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Old November 27th, 2005, 03:44 PM   #1
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Basic spaced omni/A-B technique

I want to try spaced omnis in a church, or small
to medium hall.
My original thought was to place the mikes at opposite
sides of the room, so that they'd be maybe
30 to 60 feet apart, but then I read somewhere
where someone said spaced omnis are usually
only placed a few feet apart. I'd like to
have them close together if this would be
effective, so that I could use the same recorder
rather than using two separate recorders, as I
would need to do if the mikes were on opposite
sides of the room.
Can anyone give me some guidance here?
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Old November 27th, 2005, 04:11 PM   #2
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Sorry, I don't know the answer for this, but what about this set-up makes it desireable? For your first thoughts (spaced 30-60' away) it would be nice becuase you could be runing stereo. But right next to eachother, that would be essentially the same sound feed. Sorry, I am just rambling... I'll stop

Thanks,
Max
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Old November 27th, 2005, 05:44 PM   #3
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Quote:
Originally Posted by Dave Largent
I want to try spaced omnis in a church, or small
to medium hall.
My original thought was to place the mikes at opposite
sides of the room, so that they'd be maybe
30 to 60 feet apart, but then I read somewhere
where someone said spaced omnis are usually
only placed a few feet apart. I'd like to
have them close together if this would be
effective, so that I could use the same recorder
rather than using two separate recorders, as I
would need to do if the mikes were on opposite
sides of the room.
Can anyone give me some guidance here?
Stereo techniques range from coincident mics such as "X/Y" placement where the capsules are placed as close as possible to each other with the diaphrams aligned vertically, - to spacing the mics at most a few metres apart known as "A/B" placement. But "a few metres" is one or two, not 20 <grin>. Coincident usually gives the best results. And omnis are generally not used for stereo recording - cardioid or hypercardioid are usually used to get the directivity necessary for good stereo imaging and to reduce phase problems. Schoeps has a good reference page about several different techniques online at...

http://www.schoeps.de/PDFs/stereo-re...chniques-e.pdf

If you're not actually recording stereo but instead want to pickup mono tracks from different parts of the room to get good coverage and that's why you want to place omni mics on opposite sides of the room, a spacing of 30 to 60 feet shouldn't require you to use two recorders. If you're using balanced mics and cables a 20 metre or longer cable run should be perfectly fine, especially if you're using dynamics with their generally higher output levels than condensers. Unbalanced is another story - unbalanced cables that long can pick up a signifigant amount of noise, especially if you're sending mic level instead of having a preamp at each mic to boost it to line level.
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Old November 27th, 2005, 06:30 PM   #4
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Quote:
Originally Posted by Max Liptack
But right next to eachother, that would be essentially the same sound feed.

Thanks,
Max
Well, I am trying in this situation to get stereo imaging
from using 2 omni mikes but it seems logical
that if the 2 are close together they will be picking
up the same sound ... but then I read elsewhere
that they SHOULD be placed close together. I'm
having trouble understanding this concept of how
close spaced omnis, even if they were 5 feet apart,
will give stereo imaging, seeing as you would think
they'd have to be farther apart to be taking in different
sounds.
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Old November 27th, 2005, 06:35 PM   #5
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Maybe I'm mistaken but I thought omnis (rather than
cardioids) are usually what are used in an A-B
spaced stereo setup.

Is there anyone here who has recorded spaced
omni who could tell me a good distance
apart that I should set these mikes, for a church
or small hall, for good stereo imaging?

Last edited by Dave Largent; November 27th, 2005 at 07:37 PM.
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Old November 27th, 2005, 07:46 PM   #6
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Spaced omnis can be used for stereo recording.I have not tried it but have used spaced cardiods. Most things I record have translate well to mono and spaced mics can have problems. Start with the mics spaced about 10 feet apart and move them closer together or farther apart to get the stereo image you want.

