Steel spring pendulum, inverted, vibrates at 10 Hz.

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(B)Waves - B+45: Sound Spectrum/Sources

InfraSound: not audible: A flat clock spring 40 cm. long is clamped to the lecture table. 100 gm. slotted weights can be clipped to the spring to vary the loading (and the period). The spring steel pendulum vibrates under 10 Hz,-thus is not audible, but compresses the air and produces pressure waves. Same apparatus as B+10+30
UCB Index: 
B+45+0
PIRA Index: 
3C20.16
Demo Diagram: 

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B+45+5 Giant tuning fork, barely audible, displayed with stroboscope.