ehm, what is a gradient pressure microphone?

E

earworm

New member
ok, i was reading about microphones and i'm starting to get confused,

how many types of mics are there?

i thought: Dynamics (including ribbons) and condensers (including electret mics)

now there seems to be a third kind, pressure and gradient pressure mics?
if i'm correct pressure mics are PZM's (pressure zone mic)
but i just don't know what to thinik about that gradient thing...

you guys got some nice info about that?

thanks,
earworm
 
If I may add some acoustic theory to what David wrote, a gradient is a measure of how much some observable changes as some other parameter is varied. In the case of a gradient microphone the observable is sound pressure and the parameter is distance. A gradient mic measures the pressure at two closely spaced points in space, the front and back, and outputs the (time integral of the) difference between the two. You get a measure of the change in pressure per unit distance, i.e. the pressure gradient at the point between where the two measurements are taken. (The integration with respect to time makes the measure frequency independant.)

The fundamental equation of acoustics, the differential wave equation, says that the (time integral of the) instantaneous pressure gradient along an axis is proportional to the _velocity_ of the air projected on that axis. Velocity is a vector quantity, i.e. it contains information on both the amplitude and the _direction_ of the sound wave at the point of measurement.

If the axis joining the two measurement points is at an angle with respect to the direction of the incoming wave's velocity, then the output is reduced proportional to the cosine of the angle between that axis and the wave velocity. If they are at 90 degrees, the output is zero. It is this cosine relationship that gives rise to the usual figure 8 plot that we see which shows that the mic is most sensitive to sound ariving along its axis and least sensitive to sound arriving at 90 degrees to that axis. Looking at that plot, if you draw an arbitrary line from the origin out, the distance from the origin to where that line intersects the fig 8 plot is the cosine of the angle between that line and the axis through the lobes of the figure 8. Relating that to the mic's directional sensitivity, it is that proportion of the velocity that will be measured when the axis of the mic is at that angle to the actual velocity of the wave.

To finish this off, the acoustic wave equation also tells us that the pressure at a point is in constant proportion to the velocity (of a plane wave) at the same point. The constant of proportionality is the characteristic impedence of air and is independant of frequency. (This is similar to the relationship between voltage across and current through a resistor.) So, another way of viewing the output of a pressure gradient mic is that it gives the cosine weighted pressure at the measurement point where the cosine is of the angle between the mic axis and the direction of arrival of the sound.


Bob
 
Wow...

So if I've got this right, would it be correct to think that a basic pressure transducer could be a dynamic or condenser type that does not allow sound to reach the back of the diaphragm (sealed capsule) and therefore displays a very accurate omnidirectional pickup pattern?

And...

Would it be correct to think that a basic pressure gradient transducer could be a dynamic or condenser type that does allow sound to reach the back of the diaphragm (capsule not sealed) and therefore displays some kind of unidirectional pickup pattern like cardioid or one of the cardioid variants?

And...

Would it be correct to think that a basic velocity transducer would be a side-addressed ribbon mic like a RCA 44 or something that has a figure 8 pickup pattern?

And...

Can dual pressure gradient diaphragm capsule designs really synthesize all of these different pickup patterns?


sl
 
Yes,
Yes,
Yes,
Well kinda, but not exactly.

By varying the voltage on the capsules, a dual diaphragm mic can duplicate all those patterns fairly well, but not always as well at all frequencies (or distances) as a real figure 8 velocity mic, or a pure pressure omni mic.

It has to do with capsule shadowing, vector sums, and lots of techy junk that Bob Cain can explain in more detail than I ever will. (It makes my head hurt to even think about all that stuff.)

Enough to say that a dual diaphragm condenser mic ain't "perfect" at the two extremes of it's polar responses, but it gets close enough for most things.
 
so is a 414 a double diaphragm mic?
and i bought some electret capsules and i was planning on making a microphone design and do all the electronics, knowing it is sealed it would be an omni mic, that i am pretty sure.
But would there be a way to make it into a cardiod microphone?
just a thought, omni is good too, but a lot of times cardiod is more practical and it would be nice if i could get those capsules to be cardiod.
If it involves cuting holes in the capsules, that would be a hard task as the capsule is probably less than a 1/4"
 
Yes, the 414 is a dual diaphragm mic and, no, there's no easy way to convert those little omni capsules to make them cardioid.
 
mathieujohnson said:
But would there be a way to make it into a cardiod microphone? just a thought, omni is good too, but a lot of times cardiod is more practical and it would be nice if i could get those capsules to be cardiod.
Click to expand...

