By David Herres
For years, science-fiction movies — particularly those involving robots — have created ambience by placing an oscilloscope in the background, configured to display a trace of the human voice. You can do this easily, at minimal expense, and the resulting setup can be used to perform some interesting and instructive acoustic experiments.
A loudspeaker as removed from a discarded computer, radio or audio player, is in reality a lineal (non-rotary) dc motor that is externally commutated. The cone responds to an electrical audio-frequency signal by rapidly reversing its direction of motion, moving the air to produce the sounds that we hear in all their complexity. Like any dc motor, it will also function as a dc generator, which means it can transform motion of the cone, responding to changes in air pressure, to an electrical signal that will represent those changes.
The electrical signal can be accessed by a TPP1000 probe and displayed on a Tektronix MDO3000 or similar digital storage oscilloscope. Connect the probe to the two loudspeaker terminals. You needn’t worry about blowing up the oscilloscope because neither terminal of the loudspeaker floats at some potential with respect to electrical system ground, which is connected to the oscilloscope ground plane.
Of course, the common microphone might be the best way to capture the human voice or some other audio signal for display on the oscilloscope. There are several types of microphones, but mics can be grouped into two general categories: those that require an external electrical bias and those that do not.
A common type is the carbon microphone. Carbon mics were used in early telephones. Changing amounts of air pressure compress a container of carbon dust in varying degrees, altering its resistance. The carbon is sandwiched between two metal electrodes. The conduction of the granular carbon depends on the amount of contact between the granules and how tightly they are packed together. (Thus users of early telephones would sometimes bang the microphone on a table to loosen up the carbon and get more volume out of the device.) One of the electrodes in a carbon mic is a foil diaphragm. Speaking into the diaphragm causes oscillations in the packing density of the carbon granules.
If you attach the probe tip and ground lead to this device, there will be no display because the oscilloscope is a voltmeter, not an ohmmeter. To see a signal from a carbon microphone, connect it across a 9-V dc power source and oscilloscope with a 47K Ω resistor and a 0.1 μ F capacitor in series. Depending upon the qualities of the microphone, you may have to try several different resistance values.
In a future article, we’ll take a closer look at the audio trace and see how the sound that it represents is made up of various harmonic components.