Sonometer: Resonant chamber with bowed strings (2).

Sonometer: Stretched piano wires on a sounding board. Two piano wire strings are stretched on a long sounding box. There is a tension-adjuster for each wire. A wood block (a bridge) with a sharp edge may be moved left or right to change the length of the wires. The wires can be plucked on either side of the bridge to show the relationship between the frequency of the sound, the tension, and the length.
UCB Index: 
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Demo Diagram: