Blumlein and phase

[cale w]

So I recently picked up a pair of Cascade Fatheads, and I want to record my band in a circle around the blumlein setup. I've been thinking it over, and it seems like there would be unwelcome phase cancellation happening between the lobes. For instance, if the drums were centered in the front lobe, and the bass amp centered in the rear lobe (facing each other across the circle with the stereo pair in the middle), wouldn't 80hz from a bass note and 80hz from a kick drum strike happening simultaneously cancel each other out when they reached the mics? As I understand it, the front and rear of a ribbon are opposite polarity from each other. The more I ponder it, it seems wiser to array instruments across the 'front' 180 degrees of the blumlein pair to avoid phase demons. Or am I totally off base?

Has anyone ever recorded musicians in a circle around a blumlein setup? Or for that matter, would treating the setup as a mid/side array make any difference?

Humble me with your wisdom, oh masters of things I can't quite figure out on my own...

[thethingwiththestuff]

without getting too academic about it yes, it would probably be a better idea to put the blumlein pair in front of a semi-circular band instead of in the middle of a circle. my thinking would sort of be along the same lines as what you said about cancellation, but also, blumlein is cool because the rear lobes pick up the ambient room sound. there would be too many variables when things were spread all around it.

i'd rely on an omni pattern if i wanted to stick mics in the center of a circle.

[Scodiddly]

Ah, but how do you know whether the 80Hz from the bass would even be phase with the 80Hz from the kick?

Try it out before you over-think it. If you're not doing some kind of rehearsal recording anyway, you should be.

[mrb]

I did that some days ago: two fatheads in blumlein in front of the band, more or less in a semi-circular shape. i didn't check phase as it was just a rehearsal.

I wanted to show to my sceptical colleagues you can record a whole band using just two mics and three tracks, and not 8 mics for drums, 2 for each guitar, etc. They were converted.

[Scodiddly]

To put it this way - kick drum vs. bass guitar is "uncorrelated" signal. There is no phase to talk about, because phase requires you to start with similar/identical signals coming to you at possibly different times.

It shouldn't matter.

[ofajen]

To put it this way - kick drum vs. bass guitar is "uncorrelated" signal. There is no phase to talk about, because phase requires you to start with similar/identical signals coming to you at possibly different times.

It shouldn't matter.

Precisely... no worries about different signals (drums vs. bass). The only possible problem is if somehow you end up with a lot of correlated reflected sound coming to the opposite lobe: i.e. the direct drum tone comes from the front side but you are also getting some relatively coherent reflection of the drum sound coming back off an opposing wall into the rear lobe. Same thing could go on if you use even a single ribbon mike as an overhead with the front lobe pointing down. If you have hard reflections coming back off the ceiling, you will most likely get an ugly out-of-phase effect in the signal. In that case, it's probably best to just put up an absorber overhead and kill the reflections.

I'd say just try it. it should be OK.

Cheers,

Otto

[cdixon6]

I agree with those above, and applaud them for saying what I was thinking but clearer in explanation. For what it's worth, if you are worried about phase cancellation then don't record in a room, or near a floor, or with a microphone. I guess what I am saying is don't overthink it, just try it. Recording music is better explained as an art than rocket science. It's more enjoyable that way.

[Artifex]

I agree with the above, plus I've used those mics in that setup. It sounds fantastic.

[Justin Foley]

One other thing to know - the bass drum does not produce a 80 hz sine wave. The bass guitar may produce something that more closely approximates an 80 hz sine wave but still is far from that (especially in a reverberant space). If you had two speakers in an anechoic room placed on opposite sides of a bi-directional mic sending out an 80hz wave, you could probably arrange it so that the two signals would come pretty close to canceling out at the point of incidence at the ribbon. But, for the thing you're talking about, the overtones and reflections are so complex that you're best just trying stuff, noting how it sounds and making adjustments/comparisons. You can move stuff around the room, you can move the mics around the room and you can flip the polarity on your preamp. Given all that stuff to monkey with, I bet you can come up with something that sounds good.

= Justin

[ofajen]

As I understand it, the front and rear of a ribbon are opposite polarity from each other. The more I ponder it, it seems wiser to array instruments across the 'front' 180 degrees of the blumlein pair to avoid phase demons. Or am I totally off base?