Sam
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Old November 27th, 2005, 10:29 PM   #7
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Quote:
Originally Posted by Dave Largent
Well, I am trying in this situation to get stereo imaging
from using 2 omni mikes but it seems logical
that if the 2 are close together they will be picking
up the same sound ... but then I read elsewhere
that they SHOULD be placed close together. I'm
having trouble understanding this concept of how
close spaced omnis, even if they were 5 feet apart,
will give stereo imaging, seeing as you would think
they'd have to be farther apart to be taking in different
sounds.
Wasn't clear in my previouis post. A/B is done with either spaced omnis or spaced cardioids while coincident techniques such as X/Y micing is done with cardoids or hypercardioids. Schoeps says in their reference material that stereo imaging is often poor with A/B placements and while you get a feeling of spaciousness, the image may be unstable, meaning instruments seem to "wander" about the soundstage.

The stereo image is not produced by the mics hearing different sounds as your posting implies. Think of sitting front-row centre at a concert. Your ears aren't widely separated and if you stick a finger in each ear in turn you don't lose half the instruments - each ear individually is still hearing all of the sounds. Yet you can close your eyes and point to each instrument with great accuracy. The impression of space and placement is caused by the sounds arriving at each ear at slightly different intensities and you don't need widely separated mics for that to happen. Coincident mics give good stereo on that principle - two cardioids or hypercardioids are placed together along the centreline extending out from the stage. One mic is pointed so it picks up mainly from the left side of the stage and the other from the right, usually pointed along axes diverging at about 110 degrees or so. So the a given sound arrives at each at the same TIME and thus always in phase, the mics are placed so the diaphrams are as close as possible to each other without one obstructing the other. The stereo effect comes about from the different intensities of the same sound arriving in each microphone. When mics are moved farther apart, the different arrival times start to contribute phase differences which can produce undesirable effects on playback.

Don't think you need wide spacing on the mics to get good stereo effect. Some of the most realistic stereo I've ever heard was a demonstration by Schoeps of an original master tape of the New York Philharmonic orchestra and Metropolitan Opera chorus recorded in St Paul's Cathedral, New York. The whole affair, well over a hundred muscians and singers, was recorded with only 2 mics in what is called a Blumlein array, where two figure-8 mics are placed with the axes pointing at right angles to each other and so close that the diaphrams almost touch. With eyes closed I could swear I was actually there.
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Old November 28th, 2005, 02:22 AM   #8
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Thanks, Steve, that clears up some things for me.

Too bad Schoeps doesn't list the Blumlein array
in their chart ... I'd be curious how it stacks up
for spaciousness and localization as compared
to the other methods.

Now, with Blumlein, which is an option
for me, is one mic placed right on
top of the other one?

Here's my situation. I'm mainly interested in
getting a sense of spaciousness of the room.
And I won't often be able to put the mics
in an ideal positioning, such as
down the center line from the front.
Mostly the mikes will have to be on one side
of the room or the other, not centered, so
I'm wondering if I'd be better off going
with a stereo miking setup where localization
is minimized so that the listener can't
tell so much that the mikes are off to the side.
Any thoughts on this would be appreciated.

Last edited by Dave Largent; November 28th, 2005 at 02:59 AM.
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Old November 28th, 2005, 06:39 AM   #9
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Quote:
Originally Posted by Dave Largent
Thanks, Steve, that clears up some things for me.

Too bad Schoeps doesn't list the Blumlein array
in their chart ... I'd be curious how it stacks up
for spaciousness and localization as compared
to the other methods.

Now, with Blumlein, which is an option
for me, is one mic placed right on
top of the other one?

Here's my situation. I'm mainly interested in
getting a sense of spaciousness of the room.
And I won't often be able to put the mics
in an ideal positioning, such as
down the center line from the front.
Mostly the mikes will have to be on one side
of the room or the other, not centered, so
I'm wondering if I'd be better off going
with a stereo miking setup where localization
is minimized so that the listener can't
tell so much that the mikes are off to the side.
Any thoughts on this would be appreciated.
"Stereo" with the mic off to one side just won't work AFAIK. If that's your case you'll be better off recording multiple mono tracks and mixing them in post, using the pan controls to place them in the stereo field.

