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MS stereo imaging problems with MKH30/50 combo - Sounds are swapping

Subject: MS stereo imaging problems with MKH30/50 combo - Sounds are swapping
From: "J. Mullens" ajustend
Date: Mon Oct 15, 2012 10:59 am ((PDT))
Hi, this is my first post to Nature Recordists.

I have just added an MKH 30 to my collection in order to expand into MS
recording and am experiencing very bad imaging (side swapping) with the
setup when the sound source is located in certain places behind the
microphone.

To clarify, I am *not *trying to record a surround signal using a stereo
mic setup, but I *am *interested in maintaining a correct left-right stereo
image. When I use an XY setup, or an XY mic like a BP4025, anything on the
left side of the mic (in front of it or behind) will playback out of the
left speaker. Using the MKH30/50 combo this is not the case.

Here is the test I conducted: Set the microphones up in the middle of an
open field so that I can walk in a full circle around it. I start directly
in front of the M/S setup, about 15-20 feet away, and walk in a clockwise
direction around mic calling out each position on the clock face. 12 is
directly in front, 1, is to the right of that, 3 is directly to the right,
6 is directly behind, 9 is directly to the left side, etc.

Here's the problem, when I get to the 4 & 5 o'clock postions (rear right
side) the sound source swap sides and sounds like it is coming from front
left. And the exact opposite happens to the 7 & 8 o'clock positions
sounding like they are coming from the front right.

Again, any source originating in the 180 degree arc in front of the setup
performs beautifully and the stereo image is perfect. It only happens to
sounds coming from behind the mics, and if a sound source is making a
constant tone you can actually hear it jump diagonally across from one side
of the stereo field to the other as it enters these rear right & left areas=
.


After asking around it has come to my attention that the closer your Mid
mic is to a pressure-gradient mic (figure 8 polar pattern), the more of a
rear lobe you have, and thus the more chance you have of that rear lobe
picking up sound in the opposite channel. This definitely explains how, and
why, I could be experiencing side swapping.

Since I've never used an MS setup with a pure cardioid mic I am wondering
if the same thing happens...  For those using a Sennheiser 30/40 MS setup:
does this happen? Do you have side swapping in the 4-5 o'clock and 7-8
o'clock positions?

Nature recording is, for me, one of the areas where a really accurate
stereo image in a 360 degree circle around my recording location is
critical. I am always pointing my mics at the source I am recording, but
having sounds behind the mic appearing in opposite channel is just weird.
How do the MS recordists on this forum deal with this?

Thanks!

-Justin









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