most of the time we don't need frequencies below 30hz and boosting them only make the subs amp work harder for nothing.

    Looking at the video, I would suggest putting the FOH on the floor if possible to gain stage space and the subs will disperse better.
     
  7. CherylJosie

    CherylJosie Active Member

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    Thanks. I halted the bass lessons though. Too much pain. Maybe later.

    Yes, agreed. The house music had lots of low bass in it though and the EQ seemed to help without causing issues. I was more concerned about blowing a breaker than anything. I think we were powered off a single extension cord.

    The camera is garbage. I only took video so that I could refer back to it and track my improvement. The wind is from the camera mic not the PA.

    Eventually I would like to get some wireless gear so I can mic/vid the venue properly. Not sure how to do that. Some of the performances I have mixed have really great moments that would work well for promos, but the recording needs to be good.
     
  8. CherylJosie

    CherylJosie Active Member

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    It was not intentional. I stored a 'house curve' of -1dB/octave and recalled it for both the mains and subs before tweaking.

    Thanks. I was not sure exactly how to handle it, so I figured 5dB was 4x power and if that was going to cause trouble I would probably hear it before it caused any damage. The biggest issue was the power alley.

    The gig was not loud at all. Only the kick had a mic on it. We were close up and there was a wall behind the audience reflecting some bass back. If I had to really crank the volume I would have cut it back more.

    I had some compression set up for limiting but probably the biggest risk was losing a sub on a mic drop or loose cord. The compression had some attack on it. Would not have stopped it completely.

    I should probably have just cut the bottom two bands of the graphic.

    The mains would have ended up too low.

    The spec says do not pole mount them, they have to sit on a solid surface because they are so heavy and tall.

    I discussed it with Charlie while evaluating the power alley. I wanted the subs close together but they were needed as speaker stands.

    He wants to replace the Mackies eventually with something smaller that he can pole mount, but the thing is that particular Mackie sounds really, really nice compared to the other (smaller) options...
     
  9. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    Most people don't hear 20 hz givin that the equipment can produce it, that was my point. For exemple, if your Amp can but your speaker can't you can blow up your amp without any warning.
    Not only that, most of the electric power would be drained and litle left for the frequencies people can hear.
    I saw many people pullout 20hz completly and keeping 25 half way down. (that's what I do)

    You could construct some kind of solid wood crate to put between the subs and the tops if you had the energy and motivation. ;)
     
  10. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    I used a 40Hz HPF on my subs. They sounded better and got louder.
     
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  11. dvdhawk

    dvdhawk Well-Known Member

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    I don't doubt you hear something when you boost the 20Hz, but it's going to mostly be the harmonic an octave above you hear coming from the speakers. I agree with bouldersound's approach. I would refer you to the case of die vs. trying referenced above.**
     
  12. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    I don't think anyone said that those instruments are not going into the subs and main at the same time.
    I agree a real crossover is still a great tool by todays digital mixers like the Yam M7CL and LS9 have dedicated sub output that can simplify the soundtech a lot. (by not having to go under the stage to tune the crossovers and hear the results right away from the mixer position)
    Also having a dedicated mixer output for subs meens you only send instruments that need subs in it which makes the subs job easier.
    Knowing this, I've yet to be convinced one is better than the other.

    Of course if your venue gets many people at the mixer and you are not sure of their level of knowledge, you must have a pre-tune crossover to prevent them from screwing up. That's why on many shows I work on, we use both ; crossover and dedicated sub output.
     
  13. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    I'm not sure it was clear whether or not they were at that point, so I made a general suggestion.

    I don't care whether the crossover is in the mixer or a separate device, but using a graphic eq (which I interpreted to mean a fixed frequency, fixed bandwidth set of filters) is not a good choice. If she was referring to a parametric eq with HPF and LPF filter options and graphic display, then I have no problem. It might just be a difference of terminology. High passing the tops is mandatory.

    In my opinion, having fully adjustable HPF on every channel and matched powered speakers makes aux fed subs redundant. Just run the mix into the mains using the built in crossovers and high pass your channels as needed, then use the time and energy you just saved not dealing with aux fed subs to focus on mixing the band. It's not a dissertation for a PhD, it's live music.
     
