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Pressure gradient microphone Explained

Discussion in 'Microphones' started by audiokid, Oct 28, 2011.

  1. audiokid

    audiokid Staff

    What does Transducer Principle : Pressure gradient microphone mean?
     
  2. TheJackAttack

    TheJackAttack Distinguished Member

    Hopefully Boswell will chime in. In short, a pressure gradient microphone has exposure to open air on both sides of the element and responds to "pressure gradients" rather than sound pressure levels like a moving coil dynamic. To my knowledge all old school ribbon microphones are pressure gradient though some modern ribbon microphones are not technically open on both sides.

    I'm a couple glasses into a fine Shhhhcotch so this is as good as I can do right now.
     
  3. IIRs

    IIRs Well-Known Member

    Actually I think ribbon and dynamic mics are velocity transducers, not pressure transducers.

    An omni condenser mic is a pressure transducer, ie: it responds simply to air pressure (like a very high resolution barometer) without taking any account of direction.

    A figure 8 condenser is a pressure gradient transducer, as it meaures the difference in pressure between two points: if the pressure changes are identical at both points the sound source must be perpendicular to the mic (the side null of the fig 8 pattern) and no output is produced.

    A cardioid pattern is a combination of both. <edit> Not so sure about the next paragraph now, as it seems to be contradicted by the pdf below!

    In the case of a pattern switching mic like a 414 (as I thought I understood it) these comprise seperate pressure (omni) and pressure gradient (fig 8) capsules, and the cardioid pattern is created by mixing the two in equal amounts so the in-phase front lobe of the fig 8 pattern reinforces the omni signal, while the out-of-phase rear lobe cancells the omni signal to create the rear null.

    However, the pdf below talks about back-to-back cardioid capsules instead... did I dream that part above? I was sure it came from an authoritative source. :confused:

    http://
     
  4. RemyRAD

    RemyRAD Guest

    I believe everybody is close but no cigar? Dynamics are not velocity transducers. Ribbons are the only ones. They act upon the gradient difference of the velocity of the air molecules hitting the ribbon. The pressure gradient microphone are the directional dynamics & condensers. They are directional based upon the gradient difference of the reverse back Phased different pressure from the external rear air holes. The pressure microphones are the omnidirectional ones. So a ribbon is a gradient velocity microphone. But if you have 2 ribbons back to back in a single capsule, one can be the velocity and one could be the gradient. But RCA stopped doing that by the time they got to the 77-C1 Or something like that?? My 77 DX is a single ribbon velocity microphone with an acoustical labyrinth on the back side of the single ribbon so as to turn it into a pressure gradient by closing off the air hole at the back of the ribbon (A poor truncated explanation here). And anytime you're playing around with air pressure phasing you get frequency response aberrations such as Proximity Effect. And the Omni reacts evenly from all air pressures, so no proximity effect.

    In a 414, you have 2 discrete capsules back to back. Cardioid is only front on. Omni is front and back on, in phase. Figure 8 is front and back on, out of phase to each other. And all of the other different offshoot patterns are just based upon the amount of phase differential between front and back.

    Now where IIRs Is confused, He should be because he's also right. I think Neumann has this up on their page somewhere? Yeah, the early 47's or something just prior to the 67's were done that way because they shared a common polarizing connection, between the front and back of the capsule? All of the later designs since then use separate polarizing connections. So it's really just a matter of which one is on and which one is in phase or out of phase to the other one and by how much.

    Everybody gets gold stars on their four heads
    Mx. Remy Ann David
     
  5. BobRogers

    BobRogers Well-Known Member

    Just a bit of mathematics to either muddy or clear the waters. The two basic variables in a fluid problem are the velocity (a (3D) vector field (a function of space and time) ) and the pressure (a (1D) scalar field). The pressure gradient is a vector field that points in the direction of steepest increase of the pressure. The velocity and pressure gradient are both vectors. They are not necessarily parallel, but they are in some simple situations and in those situations people sometimes use pressure gradient and velocity interchangably. Since Remy introduced it, the velocity gradient (spatial change in a vector field) would be a tensor - a 3 by 3 matrix. The normal component on the surface of the ribbon would be a vector.

    At any rate, I don't know much about the physics of real microphones other than what I've read in the kind of general literature of the kind that IIRs linked to. But the basic idea is that the vibrations of the air cause a solid to vibrate and through one of several mechanisms the vibrations in the solid is transformed into an oscillating voltage. We can make simplifications and say that the vibrations in the air are translated into mechanical vibrations through the oscillations of the pressure or pressure gradient or velocity or velocity gradient, but the fact is that in real life all of the properties of the fluid interact with the solid. So don't be surprised if it doesn't make perfect sense when someone says this mic reacts only to velocity, that mic reacts only to pressure gradient, the other mic reacts only to pressure. We're trying to pinpoint the main effect. These are only approximate statements.
     
  6. audiokid

    audiokid Staff

    What a great thread!
     

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