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Author Topic: diy parabolic reflector for wedding vow mic  (Read 1065 times)

Steve Loewenthal

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Re: diy parabolic reflector for wedding vow mic
« Reply #30 on: June 13, 2018, 08:58:38 pm »

What would be the minimum size (area) of plywood that I should use to get reasonable results?
I assume that if mounting on plywood that bigger is better, and there is probably a point of diminishing returns. Is that correct?
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Steve Loewenthal

"I'm, just the guy in a band that owns the PA and I'm trying to figure out how it works. (Been trying to learn somethin' about it for about 20 years and I hope somethin' learns me soon)"

Luke Geis

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Re: diy parabolic reflector for wedding vow mic
« Reply #31 on: June 14, 2018, 02:26:41 pm »

Actually it doesn't have to be that big. The whole thing about a PZM mic is that the surface that it is mounted on is what collects the energy to be picked up by the mic. So the surface only needs to be bigger than the mic itself by at least some amount. I wouldn't say bigger is better necessarily. The excepted norm is a 3' X 3' surface for best results, but I don't think it MUST be that big if the situation won't allow for it. In theory however, the surface that the mic is mounted on should be as big as the lowest frequency of interest. This is impractical for most applications. A 100hz frequency has a wavelength of about 11' so that would be one really big panel to put it on. A 500hz frequency is roughly 5' and this is a more realistic size that the mic can be placed on. Again however, it is excepted that a panel that is roughly 3' - 4' in size is sufficient.

The big aha that most don't think about is the dimensions of the boundary. It is actually NOT IDEAL to have a boundary with equal dimensions. Thus a circular panel is to be avoided. It is actually ideal to have a slightly rectangular panel and to also place the PZM mic slightly off center so that it is not equidistant form any one point on any edge. Imagine a 3' X 4' panel with the mic placed up and back 3" from center. This will reduce the amount of comb filtering that the mic design will pick up as a result of the boundary effect.

The fun of physics. The boundary is a reflective surface. So any two points that are equidistant on the boundary will couple at some frequencies and null at others. So you want to place the mic at a random spot that is not equidistant from any of the four sides, but is still near the middle of the boundary. What happens is that the sound is reflected off the boundary and back into the mic. This is what gives the mic its gain boost. The sound has nowhere to go once it hits the boundary and the mic collects that added energy. The problem is that the reflection is out of phase ( time ) with the original source sound. This is not as big an issue with larger frequencies because the shift in phase ( time ) is small relative to its length. It is easily within 1/4 of a wavelength for much of the spectrum. This is why 3' panels work well enough; a 3' panel is large enough to contain and reflect low end media so that it couples as needed. Now at higher frequencies this is more problematic, but you can't get nothing for free. Having the mic slightly off center reduces the possibility of being at a total null for any one given frequncy and the loss or gain that does occur is minimal.

Again experimentation will prevail, as a small panel perhaps as small as 2' by 2' would work just fine for speech. The reason being is that you are only interested in media from about 500hz and up. There is still enough low end from the direct sound to work, but the boost in output will occur in the critical midband where you need it most. Try a couple panels and see what works best for you.
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Lyle Williams

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Re: diy parabolic reflector for wedding vow mic
« Reply #32 on: June 14, 2018, 04:59:46 pm »

Clear perspex/acrylic is the material of choice for discrete PZM panels in conference settings.
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Stephen Kirby

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Re: diy parabolic reflector for wedding vow mic
« Reply #33 on: June 14, 2018, 05:47:56 pm »

Actually, if there's a way to directly mount the PZM to the ceiling of the arbor that may save some grief.  Alternately, mount it to a small panel and mount that to the ceiling.  The idea is that there is only one coherent pressure front at the element.  Small steps from a mounting panel don't really have any effect.
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Steve Loewenthal

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Re: diy parabolic reflector for wedding vow mic
« Reply #34 on: June 14, 2018, 07:27:11 pm »

Thanks for the replies. I think it tells me what I need to know from a sound perspective.
I often mislead myself into thinking about collection area when instead I should think in terms of wavelength of the frequency band.

I don't remember is whether the shelter has rafters with a "joist" type member at the bottom (ie a triangle) where I can lay the plywood on top, or if they are a simple inverted V where I will need to screw through the plywood into the underside of the rafters. I stuffed a couple different lengths (3', 4', 8') into the trailer and will hope for the best.
Not sure about the precipitation forecast either, so who knows if I will even set up the outside speakers.

And I still don't understand the desire for an outdoor wedding ceremony in June in Rockhill SC. (That is probably a better topic for the basement.)
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Steve Loewenthal

"I'm, just the guy in a band that owns the PA and I'm trying to figure out how it works. (Been trying to learn somethin' about it for about 20 years and I hope somethin' learns me soon)"
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