Topic: DIY Spaced Omni Bar - active capsules [BSC2s]

[Gutbucket]

One way which might be helpful in thinking about the "4 mics too close together complexity problem", is to consider the concept underlying the well known 3-to-1 rule, even though that rule is meant for multiple microphone placement when those mic feeds are to be mixed to mono or pan-pot stereo (close mic'd PA or studio recording stuff) and doesn't apply to stereo mic arrays.* 

The 3-to-1 rule is usually stated something like this: If two or more microphone sources will be mixed together, the microphones should be at least 3 times as far apart from each other as they are from the source they are supposed to pickup.

The intent of the rule is to minimize phase interactions between more than one mic picking up each source.  Non-coincident stereo microphone configurations are parlty based on intentional phase interactions between the micophones.  In standard stereo setups those phase interactions are carefully balanced against level differences. That's the basic priciple behind the Stereo Zoom idea.  When more mics are introduced in close proximity, the phase interactions get complicated fast and the potential for problems increases radpidly.

*The 3-to-1 rule is actually physically impossible to apply to to stereo mic arrays.

[Ultfris101]

Very timely thread. Thanks guys! I'll try making a bar and I've been thinking about the three mic setup too. Some fuzzy fur for dead rats and I'm in business!

[ArchivalAudio]

One way which might be helpful in thinking about the "4 mics too close together complexity problem", is to consider the concept underlying the well known 3-to-1 rule, even though that rule is meant for multiple microphone placement when those mic feeds are to be mixed to mono or pan-pot stereo (close mic'd PA or studio recording stuff) and doesn't apply to stereo mic arrays.* 

The 3-to-1 rule is usually stated something like this: If two or more microphone sources will be mixed together, the microphones should be at least 3 times as far apart from each other as they are from the source they are supposed to pickup.

The intent of the rule is to minimize phase interactions between more than one mic picking up each source.  Non-coincident stereo microphone configurations are parlty based on intentional phase interactions between the micophones.  In standard stereo setups those phase interactions are carefully balanced against level differences. That's the basic priciple behind the Stereo Zoom idea.  When more mics are introduced in close proximity, the phase interactions get complicated fast and the potential for problems increases radpidly.

*The 3-to-1 rule is actually physically impossible to apply to to stereo mic arrays.

This makes sense. Thank you
I still try to wrap my head around stereo = 2 channels so 4  divided by 2 is 2 or stereo. and 3 divided by 2 is 1.5 not two. but then again 1.5 is more than one whole so if 2 is one whole than 1.5 is more than one whole.

yeah... I think I am getting that. I suppose I am attached to pairs....
But I do see the how 3 is more than 2.

--Ian

[ArchivalAudio]

Very timely thread. Thanks guys! I'll try making a bar and I've been thinking about the three mic setup too. Some fuzzy fur for dead rats and I'm in business!

glad to help.

[ArchivalAudio]

That is very nice man.
I may cop something like this for a lightweight portable omni bar..
It needs to easier to transport than just borrowing Skaggs's, and wider.


 -Cheers!

Thanks - happy to inspire others...

[Gutbucket]

I was drawing little diagrams yesterday to see how complicated the interactions get as more mics are added. Drew a dot for each mic then counted how many lines were needed to connect them all to each other-

1 mic = no phase relationship
2 mics = 1 phase relationship
3 mics = 3 phase relationships
4 mics = 6 phase relationships
5 mics = 10 phase relationships
6 mics = 15 phase relationships

Complexity doesn't mean it can't work, only that things get complicated and the potential for problems becomes greater.

Didn’t mean to sidetrack your DIY omni bar thread, but you got me rolling.

[fatstratcat]

I'm a little confused; a question about the third (middle) mic: how do I assign it to be neither left or right, but middle? I use Audacity or Audition in post.

[Gutbucket]

Depends on the software program used. I've never used Audition and havent used Audacity in many years.

Some use the paradigm of an analog mixing board/desk with channel strips for each inidvidual mono or stereo input.  In that case you pan that channel's output (routed to the main 2channel stereo output bus) to center, just like you would on a physical mixer.  That's probably the easiest to grasp.

On other editors, you click boxes to select the desired output bus(s) for each channel.  In that case if you select both the Left and Right main output buses, you route the identical channel output signal to both of them, which is effectively the same as panning to center in a mixer.

Aparently on other editors, one needs to make a duplicate copy of the channel and assign one copy to the Left channel bus and the other to the Right channel bus.  Again, as long as the two channels are otherwise identical except for the output routing, that's the same as panning to center.

I see many people on this board talking about duplicating channels to effectively pan things to the center (like the Mid channel in a Mid/Side pair) but I always wonder why they do it that way.   It works but is a rather cludgy work around.  Their software must have some way to pan a mono input from left to right.  If not they couldn't pan things slignty one way or another without major effort.

Most editors allow you to do things in several different ways.  If those programs you mention allow you to use a mixing console interface, that's probably the most straightforward way to go about it.

[Ultfris101]

Interesting question. In audacity you can break a stereo pair into individual tracks panned left and right and/or make a channel mono in the same drop down. Is a mono channel not panned left or right the same as a center panned channel? I would have assumed so but want to make sure I'm not misconstruing it.

I've gotten mono board feeds before and thought about duplicating and then panning each to make it reinforce the mic image a bit but haven't tried it so don't know what the result would be like.

[Gutbucket]

Interesting question. In audacity you can break a stereo pair into individual tracks panned left and right and/or make a channel mono in the same drop down. Is a mono channel not panned left or right the same as a center panned channel? I would have assumed so but want to make sure I'm not misconstruing it.

We need an Audacity user to answer that.

I've gotten mono board feeds before and thought about duplicating and then panning each to make it reinforce the mic image a bit but haven't tried it so don't know what the result would be like.

Duplicating a single mono channel to two seperate mono channels and then panning those to any position is equivalent to a single mono channel panned center as long as the levels of each are the same and the pan positions are symetrical regardless of their position.  If the pan positions aren't symetrical (or if the levels are different), that will pan the mono image over towards one side or the other from center.

[Ultfris101]

 actually happen to be an Audacity user, I just wanted to be sure I understood what center panning equated to. Playing a little with Audacity right now and I say the answer is YES.

Ok, yes, it seems obvious now but two mono channels panned to opposite directions is indeed identical to the single mono channel not panned.

I'm a little confused; a question about the third (middle) mic: how do I assign it to be neither left or right, but middle? I use Audacity or Audition in post.

So in Audacity, choose "mono" in the drop down where you can name the track and make sure to keep the panning in the center. You have a stereo track for the omnis and a single mono track for the middle. My R-44 is usually set to record a stereo pair so in Audacity I choose "split stereo track to mono" in the same drop down and then delete the empty track.

[fatstratcat]

actually happen to be an Audacity user, I just wanted to be sure I understood what center panning equated to. Playing a little with Audacity right now and I say the answer is YES.

Ok, yes, it seems obvious now but two mono channels panned to opposite directions is indeed identical to the single mono channel not panned.

I'm a little confused; a question about the third (middle) mic: how do I assign it to be neither left or right, but middle? I use Audacity or Audition in post.

So in Audacity, choose "mono" in the drop down where you can name the track and make sure to keep the panning in the center. You have a stereo track for the omnis and a single mono track for the middle. My R-44 is usually set to record a stereo pair so in Audacity I choose "split stereo track to mono" in the same drop down and then delete the empty track.

Thanks a bunch!!! That clears it all up for me. Now, it's off to my shop to fabricate an omni bar...

[jbou]

How did you insert the wedge bolt into the bar?

[ArchivalAudio]

How did you insert the wedge bolt into the bar?

easy... they are made to do this.
one end of the ~ hollow tube  ~ (not bar)  was a bit dinged up so I used a dremel to  widen the end - or flange it open slightly.
see tube label in photo:   Abbreviated on the label : ALUM - TUBE (RND)

Then  I placed both wedge bolts in as best as I could push them in by hand.
Placed one end on concrete and used a dead blow hammer to hammer one end then the other until they were snugly in place. I tried to rotate the bar to each end, similar to tightening lug nuts on a wheel ~ not too much of one or the other.
Then by the way they are designed I placed the washer and 3/8" nut on the bolt and tightened with a wrench. I used a channel lock "pliers" in the center of the tube where the shrink tubing would go.
I didn't really care if it was roughed up there.
The action of tightening the nut spreads the "wedge" on the bolt.
if you get a set of these and read the instructions it likely would make sense.

Hope that helps
--Ian

[ArchivalAudio]

I was drawing little diagrams yesterday to see how complicated the interactions get as more mics are added. Drew a dot for each mic then counted how many lines were needed to connect them all to each other-

1 mic = no phase relationship
2 mics = 1 phase relationship
3 mics = 3 phase relationships
4 mics = 6 phase relationships
5 mics = 10 phase relationships
6 mics = 15 phase relationships

Complexity doesn't mean it can't work, only that things get complicated and the potential for problems becomes greater.

Didn’t mean to sidetrack your DIY omni bar thread, but you got me rolling.

This all does make sense and applies here.
Not really a diversion but applicable.
thanx again
--Ian