mics and feedback

[canadave]

Hi,

I'm new to recording and sound and all that, so I have a "Mics 101" question.

I was in my newly created "home studio" the other day, and had my SM58 mic hooked up and ready to go. I inadvertently hit a note on my keyboard, sound came out of the amp--and although the amp wasn't turned up all that loud, it was pointing directly at the SM58 about 6 feet away. I got some earsplitting feedback, so I quickly shut it down and unplugged the mic.

OK, no problem--I've always "known" you shouldn't have amped sound coming at an open mic that's relatively close.

But last night I suddenly began to question what I "know." I was at a live rock concert at a very small bar (the room was no bigger than about 50 feet square)...the band was so loud I could hardly hear myself think. And I realized, they have these huge PA systems right behind them blasting sound at a million decibels, the entire room is a deafening wall of sound, and the vocals mics are not too far away, right in front of the band, pointing indirectly at the PAs. So my question is: how do they manage not to have horrific feedback happen?

 

[Massive Master]

Having a good monitor engineer is a start.

Generally speaking, there aren't mics actually *pointed at* those monitors. Not that there aren't issues - Especially at higher volumes - but again, that's where a good monitor engineer is a priceless commodity.

 

[mixsit]

It would depend on what you mean by 'pointing indirectly' at the pa, but bottom line is it comes down to a signal to noise issue; signal being the mic's gain set for the vocalist and vocal being very close, and the noise being the pa. The pa's sound either crosses that line or not.

 

[canadave]

Thanks...

I guess my question really is (since I don't have the luxury of a good monitor engineer in my home studio), how can I avoid feedback from mics in an open sound environment?

As for the mics not being pointed *directly* at the amps...I see what you're saying, but I guess where I'm having trouble with the concept is that I don't understand how an open mic, oriented any way at all, wouldn't produce huge feedback in a room filled with sound jacked up loud enough to actually hurt my ears....when I'm getting feedback at home playing a couple of soft notes out of a single amp. I know an engineer would be helpful, but the contrast between the two situations is so dramatic that I think I must be doing something completely wrong, or I'm just not understanding the mechanics of how mics work.

 

[canadave]

Oh, okay mixsit...I think I see what you're saying. So, I need to lower the gain on my mic down low enough so that it will only amplify sound that's within a very small distance of the pickup?

Hmmm....the problem is that I'm using a Firebox as my audio interface and mic preamp, and I needed to turn the gain up pretty high to get it to an acceptable level to record into my computer. Not sure how to accomplish both desirables at once.....

 

[mixsit]

So, I need to lower the gain on my mic down low enough so that it will only amplify sound that's within a very small distance of the pickup?

Yes, sort of. The mics always pick everything, the difference is whatever is louder dominates. It stands to reason that if you mic from a few to several inches away (fairly common for natural tones vs a live pa situation) relatively quiet back ground noise is going to be in there.

Hmmm....the problem is that I'm using a Firebox as my audio interface and mic preamp, and I needed to turn the gain up pretty high to get it to an acceptable level to record into my computer. Not sure how to accomplish both desirables at once.....

Except in special cases you would not even want a speaker on during recording. It would be adding ambient crud and sound decay time to your track even if it isn't at out of control feeding back. Head phones (?) ..or if you or the instrument are there in the room why does it need to be on?

 

[Dani Pace]

Lower the output volume comming from your monitor speaker(s), or move the mic so it does not pick up the sound comming from the monitor or use headphones to monitor your recording. Headphones for monitoring while recording, speakers for monitoring when mixing works best for me. Stage mics are generaly very uni directional (designed to pick up from only one direction) which allows performers to work in clocer proximity to output sources. In live situations the mics are almost always beside or behind the main PA speakers to help keep feedback to a minimum.