Lets Dive Deeper = Phase, Angles & Thier relation to each other

[AcousticDreams]

For myself, it is important to completely understand the Science & math, before I delve into the process. While experimentation is still the end all-tell all, seeing the route first is paramount in achieving the desired goal.

However, I am not foolish enough to believe that Math & Science are the absolute keys though. Why? Well too often the math & science is not conducted in a proper way. Or, the instrument of notation is leaving out other most important information. If this were not true...then we could just pick out microphones based on their frequency graphs and polar patterns. No, no, I have seen too many graphs that were identical yet the microphones sounded completely different. Still, it is most certainly a good starting point.

In every case of my past professions & hobbies, Knowing the science, theories the math helped me achieve a much higher level or proficiency. (I was a Commercial Photographer, Custom Knife Maker,.... Traditional Archer who made his own wood arrows, strings)

Thus brings me to so many question I have about Phase, Angles, and their relationships to one another.

I have never been a fan of the sound of X/Y micing. Yet, I can not deny the naturalness of sound. No phase issues. Very realistic. But for my tastes verges on being a mono sound and just sounds a bit lifeless in most instances.

So the big question is, what is really attributing to the No phase issues in X/Y micing? Is it that they are so close to one another? Or is it because they are at 90 degrees to one another? After all, Does not a X right on top of Y also result in no Phase issues? As they are occupying nearly the same space and are the exact distance from the source?

I have experimented a bit mics at further distances from each other but still using 90 degrees angles to each other with very good results. I remember that Bob Womack said something to the effect that he likes to mic Snares with Two mics at 90 degrees angles(one top and one on the side of the snare). Rather than the typical one on top and on on bottom and then flipping the phase.

Here is one interesting statement

" If they are 90 degrees out of phase from each other, they are said to be in quadrature." Definition of Quadrature?
"the process of constructing a square with an area equal to that of a circle, or of another figure bounded by a curve."
This figure illustrates in-phase and quadrature components overlaid. Note that they only differ by a relative $ 90$ degree phase shift.

I am not really sure how to interpret this graph.

*So what is really going on with 90 Degree Angles in their relation to phase?
**And to be absolutely sure...Are Two microphones one on top of each other, the capsules lined up...also do not create phase issues because they are basically occupying the same space? Even if they are two different manufactures with different frequency graphs?

I will be posting a follow up question later today about Parallel processing and Phase issues. I thought it prudent to create a separate post on this because of the length of information.

[runamuck]

I believe phase problems are caused by a signal reaching two mics at different times, not because of frequency sensitivities between them being different or the angle at which the signal hits the mic.

[Doug Young]

Originally Posted by Knives&Guitars

[INDENT]

So the big question is, what is really attributing to the No phase issues in X/Y micing? Is it that they are so close to one another? Or is it because they are at 90 degrees to one another? After all, Does not a X right on top of Y also result in no Phase issues? As they are occupying nearly the same space and are the exact distance from the source?

Right. One thing to keep in mind with mic placements is that none of the standard setups were designed for close micing an acoustic guitar. The original intent was to record ensembles from some distance. Using them to close mic an acoustic guitar still works, but it's a bit like watching a TV screen designed to be viewed from 20 feet away from 6 inches away. Some aspects of the physics may be mangled or exaggerated a bit.

But with XY, the mics are theoretically in the same location, therefore all sound reaches both mics at the same time - "in phase" between the 2 mics. The reason for the angle is that due to polar patterns, mics have different responses at different angles. To keep it simple, you could assume an extreme case, of a mic that is very directional, and rejects most or all sound coming from the sides. In that case, when you have an XY mic centered on a guitar, one mic is aimed at the neck/body joint while the other is aimed at the bridge. Sounds coming off the guitar from each of those points has a different tone. So one mic picks up primarily the bridge, the other picks up primarily the neck area. Reality is more complicated because mics don't reject nearly as much as we expect from the sides. Even with a hypercardiod, it's only down a few db, and it's also not consistent across all frequencies. But it's enough to create two different sounds, and that creates stereo. One mic picks up louder in one direction, while sounds coming from other directions are somewhat quieter. If you applied this to an orchestra or string quartet, you'd hear, say, violins coming from one side and cellos coming from the other side, just as you would if you were listening in person.