I think there's a white paper on Blumlein on the Schoeps site but it might be in German but their catalog pages has a photo of the way they're set up. In any case a Google seach should turn up details. Your constraint of placing the mics to the side is going to preclude that technique however as the placement of the mic array is critical to getting good results (as it is with all stereo techniques, just more so). With Blumlein you use two side-firing figure-8 pattern mics, omnis will not work. The two are arranged vertically on top of each other with the business ends almost touching and the "8's" crossed at 90 degrees. You can see a picture here ...
http://www.schoeps.de/E-2004/blumlein-stereo.html
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Old November 28th, 2005, 07:51 AM   #10
 
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Quote:
Originally Posted by Dave Largent

Now, with Blumlein, which is an option
for me, is one mic placed right on
top of the other one?

.
No. Blumlein is a spread placement, and only works in some situations. You have to deal with rear reflections of left hitting the right mic and vice versa.
For mono mixes, this can create problems.

XY is where the mics are heads over each other, like the Rode NT4. With an XY arrangement, angling the heads from about 70 to 10 degrees will be common. Some guys go wider, I know some tapers that go much wider, for a more spatial sound, but for me...I prefer about 90 degrees.
Use omnis if you want actual bass response; uni's will exhibit proximity effect.
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Old November 28th, 2005, 08:10 AM   #11
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Douglas, about how far apart would you say I
should place the omnis, for a church gig?

And about the bass response: Is it true that
omnis will accurately record bass at a distance
from the sound source but that the bass really
drops off (at a distance) with cardioids?
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Old November 28th, 2005, 08:29 AM   #12
 
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Unidirectional mics will exhibit proximity effect, meaning that the perceived bass will be diminished by XdB depending on distance.
Omni's don't have this problem, only directional mics do.

As far as distance, I prefer A/b, where the omnis are less than a yard apart, or X/Y where the heads cross at near angles. I've also experimented (quite successfully with the baffled or "Jeklin disk" technique, and if it wasn't such a PITA, I'd use that more often.
Learn to make a Jeklin disk at http://www.maltedmedia.com/people/bathory/jecklin.html

For small rooms, the NOS formation, which is the opposite of the X/Y, as the butts of the microphone cross at near angles, is really useful.

Finally, there is the ORTF combination, where the mics cross each other mid-body at an angle of 110 degrees. This is good for small churches and other mid size rooms, and is closest to the human ear.

I don't use Blumlein, because I don't have a good matched pair of figure 8 mics/bidirectional mics.

FWIW, all this is found in my "Instant Digital Audio" book from CMP. :-) (Shameless plug)
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Old November 28th, 2005, 08:17 PM   #13
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Quote:
Originally Posted by Douglas Spotted Eagle
Finally, there is the ORTF combination, where the mics cross each other mid-body at an angle of 110 degrees. This is good for small churches and other mid size rooms, and is closest to the human ear.

I usually use ORTF at 110 degrees and about 7 to 7.5 inch separation between the capsules. It has slightly better stereo imaging than X-Y, at some sacrifice of mono compatibility. I've given up worrying about mono compatibility. It is a beautiful stereo image with small-diaphram cardoid condensor mics.

However, if you can't get your mics in the middle "half" of the room with some control over distance from the performers stereo recording techniques are mostly out the window. Block out a few seats, get the mic stand 6 or more feet in the air, and nobody should be bothered by it.

Is there a way you can fly the mikes in the right position? They don't have to be on a stand - up in the air can be fine positioning, and also gives you some attenuation of all those audience sounds (which can be VERY audible with stereo recording techniques!)
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Old November 30th, 2005, 11:48 AM   #14
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DPA has a great run-through of the common stereo pair mic'ing techniques here:
http://www.dpamicrophones.com/page.php?PID=131
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Old November 30th, 2005, 12:18 PM   #15
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Beginner's Resource

Wow, I'm having flashbacks from my Audio Field Prodcution class.

Steve, great explanations, and Bill, that web page looks good too.

For anybody wanting to start a little simpler, I came across this recently from my favorite source on the web...

http://en.wikipedia.org/wiki/Stereophonic_sound

Daniel
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