Is it true that omnis dont have phase problems?

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locosoundman
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Post by locosoundman » Tue May 06, 2008 9:59 am

lol it was 1 question that no one is truely explaining just guessing at. so dont guess at it. i know how phase differences work. quit trying to teach me about everything i already obviously know, im not a newb over here going DURRR HowW DoEs i RecErd tEh MuSIC. the question was, do you think omnis have phase issues or not, not "can u explain to me what phase is and how it works".
Obviously, you don't know what phase is or how it works and I deduce this simply by virtue of your original question. A microphone can't have a phase problem simply by virtue of its pickup pattern.

If you are referring to comb filtering or exaggerated separation effect in a stereo mic'ing situation this is one thing. If you are referring to phase cancellation in mono, this is another thing (though the two can be related), if you are referring to acoustic phase cancellation at the diaphragm of the mic because of reflections from nearby surfaces, this is yet another thing.

What exactly are you asking?
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Post by chris harris » Tue May 06, 2008 10:10 am

r0t4ry wrote:lol it was 1 question that no one is truely explaining just guessing at. so dont guess at it. i know how phase differences work. quit trying to teach me about everything i already obviously know, im not a newb over here going DURRR HowW DoEs i RecErd tEh MuSIC. the question was, do you think omnis have phase issues or not, not "can u explain to me what phase is and how it works".
are you actually reading the responses? did it sound like I was "guessing" about anything?
the question was, do you think omnis have phase issues or not
yes. any pair of microphones (INCLUDING OMNIS) can have phase issues depending on placement.

it's that easy.

you're trying to assign some magical property to omni microphones that they just simply do not have.

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Re: Is it true that omnis dont have phase problems?

Post by chris harris » Tue May 06, 2008 10:13 am

r0t4ry wrote: Ive heard that omni's have no phase issues but I wanna know what you guys think.
You've heard wrong.

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Re: Is it true that omnis dont have phase problems?

Post by D0uble Helix » Tue May 06, 2008 10:25 am

This might also seem kind of obvious but, if anything, omni's are more likely to have destructive phase cancelation since they are picking up a wider image of the room. As a result, they're recieving a lot more reflections all arriving at the mics at different times from the fundamental sound and different times from one another. So I'm with subatomic in saying that it was a coincidence with respect to where you were at the time. The same experiment somewhere else (even in the same room) might show the complete opposite results. Might be fun to try. I know that in general, people in project studios or untreated rooms find better results using cardioid patterns almost exclusively because of this.
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locosoundman
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Post by locosoundman » Tue May 06, 2008 10:55 am

Phase is a measurement of change over time and/or space.

I believe the question you are trying to ask relates to inter-channel correlation in a spaced-microphone array. This has everything to do with spacing of the mic's and distance from the source, as well as the directional pattern. For a given AB spacing, omni's should theoretically have more inter-channel correlation than directional mic's because there is no null in the pattern. With omni's, more of the random diffuse sound field is picked up equally in both mic's, and more off-axis sound sources will be represented in both channels with similar amplitude, thus maintaining the stereo illusion in a wider placement than a directional mic could. There will still be phase cancellation when summed to mono because there are time differences between the channels.

This is not to say that directional mic's cannot be used in a spaced array, but usually the spacing between mic's must be closer to achieve better inter-channel correlation in order to maintain the stereo illusion. Often with a closer spacing (and due to it), directional mic's will be angled to take advantage of their ability to generate inter-channel intensity differences by virtue of their pickup pattern.

In a coincident stereo array, if done correctly, there should be minimal time differences between the channels and so there will not be too much phase cancellation when summed to mono. However, there is usually not much point in placing omni's in a coincident array; angled directional mics are used because they will maintain intensity differences between the channels to maintain the stereo illusion without time differences.
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Nick Sevilla
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Post by Nick Sevilla » Tue May 06, 2008 1:20 pm

r0t4ry wrote:lol it was 1 question that no one is truely explaining just guessing at. so dont guess at it. i know how phase differences work. quit trying to teach me about everything i already obviously know, im not a newb over here going DURRR HowW DoEs i RecErd tEh MuSIC. the question was, do you think omnis have phase issues or not, not "can u explain to me what phase is and how it works".
To get back at your original question :

What I "think" has nothing to do with it. This is pure physics and thats all.

