Question about coincident mics

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If I mount two cardioid mics very close to each other, one facing to the left and one facing to the right, with the intention of recording stereo. If a low frequency sound comes from say the left, both mics will pick it up because the mics are not very directional at low frequencies. Then it seems the two signals will be out of phase. ( i know the correct term is polarity, i am using the term phase as is commonly used in this case. ) Does this cause a problem?

My usual configuration is to have both mics facing forward and separated by 10 to 15 feet. I wanted to try the coincident configuration, but i don't understand how the phasing works out for signals that both mics pick up if the mics are facing opposite directions.

Mark

[[email protected]]

To be more clear, i am talking about the XY configuration with the angle at 180 deg as shown here on the chart on the last page

http://www.tufts.edu/programs/mma/mrap/StereoMicTechniques.pdf


Mark

[William Sommerwerck]

If the mics are conventional cardioids (not having rear lobes), the polarity
of the signal will be the same at all points around the mic.

[William Sommerwerck]

Another point... Coincident miking depends primarily on amplitude differences,
not polarity or timing (ORTF being the exception). (ORTF was designed based on
subjective judgments as to the angle and spacing that most-closely
approximated what listeners heard in the concert hall.) Mics with a
directional pickup -- cardioid or figure-8 -- are therefore required.

If super- or hyper-cardioid patterns, or figure-8s, are used, then there's a
polarity inversion for sounds coming from the rear. This isn't normally a
consideration, as these sounds are principally ambience, and have no fixed
polarity or timing relationships with the direct sounds, or early reflections
coming from the front.

This is a terrific little reference! I've saved it.

[William Sommerwerck]

Something else I forgot (as did Schoeps) -- Tony Faulkner's "phased array",
using spaced figure-8s pointing forward.

http://www.sengpielaudio.com/TonyFaulknerPhasedArray01Engl.pdf

[Mike Rivers]

On 1/19/2014 9:58 PM, wrote:

> If I mount two cardioid mics very close to each other, one facing to
> the left and one facing to the right, with the intention of recording
> stereo. If a low frequency sound comes from say the left, both mics
> will pick it up because the mics are not very directional at low
> frequencies. Then it seems the two signals will be out of phase. (
> i know the correct term is polarity, i am using the term phase as is
> commonly used in this case. ) Does this cause a problem?

No. If the mic diaphragms are very close together, things entering both
will be "in phase." What makes them sound like stereo is that due to
reflections in the room, both mics won't get exactly the same signal.

> My usual configuration is to have both mics facing forward and
> separated by 10 to 15 feet. I wanted to try the coincident
> configuration, but i don't understand how the phasing works out for
> signals that both mics pick up if the mics are facing opposite
> directions.

10 to 15 feet is pretty wide for a spaced pair of stereo mics. You're
much more likely to have any given source be out of phase in the two
channels than you would with an X-Y pair.

Hopefully, you understand that the mics are 90 degrees apart, not 180
degrees as you said in another post. X-Y stereo really works, but
generally positions in the sound field aren't very precisely defined.
You can still tell left from right but it's hard to point to a specific
instrument or voice. The spaced mics (6-10 inches apart and pointing
outward from straight ahead) can often provide a sharper stereo image
because they work on arrival time (and hence phase) differences between
the two mics.

One advantage of coincident (like X-Y) arrangements is that if the
stereo is mixed to mono, you won't lose certain frequency ranges due to
phase cancellation between the intentionally not in phase mics. There's
a lot of theory that you can read about stereo mic setups. That DPA
article is a good start, but the best thing to do is experiment. Each
technique has its best and worst uses.


--
"Today's production equipment is IT based and cannot be operated without
a passing knowledge of computing, although it seems that it can be
operated without a passing knowledge of audio" - John Watkinson

Drop by http://mikeriversaudio.wordpress.com now and then

[Tom McCreadie]

Mike Rivers wrote:

>Hopefully, you understand that the mics are 90 degrees apart, not 180
>degrees as you said in another post. X-Y stereo really works, but
>generally positions in the sound field aren't very precisely defined.
>You can still tell left from right but it's hard to point to a specific
>instrument or voice. The spaced mics (6-10 inches apart and pointing
>outward from straight ahead) can often provide a sharper stereo image
>because they work on arrival time (and hence phase) differences between
>the two mics.

For a given mic angling, a (near)spaced pair certainly gives a wider soundstage
than that from its coincident sibling array, because the intensity- and
arrival-time factors are collaborating to give a reinforced effect. And the
near- spaced pair does tend to yield a more pleasant experience, ascribed to a
heightened sense of airiness (slight phasiness).

But with decent mics, it's seldom been my experience, Mike, that such spacing
also gave a sharpening of the stereo image. If anything, instrument sound
sources got a slightly smearier imaged sound location..
--
Tom McCreadie

[Scott Dorsey]

> wrote:
>If I mount two cardioid mics very close to each other, one facing to the le=
>ft and one facing to the right, with the intention of recording stereo. If=
> a low frequency sound comes from say the left, both mics will pick it up =
>because the mics are not very directional at low frequencies. Then it seem=
>s the two signals will be out of phase. ( i know the correct term is polar=
>ity, i am using the term phase as is commonly used in this case. ) Does t=
>his cause a problem? =20

No, if you do this, the two signals will still be in phase, because they
are in the same point in space and the wave is reaching them at the same
time.

>My usual configuration is to have both mics facing forward and separated by=
> 10 to 15 feet. I wanted to try the coincident configuration, but i don't=
> understand how the phasing works out for signals that both mics pick up if=
> the mics are facing opposite directions.

Imagine them as omnis at low frequencies, which they are.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."