(These were done by Jerina Pillert in the winter of 2002-2003).

An acoustic transmission line was constructed, following the ideas of two articles in the American Journal of Physics [1,2]. This was done by putting together a small periodic structure consisting of 4" pvc tubing. This periodic structure gave rise to 'bandgaps' in the structure.  In a bandgap, the oscillation mode frequencies which would present in a long pipe are missing. Two bandaps were found in a structure similar to this, each having a bandgap of about 300 Hz..

Figs. 1 and 2 show the entire line, starting with a speaker closest to the camera, driven by a signal generator.

There are 6 repeated secions in all, in addition to starting and ending sections. Each repeated section consists of a coupler (which looks like a collar) and a short section containing a restriction in the form of particle board having a 1/2" diameter hole in it. Fig. 3 (following) shows a short section and restriction.

The next picture shows the speaker end of the transmission line.

Finally, we show the far end of the transmission line. A small microphone is sealed in the end of the line. The microphone signal goes to an amplifier (a fist-sized object), and then to the oscilloscope.

References

1. Carr and Yu, Am. J. Phys, 70 (11), 1154 (November 2002)

2. Robertson, Ash, and McGaugh, Am. J. Phys, 70 (7), 689, July 2002.