ELEC 241 Lab

Experiment 2.2

Electroacoustic Transducers II

Equipment

Instrument Suite (scope, function generator, power supply)
Breadboard
Lab PC

Components

Loudspeaker
Microphone

Software

Volume Control program

Part 1: Microphone

So far we have two acoustic output devices, the speaker and the telephone earpiece, which convert electrical signals to sound. Now we need an input device to convert acoustic siganls to electric ones.

Step 1:

Get a microphone from the equipment cart. It has two connectors, one slightly smaller than the other.

The small one is connected to the switch to turn a cassette recorder on and off when dictating. We will use the other one.

We want to use the scope to measure the microphone's output, but attempting to connect the clip leads to the microphone connector is an exercise in futility. So ...

Step 2:

Use a BNC patch cord to connect CH1 of the scope to J1-1 of the interface board.

Step 3:

Plug the microphone into J1-4 of the interface module. Take a piece of wire about 4cm long and strip 6 to 7 mm of insulation from each end. The end of the wire should look like this:

Note The stripped length of a wire is very important. If it is too short (less than 6 mm), insulation will be forced between the contact fingers of the socket strip, resulting in an intermittent connection (or none at all). This is the second most common cause of problems in the lab. If it is too long, the bare portion of the wire above the socket strip can short to other wires.

Connect the microphone to Channel 1 of the scope by plugging one end of the wire into pin 1 of the socket strip and the other end into pin 4. The grounds are connected automatically by the interface board.

Step 4:

Set the oscilloscope V MODE switch to CH1, the CH 1 VOLTS/DIV switch to 5 mV, and the TIME/DIV switch to 1 mSEC. Set the other controls as required.

Step 5:

Speak, sing, or whistle into the microphone and observe the signal on the scope. If the amplitude is too small, you can use the magnifier to get a little more gain. Pull out the POSITION knob (the VARIABLE knob on the Leader) to increase the gain (thereby decreasing the Volts/Div) by a factor of 5 (10 on the Leader).

Diversion:

The triggering controls (Auto/Norm, Level, Slope, and Coupling (and Holdoff on the Leader)) determine the relationship between the origin of the display (t=0) and features of the waveform. In AUTO mode, the beam sweeps continuously, whether a signal is present or not and attempts to synchronize automatically when a signal is applied. This usually works well for simple signals, such as those produced by the function generator, but often results in an unstable display with more complex signals, such as speech signals. For these signals, NORMAL mode is often more appropriate.

In normal mode a sweep is started only when the signal being displayed crosses a specified threshold. The level of the threshold is controlled by the LEVEL knob, and the direction of crossing by the SLOPE control.

Step 6:

Set the AUTO/NORM control to NORM. Speak into the microphone and adjust the LEVEL control to produce a stable display. Experiment with the triggering controls and the TIME/DIV control to see what effects they have on the display.

Step 7:

Measure the amplitude of the signal. (Remember to include the scale factor if you used the magnifier.)

Step 8:

Produce a sustained vowel (a, e, i, o, u) sound. Does it resemble the signal we saw in class? If not, try different vowels and see how close you can get.

Step 9:

Continue producing a sustained vowel sound (inhaling as necessary) and measure its frequency (by measuring the period).

Step 10:

(Optional) If you are musically inclined, sing (or whistle or hum) the note "A" and measure its frequency. (If you play an instrument and have it with you, use it to produce your note.) How does your measured frequency compare with the "official" value for the frequency of A? Which do you trust to be more accurate, your sense of pitch or the oscilloscope?

Question 2:

Based on your measurements of the earpiece sensitivity and the output of the microphone, would it be possible to produce an audible sound in the earpiece by connecting it directly to the microphone?

Part 2: The Lab PC as a Signal Source

The microphone is a device that converts an acoustic pressure into a voltage. The Lab PC contains a device, the Sound Card, that converts a sequence of numbers into a voltage. This sequence of numbers could represent the samples of a physical signal that we talked about in the first lecture.

Since the computer can compute functions and the sound card can produce electrical output we could use the Lab PC as our function generator. In fact, the computer can compute much more interesting functions than the three simple ones that our Function Generator (with a capital F) produces. Another advantage is that $R_{out}$ of the sound card is less than that of the function generator, so we can connect the speaker directly with less signal loss. Let's look at a few examples.

Step 1:
Plug the sound card cable from the Lab PC (with the 8-pin round connector ) into J2-1 on the interface module.

Step 2:
Connect a BNC clip lead to J1-3 of the interface board. Connect the clips to the loudspeaker. Connect the speaker to the sound card speaker output by connecting a piece of wire between pins 3 and 20 of the interface board socket strip.

Step 3:
Also connect the speaker output to CH1 of the scope by connecting a wire from pin 20 to pin 1.

Note:
The following three steps involve playing signals through the builtin sound capability of Netscape. In the online version of the manual, they contain links to the files containing the signals. To listen to the signal, just click on the link.

Step 4:
Here's a familiar signal: Signal 1.

Step 5:
Here's another familiar sound having a more complex signal: Signal 2.

Step 6:
Finally, here's a challenge: Can you figure out the Mystery Signal?
We'll look at this signal again in a few weeks when we have some more sophisticated analysis tools. In the meantime, based on what you can hear and what you can see on the scope, can you figure out the trick?

Step 7:
Disconnect the speaker and remove the clip lead from P3 of the interface board. Unplug the sound card cable from P9.


Step 1:		Plug the sound card cable from the Lab PC (with the 8-pin round connector ) into J2-1 on the interface module.
Step 2:		Connect a BNC clip lead to J1-3 of the interface board. Connect the clips to the loudspeaker. Connect the speaker to the sound card speaker output by connecting a piece of wire between pins 3 and 20 of the interface board socket strip.
Step 3:		Also connect the speaker output to `CH1` of the scope by connecting a wire from pin 20 to pin 1.
Note:		The following three steps involve playing signals through the builtin sound capability of Netscape. In the online version of the manual, they contain links to the files containing the signals. To listen to the signal, just click on the link.
Step 4:		Here's a familiar signal: Signal 1.
Step 5:		Here's another familiar sound having a more complex signal: Signal 2.
Step 6:		Finally, here's a challenge: Can you figure out the Mystery Signal? We'll look at this signal again in a few weeks when we have some more sophisticated analysis tools. In the meantime, based on what you can hear and what you can see on the scope, can you figure out the trick?
Step 7:		Disconnect the speaker and remove the clip lead from P3 of the interface board. Unplug the sound card cable from P9.