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"Variable Fixed-Configuration Stereo Bar" - thoughts?

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melontracks:
My first foray into open taping after many years of stealthing was this past Sunday. I really enjoyed the freedom of not having to wear my recording gear. Ha! I tried to pay attention to my setup routine so i could improve it at future shows. Aside from managing to lose one of the clips that came with my brand new AT4041s, my biggest gaffe was trying to set up a stereo-pair of mics on a stereo bar using a ruler and a protractor. i wasted a solid 15 minutes dicking around trying to get it "just right." Having two variables (separation and angle) where changing one changes the other is maddening. Being brand new to this, I would like to try out many of the different configurations (ORTF, DIN, A-B, etc).

People make rigs to hold specific mics in specific configurations, but I wanted something that would let me lock the mics down quickly in a number of arrangements, depending on what i wanted to try. Last night, I built this from a piece of aluminum plate and some cheap shockmounts:



I had bought the shockmounts for another purpose, then noticed that they had a couple of threaded holes in the top of the mount that take M4-.70 bolts. I should have probably cut the plate longer, but this was just a prototype. So far, I have it drilled so I can run ORTF, DIN, NOS, and a 30cm-spaced A-B pair. I could pepper the plate with an unlimited amount of holes for a great variety of placements. I've labeled them with the config and spacing.



This thing removes the angle variable, so all you need is a cm ruler to space the capsules. It's really fast, and requires only a screwdriver and a minute or two to completely re-arrange. I need to come up with a better spacer than the miniature sockets i'm using, but that's a detail.

Aside from the fact that the mics tend to sag a little if they're hanging too far out one side or the other, i haven't identified any liabilities. I was wondering if the plate itself might present acoustic problems. I could skeletonize it, but would rather not bother. I searched around for similar homebrew stuff and was surprised i didn't find anything. What might I be overlooking here as far as potential problems?

Thanks.

One more pic:

DSatz:
That's some nice-looking metalwork. I agree that it can be difficult and time-consuming to get the angles between capsules the way you want them to be, and anything that simplifies the process and reduces errors is very welcome. That's why for some configurations, I use fixed stereo bars that set the angle and spacing between capsules on active cables so that there's no ambiguity.

--The acoustic effect of a rigid, flat surface depends on how large it is, how far your capsules are from it, and sound wavelengths. As a rule of thumb, any rigid surface or solid object with dimensions half a wavelength or larger should be kept several wavelengths away from the capsule. High frequencies are the most vulnerable to physical interference, because their wavelengths are the shortest.

How big is a wavelength? If you use 1100 ft/sec as the speed of sound in air, then a wavelength at 1100 Hz = one foot. At 11 kHz (ten times that frequency) it's one-tenth of a foot, or just 1.2 inches. So if we just consider just the central part of your mounting plate by itself (ignoring most of its length, in other words), that's already half a wavelength or more for all frequencies above about 2 kHz.

So its potential for physical interference with sound is considerable, and you really do need to keep your capsules away from it. I would suggest six inches or more so that its solid angle in the sound field around your capsules won't be large. If you build another version of this plate, I suggest that you try to reduce its depth (the dimension that looks as if it's maybe 3 to 4 inches in the current version) but most of all, keep your capsules away from the flat part.

--best regards

P.S.: If those are cardioid mikes in the center--for coincident cardioids, I would really urge you to try a wider angle between your capsules unless a near-mono recording is required for some reason. Cardioid just isn't a sharply directional pattern.

Gutbucket:
Nice work.

Consider making a few of these, or otherwise figure out a way to extend the spread of the omnis for any situation where you are able to run them wider, which will be advantageous for taper situations.  I'd double it to 60cm or better 1m, and if mixing the omnis with with the X/Y pair in the middle, 1 to 1.5m or even more will often improve things significantly.  One way to increase omni spacing to something wider than the bar itself is to point the omnis outwards in opposite directions to either side, so that the capsules are cantilevered outwards past the ends of the bar when placed in the shock-mounts.  Would be easy to set that up by making another pair of mounting holes in-line with the bar at either end.  Such a 180 degree opposed arrangement of the omnis also leverages their slight high frequency directionality in a way which helps compensate somewhat for an otherwise overly narrow omni spacing.

I commonly use heat-shrink over the aluminum bar-stock parts I make for my taping rigs.  It provides a tough, resilient, hard-rubber optically flat-black surface, decent acoustic damping, and deforms just enough when bolting through it to friction-lock both the part and screw in place so as to resist rotation.  Requires a diameter large enough to slip over the bar prior to shrinking, and works best if you shrink it onto the bar prior to drilling the holes.  I  clean it up by sanding down the heatshrink which extends past the ends until flush with either ends of the bar using a standing belt-sander.  The bar-sock I use is narrower in width yet thicker than that pictured above, which increases resistance to bending while somewhat reducing the acoustic interaction issues Mr Satz mentions.

