I was really interested in seeing more of the details of your 8x8 soundplane, but the PDF link is broken (gives me a 404). What are the other ends of the copper strips connected to? Are the grounded, or just floating?
Thanks for the note! I fixed the link.
The copper strips are just connected to the positive sides of the audio inputs and outputs. Ground is connected to the shield on each cable to a pickup or carrier from the interface. Ideally the shields would end where the copper strips themselves start. Right now there are some lengths of unshielded cable inside the 8x8 prototype. It works anyway.
The schematic for the Radio Drum (Figure 2 in the paper) shows an op-amp circuit that will work for an active version of the device.