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What type of microphone?


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#1 Ten301

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Posted 22 September 2010 - 12:23 PM

Hello Everyone,

I have an idea for an "experimental" type of parabolic microphone to try to caputure EVP's (indoors). I'm not sure it will work, but I love to build and tinker around with stuff, so I have a few questions if anyone is familiar with microphones:

What type is commonly used in a parabolic assembly, unidirectional or omnidirectional? Does it make a difference? This will be connected to a Zoom H2, so would it be advantageous to use a stereo microphone for this application, or would mono be better? Finally, does anyone have a specific suggestion as to a brand/model of microphone that would work well in this set-up?

Thanks in advance for any advice anyone can offer!

#2 CaveRat2

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Posted 22 September 2010 - 12:56 PM

If you are using a parabolic reflector then a single microphone (mono) will be required. A parabola has a single focal point, thus you will be using a single mic element. While certainly stereo operation would be advised, in order to accomplish this you will need two of whatever system you are building. If I were doing this I would build one, get it working as you intend, then construct a second just like it for stereo.

That said since you are usinga stereo recorder I would recommend you simply branch the single output into both channels of the recorder. That would still be mono, but would allow you to use both channels for now. Later when you build a stereo system simply feed one mic into each channel.

As for mics, I can't say for sure because I don't know what your experiment entails. For general purpose though I would use an electret element and include a preamp right at the source. A simple OP amp would do. Plus you can also use the circuit to provide the DC bias voltage needed for the mic element. But as I said if you have some special requirements you may opt for an inductive element or even a ceramic one. Obviously I can't recommend any brands since I am not even sure what it is you are doing.

Good luck on the experiment!

#3 Ten301

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Posted 22 September 2010 - 02:04 PM

Thanks for your advice, CaveRat!

I will readily admit that mine is not an original idea, as there is a guy on Ebay selling what I want to build (with some modification). I considered simply buying his and possibly adapting from there, but he has it up for bids, which is a process I don't enjoy. Also, there are already several bids on the item. When I do use that site, it's usually only with the "buy now" option.

This is going to be a parabolic dish that sits horizontally on a surface (a table for example) much like a bowl would. I have read online that the dish must be at least 18" in diameter to be effective, at least for the usual uses of a parabolic microphone. This would, I believe, amplify the ambient sounds in a room. There may be other ways to accomplish this effect, but I thought it would be a fun project.

The one remaining question I do have is that microphones come in different "pick-up" patterns, so would the particular pattern make a difference in this application and, if so, which would be best?

Thanks again for you advice. It is much appreciated!

#4 Ten301

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Posted 22 September 2010 - 02:19 PM

Also, failed to mention that I plan to attach a base to the outside bottom of the dish to keep it from rolling to one side, and some type of vibration-insulating foam or rubber mat to place the entire assembly on to prevent the microphone from picking-up noise from vibration.

#5 CaveRat2

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Posted 22 September 2010 - 02:19 PM

The size is important in any parabolic application. The larger the better. However it is a misconception that a parabolic mic is more sensitive. Rather it is more directional. It picks up in the area in front of the reflector. Thus your horizontal mic would increse sensitivity from the ceiling area while reducing it from the floor. Areas along the face (horizontal) would not be affected much.

With an 18 inch aperature the gain effect would be approx. 3 db at 1 kHz. Lower frequencies would be less, higher ones slightly more. The factor controlling this is the relationship between the wavelength and the area of the parabola.

As far as a directional mic, I would use a unidirectional pickup to minimize side lobe pickup. Again though as small as the reflector is the effect is minimal. Keep in mind too that even if you use an omnidirectional element you have made it directional by placing it in the focal point of the parabolic reflector. You have altered its pattern by the reflector.

#6 ourobouros2k2

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Posted 22 September 2010 - 02:49 PM

Here is a set of photos and schematic for exactly what you are looking for. I hope that the links work, as this group may require membership to view them. Basically it is a homebrew parabolic for the H2 itself. If the links do not work, simply search for the naturerecordists (one word) yahoo newsgroup/list. Here are the links...

http://xa.yimg.com/k...942466/name/n_a

http://xa.yimg.com/k...830112/name/n_a

http://xa.yimg.com/k...842974/name/n_a

http://xa.yimg.com/k...790050/name/n_a

http://xa.yimg.com/k...594918/name/n_a

Hope these links work without you having to join the group.

regards,
Andy
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#7 Ten301

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Posted 22 September 2010 - 03:17 PM

Thank you all!

I just had a "Duh!" moment when looking at the schematics, in that I may be making this more complicated than necessary. If I intend to place the dish of this horizontally (and thank you, Cave Rat, for pointing out that if placed on a table, I will more or less lose much sound that is below the dish. Perhaps the floor would be better?) I may achieve the same effect by simply placing my H2 in the dish on its stand. Even if I have to elevate the recorder a bit, it would be easier. Also, the H2's microphones are probably better quality than something I would buy for this project.




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