Recording - May 2015 - 22

If you go down to the corner music store, you'll find just about
every mic they sell is a cardioid-pretty much the most common
polar pattern for stage and studio mics these days. So why
would anyone care about any other patterns? For one thing,
there are powerful benefits to some of the other mic patterns out
there. And for another thing, sometimes a cardioid isn't really a
cardioid and an omni isn't an omni, and it's important to know
when and why.
So what makes a pattern?
You can place all the theoretical mic patterns along one line, with the omni at one
end, the figure-8 at the other end, and
wide cardioids, cardioids, supercardioids,
and hypercardioids in the middle. Why?
Because there are two different things
being sensed by the microphone, to varying degrees... and a mic's pattern comes
from that.
The omnidirectional mic is a pressure
microphone. That is, it measures instantaneous air pressure at one point in space.
There is a sealed chamber with a
diaphragm across an opening, and as the
air pressure outside changes with respect
to the pressure in the sealed space inside,
the diaphragm moves in and out. In actuality, real-world omni mics have a teeny tiny
pinhole to deal with very slow changes in
air pressure between the sealed chamber
and the outside, but that doesn't make any
difference to sound.
The figure-8 mic is a velocity microphone, and if you look at some old
Shure and Altec ribbons, they say
"Velocity Ribbon Microphone" on the
name plate. That is, they respond to air
speed, not to pressure, and speed has a
direction to it. You can imagine a ribbon
mic as being a ribbon in free space-
the faster the air goes perpendicular to
the ribbon, the faster the ribbon moves.
Air going parallel to the ribbon doesn't
move the ribbon at all. So the figure-8
has a "front and back," which isn't true
of a perfect omni mic.
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RECORDING May 2015

Now, if you sealed off the back of that ribbon mic completely, you could turn the figure-8 into an omni. This is how the variable-pattern mechanism on the RCA 77 ribbon works. If you
added a second diaphragm to the back of an omni capsule and
looked at the difference in movement between the two, you
could turn the omni into a figure-8. That's how the figure-8 pattern on the Neumann U 87 works. In both cases the results are
suboptimal but still very useful.
So what of the cardioid?
We haven't talked about the cardioid
yet, and that's because the cardioid is a
combination of the two cases described
above. The cardioid mic, a relative newcomer to the world of microphone design,
responds to air pressure and velocity.
If you took a figure-8 capsule and an
omni capsule and put them at the same
point in space and wired them in parallel,
you'd get a cardioid microphone.
Western Electric tried this with the 639A,
and it kind of works... except that you can
never get the two capsules in exactly the
same location, and so things tend to go
wrong at high frequencies.
It's all about the vents: Neumann's KM 183, KM 184,
and KM 185 pencil condenser mics have different
polar patterns thanks to the venting in their capsules.
A much better thing you can do is to
take that sealed-back omni capsule and
drill a couple of holes in its sides. This creates vents that allow air to move in and
out. The larger the vents, the more the mic
responds to velocity; the smaller the vents,
the more it responds to pressure.
This means you can often identify a mic
by the way it looks. If there are no side
vents, it's an omni; if there are vents on
the side, it's a cardioid; if there's a whole
lot of venting in the back it's a figure-8.
Oh yes, and if you block the side vents, a cardioid turns into an omni very quickly. This is why those
singers who cup the microphone ball in their hands have such
feedback problems. (Don't do that!) The vents are there to set the mic

Recording - May 2015

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