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Author Topic: Omni Mic Setup  (Read 1952 times)

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Offline CorFit Chris

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Omni Mic Setup
« on: April 22, 2015, 10:24:08 AM »
I apologize if this is already out there, but I am having trouble locating a clean, concise description of a basic omni mic setup.  I have a pair of Studio Projects C4's and would like to try out the omni caps.   I would likely only use them in small, intimate, quiet venues (coffee house, acoustic, etc).  However, I am now sure exactly to best LOCATION, SPACING, and DIRECTION to have them pointing.  I assume as close to stage/performers as possible with at least 3 feet between them.  I also have a second pair of mini-cardioids I often setup (x/y) in the middle of my SP C4's.  It would be great if someone with extensive knowledge/experience could upload photos with labels of a variety of mic configurations for 1-pair and 2-pair recording techniques in a single post.  Thanks

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Offline chk

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Re: Omni Mic Setup
« Reply #1 on: April 23, 2015, 03:59:48 PM »
There are no shortage of techniques. You need to experiment. But given the conditions/venue I would be as close to the performers as possible, but in a location that sounds good and balanced to your ears. In terms of spacing it can vary from near coincident "healy method" of around 17cm or so (with mics pointed "out" from each other /each at 90 degrees from the source), to much wider spacing of 10 feet or more. Part of it depends on how far apart the performers are from each other. The key is to find thst spot that sounds good to your ears. For on-stage recordings I have run everything from healy method spacing to 6' or more and recently have settled at around 40-45cm spacing. Gives good openess, decent imaging and doesn't suffer from sounding like there's a "hole in the middle" when played back.  Also, read "the stereophonic zoom" by williams. His guidelines suggest anywhere from 36-50cm spacing...

Generally point the mics straight ahead, but play around with that since many omni mics have some directionality at upper frequencies...something to keep in mind that you can work with if you need to.
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Offline Bruce Watson

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Re: Omni Mic Setup
« Reply #2 on: April 24, 2015, 03:12:56 PM »
However, I am now sure exactly to best LOCATION, SPACING, and DIRECTION to have them pointing.

There are as many techniques as there are people using them. However, there is a mathematical basis for a lot of this. As chk suggests, Stereo Zoom is an interesting paper. If you want an interactive and visual representation of it (different dataset apparently, but very similar results) have a look at the Sengpiel Audio Visualization Tool. For AB, pick one of the AB techniques from the "microphone system" pull down. Then play with the parameters on the right hand side. See what changes, what doesn't.

A place to start might be to set the mics pointed straight ahead (parallel), 67cm apart, 3 to 4m up, and 1m back from the conductor. But this would be just a starting point -- you'd need to make adjustments to find the sweet spot for your particular situation. There is no "one size fits all", optimum setups vary greatly from location to location and ensemble to ensemble.

If you go wider than 67cm, you start opening up the "hole in the middle" problem with widely spaced omnis. As a result, many people pull back from 67cm down into the 40-50cm range. You really should put on your headphones and listen while you make adjustments. You might be surprised by what you hear.

Offline CorFit Chris

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Re: Omni Mic Setup
« Reply #3 on: April 28, 2015, 01:17:19 AM »
Thanks Bruce and chk.  On your suggestions I have done my homework and feel much better informed.  I will try my mics with Omni caps in A/B at around 60cm spread with second pair of cardioids centered in x/y, and go from there.
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Offline bryonsos

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Re: Omni Mic Setup
« Reply #4 on: April 28, 2015, 02:37:27 AM »
If you look at the literature, you'll get a better stereo image from the omnis at a 3ft (1m if you prefer). I like running the 3ft spread with cards at DIN when I run 4 mics. ymmv
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Offline firemt66

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Re: Omni Mic Setup
« Reply #5 on: April 29, 2015, 05:08:24 PM »
Hello fellow taperssection people..I have omnis and cards and hopefully will be using them soon :) scott

Offline CorFit Chris

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Re: Omni Mic Setup
« Reply #6 on: May 13, 2015, 01:18:11 PM »
If you look at the literature, you'll get a better stereo image from the omnis at a 3ft (1m if you prefer). I like running the 3ft spread with cards at DIN when I run 4 mics. ymmv

That is how I plan to run mine.  We will see, but at least the cards should work well alone if the omni pair does not work well with the venue/crowd/etc.  Thanks
LMA: https://archive.org/details/@corfit
SoundCloud: https://soundcloud.com/orithris
Mics: AKG c460 & c480 (ck61 & ck63) | CA-14 omni | SP-SPSM-9
Recorders: Tascam DR-70d  |  Tascam DR-60d 
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Offline Gutbucket

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Re: Omni Mic Setup
« Reply #7 on: May 13, 2015, 03:23:18 PM »
Some general starting points-

Search the term "omni" in the "Microphones and Setup" section of this site and you'll find loads of info.

