Abstract:

Many of the vibration patterns of a free trombone bell show rotational symmetry, with equal numbers of oppositely moving anti-node regions of equal size. Thus it is predicted that the acoustic sources due to these patterns will cancel, particularly at low frequencies, resulting in little or no net coupling between the body vibrations and the air. This has implications for both the radiated field and the acoustic response of the instrument as seen by the player. An isolated trombone bell was driven at certain of its structural resonance frequencies by attaching a magnet chip and exciting with a solenoid carrying an oscillating current. The resulting acoustic field was probed with a miniature microphone throughout the bell. For each structural mode, the acoustic field pattern was also compared with the corresponding structural vibration pattern imaged with holographic interferometry. The results show acoustic short circuiting between adjacent anti-nodes of the structure, as expected, contributing little bell-to-air coupling. However, there are also significant asymmetries in the acoustic field that do not result in cancellation, and therefore can produce increased signals in the radiated field, as well as at remote regions in the bore. [Research supported by a UNI College of Natural Sciences SOAR grant.]