Activity 5
Voice Signals
Background Information
In this activity the students work with two transducers—devices that change energy from one form to another. One transducer is a speaker, a coil of wire wound on a cardboard form, which can move back and forth in a metal frame. A permanent magnet near the coil creates a magnetic field. When an audio signal is sent into the coil, the alternating current in the magnetic field produces an alternating force that makes the cardboard in the speaker vibrate. The cardboard vibrates the air, and the sound travels to our ears. In exactly the same way, a microphone contains a coil in a magnetic field. When the coil is vibrated by sound waves, it generates an electrical signal that is sent to an amplifier. The output of both the speaker and the microphone depends on the number of turns of the coil and the strength of the permanent magnet. In this activity, the device the students build works far more effectively as an earphone than a speaker. The side of the cup reduces room noise considerably. The side of the cup also concentrates the sound in the ear, so sound waves from the inside of the bottom of the cup reach the ear with relatively little energy loss. But what about the sound from the outside of the bottom of the cup? This sound is 180 degrees out of phase with the sound from the inside. If the sound from the outside can diffract around the cup to your ear, it will cancel much of the direct sound from inside the speaker. For low tones, with long wavelengths, that’s just what happens. This is the reason that speakers are built into boxes—to isolate the back of the speaker from the air in the room. The sides of the cup prevent this out-of-phase sound from reaching the student’s ear.
The other transducer is a solar cell, which converts light directly into electricity. Most small solar cells are made of a very thin slice of a silicon crystal. These cells have an efficiency of only about 15%, but this number could easily double if current experimental cells are produced and sold in quantity. Power for most Earth satellites is provided by silicon solar panels. If your room lights are fluorescent, they turn on and off 120 times per second. A solar cell aimed at fluorescent lights will produce alternating current, so you hear a tone on the amplifier/speaker. Pointing the solar cell at an incandescent bulb gives a different result.
The light output is indeed alternating, and at 120 Hz, but this alternating intensity is only a small ripple atop a substantial constant output. The filament of the incandescent light is hot. It does not cool down in the short time that the current is small in each cycle. Consequently, it is bright continually, but its brightness increases and decreases slightly 120 times per second.
Students shine fluorescent light onto the solar cell. They amplify the output and play it through a speaker. A speaker makes a sound when an alternating current flows through the coils, since the alternating current produces an alternating force which vibrates the cardboard. If the current were constant, there would be no alternating force, no vibration, and no sound produced. When the students hook up the solar cell to the amplifier speaker and shine a flashlight on the solar cell, the solar cell will produce a constant output, and there will be no sound. To send a message in Morse code, the students will have to figure out a way to produce dots and dashes with essentially an on-off output that sounds like a click. They might make clicks continually for one second to represent a dash. Or they might code two clicks for a dash, and one click for a dot.
Voice Signals
Background Information
In this activity the students work with two transducers—devices that change energy from one form to another. One transducer is a speaker, a coil of wire wound on a cardboard form, which can move back and forth in a metal frame. A permanent magnet near the coil creates a magnetic field. When an audio signal is sent into the coil, the alternating current in the magnetic field produces an alternating force that makes the cardboard in the speaker vibrate. The cardboard vibrates the air, and the sound travels to our ears. In exactly the same way, a microphone contains a coil in a magnetic field. When the coil is vibrated by sound waves, it generates an electrical signal that is sent to an amplifier. The output of both the speaker and the microphone depends on the number of turns of the coil and the strength of the permanent magnet. In this activity, the device the students build works far more effectively as an earphone than a speaker. The side of the cup reduces room noise considerably. The side of the cup also concentrates the sound in the ear, so sound waves from the inside of the bottom of the cup reach the ear with relatively little energy loss. But what about the sound from the outside of the bottom of the cup? This sound is 180 degrees out of phase with the sound from the inside. If the sound from the outside can diffract around the cup to your ear, it will cancel much of the direct sound from inside the speaker. For low tones, with long wavelengths, that’s just what happens. This is the reason that speakers are built into boxes—to isolate the back of the speaker from the air in the room. The sides of the cup prevent this out-of-phase sound from reaching the student’s ear.
The other transducer is a solar cell, which converts light directly into electricity. Most small solar cells are made of a very thin slice of a silicon crystal. These cells have an efficiency of only about 15%, but this number could easily double if current experimental cells are produced and sold in quantity. Power for most Earth satellites is provided by silicon solar panels. If your room lights are fluorescent, they turn on and off 120 times per second. A solar cell aimed at fluorescent lights will produce alternating current, so you hear a tone on the amplifier/speaker. Pointing the solar cell at an incandescent bulb gives a different result.
The light output is indeed alternating, and at 120 Hz, but this alternating intensity is only a small ripple atop a substantial constant output. The filament of the incandescent light is hot. It does not cool down in the short time that the current is small in each cycle. Consequently, it is bright continually, but its brightness increases and decreases slightly 120 times per second.
Students shine fluorescent light onto the solar cell. They amplify the output and play it through a speaker. A speaker makes a sound when an alternating current flows through the coils, since the alternating current produces an alternating force which vibrates the cardboard. If the current were constant, there would be no alternating force, no vibration, and no sound produced. When the students hook up the solar cell to the amplifier speaker and shine a flashlight on the solar cell, the solar cell will produce a constant output, and there will be no sound. To send a message in Morse code, the students will have to figure out a way to produce dots and dashes with essentially an on-off output that sounds like a click. They might make clicks continually for one second to represent a dash. Or they might code two clicks for a dash, and one click for a dot.