As you move the mics further apart, sound coming from one side or the other hits the mics at increasingly different times, so that while you still get sound pressure level differences, you also get phase differences. Our ears are roughly 6.5 inches apart, and part of how we recognize where sounds are coming from is that the sound in one ear may be louder, but the other is the time delay between ears. A sound that arrives in both ears at the same volume at the same time is in front of us. Sound that arrives in one ear slightly lower in volume and slightly later is interpreted as off to one side. ORTF mic placement mimics this.

By the time you get to spaced pairs, timing differences play a bigger role, and phase is a big part of why we hear the sound as stereo.

By the way, you can play games with this stuff. For example, take the exact same sound, put in the left and right channels, identically. Should sound like mono. Now add a slight delay to one channel - changing only the timing. Suddenly our ears will tell us the sound it panned slightly to one side. This is called the Haas effect.

[AcousticDreams]

While excelling in math, I was only an average Physics student. It can take me months to completely wrap my head around some theories. And even then, I often find myself still lacking in the understanding of it all.

From watching this Moving-Active Graph link below, I would suspect that there is more to the 90 degree angle and its relation to Phase.

https://upload.wikimedia.org/wikiped..._modulator.gif

Watching both the left and right diagrams: Every time axis is at 90 degrees the two waves line up as (right figure). And the internal sub squares(which are less than the whole quarters) disappear and become one whole quarter(quadrature?) on the left figure. I believe this means that anywhere in between creates less than a complete quarter in volume. Basically an inequality.

In some of my modest experiments I have notice a difference using Right angles and a seemingly phase coherent signals, even while heavily spaced.

Would this Active diagram not indicate that there is more going on with 90 degree angles and their relation with Phase alignment?

[Doug Young]

Originally Posted by Knives&Guitars

From watching this Moving-Active Graph link below, I would suspect that there is more to the 90 degree angle and its relation to Phase.

I don't think that animation is showing anything about mic angles. It's showing you phase using a different graphing technique. Phase is measured in degrees (0-360), so this graph is presenting phase within the circle. Here's another example of this kind of graph:

https://brilliant.org/wiki/phasor-diagrams/

In spite of having an engineering background, I usually find that analysis isn't as useful in musical applications as just listening. Take a pair of cardioid mics, bring up a few decent meters to monitor phase correlation and stereo width, and listen while watching the meters. It gets fairly obvious pretty quick - and how it sounds is all that really matters.

Many of the mic manufacturers do have some decent technical descriptions of various mic setups, tho, for more info. For example, here's a page from DPA where they go into time vs level differences in different stereo setups:

https://www.dpamicrophones.com/mic-u...ues-and-setups

[AcousticDreams]

Thanks Doug for the two links. Some very interesting information there.

I especially like this idea...of using two cardiods to create a type of figure of 8, except that your left and right sides would be more defined as they are each capturing one side of the body instead of the entire body. A true left and right.

Add a center channel...and then we might have that Mid side strength, with real differences from Left and right. Excited about this one Doug.

I am definitely going to be giving this a try coming up. Thanks so much. This has great possibilities for my needs.

As far as the moving 90 degree chart, you will notice that the arrows are always facing outwards on the circle(as microphones would be when they come to those 90 angles at the four points. And each time they come to the four points, the Waves on the right figure become as one. Plus, the squares go away leaving full Quarters of the circle in the first. So, to me this still would indicate that 90 degrees is phase aligned.

However, like you mentioned...and I mentioned...science & math doesn't always line up with reality. But from some of my tests there was some relationship to the 90 degree thesis.

It is the same thing for certain modifications on a guitar. What shouldn't make a difference sometimes does. In other times...it makes no difference when maybe it should.

