I had an idea for a while i finally got to test yesterday.
The idea was to improve rear (180°) rejection of a dual diaphragm mic with a k67 capsule. The same can be done with any dual diaphragm capsule.
The mic i used here has dual output. Each side of the capsule has it's own circuit and output. Think of TL Sphere, Austrian Audio OC818...
This is cardioid response of one of the sides. 0° (red) and 180° (green). Typical response in k67 based cardioids. Note that the responses are not absolute, I haven't used speaker compensation, not necessary for this test.

The idea is that if i activate the rear diaphragm, introduce some signal out of phase at just the right level, apply EQ curve so that it matches the response of the rear (green) curve but with everything cut below say 3K i should get additional cancellation from the rear and bring that HF peak down in the green line without affecting the front (red) response.
And indeed, I was right. If the signal from the rear is about 15db attenuated compared to the front, phase reversed, of the right shape, suddenly you get the purple line as the 180° response.
Considerably better rejection in the high end compared to the green line! And the best thing, it didn't affect the front response. Slight bump between 2k and 3K, but that's because of the phase shift of the HP filter. The corrective EQ curve can be tuned better to avoid this.

And this is the EQ curve I applied to the rear membrane to get the best rejection. It is roughly similar to the green line, but fine tuned because the EQ's HPF introduces phase shift.

I hope that made sense. I did some audio testing as well, and i've never heard a mic reject that well from the rear.
It is important what EQ you use because different plug-ins handle phase shift differently. With Q3 i could test different modes, and it worked best in zero latency mode.
The result would be even beter if time delay was adjusted between front and back so that you virtually bring rear diaphragm closer to the front one.
This is so different to using a hypercardioid. Hypercardioid changes the shape of the 0 degree response compared to a cardioid capsule of the same construction, and lets more of the signal through at exactly 180° in order to get more rejection from the sides. I would call this for a hybrid second order cardioid. I'm not aware of anything like this commercially available.
The idea was to improve rear (180°) rejection of a dual diaphragm mic with a k67 capsule. The same can be done with any dual diaphragm capsule.
The mic i used here has dual output. Each side of the capsule has it's own circuit and output. Think of TL Sphere, Austrian Audio OC818...
This is cardioid response of one of the sides. 0° (red) and 180° (green). Typical response in k67 based cardioids. Note that the responses are not absolute, I haven't used speaker compensation, not necessary for this test.

The idea is that if i activate the rear diaphragm, introduce some signal out of phase at just the right level, apply EQ curve so that it matches the response of the rear (green) curve but with everything cut below say 3K i should get additional cancellation from the rear and bring that HF peak down in the green line without affecting the front (red) response.
And indeed, I was right. If the signal from the rear is about 15db attenuated compared to the front, phase reversed, of the right shape, suddenly you get the purple line as the 180° response.
Considerably better rejection in the high end compared to the green line! And the best thing, it didn't affect the front response. Slight bump between 2k and 3K, but that's because of the phase shift of the HP filter. The corrective EQ curve can be tuned better to avoid this.

And this is the EQ curve I applied to the rear membrane to get the best rejection. It is roughly similar to the green line, but fine tuned because the EQ's HPF introduces phase shift.

I hope that made sense. I did some audio testing as well, and i've never heard a mic reject that well from the rear.
It is important what EQ you use because different plug-ins handle phase shift differently. With Q3 i could test different modes, and it worked best in zero latency mode.
The result would be even beter if time delay was adjusted between front and back so that you virtually bring rear diaphragm closer to the front one.
This is so different to using a hypercardioid. Hypercardioid changes the shape of the 0 degree response compared to a cardioid capsule of the same construction, and lets more of the signal through at exactly 180° in order to get more rejection from the sides. I would call this for a hybrid second order cardioid. I'm not aware of anything like this commercially available.
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