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Fig. 1. (a) gives a schematic diagram of the end of a rod with attached NdFeB magnet and detector coil. (b) gives a schematic of the circuit used to amplify the coil output. Although the figure shows supply voltages of ±15 V, a pair of 9-V batteries will also work. The 10 K offset adjustment potentiometer is used to zero the output with pin 2 shorted to ground. Without this potentiometer, the offset voltage for this circuit is typically 1 V. First citation in article

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Fig. 2. The frequency content of the signal produced by tapping the end (far from the detector coil) of a 0.95-cm diameter aluminum rod 1.045 m long. Peaks are labeled with the mode order n, and data for n = 6 through 9 have been multiplied by a factor of 10 so that these mode peaks are clearly visible. First citation in article

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Fig. 3. Normal mode frequencies for the data of Fig. 2 are plotted (as diamond-shaped points) versus mode order n. Errors are smaller than the points. The solid straight line represents a best fit to the data. First citation in article