TOPIC:

ID Code: H2-32
Purpose: Demonstrates diffraction and interference of sound waves
Description: A small loudspeaker plays music with lots of bass, but the bass is not very loud. When the speaker is held up behind a hole the size of the speaker in a board about two feet square, the sound becomes much louder, especially the bass. A loudspeaker produces two distinct sound waves: one from the front and one from the back, which are out of phase with respect to each other. In the absence of the baffle, these sounds both diffract in all directions, and, because they are exactly out of phase they interfere destructively, especially the bass. The baffle forestalls the diffraction and thus reduces the magnitude of the interference.
Availability: Available

The reference list for this demonstration has been expanded by some suggestions from Prof. T. Einstein.

Burt Brody, Destructive interference with a circular saw blade, TPT 40, 250 (2002).
H. Richard Crane, How Things Work: Never Throw a 2-inch Speaker Away: It Has Many Uses, TPT 30, 375-377 (1992).
R. Deb, S. Modak and R. Balakrishnan. Baffling: a condition-dependent alternative mate attraction strategy using self-made tools in tree crickets. Proceedings of the Royal Society B. Published online December 16, 2020. doi: 10.1098/rspb.2020.2229
Jonathan Lambert, Small, quiet crickets turn leaves into megaphones to blare their mating call, Science News, December 2020
Michael J. Moloney, Simple acoustic source radiation near a large wall, AJP 71, 794-796 (2003).
L. Prozesky-Schulze et al. Use of a self-made sound baffle by a tree cricket. Nature. Vol 255, May 8, 1975, p. 142. doi: 10.1038/255142a0