ELEC 241 Lab

Experiment 2.1

Electroacoustic Transducers I

Equipment

Components

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? Why not?

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.

Part 2: Source Loading

Looking back at the diagram in the Background section we can see what caused the reduction in signal amplitude (attenuation): $R_{out}$ of the function generator and $R_L$ of the speaker form a voltage divider. To reduce the attenuation caused by loading, we can either reduce $R_{out}$ or increase $R_L$ . But, since $R_{out}$ and $R_L$ 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. Let's look first at reducing attenuation by replacing the load.


Step 1:

 Measure the resistance ($R_L$ ) of the speaker.

Step 2:

 Unscrew the cover from the earpiece of the telephone handset. (The earpiece cover has 7 holes, the mouthpiece has 37.) Carefully remove the earpiece being careful not to damage the wires (it will remain tied to the handset).


Step 3:

 Measure the resistance of the earpiece.

Step 4:

 Set up the function generator to produce a 1kHz sine wave with a peak-to-peak amplitude of 0.5V. Since 20dB is equivalent to a factor of 10, pressing the 20dB attenuator button IN will reduce the previous amplitude of 5V to 0.5V with no further adjustment.

Step 5:

 Connect the earpiece to the Main Output of the function generator using the BNC clip leads.

Step 6:

 How much does the signal amplitude drop in this case? How does this compare to the speaker?

Question 1:

 Based on the source-load model above and your measurements, what is $R_{out}$ of the function generator?

Diversion:

 Note that even though the applied voltage is about the same, the loudness of the resulting sound is less, i.e. the sensitivity (in sound level/volt) is lower for the headphones than for the speaker.

Nevertheless, since the design of the handset places the earpiece close to the ear, the perceived loudness (to the intended listener) is comparable (and that perceived by nearby, unintended listeners is much less) so it may be considered an improvement in many circumstances.

Step 7:

 Replace the earpiece in the handset and replace the cover.