A recent note in this journal1 presented a demonstration of standing waves along a cord consisting of two parts having different material densities, showing different sized wavelengths in each part. A generalization of that experiment to a continuously varying linear mass density is to vibrate a strip of material with gradually varying width (mass per unit length), in order to show that as the strip gets wider, the wavelength becomes shorter. Figure 1 shows the setup. It consists of a mechanical driver fed by a signal generator, and a strip of flexible fabric connected at its narrow end to the driver, nailed to a suitable piece of wood at the wider end. The strip is in the shape of a trapeze, 2 cm and 10 cm at its ends and some 50 cm long in its middle. It may be cut from the inner tube of a tire, or from body‐building elastic strips.

1.
John
Welch
, “
Demonstrating wavelength dependence on medium density
,”
Phys. Teach.
47
,
476
(Oct.
2009
).
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