Reducing amplifier sound bleeding into mics in a theater setting
I'm having a problem in a live staging setup, in which I have 3 guitar amps and 2 bass amps, all miked, feeding back on on-stage rifle mics. Just to specify, there isn't any way to line out from these amps, for reasons unimportant to this question. What I want to know really is what acoustic shielding techniques are used for containing the sound to their own microphones?
EDIT:
Another thing I want to bear in mind is the heat of the amps, because they're positioned under the staging which is always covered by the lighting system, thus meaning they will get hot. I really don't want anything melting with the heat, especially coming from the valves in the amp.
EDIT:
In terms of the staging, it's for a production in a theatre with a live band. There is staging that rises up to two balconies which the band play under. The amps are situated with the band, and the microphones in question are 2 hanging from the rigging in the ceiling, and 2 at the front of the stage. I think a large part of the bleed is coming from the closest rifle hanging from the ceiling which is probably around 7-8m away from the amps. Also, there are fallback monitors at the front of the stage and a set in with the band.
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My first questions would be 'why rifle mics?' & 'how far away are they?'
As Doug said, an SM58 tight in front of one of the cones has been used with reasonable success for decades - play with axis to get the sound as you'd like it.
It may not be the ultimate solution, but it really is a working solution.
When you say they're under the staging, you mean they are in an enclosed environment? In that case, yes, you might need to add shielding, though I'm guessing your feedback in that kind of setup is going to be low frequency & therefore much harder to shield.
Edit - maybe I'm reading this wrongly... are the rifle mics on the stage, the amps below? If so, & the feedback is low frequency, then it's probably through the mic stands rather than in the air & would require you to isolate the stands from the stage.
Edit2. OK, after your 2nd edit, I now get it - the term 'feeding back' was totally throwing me. Are we not talking feedback at all, simply spill?
If it's spill you need to contain, then you really are going to have to try isolate the amps from the theatre sound. Without building further structure, simple choices would be, turn the amps round, mic from the back; or get them out of the auditorium altogether & send the guitarists separate feeds, to headphones or small monitors, placed much closer to them.
Are the mikes on the amps feeding a PA which feeds the audience ears, or are the amps making the sound for the audience themselves ?
If they're going through a PA one way is to turn the volume of the amps down a bit (so they don't saturate the room & bleed onto each other), face them away from each other or use acoustic shields to isolate them (or just have them far away from each other), and have the mikes close up to the amp so that the amp is much louder per mike than everything else.
I have done this quite successfully (without any shields) with SM-58 mikes on guitar & bass amps, as they seem to handle having the sound source very close up quite well, and the cardiod 'catchment area' means they're not going to pick up too much whcih isn't nearby.
I daresay the acoustic shields end of things is a separate answer in itself. I havne't used these but I gatehr they can be quite effective.
Acoustic shielding is difficult. The effectiveness of any shield is a function of the ratio of it's size to the wavelength. That means that shields are good at controlling high frequencies (= small wavelengths) but do almost nothing at low frequencies. Wavelength at 100 Hz is 3.40m.
So a shield can eliminate high frequency pick up but you still end up with all the mid and low frequencies which (on their own) sound muddy and indistinct. Shielding the high frequencies may actually make it worse.
By far the best setup would be to put the band on e-drums, modelling amps and headsets and run it all into the board and through the main PA. Nice and clean. The band sound can either be mixed at the main board or you could use a small submixer type thing like this jamhub.com/what/tourbus.html where the players can individually adjust what they want to hear. (disclaimer: no endorsement or advertising intended). Second best would be to get the amps as small and low level as possible and mic or line-out them into the board.
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