Wind chime tongues are tuned so their mechanical bending matches the tube’s air resonance.

Evidence

The familiar bamboo wind chime is a half-open tube with an extending tongue. The length of the tongue is adjusted so that the mechanical vibration of the tongue, acting rather like a tine of a tuning fork, is 1:1 resonant with the quarter-wave air resonance of the tube. This is yet another example of the NFC mechanism. The bamboo tube is closed at the top; the beginning of the half-pipe or “tongue” section marks the end of the half-open bamboo tube, which has the usual fundamental quarter-wave air column mode, and the higher modes (see figure 16.5). The tongue is made just long enough that the bending frequency of the tube plus tongue equals the quarter-wave air resonance frequency. This makes the arrangement like a one-piece Savart cup or marimba: the energy of the bending is considerable when struck but is a weak, dipole emitter of sound. The resonator acts as monopole emitter with a shifted phase, at least partly overcoming the short-circuiting of the tongue oscillation. — Eric J. Heller, Why You Hear What You Hear- An…, 1905

Categorization

• XREF: Directly extends the NFC (near-field coupling) mechanism developed in preceding chapters; the tuning trick parallels chapter 16 tube modes.
• DEFINE: Explains the mechanical-to-acoustic coupling in a bamboo wind chime, and resolves why a mechanical striker needs a resonator to radiate efficiently.
• CANDY: The tuning-fork tongue matched to the tube’s quarter-wave resonance is a neat, tweetable bit about a quotidian object’s physics.