FOH howl from mic

[The Boogie Man]

Hi troops,

 

I've been having a few problems with a couple of rooms in pubs (very square, very reflective) and whilst I know the potential culprit for howlaround is most likley further along the chain, I was curious if there might be a problem with the source arrangement before even reaching the mixer.

 

The chain is: sm58 to berhinger ultravoice to digitech quad then the mixer.

The behringer is set not to cip the digitech and is adding only a little comp and eq. The digitech is balanced in then pitch shift, chorus, delay, then out panned hard left and right to the desk. flat eq (or cut, no boost ever) The desk is never clipped either. Could there be a problem with that arrangement over straight to the desk with the 58 then send and return for effects?

[Rob_Beech]

There are a couple of things to look at.

 

When trying to eliminate feedback alot of people go straight for the EQ. This is NOT the way to do it. First of all you need to look at WHY its feeding back. Speaker placement, mic placement and musician placement are the key. Try to work out the path the sound takes when it comes out of the speakers, if its hitting walls too early then this is going to cause problems with the sound, it may not contribute towards the feedback but it won't do it any good. Mic placement, look at the pattern the mic picks up, if this is picking up a direct sound from the speakers this is going to cause a problem. move one of them where possible before reaching for the eq and other things.

 

Secondly, you need to understand how a compressor works, when a compressor is working it is reducing the gain of the signal which means that when a signal is present you can often increase the level somewhat before feedback. As soon as that signal goes, the gain reduction stops and in effect the gain rises automatically, this can often cause feedback if you are not careful.

 

You need to establish the feedback frequencies and see what each unit is doing to the sound. Take each out of the chain and see if it makes a difference. try difference mic and speaker placement. and if all else fails, head for the eq.

 

Rob

[The Boogie Man]

Hi Rob, thanks for the input.

As I said in the post I'm aware that there are a lot of possibles that may contribute to the problem, but my first port of call was just to question my own set up of the mic signal path.

I usually use the tried and tested sherlock approach of removing until....

I was just curious if this signal set up path was in anyway known to be "problematic"?

(not a criticism of your advice, by the way, gratefully received as always)

[Captain]

what sort of sound are you trying to amplify? One man and a guitar or full band type affair?

 

When you are experiencing feedback issues, is it when your trying to get any old level out of the system or is it when your fighting to amplify a vocal above the level of band?

 

In addition to robs suggestion about placement its maybe worth thinking a little about your microphone/speaker selection.

 

He very briefly touched on mic pick up patterns in his post. I've had good results (for bands in small bars) by switching to shure beta's 57's / 58's, which have a tighter, hyper cardoid, pattern. Most of my feedback issues were caused from having a compact stage, meaning the vocal mics were picking up a lot of spill from everything else, and therefore amplifying too much chaff and not enough wheat.

 

Think along the same lines about your speakers. Ultimately feedback results from either speakers projecting sound into an area you don't want it, or by microphones picking up sounds you don't want.

 

Placement and equipment choice is the key. The extra bits and bobs in the chain pre mixer shouldn't cause feedback problems. (but if setup incorrectly could exaggerate it).

 

See if you can beg/hire/borrow/"try before you buy" a few different microphones. (begging/hiring/borrowing a different PA may be beyond what budget allows/ is practically possible) Experiment with placement and see what works for you.

[jamesperrett]

I would probably start by taking the Behringer and Digitech out completely. I would then repatch the Digitech so that it was fed by a post fade auxilliary send on the desk and its outputs returned to a couple of spare channels on the desk. From what you've already told us, all of the effects that you use the Digitech for can be obtained by using it with an aux send rather than with it inline - just turn down the dry setting completely and set the wet to 100%. You can control how much of the effect is audible by the aux send and channel return levels on the desk. If I remember rightly, the Digitech is a fairly old unit and I wouldn't send anything too important through it as an insert effect as the A/D convertors in modern units are far better.

 

Now that you have a bare minimum of stuff in the signal path you will probably find that things sound better and feedback is less of a problem. Once you are used to running like this, you could try patching the Behringer back into the system to see if it improves the sound.

 

Cheers

 

James.

[The Boogie Man]

Cheers James,

 

yes that was the plan, as experimenting is everything. The reason for the inline set up as opposed to send and return was the ability to keep the vocal effects when doing festivals, ( no mixer, so foh used to get the finished vocal sound plugged straight into the stage box.)

I'll go back to the old way then move further down the line for culprits.

[Rob_Beech]

The trouble with giving FOH the finished sound is what if it doesn't work for them? What if they can't get a useable sound out of it due to it being a completely difference concept with regards to systems in use. It would be a bit like giving them a single channel for the drum kit, you've mixed it yourself and then they are given an overall level.

 

Keep the options open. Have the necessary leads and what not to do both. If you gave the 2 units to FOH and said can you run these from a post fade aux for the vocal channel, if its possible then there is no reason why they'd say no. And if its not possible, they would still run it inline, but at their end so they still had control.