[Doug Young]

I'm not sure what they're trying to show with that animation, really. Is there a site associated with it?

Phase gets kind of confusing. You can only really talk about it in relation to a specific frequency. Sine waves make things look both easy and confusing, because something 360 degrees off appears to be in phase. 720's in phase too, and so on. You can have 2 sine wave signals that are delayed by hours but seem to be "in-phase", as long as the peaks and valleys align. But in real music, where the signal isn't a neat little sine wave, we're really talking about delay overall, and phase only between certain frequencies. And if you have two mics that aren't exactly in the same location, a full-spectrum sound hitting them is going to have some frequencies in phase, and others won't be. Even the components of the sounds coming from the guitar aren't all in phase, since the sound is coming from different locations, and the tops of guitars don't vibrate in a single phase. It all gets too complex to wrap your head around. So people drop back to sine waves to explain the concept, which may or may not be all that helpful in dialing in a sound or mic placement.

That left-right mic setup's worth trying, tho the idea is basically like the 2 mic demo I posted earlier in the week - a mid mic, plus two side mics. The side mics are just aimed hard left and right instead of separated and aimed at the guitar. Placement will make a difference of course. I'm not sure why this would be more attractive than MS.

I think part of the beauty of MS is that it actually achieves true coherence and mono-compatibility. With an XY setup, theoretically, the capsules are in the same place. But in the real world that's impossible. Close, but not identical. And the mics may be close to "matched", but not identical. The "mid", in-phase component of XY is what you get if you mix XY to mono. In the real world, that mix will include some phase cancellation and also be affected by any differences between the 2 mics. Maybe not enough to matter, but it's two different mics in not exactly the same place.

In MS, the coherent part is coming from the same exact capsule, so it's perfectly in the same place, and the mid mic is perfectly matched with itself!. The side part of the stereo image is also close to theoretical perfection, since again, the sound of both sides is being picked up by the same exact capsule. Put them together, and there's something really stable and solid-sounding about MS, while still being able to be as wide as you like. I don't hear that, even with XY.

[Doug Young]

I just tried that left-right facing pair quickly. I guess what I'd compare it to is just XY taken to an extreme. Instead of 90 degrees, or 110-120 as some people do if they want wider, these are XY at an angle of 180 degrees. The mics are quite out of phase, as you'd expect, but nothing like the sides of MS. More of an ultra-wide, poorly phased stereo sound. Mixing in a mid mic, reduces the width of course. The mix of the 3 sounds fine, but nothing especially exciting to me.

I'll play with it some more and send you a demo if I get results I'm sure are correct...

[Doug Young]

Oh, I went and read the text around that image you posted - I was missing a key aspect. They're suggesting that you swap the wiring on one of the mics, combine them together and send them to the same channel. That basically creates a figure-8 mic. So this is just a way to create an MS setup if you don't have a figure-8 mic, but do have 3 cardioid mics, with 2 of them reasonably matched.

UPDATE: tried it, and sure enough it works. It's basically identical to MS, tho a lot more work with a 3 mic setup, and then either a custom cable and and some routing to get both mics in 1 channel, or recording to 3 tracks and setting up the routing to phase reverse and blend before sending to MS decoding, as well as getting all levels set appropriately. Here's a short noodle, first with real figure-8 MS using a Brauner VM1 as the mid mic and the Scheops MK8 as the figure-8, followed by the faked figure-8, Brauner as mid, 2 Schoeps MK41s facing each other (and then one phase reversed, mixed to mono, then sent to the MS Decoder along with the Brauner mid mic). Pretty hard to tell apart, and I'd say any differences are due to mismatched levels between the MK41s and MK8 and/or slightly different mic placement.



Interesting....

[Bob Womack]

Originally Posted by Knives&Guitars

https://upload.wikimedia.org/wikiped..._modulator.gif

I think this graphic is illustrating the relationship between absolute phase, aka polarity, and time alignment.

