Electret Microphone/Oscilloscope Demo
Overview of Demo:
In this demo, an electret microphone is connected to an oscilloscope, allowing students to “see” sound
waves. This demo is primarily designed to illustrate principles in acoustics, but points can be made
pertaining to electronics and electricity as well.
The Setup:
In order to set up this demo, the following parts are needed:
 A power supply
 An oscilloscope
 An electret microphone
 A 22kΩ resistor
 A breadboard (optional)
 Various cables and connectors
(See below for notes on part locations)
Diagrammatically, the circuit setup looks like this:
The power supply is connected across the resistor and microphone in series, and the oscilloscope
measures the voltage across the microphone.
A few notes:
1. The electret is rated for a maximum voltage of 10V. In this setup, about 4V should be dropped
across the resistor. Use a power supply voltage of somewhere between 6 and 10 volts (8 is good).
2. The polarity of the microphone matters. By convention, the negative lead is connected to the
chassis, as demonstrated in this diagram:
3. The oscillations will be on the order of magnitude of 60mV (millivolts), so choose a vertical scale
on the oscilloscope appropriately (20mV per division should do). Experiment with the horizontal
scale so that the wave form looks good.
4. The negative terminal of the 40 amp BK precision multimeter is connected to ground through the
power outlet’s ground pin, and the same is the case with the oscilloscope. If the scope is being
used with this power supply, both negative leads have to be connected to the same node (i.e., you
can’t move the leads of the oscilloscope to measure the voltage across the resistor as shown in the
diagram).
Once everything is set up, it should look like this:
Part Locations:
The oscilloscopes are located in a lower cabinet in the physics shop labeled “OSCILLOSCOPES”. The
power supplies and breadboards are located in cabinets 19A and 13C in the lab. The electret microphones
and resistors are located in the electronics cabinet in the shop. In the diagram above, 2 banana-alligator
cables are used on the power supply, and one BNC to test cable is used on the oscilloscope. Both of these
cables can be found in the gray cabinet drawer labeled “BANANA ALLIGATOR” in the lab.
Suggested Demonstrations:
Once the demo is up and working, there are any number of things the demonstrator can try. These are a
few suggestions:
1. Hold a tuning fork to the microphone, and observe the sinusoidal output. Then make a vocal
sound. The waveform from a human voice will not be sinusoidal, and will change with the
quality of the sound (e.g., if the vowel is open mouthed, the waveform will be jagged, if the lips
are closed, the waveform should become smoother. From this we can conclude that the closed
lips mute the higher frequencies—see the next suggested demo for more on this)
2. When a tuning fork is struck sharply, it emits two tones, one higher pitched that can be heard at
arm’s length, and one that is lower pitch that can be heard when the fork is held up to the ear.
When the fork is struck this way and held up to the microphone, the waveform will be a low
frequency sine wave, with a higher frequency wave superimposed on it. (Note: This higher pitch
tone is called the “clang tone” of the tuning fork. One page on the internet that shows visually
how this comes about is: http://hyperphysics.phy-astr.gsu.edu/HBASE/Music/tunfor.html)
3. Two tuning forks with different frequencies can both be struck and held up to the microphone at
the same time, and the superposition can be observed.
Electronics:
The electronics of the demo can also be explained, and may be interesting to demonstrate along with the
acoustics. The way the setup works is that the microphone capsule acts as a current regulator in that the
current passing through it doesn’t depend on the voltage across it. It does, however, change slightly as
the pressure waves of sound hit it. The current through the device is something like 200µA, and varies by
something like 3µA as sound is picked up. An experimental setup to confirm this is shown in the next
diagram:
Here, a power supply is connected across the microphone with an ammeter in place to measure the
current. As the voltage is varied, the current should remain very nearly constant (around 200µA). If the
demonstrated decides to do this, it should be kept in mind that
1. The microphone is rated at 10 volts max—it’s probably safest not to exceed 9 and
2. The polarity of the microphone matters
The average value of the current through the microphone motivates the choice of resistor of 22kΩ,
because 200µA × 22kΩ ≈ 4V, which is about half of the supply voltage (8 volts). If the supply voltage
were different, then a different resistance value would be appropriate.
The electret microphone capsule contains two parts: (1) the electret microphone proper, and (2) a JFET
(Junction Field Effect Transistor). A diagram of how this connection is made can be seen on the electret
Wikipedia page: http://en.wikipedia.org/wiki/Electret_microphone
The electret microphone proper creates voltages that vary based on the arrival of pressure waves. The
JFET helps to amplify the signal by acting as a voltage-controlled current source; the current flowing
through it flows through a channel of n-type semiconductor. The size of the channel grows and shrinks
based on the voltage at the gate (connected to the microphone), so the current depends on the pressure
waves arriving at the capsule. Because increased voltage across the terminals has the effect of closing the
channel, the current through the device is not affected by a change in voltage across it. This is why the
microphone current has little dependence on the applied voltage, but does respond to sound pressure
waves. For more info (including pictures) pertaining to the JFET, see the Wikipedia page:
http://en.wikipedia.org/wiki/JFET
The microphone is able to change its voltage based on pressure waves, because it contains two
membranes that act as the plates of a capacitor. As the sound waves hit, the distance between the plates
changes, and the voltage changes. The charge on the capacitor plates is a permanent electrostatic charge
in the material that forms the membrane. A good explanation of this is given in the electret mic section of
the condenser microphone Wikipedia page:
http://en.wikipedia.org/wiki/Condenser_microphone#Condenser.2C_capacitor_or_electrostatic_micropho
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