I answered that YES, there will be phase issues. this goes for any two or more microphone recording setup. OMNI or other patterns.

This is not a "guess". It is fact. Check with some books on the topic, if you like. John Eargle is a good author to check with.

Hopefully, this is a short enough answer for you.

Cheers
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r0t4ry
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Post by r0t4ry » Tue May 06, 2008 2:21 pm

thanks locosoundman

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blackdiscoball
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Post by blackdiscoball » Sat May 10, 2008 6:05 am

Now if I remember correctly (and I could easily be wrong) PZMs do not have phase issues if used with other mics?

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Post by chris harris » Sat May 10, 2008 7:07 am

blackdiscoball wrote:Now if I remember correctly (and I could easily be wrong) PZMs do not have phase issues if used with other mics?
where are you guys getting this stuff?!?!

microphones just capture the sound waves. if the two captured sound waves are out of phase with each other, there will be phase issues. it doesn't matter what kind of mic you use.

there are no magic mics that will correct phase issues on the fly when used with another mic.

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Post by locosoundman » Sat May 10, 2008 7:41 am

I think I am developing phase issues...
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Post by locosoundman » Sat May 10, 2008 8:06 am

Sorry - I guess that doesn't really answer your question.
Phase is a measurement of change over time and/or space.
Unless two microphones are in the exact same spot, there will be some difference in phase between them. Think of it: if they are receiving the same sound wave at different times, the wave will be at different points in its cycle when they pick it up. It does not matter what kind of microphones you are talking about - this is a law of physics.

Think of two boats getting hit by the same wave: if they are in the same place, they will go up and down at the same time; if one is further away from the source of the wave, they will go up and down at different times. It doesn't matter what kind of boat it is.

The only phase issue a PZM is immune to is the situation where the direct sound coming from the source and a reflection of that sound (from the floor or a wall or something) combine at the diaphragm of a single microphone. Because the reflection has to travel a longer distance to reach the microphone, it will be delayed and, at some frequencies, the reflection will be "out of phase" with the direct sound causing some cancellation at the diaphragm of the mic.

A PZM (aka boundary mic) is generally mounted on the wall or floor or some large surface which would reflect sound (piano lid, ceiling, etc.). Because the distance from the "boundary" (reflecting surface) to the diaphragm of the mic is extremely small, there are no severe phase problems between the direct sound and the reflection, since they arrive at the diaphragm virtually at the same time.
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blackdiscoball
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Post by blackdiscoball » Sat May 10, 2008 8:28 am

So I wasnt a complete idiot. Im at work and cant get to any of my sound books but I knew there was something special about pzms and phase. When I get home ill have to read back up on that.

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rhythm ranch
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Post by rhythm ranch » Sat May 10, 2008 9:18 am

In boundary mics, the element is located so close to the surface it's mounted on, that boundary phase issues are shifted far enough beyond 20Khz that they become inaudible.

See the Crown whitepaper on PZM technology here.

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Post by Nick Sevilla » Sat May 10, 2008 7:36 pm

blackdiscoball wrote:Now if I remember correctly (and I could easily be wrong) PZMs do not have phase issues if used with other mics?
You are remembering absolutely wrong.

Here's the rule of thumb:

2 or more mics of ANY pickup pattern will invariably exhibit phase issues somewhere along the frequency spectrum. There is no "magic fix" and there are no two mics on the planet which do not do this. Why?

PHYSICS. The phase issues are NOT, I repeat NOT caused by the microphone itself, but by the SOUND WAVES in the room, crashing into each other. The SOUND WAVES.

The mics just pick this up, in varying degrees, depending on their POSITION in this room.

The only way you could achieve NO phase issues, is to not play anything at all, and record this silence.

Cheers
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Post by Spark » Sun May 11, 2008 8:36 am

Just throwing this out there:

For those that have been asking if omni mics/pzms are without phase issues, are you confusing phase and proximity effect? Omni's generally have a lot less proximity effect.

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