In an acoustic model, phase can be defined by two characteristics: polarity and time. Polarity is set by the mic's physical relationship to the source. If there is a mic pointed directly down at the top of a drum head and another a pointed directly up at the same head, when the drum is struck, the bottom head first receives the effect of the downwards flexure of the head towards it as an increase in pressure. The top mic first receives the effect of the head flexing away from it as a decrease in pressure. They are 180' out of phase. The effect of time is slightly different. If you set one mic at certain distance and another at another, the sound arrives at each mic at a different time. If the distance is equal to one-half the wave length of the fundamental pitch the mic is picking up, the sound can be said to be 180' out of phase.

By the way, this is the difference between a phase shifter effect and a flanger effect. Both blend an effected copy of the sound with the clean sound but they affect phase differently. Phase shifters have a network that changes the polarity of the incoming wave in real time. Flangers put a delay on the effected side to set the phase relationship between the clean and effected copies. The left side in your pic illustrates the effect of polarity change (a phase shifter's effected side) and the right illustrates time change (a flanger's effected side) and shows how the two, absolute phase vs. time shift, are related.

Bob

[AcousticDreams]

Originally Posted by Doug Young

Phase gets kind of confusing. You can only really talk about it in relation to a specific frequency. Sine waves make things look both easy and confusing, because something 360 degrees off appears to be in phase. 720's in phase too, and so on. You can have 2 sine wave signals that are delayed by hours but seem to be "in-phase", as long as the peaks and valleys align. But in real music, where the signal isn't a neat little sine wave, we're really talking about delay overall, and phase only between certain frequencies. And if you have two mics that aren't exactly in the same location, a full-spectrum sound hitting them is going to have some frequencies in phase, and others won't be. Even the components of the sounds coming from the guitar aren't all in phase, since the sound is coming from different locations, and the tops of guitars don't vibrate in a single phase. It all gets too complex to wrap your head around. So people drop back to sine waves to explain the concept, which may or may not be all that helpful in dialing in a sound or mic placement.

same place. But in the real world that's impossible. Close, but not identical. And the mics may be close to "matched", but not identical. The "mid", in-phase component of XY is what you get if you mix XY to mono. In the real world, that mix will include some phase cancellation and also be affected by any differences between the 2 mics. Maybe not enough to matter, but it's two different mics in not exactly the same place.

Originally Posted by Bob Womack

I think this graphic is illustrating the relationship between absolute phase, aka polarity, and time alignment.

In an acoustic model, phase can be defined by two characteristics: polarity and time. Polarity is set by the mic's physical relationship to the source. If there is a mic pointed directly down at the top of a drum head and another a pointed directly up at the same head, when the drum is struck, the bottom head first receives the effect of the downwards flexure of the head towards it as an increase in pressure. The top mic first receives the effect of the head flexing away from it as a decrease in pressure. They are 180' out of phase. The effect of time is slightly different. If you set one mic at certain distance and another at another, the sound arrives at each mic at a different time. If the distance is equal to one-half the wave length of the fundamental pitch the mic is picking up, the sound can be said to be 180' out of phase.

Thanks so much guys. Your explanations have helped immensely.

Phase is so confusing. And yes, it does take me hours and hours to wrap my head around something that can be rather complex.

I am pretty good at seeing things in my head. But sometimes there is no substitute for mapping things out on paper. Well in this case, computer.

After long and careful study of the moving diagram...I see that in each quarter the arrow tips are always at right angles. It never really shows 180 degrees.

In the graph, they are outwards. And with my experiments I point them inwards.

Both are 90 degrees but in reality, they are covering different territories. Therefore, they are entirely different in how they might effect the signal.

In Fact, the graph does not take into account a polar pattern at all.
Here is what the graph is showing if you were to add a polar pattern.
[/INDENT]

And here is a diagram of some of the mic positions I have previously experimented with. As you can see, while both are 90 degrees, yet one might say that the inward pointing mics will cover a smaller area. (Duh? why did I not see this at first?) As the methods of pointing direction are opposite.

