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2 cardiod back to back=figure 8


rosa

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From what I understand, 2 cardiod capsules, back to back, and out of phase, create a figure 8 pattern. If this is correct, which way is front and back of the created figure 8? Looking at the graphs I would think the sides of the cardiods are now front and back of the figure 8, and the two fronts of both cardiods, being out of phase, cancel each other out.

Is this correct?

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It's exactly the other way round. The cardioids' fronts make up the front and rear lobe of the fig-8, and the sides of the cardioids cancel each other out.

Sound that hits both cardioids from 90° is equally loud in both, but one of them has polarity reversed and thus the sound will cancel itself out.

Mixing the two cardioids at different levels enables you to continuously vary the pattern from cardioid to fig-8 - or from cardioid to omni if you don't reverse one cardioid's polarity. This is how the MKH 800 TWIN basically works. Switchable (dual-diaphragm) multipattern mics, like TLM 170, create their patterns not by mixing the signals in different ways, but by different polarization voltages for each diaphragm.

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You *can* create a cardioid that way, yes.

The more common way, however, is to have one capsule that has an acoustic "delay line" built in a way that sound from the front hits the diaphragm's rear later than its front (large pressure gradient --> large diaphragm motion), and sound from behind reaches both sides at the same time and with same polarity (no pressure gradient --> no diaphragm motion - same as 90° on a fig-8)

Without the acoustic delay line you'd get a fig-8, with a closed back you get an omni.

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Yes, I was reading parts of it earlier today. Thanks to both for all the information. 

What I don't get is why certain microphones use two cardioids for figure 8 rather than a single capsule without the delay line?

 

I am sure there is a good reason for it.

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  • 4 months later...

to me it seems like if i were to put two cardioids back to back i'd get an omni type feel(with proximity effect unlike omnis). i feel like the null of figure 8's are quite extreme. specially on ribbon mics. i've got a 77dx and a 44bx and you wouldn't believe how much the signal drops if you put the sound source in the null. one of my friends was telling me that radio broadcasters used this to their advantage. you'd have the person talking and reading the paper which was right by the null and you would never hear the paper sounds at all. i doubt you'd get that kind of behavior with two cardioids back to back

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  • 3 months later...
On 2/9/2017 at 9:48 PM, ivanovich said:

to me it seems like if i were to put two cardioids back to back i'd get an omni type feel(with proximity effect unlike omnis). i feel like the null of figure 8's are quite extreme. specially on ribbon mics. i've got a 77dx and a 44bx and you wouldn't believe how much the signal drops if you put the sound source in the null. one of my friends was telling me that radio broadcasters used this to their advantage. you'd have the person talking and reading the paper which was right by the null and you would never hear the paper sounds at all. i doubt you'd get that kind of behavior with two cardioids back to back

You're correct if you simply put two cardioids back to back. As the original post stated though, flipping the phase on one of the cards (the "rear" mic) is what gives the figure-8 pattern as opposed to an Omni-like pattern. 

The coincidence of the diaphragms also determines how accurate the pattern is. That's why single diaphragm figure-8 mics like the MKH30 are so highly regarded. Ribbon mics also use a single element for both sides of the pattern, hence the deep null. Simply putting two cardioid mics back to back will never get the diaphragms close enough to give an accurate figure-8 pattern.

-Mike

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