I don't think you need to worry much about resonance of the bar, especially with the mics suspended in shock mounts.

Gutbucket:
I strongly concur with Mr Satz's post script comment on increasing the X/Y angle using cardioids to greater than 90 degrees as long as your intent is use of that pair on its own.  And I strongly recommend your own experimentation with other stereo configurations to learn what they sound like in the situations in which you record.  That's the best way to really learn about two-channel stereo microphone arrangements.

But be aware that complications arise when mixing two or more two-channel microphone arrangements together, especially if they are in close proximity to each other.  That pretty much throws much of what works for 2-channels alone out the window, and X/Y angle is a good example of that.  If mixing with omnis, I usually prefer a center pair X/Y angle which is 90-degrees or less, in combination with a wider omni spacing.  The near-mono (but not quite) nature of that center X/Y configuration blends nicely with the omnis without conflict, providing tight X/Y stereo imaging across the center of the image while the wide omnis provide an immersive ambient "stereo-ness" which a cardioid X/Y setup tends to lack regardless of angle.

Because of that, if you wish to play with mixing both microphone pairs at some point, you might consider positions for 45-degree X/Y for combination with wide omnis, as well as 90-degrees, and say 110-120-degree X/Y for a pair of stereo cardioids on their own.   

At that point, a productive approach that works especially well for typical taper recording positions which are not close to the stage is to choose whichever angle places your cardioid or supercardioid X/Y pair closest to on-axis with the main PA speakers.  Then afterwards while mixing with the omnis, re-adjust the stereo width of the X/Y pair to whatever works best by ear, using a stereo-width-adjustment plugin that does L/R>Mid/Side>L/R type adjustment.

Apologies for jumping ahead, just attempting to lay out something of a roadmap to keep you out of the ditch.

melontracks:

--- Quote from: DSatz on May 16, 2019, 12:27:22 PM ---So its potential for physical interference with sound is considerable, and you really do need to keep your capsules away from it. I would suggest six inches or more so that its solid angle in the sound field around your capsules won't be large. If you build another version of this plate, I suggest that you try to reduce its depth (the dimension that looks as if it's maybe 3 to 4 inches in the current version) but most of all, keep your capsules away from the flat part.

--- End quote ---

this bar is about 6cm deep. I think after I make others, I'll cut this one out to suit some specific configurations. My options for bar shapes are pretty unlimited, as I'm cutting them from a large plate of aluminum with a jigsaw. I'll put more specific thought into future designs. I went with 6cm on this one because it was pretty much the narrowest bar that would still permit a parallel pair of A-B mics with the threaded holes in the shockmounts being where they are (41.2 mm apart).


--- Quote from: Gutbucket on May 16, 2019, 12:49:21 PM ---I commonly use heat-shrink over the aluminum bar-stock parts I make for my taping rigs.  It provides a tough, resilient, hard-rubber optically flat-black surface, decent acoustic damping, and deforms just enough when bolting through it to friction-lock both the part and screw in place so as to resist rotation.  Requires a diameter large enough to slip over the bar prior to shrinking, and works best if you shrink it onto the bar prior to drilling the holes.  I  clean it up by sanding down the heatshrink which extends past the ends until flush with either ends of the bar using a standing belt-sander.  The bar-sock I use is narrower in width yet thicker than that pictured above, which increases resistance to bending while somewhat reducing the acoustic interaction issues Mr Satz mentions.

--- End quote ---

I like this idea! Man, the bar I have has 3/8 thread on the thumb screws, so i had to use 3/8 to 5/8 adapters to connect to the stock mic clips, and BOY did they want to spin. Ha!


--- Quote from: Gutbucket on May 16, 2019, 01:36:18 PM ---I strongly concur with Mr Satz's post script comment on increasing the X/Y angle using cardioids to greater than 90 degrees as long as your intent is use of that pair on its own.  And I strongly recommend your own experimentation with other stereo configurations to learn what they sound like in the situations in which you record.  That's the best way to really learn about two-channel stereo microphone arrangements.

--- End quote ---

Thanks for all this good advice. I guess I need to read up on the X-Y 90-degree card pairing. I was under the assumption that you wanted the capsules to be as perfectly coincident as possible to avoid potential phasing. As far as having four mics on the stand at the same time, I probably should have posted different photos; i don't really foresee doing that much, if at all. Mostly, i just wanted to see how much junk I could mount after i built it, and get a feel for what would physically interfere with what, and when spacers would be needed. I expect to build a few of these for different general purposes.

Thanks again for the input!

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