Soundfield Mic Discussion

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arcanemethods said:
Whatever. I was trying to address people that want to know how it works, not just that it does.

That's the problem with oversimplification. It's often wrong. Read it again.
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Okay Bob, you got me!! LOL. Make it "any kinda cardioid - hyper, wide, sub, super, etc."

Well, since we're each trying to do the same thing, a little (over)simplified version of some of the terms might be in order:

"Zero order", "pure pressure" refers to omni mics (or the omni portion of a cardioid mic) that responds only to air pressure changes, not direction. "First order", "vectors", "pressure gradient", etc. refers to type of mics and patterns that have some directional pickup. These also respond to air motion rather than just air pressure.

When you combine a pressure gradient pattern (like a figure 8) with a pressure pattern (like an omni), you get something in between - a cardioid.
Vary the level of the omni portion to change the cardioid pattern to something else.
Increasing the omni level slightly makes the cardioid pattern wider, called a wide cardioid or subcardioid.
Decreasing the omni level slightly changes the cardioid to a narrower pattern, called a hypercardioid or supercardioid.

So when you read the stuff about the Soundfield; if they're talking about "pressure" or "zero order", they mean the omni part of the signal. If they're talking about "pressure gradient" or "first order" or "vectors", that mean the directional part of the mic's signal - the figure 8 part.

With the decoder, it's like having 3 figure 8 mics: two like an M/S setup (with two figure 8 mics), plus another figure 8 mic aimed at the ceiling and the floor, and another almost perfect omni mic. The decoder lets you play with the various combinations.
 
alanhyatt said:
Here is the problem that I see with this. Height and spacing is very important when dealing with this kind of application. Trying to adjust for differences between the capsules would effect frequency. Time and distance and its effect of frequency are known as Doppler Effect. Trying to control this from a decoder without it being controlled at the capsules as well is probably too much trouble.
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Sorry, Alan, I misunderstood your point here. Yes, for such a system it would be _very_ important to control capsule spacing and orientation in order for a decoder to have a chance at it. Pretty small deviations from what an encoder assumes the geometry to be will result in really signifigant errors in its output.


Bob
 
Bob

Does SPL Atmos work on the same principle to Sounfield? The mic has 5 non-coincident capsules and their pattern is individually variable from * to omni. Other then a massive price difference are the two systems more or less similar in the way they operate? What are the advantages/disadvantages of the coincident (soundfield) and non-coincident(SPL) arrangement?

I hope this is not a too stupid question

Thanks

Yuri
 
YuriK said:
Bob

Does SPL Atmos work on the same principle to Sounfield? The mic has 5 non-coincident capsules and their pattern is individually variable from * to omni. Other then a massive price difference are the two systems more or less similar in the way they operate? What are the advantages/disadvantages of the coincident (soundfield) and non-coincident(SPL) arrangement?

I hope this is not a too stupid question

Thanks

Yuri
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No stupider than the answer. :-)

I don't know. I've not seen this mic. Do you have a link to information on it?


Bob
 
arcanemethods said:
Nah, with the right DSP it's a piece of cake.
BTW, the Doppler effect pertains to moving sound sources like, ostensibly, loudspeaker cones and, if it exists, is a non-linear effect. With microphones, if uncompensated, time/distance factors result in comb filtering, a linear effect.


Bob
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It seems you are certain it is a piece of cake. If it was, a product would be out by now, so I still see problems with this type of setup.

I mean no disrespect, but formulas and theory are fine, but until they are proved, they mean nothing. I maintain that the method OneRoom speaks about will cause too many issues and problems.

By the way, singers do not stand still... :)
 
DJL said:
I tihink that would be cool... and with enterchangeable capsules for different tones and etc... oh, and a cool pre-adjusted soundfield quad mic holder. :D
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Interchangable capsules would yet cause another problem. What is a cool pre adjusted soundfield quad holder? I am curious...what applications do you have for this?
 
I don't think there is one yet...

But here's what I was thinking.. it might be cool if there was a quad mic holder for holding four incidental mics of the users choice... and an encoder with a selector and etc... or would that not someday be possible?
 
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arcanemethods said:
The XY and Z "velocity" components are formed by summing the four capsules in the three combinations that exist where two of them appear negatively in each sum. Since two are positive and two are negative in each sum, the omni components, being non-directional, simply cancel. The summing of the figure eight components has to take into account the direction from which sound arrives at the mic and it turns out that, due to the angles involved, the result of each sum is another figure 8 pattern such that each lobe has two postive and two negative capsules on each side of it and the axis of the lobe goes straight between them. This gives three figure 8 pattern each of which are orthogonal (at right angles to) the other two and can be aligned to up/down, left/right and front/back.
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I gotta say I don't have a clue as to what all y'all are talking about here, but why not use 6 capsules arranged orthagonally, as on the faces of a cube, and hook 'em up to a circuit like this one. Seems it would make the decoding easier.
 

