Choir Microphone setup for Webcast

Conversations around originating a webcast for conference, including cameras and mixers.
quintonrhq
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Choir Microphone setup for Webcast

Postby quintonrhq » Wed Sep 08, 2010 3:23 pm

Our buildings have very little room between the choir, piano and organ, and the seating for the conducting officers. The ceiling is much too awkward to drop wires from. One of our theatre sound fellows suggested using boundary microphones. There was an Audio Technica AT871R on hand. These are usually used on the front of a stage. Because there was no effective floor space to place the microphone, he devised a 12 inch by 12 inch Plexiglas plate that attached to a microphone stand. See the attached picture. These were placed about waist high to a standing choir and tipped at an angle. The Plexiglas reduced the visual impact and the plate helps reduce the sensitivity of the microphone to low frequency sounds. It worked well enough that another was added. The microphone is stuck on with double sided tape.
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russellhltn
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Postby russellhltn » Wed Sep 08, 2010 4:54 pm

I guess I don't understand why the boundary mic rather then a common condenser stand mic (of course you only have what you have).
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quintonrhq
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Postby quintonrhq » Thu Sep 09, 2010 11:08 am

The handheld microphones we are used to using in the church have a cardioid response pattern. This boundary microphone has a half-cardioid pattern. This allows greater gain before feedback occurs. There is also a multi-path filter effect that is avoided with this kind of design. Here is a link offering a nice explanation.
http://www.blue-room.org.uk/wiki/Boundary_Microphone

russellhltn
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Postby russellhltn » Thu Sep 09, 2010 10:59 pm

I can see the difference in response pattern being attractive.

I'm not quite buying into the "multi-path" explanation. I understand it as it applies to normal applications: Sometimes you have to place a mic close to a surface. In that case the PZM is good because by placing it ON the surface, you can reject the phase cancellation created by reflections from it.

But I don't see how you have any "multi-path" benefit when you put it up on a stand. I just find it odd for the PZM mics to be mounted on a stand. I haven't seen it before.

But, an old saying applies: If it's stupid, but it works, it's not stupid. :)
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quintonrhq
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What the microphone heard

Postby quintonrhq » Thu Sep 23, 2010 2:19 pm

Oops, there is a missing word in the original thread posting. It should read “The Plexiglas reduced the visual impact and the plate height helps reduce the sensitivity of the microphone to low frequency sounds.”
Here is an example of the frequency response for a microphone placed over a hard surface in an outdoor setting. The experimental setup has an 8 inch bookshelf speaker placed 5 feet above the ground. The microphone is placed 9.5 feet away. The software used to produce this is a free product called Room EQ Wizard from www.hometheatershack.com/roomeq. 1/6 octave smoothing is used.
See the attached chart.
The purple trace is from a Behringer ECM8000 microphone placed on the ground. Nice uniform response.
The green trace is the same microphone raised 2 feet in the air. There is a destructive resonance point at about 300 Hz common to all positions at 2 feet of elevation. Approximately 17db of ripple in the mid-band.
The blue trace is an audio-technica AT871R boundary microphone sitting on a small peg at 2 feet of elevation. The case is about 3 inches across. Approximately 10db of ripple.
The red trace is the AT871R again but placed on a 12x12 inch plate at 2 feet of elevation.
The orange trace is the AT871R placed on a 24x24 inch plate at 2 feet of elevation.
It appears that once a plate dimension of about 12 inches square is reached, further plate increases are not effective. Approximately 6db of ripple. The general slope could be cleaned up with a mixer.
If there is no floor surface to use then a small artificial floor is an option.
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craiggsmith
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Postby craiggsmith » Sat Oct 09, 2010 11:19 am

This is very interesting. It's nice to see that some people are actually mic'ing the choir. At my last stake we had Shure MX202 hanging mics, but I didn't particularly like them, and hanging mics is rarely convenient. We also need some mics for roadshows and such. I've been debating between boundary mics and stand-mount condensers, or a mix of both. Other pro audio forums seem to favor the latter.

But these charts are interesting. My understanding of boundary mics is that one of their weak points is low frequency response, and the larger the surface the better. But maybe that only works at ground level. And in this case, the response seems actually greater than the Behringer. And I also thought one of their main points was to eliminate phase cancellation, which it doesn't seem to do much better than the Behringer.

And I'm confused -- it seems the plate smooths out some of the higher frequencies but makes the low frequencies worse. So everything is opposite of what I expected. So now I'm not sure what to get or do. I was thinking of putting a boundary mic on the podium or perhaps one on each side on the rostrum wall or whatever you call it. I'm not sure how reflective the carpet is, but hopefully the cancellation won't be as bad as this. Or do I go with a condenser or two in some configuration? Perhaps something under a condenser?

If I remember correctly you will get primary cancellation at the frequency where the height is half the wavelength, which seems to match your testing. I was thinking the 300 Hz notch would cut out a lot of desirable frequencies, but I guess that's close to middle C and which might be the best place to put it. Note that it might be easier to read the chart with just 2 or 3 lines at a time.

Thanks for the info.
Craig
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South Jordan, UT

quintonrhq
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Postby quintonrhq » Sat Oct 09, 2010 10:01 pm

The response curves are admittedly dense. A curve for the AT871R on the ground was omitted because it was the same as the Behringer on the ground. The vertical curve positions should not be taken as absolute since they have been slid a bit for clarity, only the relative changes are meaningful.
There are two filters here, one caused by the height and the second by the plate area. We were trying hard not to pick up the organ speakers which are very close; hence the low frequency loss was a plus.
Another run was made at a 4 foot height which moved the low frequency cutoff lower with the same plate responses. Having a microphone at about a foot off the ground would be really horrible since the cutoff would move into the vocal range.
The measurements were made out of doors to more clearly tease apart the various effects.
Incidentally, the organ service representative has identified where we can grab the electric organ signal on our older electric organ. As others have mentioned, having a separate organ signal should give us a better prelude and congregational singing signal to the other buildings.

craiggsmith
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Postby craiggsmith » Mon Oct 11, 2010 5:21 pm

Thanks. I was wondering if the verticle positions were absolute or relative. Yeah, I thought about putting it at 4 feet but I think they want it as invisible as possible.

Interesting, I hadn't thought about a separate organ feed. I'll have to see if I can find a way to do that. But won't there be a delay problem with the electronic feed and the sound being picked up from the mic (which seems inevitable)? I could always delay it.
Craig
STS
South Jordan, UT

rmrichesjr
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Postby rmrichesjr » Mon Oct 11, 2010 7:30 pm

hemismith wrote:Thanks. I was wondering if the verticle positions were absolute or relative. Yeah, I thought about putting it at 4 feet but I think they want it as invisible as possible.

Interesting, I hadn't thought about a separate organ feed. I'll have to see if I can find a way to do that. But won't there be a delay problem with the electronic feed and the sound being picked up from the mic (which seems inevitable)? I could always delay it.


Assuming the direct feed from the organ is in-sync with what is sent to the organ's speakers, the delay would be at most a couple of tens of milliseconds, about one millisecond per foot the sound travels through the air. That won't be noticed by human listeners as a delay. Phase cancellation _might_ be measurable with laboratory instruments but shouldn't be a very significant problem.

Now, if the choir is slow relative to the organ by a quarter of a second or more, that's a different matter. :-)

craiggsmith
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Postby craiggsmith » Tue Oct 12, 2010 5:56 am

True, thanks.
Craig
STS
South Jordan, UT


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