Chladni plates, invented by the physicist, musician and musical instrument maker Ernst Chladni (1756-1827) in the late 18th century, are used to demonstrate the complex patterns of standing wave vibrations that can occur in two-dimensional objects. The Whipple's collection contains two examples from King's college London, made and used in the laboratory of the physicist Sir Charles Wheatstone (1802-1875).
The Whipple's plates are made of iron and are caused to vibrate by stroking with a violin bow. When stroked, a given plate will resonate at one of its natural frequencies. The experimenter then sprinkles fine sand, which bounces about on the plate until settling at nodal points (areas of zero movement) thereby producing intricate patterns such as the one shown in image 2. This technique of visualisation was invented by Chladni and documented in his book Die Akustic (published 1802).
Theoretically, any plate has indefinitely many possible vibration modes each corresponding to a specific frequency of sound. Each mode produces a unique pattern, the complexity of which increases with the frequency of the vibration. The shape of the patterns produced on a given plate depends on other factors, including the shape of the plate itself. One can get a sense of the variety of possible patterns from the accompanying illustration from Chladni's book (image 3).
Chladni plates have been used for serious research and are instructive as learning tools, but modern researchers are interested in the vibrational behaviour of more than just sheets of iron. Stringed musical instruments like the violin and the guitar rely on the resonance of their wooden bodies to amplify and colour the sounds produced by their vibrating strings. Chaldni's methods can be applied to violin and guitar bodies and used by instrument builders to 'tune' the resonances of the instrument. Image 4 shows professor Jim Woodhouse of the Cambridge University Engineering Department demonstrating Chladni patterns on a violin back plate.
Torben Rees, 'Chladni plates: the first step towards visualizing sound', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2009 [http://www.hps.cam.ac.uk/whipple/explore/acoustics/ernstchladni/chladniplates/, accessed 24 July 2016]