| http://www.w3.org/ns/prov#value | - As mentioned above, the panel 18 must be able to vibrate, and it may be made of any suitable rigid, but resilient, material, such as plastics, wood, card, cardboard, or a composite material consisting of two lightweight skins of high stiffness (Young's modulus) separated and connected by a lightweight core of either an open or closed cell.
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