Feedback issues

[Felipe Ribeiro]

Hope someone can help me to understand the following. I had mic feedback issues in two concerts and this is bugging me. During sound check (no audience in the hall), the overall level was quite good. However, when we start playing during the concert itself (with audience), there was feedback. Since I was in charge of keeping track of the equipment, I can assure that no parameters were changed, so the only difference was the presence of the audience. What could have happened? Shouldn't the amount of reflections decrease with more people? I also though about the humidity of air increasing and changing the speed of sound... Any clue? Thanks.

[J-listens]

Were there any costume wardrobe changes between sound check and show time?. A hat could do it for sure.

[rcrowley]

Yes, at first view, this would appear to be counter-intuitive.

However typically overall levels are raised once an audience fills the hall because the people absorb some of the sound. So higher gain (to compensate for the audience) could easily explain the feedback problem.

[edva]

there is generally a huge change in humidity with a full house, and also some absorbing of various frequencies, both of which can cause problems. Not sure if that is your issue, but certainly might be a factor.

[bajde]

What kind of sound was it? High pitched, low???

[Jim Williams]

When I did live sound once for Dwight Yoakum, we set monitors and then re-set them after he put on his trademark cowboy hat.

It was a wardrobe "malfunction".

Me, I always have issues if I don't get feedback from my guitar, it's my trademark.

[Felipe Ribeiro]

Thanks for the quick reply. It was a solo cello plus live-electronics "classical" music concert. No parameters changed before or during the concert, just the amount of people. The only things that I can think that could have changed the sound are the room's acoustic response and the humidity of air. But the tricky thing is that I used to think that people's clothes would absorb sound, instead of reflecting then back to the microphone... Any other clues?

[hbphotoav]

One other thought...

Rarely does anyone in the band "bring it" during soundcheck/rehearsal. Well... maybe the drummer... but they're freaks of nature anyway... :-)

It makes sense when there's no crowd to get the adrenalin flowing. But when nominal levels in soundcheck are 3-6dB lower than showtime, and when the lead grabs the 58 in a rapstar death grip up over the bottom of the ball... well, then. Trouble will usually ensue.

No way to "force" singas and playas to "bring it"... so we gotta be hip to their tricks and ready on the knobs. Sucks, sometimes... but, there it is.

[bajde]

When you turn on el.devices, ee, they need two hours minimum to warm up, to get in the groove. +as said above, adrenalin. What did you use? mics.pos.pa..

[Felipe Ribeiro]

To clarify, the concert was not a rock full of adrenaline concert, i.e. it was a classical music concert, with lots of 'mp' dynamics... Everything fragile. No 58s, just Neumann 184 cardioid, a Soundcraft mixer, Apogee converters, computer with MaxMsp, and Meyer loudspeakers. I am very aware of the parameters used and the overall sound of the cellist. If we assume that the sound during soundcheck and concert are absolutely the same, what could have gone wrong with the acoustics of the hall? Does anyone had similar problems?

[hbphotoav]

Quite possibly, in the scenario you describe, as the ambient noise in the room due to a crowd increased, air handling came on, etc., you ended up at least 2-3dB "hotter" than soundcheck. It's also quite possible that the performer was generating a bit more level as his adrenaline got moving in performance. Either of those circumstances might get you into exciting a problem frequency, and begin to ring.

I'm also going to guess (though I'd not mind being wrong) that, as it sounded "fine" at soundcheck at a certain level, you didn't then run the signal up to find just where the mic, instrument and player began reacting with room nodes, found the freqs, and did a couple of minor, precise cuts on the primary and secondary rings "just in case" the level had to be hotter than soundcheck. One or two dB is enough to get a node going, and, with the changes previously discussed, and no idea exactly where the "troublesome" freqs lived... well, then.

[studer58]

The appearance of feedback in a concert is not a big deal in itself, depends how it manifests itself and how quickly ! A low growl around 240 or 440 that slowwwly builds lets you grab it quickly on the FOH graphic and kill it rapidly, then turn to the individual offending desk channel and eliminate it there too....result is often that you can then carefully, slowly return offending frequency back (or at least some of it) into the main mix via the FOH graphic eq (because you've attenuated it at source on the mic in question)

Feedback isn't bad or unprofessional per se...but the mark of a professional is how quickly they identify and kill it in the mix. More than a couple or 3 of feedbacks (especially at the same recurring frequency) in a show marks you as somewhat of a newbie with some learning to do.....YMMV of course.

[12ax7]

.

To simulate this effect, do a sound check, and then place a fully dressed mannequin in every seat in the hall.

Did this result in any acoustic changes within the room?

(Might acoustic changes in a room effect feedback?)

.

.

[studer58]

If you have the appropriate software and hardware, you could send a short blast of 85dB white noise through the system post soundcheck and capture it via a measuring mic at your mixing console and analyze it's spectral content. Save the screenshot. Do the same just before the concert with audience seated (they won't notice, it'll be over before they know what's happened) and compare the 2 spectrum graphs. Will they be different .....almost certainly ! Will they give you information which can be used for feedback avoidance....maybe ?

[chris319]

I picked up a used Sabine feedback unit on ebay a while ago to address a problem of ringing. Well, ringing and howling are two different issues. The Sabine took care of the howling. The ringing, not so much. That might be an option for the OP, but it doesn't answer the original question.