I can't say for sure but that's a lot of foam. Isn't the idea to duplicate the human head? We aren't made out of wood or plastic but maybe inserting something hard inside to duplicate bone might work. Of course our skin isn't like foam either. Maybe a latex rubber coating?
I like a baffle to be inert, neutral, as an attenuator between close-to-coincident omni's.
I think it depends on what you're trying to do. A dummy-head/HRTF type baffle tries to emulate the human head in shape, density, resonance and reflectivity. Guysonic's baffles are made of Sorbathane I think, which is a soft polyurethane material used for shock absorbing applications that supposedly is pretty close to the density and sound reflectivity of water-dense human tissue. I'd guess the Neumann head, the Head Acoustics head, the old Sennheiser head and the Achen dummy heads use something similar. I got some Sorbathane samples at work recently recently for another application and it's cool stuff but pricey.
For stereo sphere mics like the
Schoeps KFM-6 &
KFM-360 spheres, the
T.H.E. BS-3D hard wooden sphere or the original 50's Neumann (or was is Sennheiser ?) aluminum sphere, the idea is quite a bit different as they are not trying to emulate a human head. The idea in that case is to have a hard, evenly curved surface for the sound to diffract smoothly around (like turning a mixer pan-pot). They are using the sphere shape to avoid the irregularities of human head geometry and are using the boundary effect of the hard surface with the mic capsules flush to the surface (purposefully creating a reflective surface) vs using the material of the head or baffle to absorb the reflected sound. They use mics with the same amount of high frequency boost on-axis (90 degrees to the side) as the sphere attenuates those frequencies for sounds arriving from the opposite side of the sphere. Supposedly the combination of both mics then sum evenly for sounds directly ahead.
For a Jecklin disk I think you'd want to have an even, broad spectrum absorber like Moke says, since the mics are positioned a short distance away from the baffle and you just want the acoustic shadow effect and zero reflection off the surface. The only thing I differ with Moke about is that I like the omnis spaced farther apart if possible. The problem is with more mic spacing is that you then need a bigger baffle. I built a bigger one but have never had the cajones to fly it.
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With smaller baffles you have to keep the mics close to the baffle to get enough acoustic shadowing, but smaller baffles are more useful in most cases because you can actually use them.
Jorg Jecklin himself eventually settled on a bit larger disk with more mic spacing than in his original description with a disk of 36cm and mic spacing of 35mm - maybe I have that backwards? seems pretty exacting in either case with a 1cm difference?! I met Ray Kimber last spring and asked him a bunch of questions about his
gigantic Iso-Mic monster Jecklin disk derivative that is as big as my living room and allows the mics to be spaced quite far apart but he was a bit tight lipped at the time. I've wanted to play with using
two disk baffles so that I could keep each mic closer to the disk surface and keep the baffles small, yet space the mics farther apart. I could also mess with angling the baffles in that case. Still not very low profile though so I haven't pursued it.
More thoughts on baffle size here.