I wound the detection coil on the rod using five turns of enameled 22-gauge magnet wire. To prevent rapid damping of the excited vibrations, the wire should not grip the rod tightly. With a properly adjusted coil on a metal rod, the vibration signal typically decreased to one-fourth of its initial amplitude during the 0.2-s recording time. The coil was connected to the LF411 amplifier with 2 m of twisted pair 26-gauge wire to reduce noise pickup. As shown in Fig. 1(b), the amplifier gain was 10⁶ / 510 = 1960 in these experiments. This gain should be adjusted so that a light tap on the rod produces a signal of more than 1 V at the amplifier output.

I made a small metal hammer (total mass 0.037 kg) using 3/8-in diameter steel rod to tap the ends of rods. Because the detector coil was relatively loose on the rod, it is best to use a small hammer that transfers a relatively small amount of momentum to the rod. Jerking of the rod can produce a low- frequency offset contribution in the recorded voltage signal. This can be removed with a high-pass filter placed before the amplifier, but I found the added complication unnecessary. A hard steel hammer produces a short (time duration) impact with a metal rod that excites more higher frequency modes than a softer wood hammer. The frequency spectrum of the driving force (impact) determines the relative intensities of the normal modes initially excited. A short-duration impact from a hard hammer includes high-frequency components and excites high-frequency vibrations.

For students interested in music, this apparatus can be used for an added set of experiments. The principle discussed above applies to percussion instruments such as the xylophone. The hardness of the hammer used affects the frequency content of the sounds produced by striking. Using this apparatus with metal, wood, and hard rubber hammers striking an aluminum rod provides a nice experimental demonstration of this principle. With a wood hammer, mainly the n = 1 mode is excited, while with a rubber hammer it is very difficult to excite even the n = 1 mode. For a material like nylon, which is easier to compress, the wood hammer is nearly as effective as the steel hammer in exciting higher frequency modes, and a hard rubber hammer easily excites the n = 1 mode.