Oscillators, Speakers and Sound
Prelab
1. Adjustingtheinputtosomethingbasedonwhatitsoutputisdoingisanexampleof.
2. Why is it necessary for a speaker to have both a permanent magnet and an electromagnet to produce sound?
3. In Experiment 2 (The Lighthouse), what circuit elements can be changed to vary theoscillator’sfrequency?
4. How is the disc capacitor different than the electrolytic capacitors that were used in previous labs (other than capacitance value)?
Oscillators, Speakers and Sound
EXPERIMENT 1:HOW’S YOUR HEARING
Objective :
- Determine the limit of human hearing
MATERIALS : amplifier, function generator, sharp ears
Change the frequency setting on the function generator to produce different frequencies through the amplifier (an instructor will help).
What is the lowest frequency you can hear?______Hz
What is the lowest frequency your partner can hear? ______Hz
What is the highest frequency you can hear? ______Hz
What is the highest frequency your partner can hear? ______Hz
Find an instructor, or other person who spent too much time at rock concerts in their youth.
Highest frequency for an older person ______Hz
Lowest frequency for an older person ______Hz
Discuss what you think happens to the ability to hear as one gets older, including whether it primarily affects higher frequencies, lower frequencies or both equally?
EXPERIMENT#2:THELIGHTHOUSE
Electric circuits can make use of the principles of Faraday’s and Lenz Law by using a wire coil as a circuit element. This coil is called an inductor. When a current passes through the coil, it generates a magnetic field. As long as the current does not change, the magnetic field does not induce a current of its own, and the inductor does nothing more that act as a wire, with negligible resistance. However, when the current is turned on, turned off, or if a capacitor in the circuit charges and discharges the changing magnetic flux through the coil induces a current that opposes the change. An inductor is often used with a capacitor and a resistor to createacircuitcalledanoscillator. Ituses feedback.Feedback iswhen youadjusttheinputtosomethingbasedonwhatitsoutputisdoing. The inductor either blocks or enhances current, depending on how the capacitor is changing the current. The capacitor, in turn, charges or discharges based on the behavior of the inductor. The next circuit is an example of an oscillator. In this case, the primary coil of the transformer is used as the inductor. The secondary coil is not used in this circuit. However, it will be used in the following circuit, when we use the oscillator circuit to drive a speaker and make sound.
Parts Needed
breadboard
switch
9Vbattery
oneLED
one 1 MΩ resistor
one 100KΩ resistor
one 100 μF capacitor
one 10μF capacitor
one transistor
one transformer
short and longwires
Notethatthe wire to the middle tap point of the primary side and the wire to the “top” pin of the same coil arenotconnectedalthoughtheirwirescrossinthecircuit diagram.
Buildthecircuitusing the 100μF capacitor, press the switch and wait for 10-12 seconds.If you have wired the circuit correctly, the LED should blink. As long as the switch is closed, the LED blinks every 10-12 seconds. How does this oscillator work? When the switch is first closed, there is no potential difference (voltage drop) between the base (B) and the collector(C). The transistor acts as an open switch and does not allow current to flow through the LED. The capacitor begins to charge and soon there is enough of a voltage drop (approximately 0.6 V), between base and collector to allow current to flow between collector and emitter (E). The LED lights up. The inductor opposes this change, which acts to rapidly discharge the capacitor.This removes the voltage drop between base and collector, and the LED goes out. This restarts the cycle.To change the frequency of the oscillator circuit, we can change either the capacitance, or the resistance.
Q1: Replace the 100μF capacitor with the 10μF capacitor. Does decreasing the capacitance increase or decrease the frequency? Explain why:
Q2. Replace the one 1 MΩ resistor with the 100KΩ resistor. Does decreasing the resistance increase or decrease the frequency? Explain why:
SPEAKERS
Now it’s time to make some noise.Todothisweneedaspeaker.In order to translate an electrical signal into an audible sound, speakers contain a metal coil (2) which becomes an electromagnet when an electric current flows through it. Reversing the direction of the current in the coil reverses the poles of the electromagnet. Inside the speaker, the coil is placed in front of a permanent magnet (3). The permanent magnet is fixed firmly into position whereas the coil can move. The oscillator circuit rapidly changes the direction of the current, in the coil. This reverses the north and south pole, and the coil is in turn attracted to and repelled from the permanent magnet, vibrating back and forth. The coil is attached to a cone or diaphragm (1) made of a flexible material such as paper or plastic which amplifies these vibrations, pumping sound waves into the surrounding air and towards your ears.
EXPERIMENT#3: MY FAVORITE MARTIAN
Parts Needed
breadboard
switch
9Vbattery
onespeaker
one33KΩ resistor
one 470Ω resistor
onevariable resistor
one.047μF(473)disc capacitor
one.10μF capacitor
one.100μF capacitor
one transistor
one transformer
one tuning fork (share!)
rubber or wood for tuning fork
short and longwires
black/red alligator leads
blue and white multimeter
Build the circuit as shown. The .047 μFdisc capacitor labeled “473” is different than the 10 μF and 100μFelectrolytic capacitors in that there is no positive/negative polarity. It can be connected in either direction, like a resistor. You will not be using the larger 100μF yet. While holding the switch down, turn the variable resistor knob. If you watch old sci-fi movies, you may recognize the sound as being a popular sound effect for Martian ray guns and tuning into alien radio stations.
Now add 2 loose long wires to this circuit. Each loose wire should be in the same column as one of the speaker wires (columns 26 and 30 in the wiring diagram). Use these wires to connect with the leads of the multimeter. Measure the frequency of the sound by doing the following:
1. Put the red lead in the VΩ hole and the black lead in the COM hole. Connect the leads with the loose long wires using the alligator end.
2. Turn the dial on the multimeter to Hz (it reads in KHz).
Q1 What is the frequency in KHz when the variable resistor dial is turned all the way to the left? ______
Q2. What is the frequency when the variable resistor dial is turned all the way to the right? ______
Q3. Using the relationship between resistance and frequency, from the previous circuit, which way is the resistance at a minimum, when the dial is turned to the left, or to the right? Explain your reasoning:
Obtain a tuning fork with a frequency between the minimum and maximum frequencies measured.
Note______F______Hz.
Find the frequency of the tuning fork on the multimeter (as close as possible). Strike the tuning fork against a piece of hard rubber or wood (not the lab desk please!)
Q4: Based on this, would you agree that frequency determines the musical note?
Use the long loose wires to measure the voltage across the speakers by doing the following:
1. Turn the small switch from DC to AC (the oscillator emulates alternating current).
2. Turn the dial on the multimeter to 20 V.
Q5. What is the voltage reading when the resistance is minimized? ______
Q6. What is the voltage reading when the resistance is maximized? ______
Q7. Do you think this transformer is acting to step up voltage, or step down voltage? Explain your reasoning:
Q8. Replace the 10 μF capacitor with the 100 μF one. Explain why the sound persists after the battery is disconnected when the 100 μF is used:.
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