is it possible to make a directional mic out of an omni?

Hey all;

I've got a couple mic elements that are omnidirectional. It would be better
for me if they were directional (like a cardioid or something). How do they
make the directional pattern? Is it something in the electronics, the
casing, or the element itself? Is it possible to make the omni uni?

Thanks

 

The directional capsules are different, they have a soundpath on the
backside, where the omnidirectional ones have only a tiny capillary hole for
air pressure changes.
It is possible to convert an omni to directional by making an acoustical
lens in front out of a bundle of plastic straws, John Woodgate had posted
some pic not so long ago. You could also use a parabolic reflector with the
mike mounted in the focus point.
Another possibility is to electrically combine the signal of several
capsules in form of a phased array. This is usually defined with the
capsules equidistant in a line. Also you can make a nice stereo-mike this
way with 9 or 13 capsules. I do not know how to calculate the positions and
damping factors though, but I think K. Aylward has some software to do this,
it should be similar for sound and antennas.

 

It's the casing, believe it or not. You mount the mic element at the end
of a tube, and put holes in the sides of the tube, and baffles to increase
the length of the acoustic path along the tube. That way, the sounds that
come from the front, that miss the front of the mic element but travel
alongside the tube, go through the holes in the tube, and back forward to
the back of the element, arriving 180 degrees out of phase, reinforcing
the signal from the front. Think of a transmission line 1/2 wavelength
long. Sounds that come from the sides or the back arrive at the element
out of phase, and so cancel out.

It's theoretically possible to do this at home, but if your time has
value, it'd be cheaper to buy one.

Hope This Helps!
Rich

 

Following up on Rich's comments, you might want to Google
for "Shotgun Microphone" if you are looking for super
directionality and don't mind fiddling a bit. There used to
be perennial articles in Popular Electronics and the like
back in the '60s about how to build your own, but it all
boiled down to just what Rich says: A pipe with holes in
the sides. It won't be small, but it will be directional!
You used to see these on TV shows where the host
would point it at a member of the audience to pick up
one voice from a crowded room.

However, most small "unidirectional" mics are nowhere
near this directional. They are simply cardioids: The
response pattern looks like a figure 8 with the front lobe
bigger than the back. Also note that the more directional
a mic is, the more dips and peaks in its frequency response.
If you are using it for music recording, it's often better to
use an omni up close instead.

Best regards,



Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

 

OK
Thanks for the info guys. I guess I'll just leave it the way it is.