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MC208 30 May 2018

Working in a finished residential home basement shooting music instructional videos. I have a Shure SM58 on a mic stand just above the picture frame, aimed down at the teacher's mouth. I also need to have speakers on for the two musicians to hear their instruments (guitar and keyboard). I positioned the speakers on the floor pointing in the opposite direction of the video frame, i.e. 180 degrees from the direction the mic is pointing and about 8 feet or so from the mic.

I thought that being a cardioid mic, I wouldn't have any sound from the speakers being picked up by the mic, but I do. Someone I was talking to suggested that I'm getting acoustic reflections. Which I understand to mean the sound from the speakers is bouncing off the back wall, to the other side of the room and then into the mic. Is this a possibility? Is there an easy way to test to see if this is the problem before I start trying address acoustic reflections? I really know nothing about this subject, saw some youtube videos showing that it makes a difference. Thanks for any advice :-)

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pcrecord Wed, 05/30/2018 - 18:25

As friends said, it won't eliminate bleedings. But there is one technic I never tried that can help. Put a mono speaker as a monitor and record the performance. Then record the song again with just the speaker's sound (without changing anything) with the person standing silent where he or she performed. Then put both tracks aside and invert polarity of the speaker alone track.. Phase should cancel a great part of the bleeding. Where it can fail is if the person moves a lot which will change some reflexions.

paulears Thu, 05/31/2018 - 01:18

We use the polarity reverse trick on stage, with two monitors - BUT - success is very variable. It's not very effective with cardioid mics because their best null is immediately behind, so putting two speakers either side of the mic stand, increases bleed, then the polarity reversal decreases it. If you use a hyper-cardioid mic, they have less of a null straight on, but one either side - and sticking one monitor in each one, polarity reversed one side, works pretty well. It get wrecked if the singer moves the mic boom left or right, and feedback can be vicious. So if you can control the users, it can be effective. Flipping the polarity on one speaker by swapping the cables at the rear is a quick job and as long as you mix in mono for the recording, is worth a go. Can't make it worse!

bouldersound Thu, 05/31/2018 - 09:24

There are two different polarity reversal tricks being discussed here.

The first is a two-pass method where you do the normal recording, then do a second one with just the speakers running the monitor mix. Keep the mix and the positioning identical. Then combine the main recording with the bleed-only recording with one inverted. The bleed should cancel out.

The second is the one pass method, using two speakers in a symmetrical configuration with the mic. Invert one speaker so the sound cancels at the mic. There are two problems with this. One, it's likely to cancel the sound at the performer's ears nearly as well as at the mic. Two, the room reflections also have to be symmetrical, which is unlikely.

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