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They need a bit of help at lower fq, so you could hide some small speaker boxes or subs in the room. ___
This French guy went one step further and put two panels together in a thin box that looks to be about 5cm thick. The shape seems to combine those of a piano and a guitar. I think the front panel emphasizes higher frequencies and the back panel emphasizes lower frequencies. The box and various vents will boost bass and provide some level of tuning.
I attached transducers from Clark Synthesis to the soundboard of an old upright, and set my Kawai MP11 in place of the old action. The transducers were expensive, but they have great bass response, and the upright was free.
It sounds bass heavy. Analysis with pink noise and an RTA shows it has pretty flat response up to about 1.5 Khz, and then it gently rolls off on the upper octaves. I plan to add a couple smaller transducers through crossover and separate amp to take care of the high end. But even with the high end deficiency it has now, it still sounds better than most speakers, simply because the surface area of the sound board is comparatively huge. It sounds much closer to the real thing.
I've been experimenting myself, that's why I knew the video! It's very useful.
Materials: It's cool that inexpensive end-grain balsa, extruded polystyrene and palette cardboard were excellent sound boards. I'd love to hear what the piano re-builders think about using End-Grain balsa vs. normal Edge-Grain wood. FYI - if you go to Home Depot, the extruded polystyrene is also listed under "Foamular".
Shape: I still think a circular shape will give good acoustic performance but I haven't had the time to get my materials cut properly.
Pricing + Amplifier: $30 is a bit misleading if you don't consider the amplifier. My setup was about US$150 which included a matching amplifier + 2x transducers and several different materials for sound boards: