Omni to Cardioid for ambisonics

Question

Some friends and I are trying to build a DIY ambisonic mic using mems mic. The capsules say that the polar response is omni but in order to get the directionality we need to capture 4 cardioid signals. (first order) Does anyone have ideas for turning the omni into a cardioid response. We are basing our design on this http://www.core-sound.com/TetraMic/1.php

Thanks

Edit: The enclosure that we designed was printed three dimensionally using a Mojo. As you can see from the design the back side of the capsules have gaps. We are not sure if this is something that would help with the phase cancellation or if we need to close this off and push the boards far inside the cylinder. Not sure what model the capsules are but they are about 2mm by 3mm and omni in response.

Answer 1

An omnidirectional capsule is a pressure receiver whereas all directional capsules are pressure gradient receivers. That means that you would need to drill the right amount of holes/sound paths behind the membrane to get a proper cardioid capsule. Going the other way is much more realistic (namely closing the sound paths to the back of the membrane).

You can try working with sound shields, but the resulting directivity will have sharp changes in the polar pattern rather than the gradual change of an actual directional capsule.

So in short: I don't think your proposed material is suitable for the task.

Answer 2

It you look at a classic microphone, like the SM-58, you will realize that the capsule itself is somewhat omni. What creates the directivity are tiny accoustic pathways (tunnels? Not sure how to call them in english) that cause the sound to enter the back of the capsule with a slight delay, causing phase cancellation in the back. I have honestly no idea how they are shaped to get that effect, but they are definitely working because the capsule becomes omni as soon as you cup it in your hands (and this is also why sound engineers hate it when you cup your vocal mic on stage).

I highly doubt it would be efficient to try and come up with your own phase cancelation system. What could be an option though, is to use two capsule for each mic and combine their signals to create the pattern you want. I don't know about the physica shape of the capsules you found, but it's the basic idea behind multi-pattern mics. IIRC, it a combination of a figure 8 capsule and an omni one, but you should google it and confirm that.

I'll try to find an article and I'll update my answer when I do.

Answer 3

The classic way of creating cardioid pickup pattern was to couple an omni with a figure 8 capsule, where the + side of the figure 8 is pointing forward.

So I guess in theory you could add 4 figure 8 mics to the setup, and end up with cardioid pickup patterns, but I'm not sure if this would be the most cost effective solution.