Wednesday, April 10, 2013

Multiple Pattern Microphones

Even if you're familiar with some of the mics out there with switchable patterns it may never have occurred to you what exactly is going on inside when you flip that little selector switch. Personally I find it fascinating that one mic can have multiple pickup patterns at the flick of a switch. I finally got around to digging in to how it's done.

Your typical multi-pattern is a condenser. I'm not aware of any dynamic mics that do this trick but hey, it's the internet, someone is bound to come up with an example almost instantly if I'm wrong on that. Actually now that I think of it I remember reading about an old mic that had screws you could put in or take out of the bottom to make it a cardioid or an omni. So there you go, beat you to the punch.

In a condenser mic you've got a very thin membrane for a diaphragm that's covered in an ultra thin coating of metal. That metal forms one half of a capacitor, the other half is a metal plate that holds still and when a charge is applied across the two, the movement of the diaphragm causes the value of that capacitor to change. As part of a little preamplifier circuit that capacitor gives you useful audio at the output. Multi-pattern mics have a pair of capsules in close proximity, usually sharing the same back side. The way that these two capsules are connected to the circuit determines what pattern the mic will have. 
Here's a little chart. 
O-In Polarity
Ø-Out of Polarity 
(-dB) Reduced Level
The front of the mic is facing to the left <--

Omnidirectional - Both capsules are combined in polarity   
<--  ||  ||
      O O
Each capsule picks up half of the space and they hear pretty well all around.

Figure Eight - Capsules combined with one having reversed polarity  
<-- ||  ||
     O Ø
The phase differences cause very high rejection in between two cardioid pickup patterns.

Cardioid - A single capsule is used  
<-- ||  ||
     O X
A hot side and a side that rejects fairly well.

Super/Hyper Cardioid - One capsule normal and the other reversed polarity and blended in 
<-- ||  || 
     O Ø(-dB)
 The tightness of the pattern can be adjusted by how much of the out of polarity capsule is blended in.

There you have it Brethren of the Knob and Fader. One more tidbit of the rocket science we call audio. What you choose to do with it is your business. Use it well.

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