Jump to content

Techniques for feedback elimination


Recommended Posts

  • Members

 

I was reading the other thread about automatic feedback detection, and PSG's comment made me wonder what real sound guys do.

PSG said, "A parametric (PEQ) is about as precise as it gets for instant control. Most do not use parametrics that I have seen in 35years."

 

If most people don't use PEQs for feedback elimination, how do they eliminate feedback, and what do they use PEQs for?

 

I'm not a sound guy, I don't have really experienced ears, but when I set up the small PA for my band and have a feedback problem, here's what I do (after checking mike and monitor placement) --

  • Turn channel level down

     

  • Turn sweep level to +15

     

  • Turn sweep frequency to somewhere near where I think the feedback is

     

  • Turn channel level up until it *juust* starts to ring

     

  • Turn sweep knob to make the feedback as bad as possible, while tempering with the channel level to keep the system from running away

     

  • Once I have identified the worst position on the frequency knob, turn down the sweep level to -3 or maybe even less

     

  • Turn channel level back up

     

Is there a better strategy to do this when the problem is on only one or two mikes?  My problem is most often the kick mike.  The practice PA does not have an EQ, but I can't see EQing the entire mix for this?  Do large-venue operators run a separate graphic EQ for each monitor?

Link to comment
Share on other sites

  • Members

The challenge with live sound and feedback control by notching is to not cut anything but the feedback itself. You would like to minimize any loss of overall tone in the process. And sometimes feedback management is more important that tone.

 

Since feedback always starts as a single frequency sine wave using the narrowest filter possible should give the best results. Here's where PEQs are a much better tool than 1/3rd octave graphics that typically are one octave wide filters.

 

Your strategy is probably reasonable given fixed filters. But by using dynamic filters ( feedback eliminators) that can move according to the current conditions you can achieve more ( although depending on conditions not always a lot more). The thing about feedback frequencies is that if you move the mic the potential feedback frequencies change. This is mainly caused by because for feedback to occur you need the offending frequency to be in phase. As you move the mic around you get comb filtering. Feedback never occurs in the notch frequencies of the comb. So moving the mic may heal a frequency that you already carved out while introducing a new frequency problem that wasn't there when you rang out your system using your method. So now you have removed what could be useful signal that is not helping manage your feedback depending how you set it in the beginning :)

 

Link to comment
Share on other sites

  • Members

I am not a "big boy", but I have certainly had my issues with feedback over the years.

My ultimate "cure" to feedback was to get rid of the wedges and use all IEM's.  I haven't had feedback in years.

When I did have wedges, I had as much problems as the next guy.

Here are the things I did, in no particular order:

  • Replace cardiod mics with super cardiod mics
  • Gate dum mics using gates that have a frequency filter for the trigger
  • IEM for the drummer (since there are so many microphones there) and eliminate at least the drummers wedge.
  • pEQ on any remaining monitor sends along with some kind of tool that lets you know which frequency is feeding back.
  • Here is the big one ..... bring up the sound levels with all micropones in use until you get feedback then back off your volume about 3db.  This is as loud as you can get if you don't want feedback IME.

There is a general debate on what I am about to say, but I have found it to be emperically true.  Better speakers don't feedback as soon as crappy speakers do.  Placing a 15" JBL EON anywhere near my stage instantly produces feedback in a system that never has feedback as an example.  

If you have monitors, good ones will help keep your feedback in check IME.  This is also true for mains, but assuming that you have your mains well in front of your microphone plane, it should take quite a bit to make your mains feedback in the absence of monitors.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...