Upon further reflection, you should probably know that it may be wise to keep musicians out of the "side quadrants"... i.e. between "in front" and between "in back" are OK, but the side quadrants are iffy at best. The problem is that the direct sound will mainly reach the forward lobe of one mike and the rear lobe of the other and will have comparable levels with that polarity flip thing going on between them. It's best to only have diffuse, reflected sound coming from the sides, if possible. I hope that helps. Sorry I didn't mention it earlier. You were wise in raising the question.

Cheers,

Otto

[Scodiddly]

The sides of a figure-8 are (in theory) 100% cancellation right in the middle. Much deeper cancellation than you can get at the back of a cardioid. That's what makes figure-8 useful quite often, in that you can put noise sources (or other instrument) to the sides. I even do that live now and then - one mic over a vibraphone in front of a jazz big band, and with a figure-8 pattern (and a high enough ceiling) it actually works.

[ofajen]

The sides of a figure-8 are (in theory) 100% cancellation right in the middle. Much deeper cancellation than you can get at the back of a cardioid. That's what makes figure-8 useful quite often, in that you can put noise sources (or other instrument) to the sides. I even do that live now and then - one mic over a vibraphone in front of a jazz big band, and with a figure-8 pattern (and a high enough ceiling) it actually works.

Totally true. The null of a single figure 8 is fabulously useful.

We mustn't forget however, that here we're talking about the stereo response of crossed figure 8 mikes. The four "quadrants" are separated by the axes of the lobes. Each mike's lobe axis is also the null axis of the other mike.

The front quadrant is the range of angles that are in the interior 90 degree angle of the axes of the two front lobes. You generally want all the music in that quadrant. The back quadrant is the range of angles that are in the interior 90 degree angle of the axes of the two rear lobes. Generally you want uncorrelated, ambient sound in this quadrant, but it is OK to have music there, just realize that the stereo image will be reversed from what you actually hear from behind you.

The side quadrants are the other two ranges of angles that are in the interior 90 degree angle ranges of the axis of one mike's front lobe and the other mike's rear lobe. In those two quadrants, the direct signal is partially gathered by one mike's front lobe and the other mike's rear lobe.

You want to minimize all signal in those side quadrants, but particularly you want to have no direct signals (music) coming from those ranges, because the stereo image will be totally messed up. A direct signal coming from an instrument along the exact midline of each side quadrant, for example. will end up with precisely equal amplitude and opposite polarity in L and R channels, which is a total imaging disaster. Hence the need to keep all direct signals out of those regions.

I hope that clears it up. It is much easier to get when you can look at a few good diagrams. "The New Stereo Soundbook" by Streicher and Everest has good pictures and spells all this out.

Cheers,

Otto

[emrr]

The old radio production textbooks all show figure 8 null being the spot the producer stood while giving spoken cues to voice talent facing one another across an RCA 44 during live broadcast.

There's at least one textbook that shows all the variations of mid/side, the resulting patters, and how they correlate to other techniques, including Blumlein. Worth finding and studying.

[Scodiddly]

"The Microphone Book" by John Eargle is a great book for microphone theory. Doesn't tell you how to build one, but otherwise full of really good info about microphone theory, stereo techniques, arrays, etc.

[cale w]

Thanks for all the heads ups guys. I realized pretty quick while experimenting with my blumlein setup that the sides don't like direct sound very much. I was my plan to essentially put the drums centered int he front lobe, bass amp centered in the rear lobe with guitar amp and leslie on either side of the bass amp to move them a bit off to the sides. I'll give it a whack in a couple days and let you know how it goes.

After reading all your great responses, I've been experimenting with my Fatheads and really diggin the flexibility of a) that SWEET null spot and b) the rear lobe that can add so much character to sounds. I love the negative-space balance of thinking where those side-nulls are pointed when doing multimic setups. Too much good fun.

Not to derail mt own thread, but I'm finding a lot of sources that the bloomy low end of those fatheads really compliment. Anyone have the same experience with fatheads or ribbons in general? A friend of mine is really put off by all that chub, but I think it's incredibly useful in the right context.