rosa

2 cardiod back to back=figure 8

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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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Thanks, and a cardiod pattern itself is created by mixing omni and figure 8, correct? So in the case you mention, how are the two cardiods created to start with? Is it Physically or electronically?

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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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