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[Jay Barracato]

Once the sound is loud enough they will hear it from bleeding into other mics. I find the longer feedback goes the harder it is to trace the source.

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[Art Welter]

We were using EV ZLX-15P that has a built in crossover.  I just looked no specs on it in the manual.  I assume it's a 3rd order Butterworth, that's a bunch of assuming.  What concerns me is my fundamental assumption that this crossover was 3db per octave which I now see is wrong.

Something I thought I knew and would have argued to the death, major sigh

Please feel free to tell me that I am all wet on my bass trap theory, that is also a damn good point on the kick gate.  Although from what these guys described this bass wave on stage was visceral and building very fast. 

Scott,

Yeah, when a drum head starts to resonate from a feedback loop, the amplified output from a double 18" can get visceral very fast.

At any rate, the crossover contained in the top cabinet simply crosses between it's woofer and tweeter, on the EV ZLX-15P that is somewhere around 1500 Hz, so it (or it's slope) has no bearing whatsoever on your kick drum's acoustical/electrical feedback, which was caused by too much gain, then masked by the room response and compression making you unaware of it.

BTW, a "3rd order Butterworth" electrical crossover (nominally 18 dB per octave) can result in anything from around 12 to 36 dB per octave acoustical crossover. You are going to have to learn a lot before you will be able to properly set up your DriveRack unless there happens to be some pre-sets already in place for the tops and subs you plan to use.

Unless you had set the EV ZLX-15P to "speech" mode, it has response down to 55 Hz, so would be overlapping with the sub. You don't even mention whether the sub was crossed over, though even crossed over properly, too much gain (whether from either sub or top or both) would still result in the same problem you experienced.

Also, the overlap output of  the sub and mains is most likely not in phase (the DSP used in the ZLX-15P and it's tuning frequency would reult in a different phase response than the 2 x 18"), which can "steer" the response of the LF in directions other than forward, you may have been sitting in a cancellation zone caused by the speaker's interaction or room modes.

Anyway, you are learning- brings me back to the first time I experienced a similar problem (only on a floor tom) back in 1977, some things like room acoustics, drum resonances, and physics never change.

Have fun!

Art

[frank kayser]

I'll chime in here as someone relatively new with the electronic analysis tools.


I agree the tools like SMAART and others are great tools for the live engineer.  That said, my experience is the various tools are quite addicting visually.  Of course, that is my problem.  As the night settles down and as I tire, my attention to the band can waver - being replaced by my attention to "all the pretty colors".  Yes, I'm making lite of this a bit, but I know me.


I know I need to make a conscious effort to limit the time of my gaze/analysis of the electronic aids, and concentrate on the band's performance.  Even when I'm frantically trying to solve a problem, or fire the band up in front of an audience cold (no line check, no sound check, no ringing of the system), no matter how frantic, I have to look for queues/problems from the band.  I find one simple trick to help a bit : put the monitor high enough in your field of view such that it is just below your view of the performance and practice using your peripheral vision for both the screen and the band.


In a nutshell I'm saying it is easy to get heads down in the computer work - you have to be aware that is a problem, and work at it. 


As far as that low feedback, I have a problem with that at times - I find it hard to pick out of the mix, seems to "hover around the ankles" (I know it doesn't but that is how it seems).  I find it the hardest for me to locate and attack. 


frank

[Scott Holtzman]

Good stuff Art, couple of things.

I know that the Butterworth is an analog filter design however I thought that DSP's tried to simulate the characteristics of the complex LCR networks.  For example an older Peavey digital crossover has a 3rd order Butterworth or a Linkwitz Riley curve selection.

With regard to the ZLX's they indeed do have a crossover for the subs, in the menu you can select three crossover points of 80, 100 or 120 and lastly it has a mode for the EV ELX-118 sub.

I also agree that there was some cancellation, certainly attributable to phase discrepancies in the crossover overlap zone between the subs and the tops.  Additionally, the band likes the standard sub on either side of the stage deal so I never get any stacking gain or the phase coherence of locating the subs together.  Also the KW112 and KW181 we usually use seem to cover more sins than these older Mackie dual 18's with the EV's on top.  The Mackies were also modded with a thermostatic fan kit across the entire heat sink, all the heat sinks were stripped and high quality compound used and the factory tin solder was sucked off and reflowed with silver solder.  The critical caps were replaced with higher quality components.  The have served well.  Looking to replace them, that's another thread.

[Art Welter]

Good stuff Art, couple of things.

I know that the Butterworth is an analog filter design however I thought that DSP's tried to simulate the characteristics of the complex LCR networks.  For example an older Peavey digital crossover has a 3rd order Butterworth or a Linkwitz Riley curve selection.

With regard to the ZLX's they indeed do have a crossover for the subs, in the menu you can select three crossover points of 80, 100 or 120 and lastly it has a mode for the EV ELX-118 sub.