Optimal omni spacing is very much a personal preference thing, but is also strongly dependent on what you are recording (the type of music, type of ensemble, if there is PA reinforcement or not and the nature of that reinforcement), the environment in which you are recording, and the recording position within that environment.   You'll get wildly varying suggestions on what spacing is appropriate, more so than any other microphone technique, even from the 'by the math' type resources.

Generally, the farther away the recording position is from the source, the wider you'll want to space the microphones..  until you get very close, like on-stage or up against the stage-lip, where it often works to go wider again (for different reasons), or narrower (for different reasons).

If you are planning on simply comparing the two resulting recording to decide which you prefer- one made with omnis and the other made with cardioids, use a narrower spacing for the omnis such as other posters here have suggested above.  If you plan to play around with mixing them together, it's better to use a much wider spacing on the omnis, say double whatever spacing you find you like best when using the omnis alone or more.  The other pair takes care of any potential 'hole in the middle' making that problem non-existant, replaced instead by potential 'conflilcts in the middle' when you have 4 microphones in close proximity mixed together.

I start 3' wide and adjust narrower or wider from there for a pair of omnis alone.  I start at about 5 or 6' wide if I have a third microphone or some other array in the center, with the intention of using them in combination.  But that's just me.

Americans typically go much wider than Europeans.  ;)  Not just in the waistline.

>You can more safely 'go wider' when there is either only one sound source in the center, or multiple sources with plenty of sound from the center.
>You can more safely 'go really wide' from farther back when there is a big PA, since the PA will be mostly mono, countering tenancy towards 'hole in the middle' (even though the PA speakers may be widely spaced, the apparent sound source is strongly centered.  By "sound source" I mean "where the sound seems like it is actually coming from" not where you see the instrument, musician or speaker that is actually producing it.  Close your eyes and listen to determine actual sound source directions.)
>Safer to 'go narrower' when there is no sound source in the middle (say no PA and a 2 or 4 piece act with the mics at stage-lip, no sources coming from center) and no other microphone(s) in the middle.

There are other omni techniques besides A-B spaced pair: 
>You can place a baffle between two omnis, which allows (and requires) you to put them much closer together, potentially even right up against the baffle.  The baffle technique typically won't work if the mics are more than a couple feet apart since the baffle is then too small to do anything.  Search "Jecklin-disk",  "J-disk" and "baffled omnis" for info.
>Sometimes that baffle can be your own head.  Search "HRTF"
>You can "boundary mount" them directly against a hard flat surface which provides unique characteristics (A-B spaced on the back wall, or spaced on the stage floor) or onto several smaller surfaces with an angle between them, making them hemispherically directional from each surface in the higher frequencies (say onto two adjacent sides of a wooden crate or taped to an unused stage monitor)  An old PZM trick was fixing the mics to rigid plastic sheets arranged in a 'V' shape producing an omni technique somewhat more like near-spaced directional microphone configurations.
>You can use more than two omnis, three four or five arranged in a line, three in a triangle, or four in a diamond shape.  Search "Decca tree".

Have fun!
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Offline Gutbucket

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Re: Omni Mic Setup
« Reply #8 on: May 13, 2015, 03:38:10 PM »
One more thing-

It's common for recordists to think about different omni spacing affecting the sense of "openness, space and air" in the recording, how solid and compact verses sparce and wide the playback image is, if the sounds seem to be spread evenly between the speakers verses coming directly from each speaker separately with a hole-in -the-middle, and other stuff like that.

It's not as common for recordists to think of different omni spacings having strong effects on the frequency response, spatial and tonal qualities of the bass and other low frequency information in the recording.  But the effect of various spacings on those things can be quite strong.
volition > vibrations > voltages > numeric values | numeric values > voltages > vibrations> virtual teleportation time-machine experience
made easy- >>Improved PAS table<< | made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

 

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