Fixed problems with live feedback, can't figure out how?

[funkymunk]

We had a gig on Friday, soundchecked two of the three bands, all good. The first band played, no problems. The second band played, no problems until halfway into the set. The lead mic suddenly started feeding back for no apparent reason and continued to do so until a few songs of the third bands set. I found some frequencies on the eq that when pushed up started feeding back and pulled them but the mic kept feeding back. The lead singer ended up using the backing vocalists mic as the gain before feedback was very low on the lead mic. Finally we mucked around with the limiter on the compressor and for some reason the feedback stopped. Me and my boss couldn't figure this one out and was wondering if anyone else had experienced this and can explain why using the compressor killed the feedback. Maybe the mic just stopped feeding back on its own? Help is appreciated

[7rojo7]

I hate problems that fix themselves and watch my back for Murphy.
a compressor won't cause feedback per se, but it will change the way feedback will accumulate and present itself. Compression for live soundshould be approached differently than compression in the studio.
Was there a difference in how the singers held or appeoached the mic? If you cup a mic it becomes an omni, great way to ring out a mic, horrible way for a singer to hold one. Unfortunately, MTV has been an example to many and not many singers have any mic/stage savvy but they sure look good.
If you put your hand in front of a mic and vary the distance, you can produce feedback at some distances and at others no. The hand becomes a reflective surface thus creating a loop for the mic/monitor relationship. If you put a head in front of the mic you have much the same situation. Most feedback is the fault of the performer being a reflective surface and not knowing how to position himself to avoid it. In high volume situations, a singer must get right on the mic and move away from it or stay on it when he is not singing, anything inbetween is going to bring trouble.
They'll lok at you like you're an incompetent asshole when it's really they that are, if you try to explain to them they're lack of stage mic technique they'll think you're just trying to pass the buck on them caus' you's a lazy so and so.
IEM for the singer is a really good idea, F*&< off Murphy.

[mixedupsteve]

There are several settings on a compressor and you didn't say what you changed. My guess is you used less comp. and combined with the mic change and EQ settings you fixed it.

To learn from a problem solving attempt you should try to keep notes on what changes are being made so you will learn what helped you.

[David Rick]

Compressors used in live rigs can present "hidden gain" that arrives to bite you just when things seem safe. Here's a typical scenario:

You patch in a compressor during sound check with a significant gain reduction ratio and a slow release time. With the vocalist really belting it out, his mic channel may have 6-10 dB of compression applied. During the set's climax, you push his channel fader another 3-5 dB to carry him over the band. Then he stops singing and the compressor finally has time to release. Whammo! You've now got an extra 9-15 dB of gain on the vocal channel, putting the system way past its stable gain. Pulling the fader back to its sound-check position may not be enough to restore stability, because you've stil got all that "hidden" gain from the compressor.

David L. Rick
Seventh String Recording

[M4-10]

With live compression the solution (after ringing out and mic/monitor placement) is not to place compression on a vocal channel being sent to monitors. If your vocal is in channel 1, use the monitor send on that channel, but keep that channel out of the stereo bus. Use the direct out or a group bus to get the vocal to another channel where you can insert the compressor and put it in the FOH. So vocals in the monitors are uncompressed, and vocals in the FOH are compressed, and you shouldn't have feedback (or at least feedback due to compression). If you use heavy compression and the FOH speakers are placed poorly or if the room is small and reflective you may still get feedback between the mic and the FOH speakers.

My mixer doesn't have direct outs or inserts on the groups, so I'll bus the vocals to group 1-2 (I'll pan hard left so it all goes to 1) and then take the group 1 out and put it in a new channel and then insert a compressor on that channel.

[jude]

as for what M4-10 said. i do the same kinda thing, but in a different way (if that makes sence?)

i "Y" split all vocal mics so each vocal mic comes up on 2 channels on the desk. so my vocs to FOH will be ch # 17-20 and my vocs to monitors will be 29-32. this way i can add as much compression as i need to foh without upsetting the foldback balance


that said i have done shows as a montior engineer where i have used a truckload of compression on vocal mics to foldback (wedges that is) my advice here is to use the shortest release time your compressor offers

[7rojo7]

sound advice

[funkymunk]

Thanks for all the responses! We usually run the FOH in mono and pan the vocals hard left so we can compress the vocals as a group. We were doing it that night and still got the feedback. As far as the vocalist holding the mic, we thought it might be that as he was cupping the mic but the previous band, where the feedback started had the lead vocal on the boom stand as he was playing guitar. The settings on the compressor were probably about 3:1, attack at about between 1/4 to 1/2 way up and release about the same. Sorry I can't be more specific as I'm still learning it. I know where to set it and what sounds good while I'm doing it but I still haven't learned the specifics of what I'm dialling in. The gain wasn't very high. For some reason pushing the limiter up did the trick. I appreciate the advice and will keep it in mind for other gigs just in case it happens again in other situations but I still can't firgure out how it happened this time !

[valleysound]

Maybe the mains were feeding back.

[Shim]

Originally Posted by M4-10

With live compression the solution (after ringing out and mic/monitor placement) is not to place compression on a vocal channel being sent to monitors.

This is great advice. On top of pushing the potential for feedback, it also screws with the performers head. They sing loud, hear the compression, sing louder to compensate....etc....etc.... It's a vicious cycle that can ruin a performance.

This is why I always bring extra "Y" Mic cables with me to gigs. Usually there's an extra channel or two that you can use as monitor-only channels for vocalists (and preferably anything that you want to insert a comp on).

Compression in monitors= big no-no tutt

[Shim]

If it was the mains feeding back... you should look at the speaker placement, # of open mics on stage, and the overall levels.

Subs also have a tenancy to ring because of their lack of directivity. In this case, it's best to use a HPF on the vocals or to decouple and stabilize the mic stands a bit with some carpet (or weight them with a sandbag). But that doesn't really sound like the problem you were experiencing.

EDIT: I totally missed the OP's last post. Sounds like all of my suggestions are out of place! Oh well, glad you fixed it. Sometimes, Murphy just wants to bite you in the ass because he can.

[funkymunk]

Yeh, sounds like this one was a case of murphy's! Maybe the mic just stopped feeding back on its own. I don't know...