I also agree that there was some cancellation, certainly attributable to phase discrepancies in the crossover overlap zone between the subs and the tops.  Additionally, the band likes the standard sub on either side of the stage deal so I never get any stacking gain or the phase coherence of locating the subs together.

BW, LR and other crossovers that originally used capacitors and coils (each additional cap or coil is an "order" of 6 dB per octave) can all be emulated in the digital domain.

I missed that the ZLX's have a crossover for the subs (the spec sheet doesn't mention it, the manual does) but it is probably 24 dB per octave (the manual doesn't say), so any cancellation or build up would be limited to around 1/3 octave centered around whichever crossover frequency you chose, so you not hearing the feedback probably was due to the limiter and room modes.

There are advantages and disadvantages of either center or L/R subs. In some rooms, putting both subs on one side or the other may be the best compromise. Generally, I prefer mains above subs so the crossover alignment always remains the same.

If you are having trouble keeping the kick drum from feeding back, locating the subs closer (as in downstage center) to them would not be advisable  .

[Mike Kirby]

If I may I would like to offer a couple of ideas here. First of all depending on the volume of the system you should have the front line mics behind the rear of the drivers in you FOH cabinets, this is one thing I have learned from doing high volume gigs.

The second thing would be to have a look at an external source for the resonating frequency, I have had this happen to me on more than one occasion and found that the inhouse lighting bars were resonating and creating a feedback loop.......gaffing up the ends of the pipes fixed the problem.

If the subs/top cabs are touching or located on the stage and the stage is hollow timber sealed (carpeted or not) on all sides this can also create a problem with resonance, relocating the cabinets will fix this problem.

Aside from that I would be looking at the tom mics and more than likely it will be the floor tom creating the resonance, If there is a compressor on the kick/snare as someone else has mentioned then this can create a problem if it is set to hard, when the compressor lets go there will be a surge of gain that could be the source.

My advice is to pull back the channels that you may suspect as being the offenders one by one till you find the source.

I can see that you have great technical knowledge and I am not trying to be negative here but practical experience in a live situation is paramount, if you do not have the experience it can lead to problems such as you experienced. I acknowledge that everyone has to start somewhere and experience is gained by practical application, I commend you for coming into a forum like this and asking for advise as you will learn much faster from the experience I have seen so far in here.

As you do more shows you will start to recognise these problems and find it much easier apply a solution. Try and train your ears to focus on one instrument/vocal at a time, once you master this you can efficiently block everything else out and just hear that one channel you want to hear, this is also a very valuable tool for trouble shooting 

PS: I also use Smaart during the show and find it to be invaluable in some cases

[Scott Holtzman]

Mike - Those are all great suggestions, and also would have been by "go to's" in addition to channel mutes.

The discussion was originally not so much technical as to how I missed the feedback from my position and failed to react to it.   If I had heard the feedback I am sure I could have solved it.  If my head wasn't buried in the mixer I would have seen the look from the stage and ran up with the iPad and solved it. 

No matter what the crowd situation is and how rushed things go down I have to efficiently multi-task and move around the room. 

I appreciate all the input.

[Mike Kirby]

From experience I understand what you are saying and yes if you are not concentrating on what is happening out front it is quite easy to not hear what is going on on stage, The suggestion a few posts up to have a set of noise cancelling headphones is also a great idea as you can usually hear a the offending frequency when you select the channel causing the problems.

Walking the room is a must as it can sound very different around the room from where you are situated. It can sound perfect in the centre and then you move to the right and the sound will change, then you move to the left and hear another inadequacy and by knowing what is not right on both side but sounds good in the centre you can then balance the sides to achieve the same result as you move from left to right.

Live mixing can be quite overwhelming at first as there are just so many variables to consider but as with anything practice makes perfect

[Scott Holtzman]

as with anything practice makes perfect

Done plenty of live mixing but the club venue knob twister for hire is a whole different animal.  It's hard to hone your craft when the guitar player is ready to beat you over the head with his tele :-)

This year has been an education at a corporate or HOW gig I have never had to go find a stoned CEO. 

Ask the band eating pizza on stage to try and not get grease all over the microphones

Clean vomit out of a monitor

Have my wife look after a musicians G/F puking outside

I have to be forgetting something.

I love it though, the energy and immediacy of the feedback from the crowd.  It keeps me young. 

[Rob Spence]

on what is happening out front it is quite easy to not hear what is going on on stage, The suggestion a few posts up to have a set of noise cancelling headphones is also a great idea as you can usually hear a the offending frequency when you select the channel causing the problems.

Not noise canceling but good isolation.

Many years ago a friend tried a pair of Bose noise canceling headphones to monitor a record mix. The recording was a disaster because of the coloration the cans introduced.

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