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  14. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    Very good point !
     
  15. Boswell

    Boswell Moderator Distinguished Member

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    You have to be really quite careful about using parametric EQs or (worse) graphic EQs for speaker crossovers, as they are likely not to have phase continuity through the crossover region. An LPF at the crossover will be have a phase lag at the 3dB point, where an HPF set to the same frequency will have a phase lead, the result being double the phase difference between sub and mid. It's one of the reasons that Linkwitz extended Riley's cascaded Butterworth filter for crossover use (Linkwitz-Riley filter) to maintain phase continuity through the crossover frequency. The L-R crossover uses 24dB/octave filters arranged so that they are 6dB down (not 3dB) at the crossover point.
     
  16. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    It's standard practice to use delay to compensate for acoustic and electronic timing mismatches. I'm pretty sure I brought this up earlier in the thread, or in this poster's thread on another forum. But given the geometry of a PA system there can only be one spot where things are actually in phase. so getting too picky is futile.

    Steeper slopes do help minimize the fact that areas either side of the crossover will be out of phase, because the relative levels are more different. With digital processing it's theoretically possible to use FIR filters which have no phase effect, but they have other disadvantages (ringing, higher latency).

    In my opinion, live sound is about making it "good enough +10%" rather than technically perfect. If you get wrapped up in the technical you lose sight of the artistic and social aspects. It's about people having a good time, and technical perfection isn't required. More than adequate is more than adequate.
     
  17. CherylJosie

    CherylJosie Active Member

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    Apologies for the delay getting back to this.

    I think that the reason I ended up without any HPF in the graphic for the subs is because the frequency overlap in the 20/25Hz bands tweaks higher frequencies somewhat too. I do not think it was harmonic distortion I was hearing, though I cannot swear to that given my distorted hearing etc. The gig was not loud at all and I did not hear any bite out of the subs but I did notice the bottom drop out a little when I cut 20/25Hz.

    I mixed another indoor gig for East Crescent:

    The camcorder was in a bass null in the rear quarter of the room. The bass was far stronger than the camcorder lets on, and somewhat muddy on this gig. The bass guitarist was not happy with his sound and neither was I, plus the kick sounded flabby, but that was primarily due to me being in charge of mains and monitors mix plus full initial mixer setup for this gig. Charlie's iPad was discharged and I had not done any preliminary setup for this latest gig so I customized his last practice session off the mixer.

    After reading your helpful comments today, I added the subwoofer 30Hz HPF so I can independently tweak the LFE cutoff frequency.

    The new subwoofer parametric, with 30Hz HPF and 80Hz LPF:
    IMG_0384.PNG
    IMG_0384.PNG

    The graphic EQ, with -3dB correction at 80Hz crossover (-1.5dB for one band either side of crossover also) overlaid on the -1dB/decade house curve template i.e. -0.3dB/band (this same template is used on both the subs and top cabs):
    IMG_0385.PNG
    IMG_0385.PNG

    The top cabs parametric, with 80Hz LPF and 15KHz HPF (I kill everything above 15KHz because my hearing tops out at 10KHz and I cannot hear issues up there so I would rather just cut it entirely than rely on bleeding ears in the audience to notify me of a problem):
    IMG_0386.PNG
    IMG_0386.PNG

    The vocal subgroup proximity cut EQ (I did cut another 2dB or so and it helped with the nasally sound):
    IMG_0387.PNG
    IMG_0387.PNG

    I am thinking I might move the crossover down to 70Hz for the next gig too. I am still undecided on that. 70Hz seems a little low, but it would help assure that the guitars have full bandwidth.

    The Mackie top cabs and the QSC subs are not compatible regarding built-in crossover. Neither has the HPF. The least expensive option is the board so that is what we are doing at this time.

    I think I have this crossover pretty well figured out now, except the delay, and I will work on that once I have the correct mic and PC to measure delay with. Until then I might try experimenting a little by ear.

    The dissertation comment is apt. I do overthink these things, at least until I understand them and set up a rule of thumb. Known issue, personal style, unlikely to change any time soon.

    This forum has been wonderfully responsive and helpful. Thank you all.
     
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    The New AT5047 Premier Studio Microphone Purity Transformed

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