I'm conducting a school production of Chicago, and the drummer's part requires just 2 notes from a xylophone, and 3 notes from a vibraphone to create the 'NBC Chimes' effect - fitting an entire xylophone and vibraphone into an orchestra pit just for those 5 notes would be overkill so I took it as an opportunity to build these little units.
Both use tuned chambers (Helmholtz resonators) which have a fundamental frequency to match the pitch of the tonebar, determined with an equation which is a function of volume, porthole size and thickness of the top. It's the same science as blowing over the top of a bottle to get a note...

The 2 note xylophone uses a couple of tonebars borrowed from a spare xylophone. The 3 note NBC chimes were built from scratch using flat aluminium bar from Bunnings - the frequency for each bar follows an equation that is a function of length, thickness, Young's modulus and density. The mounting points are the nodes, which is another little bit of maths.

The NBC Chimes are based on these originals made by Deagan - which are similar to dinner chimes "marketed both as musical instruments (they could be used to play military bugle calls) and posh summoning devices for homemakers, theater ushers, and dining car operators"

A fun little project!

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