ELEC 243 Lab

Experiment 2.2

Electroacoustic Transducers

Components

Loudspeaker
Microphone

Part 1: Listening to a Signal


Step 1:		Set up the function generator to produce a 1kHz sine wave with a peak to peak (p-p) amplitude of 5 volts.
Step 2:		Using the BNC clip leads, connect the output of the function generator to the speaker. What do you hear?
Step 3:		With the speaker still connected to the function generator, measure the amplitude of the voltage across the speaker (use a second BNC clip lead). Is it the same as in Step 1?
Step 4:		Using the controls on the function generator, vary the amplitude, frequency, and waveshape (i.e. sine, triangle, or square) of the signal. How does the nature of the sound change as these signal parameters change?
Step 5:		Disconnect the speaker.

Source Loading

If we consider the following circuit:

$\includegraphics[scale=0.650000]{../241/fig4.ps}$ we can see what caused the reduction in signal amplitude (attenuation): $R_{out}$ of the function generator and

of the speaker form a voltage divider. An ideal voltage source would have $R_{out}=0$ and there would be no problem. However, any real source will have non-zero $R_{out}$ . To reduce the attenuation caused by loading, we can either reduce $R_{out}$ or increase

. But, since $R_{out}$ and

are actually parts of the source and load respectively, that means we would have to replace either the source or the load with a "better" one. If this is not feasible, we can isolate the source from the load by placing an amplifier between them.

Part 2: Viewing an Acoustic Signal

The physical phenomenon behind the operation of the loudspeaker works in both directions: in addition to converting an electrical signal into an acoustical signal it can also convert sound to electricity. In techspeak we say that the loudspeaker is a bilateral transducer.


Step 1:		Using the BNC clip leads, connect the speaker to `CH1` of the oscilloscope.
Step 2:		Set the `VOLTS/DIV` switch to `20 mV` and the `TIME/DIV` switch to `2 mSEC`.
Step 3:		Speak into the loudspeaker and observe the waveform on the oscilloscope. If necessary, adjust the oscilloscope to produce a satisfactory trace. Note the amplitude of the signal.

Part 3: Microphone

Although we can use the loudspeaker as both an electricity to sound transducer and a sound to electricity transducer, the fact that its construction has been optimized for producing sound rather than accepting sound means that its performance is suboptimal for the latter task. When we reoptimize for conversion of acoustic to electrical signals, we get a device called a dynamic microphone.

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 module.

Step 3:

Plug the microphone into J1-4 of the interface module. Take a piece of wire about 4 cm 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. Sketch one or two of the more interesting waveshapes.

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?

Step 11:

Whistle softly into the microphone. (For this step, pitch is not important, so any note will do.) Observe the waveform on the oscilloscope. Is it sinusoidal?

Question 3:

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