Another Blumlein question - stereo bars

Discussion in 'Location Recording' started by foldedpath, Mar 1, 2005.

  1. foldedpath

    foldedpath Guest

    If you're using two figure-8 mics as a Blumlein array, and they're lightweight enough to mount together on a vertical stereo bar.... does the bar itself cause any interference? Is it better to just use two booms for mic placement and leave the rear field open?

    I know the bar is sitting in the back null, but I didn't know if there would be any local reflections that might cause problems.

    Mike Barrs
     
  2. DavidSpearritt

    DavidSpearritt Well-Known Member

    Yes if they are small diaphram condensers, more difficult if they are a couple of Coles 4040's.

    Not really, however its good to keep the capsules clear for at least a couple of inches away. You are trying to avoid diffraction of HF sound around the structure.

    I will post a photo of my carbon fibre array to hold two KM120's in Blumlein, it works very well.

    The back of a blumlein array is certainly not a null, it is a full gain copy of the front but 180 out of phase. The nulls are at +-45 off axis and up above and below.

    We made a recording of a vocal ensemble recently that required an accompaniment track, we used a Blumlein array and stuck the little foldback speaker at the base of the mic stand holding the Blumlein pair. Because it was in the null of the array, we could jack up the foldback volume at hardly any came through the recorded signal, but it was still really close to the singers. Great system.
     
  3. DavidSpearritt

    DavidSpearritt Well-Known Member

    Here is three views of it. Again it is carbon fibre kite spar, thin for the arms and thick/hollow for the vertical support. It has a 3/8 joiner glued into the base that then screws nicely onto the top of the manfrotto.

    Took about an hour to make. All held together with SuperGlue, which is ferocious on kitespar, will never break. Standard Neumann clamps on the ends of the arms, bolted to little plate sections of old circuit board drilled out.

    http://www.lodestarrecordings.com.au/km120/KM120A.jpg
    http://www.lodestarrecordings.com.au/km120/KM120B.jpg
    http://www.lodestarrecordings.com.au/km120/KM120C.jpg
     
  4. Cucco

    Cucco Distinguished Member

    Dave,

    Perhaps you misunderstood what foldedpath suggests, but where he is talking about is technically the null.

    Since, in the blumlein, the left mic is pointed 45deg's off center axis to the left and the rear lobe is now centered at 45 off center axis the right and behind and the right mic is the transverse of this, the position directly behind (especially within only a couple inches) is the very definition of blumlein's null position. True, once you move out enough that the two lobes overlap, the null disappears, but at a distance of only a few inches, this hasn't taken place yet and the stereo bar would hence not get in the way.

    J...
     
  5. DavidSpearritt

    DavidSpearritt Well-Known Member

    This is incorrect. The nulls of the individual mics when positioned in the array are at +-45 degrees assuming zero degrees is straight ahead. "Directly behind" is -180 degrees which is just 45 off axis to either mic, certainly no null, and in fact is the full gain sum when the mics overlap. A re-examination of the beautiful Blumlein diagram might assist. See Figure 2 in this PDF ...
    http://www.wesdooley.com/pdf/Surround_Sound_Decca_Tree-urtext.pdf
     
  6. DavidSpearritt

    DavidSpearritt Well-Known Member

    I am not sure what you mean here. There is no distance dependence on directivity, a null is a null right near the mic, or a few inches away, directivity does not change with distance from the mic. None of this is influenced by the location of the stereo bar.
     
  7. Cucco

    Cucco Distinguished Member

    Dave,

    I don't know where to begin here, so here goes:

    First - directivity and distance are directly tied together. Think about your statement. I'll explain more in a second.

    Second - if you were simply goind by the illustration in Mr. Dooley's publication, then I could understand your logic. However, very few figure 8 microphones are truly linear in two perfectly circular lobes. It more often resembles a narrowed parabola.

    The angle of response for these types of microphones exists between two points in space at fixed degrees, thus creating a rather 'V' like shape on either side of the microphone. Therefore, if you are off axis and close (Let's say 70 degrees and 4 inches), you will be in a point where signal is attenuated. However, you can move further from the microphone in a straight line and remain at the same level of attenuation, thereby "cheating" the law of inverse squares by simply moving into the "hot spot" (This actually isn't cheating the law of inverse squares, rather it's exploiting it, but because we are working with a microphone that is directional, it appears to be cheating it.)

    If the pickup angle were truly circular as Wes' illustration implies, at some point in space, you could have one person sitting directly next to another and he would not be picked up or would be considerably attenuated because he moved slightly outside the pattern (And I don't mean into one of the nulls, I mean at the far reaches of the pattern, where it apparently is circular...) To imagine that a mic's pick-up patter could be circular on all boundaries is absurd - physics won't allow it.

    Another illustration would be -
    Stand directly in front of the ribbon element at 0 degrees. Sing a constant tone. Now move slowly and consistantly to a point 90 degrees off axis. Does your voice remain the same until magically it virtually disappears, or does it begin to fade until it's virtually non-existant? With a good ribbon, it should kind of do a combination of both, but there will be definite fading.

    This also proves that the illustration is fundamentally flawed.

    So, to reitterate, the stereo bar that we were discussing - if it were placed only a couple inches away from the ribbon and at the same point transversely within both mic's pick-up pattern, the effect of the stereo bar would be either minimal or even non-measurable.

    J...
     

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