Demonstration Manual

Electricity and Magnetism

DEM14A - Piezoelectric Speaker

Objective:

- To show that a piezoelectric crystal subject to stress produces a voltage.
- To show that when an alternating voltage is applied to a piezoelectric crystal the crystal vibrates in such a way that it may serve as    an audio speaker.

Apparatus:

• Arbor Scientific model P6-2110 piezoelectric demonstrator mounted on a lucite stand
• neon bulb
• Function Generator
• two banana-banana leads
• BNC-banana adaptor

Caution:

Do NOT manually flex the piezoelectric crystal while the crystal is connected to the function generator. The crystal can produce voltages high enough to damage circuit components within the function generator.

Method:

A: Manual Flexing

The neon bulb should be connected to the output terminals of the piezoelectric crystal. Hold the crystal assembly in both hands with your fingers under the lucite base and both thumbs on the brass cover of the crystal (that is, hold in an underhand hamburger grip). Gradually press down with your thumbs and note which pin of the neon bulb produces light. Gradually reduce the thumb pressure and note that the opposite pin in the neon bulb produces light.

The above process has demonstrated that flexing the crystal produces a voltage and that the polarity of the voltage changes with the direction of flexing.

Comments:

1. An alternative method is to leave the crystal assembly sitting on a flat surface and simply press down on the brass cover with a finger or thumb.


2. The neon bulb does not produce much light. The room may have to darkened for viewing at a distance. One could pass the apparatus around allowing the student to flex the crystal.

B: Piezoelectric Speaker

Connect the function generator to the banana jack terminals of the crystal assembly. (The neon bulb can remain in place. The generator voltage is not sufficient to fire the bulb hence the presence of the bulb will have no effect.)

The recommended settings are:

• dial: 3.0
• frequency multiplier(Hz): 1K
• mode: CONT
• function: square wave (IMPORTANT)
• amplitude: start at about 1/4 power.

Turn on the function generator and adjust it's amplitude to achieve a suitable sound level. One could also adjust the frequency dial over the audio range. The crystal's efficiency is at a maximum at about 3000 hz. One may have to adjust the function generator amplitude as other frequencies are used.