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.
B.A. says
The 47K resistor and 0.1uf cap in series – are these in series with each other, or are these two in parallel with each other and in series with the carbon mike? Or is the resistor in series with the mic and the +9V battery, and the capacitor is in series with the scope probe? Can you just capacitively couple the scope and just use the resistor/mic?
David Burrage says
I once made a carbon microphone out of a “D” cell battery just to see if it could be done.
I used the two end plates as the diaphram carbon powder in the battery as the carbon for the microphone.
It worked but was a bit dull. I’m sure it could be improved but it showed what you can do when you know how thing work.
Regards
David Burrage
Laboratory Technician
Chairo Christian School
Science Department
David Herres says
Everything should be in series. The purpose of the resistor is to reduce the voltage. The capacitor is needed to block the dc so that it doesn’t overwhelm the audio signal. Without the capacitor you would just get a straight horizontal line (dc voltage) in the display. The oscilloscope is autoranging so when you push Autoset you should get a good display. Depending on the characteristics of the microphone you may have to adjust the values of the capacitor and resistor.