A lot of camp sites have these metal triangles as fire alarms. On hitting the triangle, a stationary wave should with a node in the middle and an antinode at each end. Thus the length is equal to half a wavelength. Say the length of one side is 20cm, then one whole wavelength would be 1.2 metres. The wave equation says c = f x wavelength. If the speed of sound in air is 330 m/s then the frequency of sound would be 275 Hz. Middle C is 261 Hz so the note must be close to middle C.
