Audio Engineers: Should I use a phase inverter with this multiple mic setup?

I have ditched the whole multichannel, multi-mic setup for live performance in favor of using high-end variable field large diaphragm condensors. Currently no monitors, although we’re going to in-ear mains feeds in the fall.

Currently we are using one microphone, and the group has become fairly adept at stepping in and out of the mic pattern for solos and vocal harmonies. I’m not pleased with the fact that it is a mono send, however, and would like to add a second identical mic, on a t-bar, with each mic angled at about 20-30 degrees outwards, and each channel soft-panned left and right. This would allow a more open audience-friendly stage setup, and of course give us a true stereo feed as well.

The mic has a nice variable field pattern, which is currently being used in an extended hypercardioid pattern, with just a touch of omni in there to get a fuller room sound.

If I add a second mic, should I use a phase inverter to avoid cancellation, or will that actually create more issues than it would solve?


What are you micing?
I assume this is for reinforcement, and you’ll need to keep the mic’s patterns in cardiod or hypercardiod, right?
How far away from the mics are the main speakers? What type of venue are you using this in? Very live, dead, or in between? How far are the mics from the performers.
How much, how hard, do you have to “push” out to the audience?

This is a very complicated question and depends on a lot of factors…

There is a micing techique that involves putting a mic out of phase with the other or others - “Blumenfeld Pair” (??? correct me if I’m wrong), but it’s not as simple as one mic out of phase. I believe this is a real old technique and not used much anymore.

I’ve had very good results with large acoustic ensembles (choirs, orchestras, bands) with spaced omnis (LCR), but that is only in a venue that can allow that and you really are “reinforcing” and not “amplifying.”

Probably the best thing to do is to try different arrangements and see what sounds best.
In a lively acoustic venue where feedback can be a problem, large-pattern cardiods in a coincident or near-coincident configuration can work well.

Hey Fritz

This is mostly a reinforcement setup, for a four piece bluegrass group, with two lead instruments and four part vocals, with everyone sharing lead vocals and harmonies depending upon the tune.

Room size is variable as all hell - although our weekly gig is in a fairly large bar with decent acoustics, we’re touring through a variety of venues and pa systems.

I went to this setup for multiple reasons, not least of which is that on tour we have found that most engineers do not have a lot of experience mixing acoustic music, let alone bluegrass, and they can’t get it through their heads that the reason the mandolin player is singing soft in this particular song is because he’s backed off the mic to let the harmonies blend better. And so on… We record every show, and it’s ridiculous how much some of these engineers just keep monkeying with the levels throughout the show.

So I decided to eliminate the problem by sending them one (or two) signals. The sound quality is pretty impressive, and the harmonies are much more unified, because we are both hearing the natural acoustics and not being misled by off-kilter monitor mixes etc.

Currently I am using the one mic, at just below chin height, on a boom in front of the carpet we perform on. The soloists (me on guitar/banjo and the mandolin player) are on the outside, with the rhythm guitar and upright are on the inside.

We have set tape marks to delineate best positions relative to the mic for each of us: We each have a mark for singing lead, and one for just playing, and we lean in & out and/or change our vocal dynamics for harmonies. These are all based off of the input gain set at unity on the board, so we’ve got a benchmark for distance. At rehearsals we run through a song and record it, and then adjust our positions and dynamics as needed after hearing the playback.

It works pretty darn well, and the only thing that’s been an issue is that the more reinforcement we need in a room the tighter in we need to be to get acceptable input strength without causing feedback as we crank the output on the power amp.

Anyway, here is the setup we’re currently using, followed by the one I hope to go to.

I can't make the diagram come out even, but we're in a pretty accurate semicircle

      G/B            M
            RG    B
What I Want to Do

                X X
  G/B                       M
           RG        B

In this configuration, I’d like have each mic angled out about 20-30 degrees, with overlapping coverage from both mics (dialed to high gain supercardioid) catching the RGuitar and Bass. And that’s where the phase question comes in - will the overlapping areas of coverage cause phase cancellation?

This should give a nice subtle stereo feel, and allow us to spread the stage a little more, so that we’re not all crammed into 1/3 of the useable stage area. This will allow allow myself and the mandolin to face the audience a bit more during solos, rather than the mic.

The mic inputs are going to a little mackie 6 channel, with a compressor inserted on the aux send and a stereo eq patched inline before the power amp. Even though my current setup is a mono signal, I still send it left & right to have independent gain controls to each side.

Bear in mind that in the fall, we’re all going to be using in ears for a mains feed, and later, as I can afford a full time tech I’m going to send line signals for each instrument as well (but only when MY tech is on the board)
Wow - that was long and involved. Many thanks if you can shed a little light on this, or suggest improvements. I just want maximum control of our sound, and the lightest darn touring rig I can come up with.

As a general rule, the more open mics you’ve got, the less gain you can have in the system before hitting feedback. If this is already an issue, going to two mics is more likely to make things worse rather than better. If you have to stand close to the single mic to avoid feedback, you’ll have to stand even closer with the two mic setup. You might want to look into a feedback suppressor of some sort if this is frequently a problem.

And I’m certainly not an expert, but my gut instinct tells me that inverting the phase on one of two mics that have overlapping patterns is going to sound roughly like inverting the phase on one of a pair of stereo speakers. Not what you’ll want to do, especially given that you’re panning them on the console. But hey, what do I know?

I appreciate the feedback (here, not on stage…bah dum bum crash…)

I’m back out on the road here in about 20 minutes, but I’ll check back in on Monday afternoon.


ACK! It’s Blumlein pair…
[sub]I knew there was something wrong with “Blumfeld” when I typed it last night.[/sub]

And the technique which involves putting a mic out of phase to the other is called MS or Mid-Side. Here’s a good explaination
Both techniques are far more relevant to recording than reinforcement.

I think a near-coincident (card or hyper card) stereo pair will work pretty good in most places - especially if you can keep the performers grouped pretty tightly.
I would also probably avoid panning hard left and right.

Every venue is different, though. It would be impossible to predict success with this everywhere.