If you use two capsules back to back and a few opamps you can make a very rough approximation of a variable pattern condenser mic. Search google as people have already gone to the trouble of designing and testing this idea.
 
snow lizard said:
Wow...

Would it be correct to think that a basic pressure gradient transducer could be a dynamic or condenser type that does allow sound to reach the back of the diaphragm (capsule not sealed) and therefore displays some kind of unidirectional pickup pattern like cardioid or one of the cardioid variants?
Click to expand...

Almost. A pure pressure gradient transducer would just give you a figure 8. In the design of the other variants, a pressure component is also integrated into the capsule and summed with the gradient component. The ratio of the gradient component to the pressure component determines the pattern. If they are equal it is a cardiod.

And...

Would it be correct to think that a basic velocity transducer would be a side-addressed ribbon mic like a RCA 44 or something that has a figure 8 pickup pattern?
Click to expand...

Yes, this type of transducer is pure gradient. It is the difference in pressure on the two sides of the ribbon that is given as an output. The time integral I mentioned which keeps its respnse flat in pressure is approximately effected by the mass of the ribbon. The imperfection in the approximation appears as low frequency rolloff.


Bob
 
amazing replies, read them twice and gonna read them a few times more,,

i'm also reading the posts in that sticky we got up here,but takes some time..

so a 414 has two diaphragms, they both are cardiod,right? and by mixing them together you can get the hyper,omni and 8 directivities?

a 414 has two diaphragms, but does that make it a gradient pressure mic when its switched to figure of 8 ?
a gradient pressure mic is a figure of 8 mic,
and are all figure of 8 mics gradient pressure mics?

if not, then a gradient pressure mic is something else, and ..;what would the use be of such a mic?
 
ehm, sorry for asking, but a gradient pressure mic, has only one diafragm, and the air and soundwaves touch the diaphragm on both sides, (with a delays and stuff)
so this one doesn't have two diaphragms, only one?

and a "normal" pressure mic (the barometer thing) only touches air on one side...thats the big difference between the two
 
Actually, a ribbon bi-directional mic is a "velocity" mic that responds mainly to directional air motion, not air pressure. A pressure mic responds primarily to air pressure, not air motion. A pressure gradient pattern is a combination of the two.

Not that's kinda the "techy" version, but like lotsa things, it helps visualize what we're talking about , but it ain't completely true; just true enough to get the idea across.

For example, there are pressure mics that are made to be flat off axis (in the "diffuse" field), but on axis, they have a rising high end, which makes it a little directional at high frequencies. If it's made to be flat on axis, it will roll off the high end as it goes off axis. Sorry, but that's how the physics works. B&K make nose cones and other add-ons to help make their mics a little flatter in all directions, but those things add their own problems.

The physical size (and the width) of the capsule, ribbon, body, grill, housing, etc. all contribute to the overall curve and polar response.

The wavelength of a 7,000Hz note is roughly 2 inches - a 1/2 wavelength of 7kHz is roughly 1 inch. See any dimensions there that sound familiar? Gee, I wonder what happens to a 1" long note when it hits a 1" wide blockage?

A condenser capsule is built kinda like a drum, isn't it? Does a drum have a natural resonance? Does a condenser capsule have a natural resonance? Can you tune the drum a little by changing the head tension? Can you tune the capsule a little by changing the diaphragm tension? Does the drum stop resonating when you tune the head? Does the capsule stop resonating when you tune the diaphragm?

Mics are complicated little beasts; they're a compromise at best. They hafta obey the laws of physics, just like we all do, but there are "fudge factors" you can use, to kinda sneak around the obvious laws.

But in the end, you pay a price for trying to get around those laws. You sacrifice something a little less critical, or you pay a price for hand tuning something to "trick it out", but the price is always there. Break a law "here", pay for it "there".

That's what designing a mic is all about. And that's why there are no simple answers.
 
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