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Harvey Gerst said:
Okay Bob, you got me!! LOL. Make it "any kinda cardioid - hyper, wide, sub, super, etc."
Click to expand...
:-)
Well, since we're each trying to do the same thing, a little (over)simplified version of some of the terms might be in order:
Click to expand...

I wouldn't call what you wrote oversimplification, but rather clarification, and thanks. You have a much better idea, having been here a long while, what the technical levels of the members are.

For those more technically inclined, the first order, figure 8 component that is logically contained in each mic in the cardiod family has a (normalized) sensitivity as a function of angle given by:

cos(theta)

The omni component has a (normalized) sensitivity of 1. The ideal cardiod family of mics have the sensitivity as a function of angle given by:

(A + cos(theta))/2

Where A is the gain of the omni component relative to that of the figure 8 component on its axis (theta=0). If A is one it is a cardiod, if A is greater than one it is a wide or sub cardiod and iif it is less than one it is a narrower super or hyper cardiod. The reason that A=1 gives a cardiod is that from the front, when theta=0, then cos(theta)=1 and the sensitivity is 1. From the back, when theta=180, then cos(theta)=-1 so the sensitivity is 0.


Bob
 
YuriK said:
Bob

This all the info I could get. There are some reviews on the site, but nothing too detailed

http://www.soundperformancelab.com/Atmos/in_detail.html


Yuri
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This mic seems more akin to an ORTF stero setup but with one mic each for the separate surround channels. I don't think there is any matrixing of the mic outputs but rather they go directly to the surround channels on playback. It probably gives pretty pleasant results but I wouldn't expect it to be as accurate in imaging as what you could get from planar Ambisonics (two crossed figure 8's plus an omni.)


Bob
 
crazydoc said:
I gotta say I don't have a clue as to what all y'all are talking about here, but why not use 6 capsules arranged orthagonally, as on the faces of a cube, and hook 'em up to a circuit like this one. Seems it would make the decoding easier.
Click to expand...

Here we go with the tecnical stuff again but I just don't know any other way to tell the story. Maybe Harvey can clarify in easier terms.

The configuration you show will result in a lumpy sensitivity surface for the directional component and no way to extract a purely non-directional omni component. The sensitvity surface of the Ambisonic mic with ideal cardiod family mics after encoding is a smooth spherical surface. No matter what direction a sound source is relative to the mic, as long as it remains at the same distance, then we will have the property that the outputs X, Y and Z will be related as:

sqrt(X^2 + Y^2 + Z^2) = constant

This fact can be put to excellent use in decoding the outputs back to speaker feeds for reproduction.

With the configuration you show, there is no way to matrix the results to maintain that relationship and instead of a constant in above expression you would get a value that is dependant on the angle. This will give a lumpy sensitivity surface which adds the requirement that you need to know the angle of incidence to properly decode it to speakers. This is not possible.

That's not to say that the configuration and the matrix that is shown in your diagram might not sound pretty decent if fed to speakers properly and if the speakers are in the proper arrangement but the beauty of the Ambisonic system is that it can both in theory and in practice be used to derive the correct outputs for any number of speakers arranged somewhat arbitrarily so long as they meet a few requirements.


Bob
 
arcanemethods said:
No matter what direction a sound source is relative to the mic, as long as it remains at the same distance, then we will have the property that the outputs X, Y and Z will be related as:

sqrt(X^2 + Y^2 + Z^2) = constant
Click to expand...
Well, I recognize the equation for a sphere of radius "constant" in Cartesian coordinates. I guess this means the output of the mic is constant for a sound source if equidistant, in all directions. Then I guess decoding this information and outputting it thru an appropriate speaker system will place that sound source in that same virtual location for a listener at the virtual mic location?
 
crazydoc said:
Well, I recognize the equation for a sphere of radius "constant" in Cartesian coordinates. I guess this means the output of the mic is constant for a sound source if equidistant, in all directions. Then I guess decoding this information and outputting it thru an appropriate speaker system will place that sound source in that same virtual location for a listener at the virtual mic location?
Click to expand...

Well said. Yes, that's the idea.


Bob
 
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