Demos: 4A-02 Sound Production with Rotating Slat

A simple, homemade device commonly referred to as a “Bull-Roarer” is used to produce a low-frequency sound. It is spun at the end of a string and as it rotates it produces a series of compressions and rarefactions.

Directions: The string is attached to the flat board slight off center so that when the board is whirled around in a horizontal circle, it spins rapidly about its axis. A dull roar can be heard, indicating a low frequency.

Suggestions for Presentation: Suggest that just as a flipping card can be made to produce sound (Demo 4A -01), a rotating slat will alternately produce compressions and rarefactions. (Note: As a child, I made the same apparatus except that I didn’t tie it to a string. I held the two edges between my fingers and when I threw it forward, releasing the bottom edge first, it started spinning and made a low frequency “roar.”)

Applications: Propellers, other spinning shafts with protrusions.

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