Week 9

Blogsheet week 9

1.       Measure the resistance of the speaker. 
      
       The resistance value of speaker is 8.1Ω.

2.       Build the following circuit using a function generator setting the amplitude to 5V (0V offset). What happens when you change the frequency? (video) 

Figure 1: Test setup for the speaker.

Video 1. Different pitch with changing frequency

As the frequency increase the sound coming from the speaker increases to a higher volume, and a higher pitch.

Fill the following table. 

Frequency (kHz)	Observation
500	Lowest level of sound
700	Slightly louder
900	A little bit loud, the pitch also increases
1100	Even louder, and higher pitch
2000	Highest level of sound, and pitch

Table 1. Observations of different frequency

       By observation we can see that with higher frequency signals, the pitch of the sound will be higher, also the loudness.

3.       Add one resistor to the circuit in series with the speaker (first 47 Ω, then 820 Ω). Measure the voltage across the speaker. Briefly explain your observations.

Fill the following table. 

Resistor value	Oscilloscope output	Observation
47 Ω	pp - 880mV	Higher sound, and pitch
820 Ω	pp - 8.8mV	Lower sound, and pitch

Table 2. Outputs of 47Ω and 820Ω in series with the speaker

By observation we can see that the sound and pitch will decrease with higher resistance.

4.       Build the following circuit. Add a resistor in series to the speaker to have an equivalent resistance of 100 Ω. Note that this circuit is a high pass filter. Set the amplitude of the input signal to 8 V. Change the frequency from low to high to observe the speaker sound. You should not hear anything at the beginning and start hearing the sound after a certain frequency. Use 22 nF for the capacitor.

Figure 2: Test setup for the high pass filter.

a.       Explain the operation.  (video)

Video 2. Operation of high pass filter

       If there is low frequency signals, then we can not hear anything; if there is high frequency signals, we can start to hear sounds.

b.      Fill out the following table by adding enough (10-15 data points) frequency measurements. Vout is measured with the DMM, thus it will be rms value.

Table 3. Vout/Vin table of high pass filter

Table 3 shows the data points we collected by changing the frequency, but the data are not what we expected (cutoff frequency is different than the calculation). The reason may due to the points we collected are still not enough.

c.       Draw Vout/Vin with respect to frequency using Excel.

Figure 3. Vout/Vin v.s. Frequency of high pass filter

      The plot shows the changing of outputs with different frequency.

d.      What is the cut off frequency by looking at the plot in b?

 From the plot the cut off frequency looks to be around 40 kHz

5.       Design the circuit in 4 to act as a low pass filter and show its operation. Where would you put the speaker? Repeat 4a-g using the new designed circuit.

Video 3. Operation of low pass filter

      To create a low pass filter we connected a capacitor and a resistor in series. Then we connected the speaker wire across the capacitor. 

6.       Construct the following circuit and test the speaker with headsets. Connect the amplifier output directly to the headphone jack (without the potentiometer). Load is the headphone jack in the schematic. “Speculate” the operation of the circuit with a video.

      

                                       

Video 4. Speaker test

      The 5k resister and the capacitor act a filter for the signal coming from the microphone. The op-amp amplifies the signal to the audio jack so that we can hear the sound in the speaker (we use speaker instead of headphone since speaker can make louder voice and the sound can be easier to be took in the video).