Depending on the distance you spread the microphones from each other, and the closeness to the guitar, how the polar patterns interact could be the answer. I would have to assume that there would be some value as Decibel levels overall combined Off Axis response decibel levels, via distance, should be considered.

[Doug Young]

Originally Posted by Knives&Guitars

Depending on the distance you spread the microphones from each other, and the closeness to the guitar, how the polar patterns interact could be the answer. I would have to assume that there would be some value as Decibel levels overall combined Off Axis response decibel levels, via distance, should be considered.

For sure, all of that is why XY works. It depends on the cardioid pattern, and on-axis vs off-axis response. If you set up omnis as XY, you're going to get mono (assuming perfect omni mics). You're using wide cardioids, tho, right? I'm not sure you'll like the XY sound from those. Maybe, but they're closer to omnis, so the stereo effect may be greatly reduced.

I'm not sure in your diagrams if you're indicating different directions or different overlap. When people say 90 degrees with an XY mic, they mean something like this, with the capsules as closely aligned as possible:



and here's how the polar patterns interact:

[AcousticDreams]

All Righty then,

I think now I am realizing what is really happening. Why 90 degrees is working for my application..

First you should know that I have the mics spread out at a good distance. About 30 inches actually. While my mics are still seemingly are only 11 inches from the guitar, hitting two different areas of the guitars....But are they really?

The Centers of of the Wide Cardiods are pointing to the same location.
So what might really be happening is that there is little delay between the Mics because in actually, they are the same distance thus good phase alignment. In this diagram, the centers are still pointing towards the same place.

[alohachris]

Aloha Knives & Guitars & Friends,

Interesting discussion thus far.

When I was REALLY into high-end mic buying, trading & selling, I REALLY tried to do my due dilligence to tried & learn all I could about the science of mic's & how they work ensemble in patterns & combinations, particularly in stereo. The theories made sense but I got REALLY bored with them after a few years because in the end, they were negated or trumped by two key elements:

- Variations in your ears (even educated ears) on a given day,
- Variations in the space in which you play & record on a given day (like the outside weather effecting its atmosphere & humidity).

At that point (as a mostly, non-commercial recordist), I tried to understand mic's through the filter of those elements & not to worry as much about something like off-axis performance variances or phase issues I could not hear. If I heard or "felt" an issue, I paid attention - with my ears.

All those crazy mic placement experiments I conducted literally in every place of every house I lived in for all those decades taught me that it's ALL ABOUT THE EARS! To me, the results I could hear & create mattered more than the science (undergraduate science major/former luthier, & longtime gigger here).

Many theories I had to throw out the window when mic placement experiments - that should have never worked due to anticipated phasing issues - turned out to yield some of the best miking results I've ever heard/achieved. Like an SDC pair close-miked in X-Y combined with a widely-spaced LDC AB pair about 3-4 feet out from the guitar (in a treated space of course).

The theories & physics do become ultra important when trying to apply them to something like miking an orchestra, or tap-tuning a guitar top. But then, the space or mic placement again can put the kibosh on the absolutes & importance of those theories when listening day to day.

So as interesting as this discussion has been, I guess it's more for those who give a hoot. Why eff up the magic of playing & listening to music by moving the experience from the creative into the left side of your brain?

alohachris

[AcousticDreams]

Originally Posted by alohachris

Aloha Knives & Guitars & Friends,

it's ALL ABOUT THE EARS! To me, the results I could hear & create mattered more than the science .
Why eff up the magic of playing & listening to music by moving the experience from the creative into the left side of your brain?

alohachris

Quite possibly, the wisest and truest words spoken yet.

Doug has also said something similar to this on many posts.

And one of my good friends who is a pro recording engineer... has said to me when I have asked him of the science of recording, " I don't know about those things...I use only my ears "

In the end, I know you are absolutely right. We must all trust our gut before anything else. The knowledge of science we know right now...will more than double is less than 5 years. So what is true today, may not be true tomorrow.

In reality, I will probably always be hindered in some ways by my quest for truth. It is something that is part of my DNA.