Experiment 24 showed that voltage can create a magnet. Experiment 25 has shown that a magnet can create voltage. We’re now ready to apply these concepts to the detection and reproduction of sound.
Figure 5-22. Because inductance increases with the diameter of a coil and with the square of the number of turns, your power output from moving a magnet through the coil can increase dramatically with scale. Those wishing to live “off the grid” may consider this steam-powered configuration, suitable for powering a three-bedroom home.
Figure 5-23. Using a diode in series with a capacitor, you can charge the capacitor with the pulses of current that you generate by moving the magnet through the center of the coil. This demo illustrates the principle of rectifying alternating current.
Experiment 27: Loudspeaker Destruction
I’d like you to sacrifice a 2-inch loudspeaker, even though it means wasting the $5 or so that it probably costs. Actually, I don’t consider this a waste, because if you want to learn how a component works, there’s no substitute for actually seeing inside it. You might also have such a speaker already, part of a piece of cast-off personal electronics or toy you have in your basement.
You will need:
Cheapest possible 2-inch loudspeaker. Quantity: 1. Figure 5-24 shows a typical example.
Figure 5-24. A 2-inch loudspeaker can be instructively destroyed with a utility knife or X-Acto blade.
Procedure
Turn the loudspeaker face-up (as shown in Figure 5-25) and cut around the edge of its cone with a sharp utility knife or X-Acto blade. Then cut around the circular center and remove the ring of black paper that you’ve created. The result should look like Figure 5-26: you should see the flexible neck of the loudspeaker, which is usually made from a yellow weave. If you cut around its edge, you should be able to pull up the hidden paper cylinder, which has the copper coil of the loudspeaker wound around it. In Figure 5-27, I’ve turned it over so that it is easily visible. The two ends of this copper coil normally receive power through two terminals at the back of the speaker. When it sits in the groove visible between the inner magnet and the outer magnet, the coil reacts to voltage fluctuations by exerting an up-and-down force in reaction to the magnetic field. This vibrates the cone of the loudspeaker and creates sound waves.
Large loudspeakers in your stereo system work exactly the same way. They just have bigger magnets and coils that can handle more power (typically, as much as 100 watts).
Whenever I open up a small component like this, I’m impressed by the precision and delicacy of its parts, and the way it can be mass-produced for such a low cost. I imagine how astonished the pioneers of electrical theory (such as Faraday and Henry) would be, if they could see the components that we take for granted today. Henry spent days and weeks winding coils by hand to create electromagnets that were far less efficient than this cheap little loudspeaker.
Figure 5-25. Loudspeaker ready for creative destruction.
Figure 5-26. The cone has been removed.
Figure 5-27. The neck of the cone has been pulled out. Note the coil of copper wire, which fits precisely in the groove between two magnets in the base of the speaker.
Background
Origins of loudspeakers
Loudspeakers utilize the fact that if you run a varying electrical current through a coil situated in a magnetic field, the coil will move in response to the current. This idea was introduced in 1874 by Ernst Siemens, a prolific German inventor. (He also built the world’s first electrically powered elevator in 1880.) Today, Siemens AG is one of the largest electronics companies in the world.
When Alexander Graham Bell patented the telephone in 1876, he used Siemen’s concept to create audible frequencies in the earpiece. From that point on, sound-reproduction devices gradually increased in quality and power, until Chester Rice and Edward Kellogg at General Electric published a paper in 1925 establishing basic principles that are still used in loudspeaker design today.
At http://www.radiolaguy.com/Showcase/Gallery-HornSpkr.htm you’ll find photographs of very beautiful early loudspeakers, which used a horn design to maximize efficiency. As sound amplifiers became more powerful, speaker efficiency became less important compared with quality reproduction and low manufacturing costs. Today’s loudspeakers convert only about 1% of electrical energy into acoustical energy.
Figure 5-28. This beautiful Amplion AR-114x illustrates the efforts of early designers to maximize efficiency in an era when the power of audio amplifiers was very limited. Photos by “Sonny, the RadiolaGuy.” Many early speakers are illustrated at www.radiolaguy.com. Some are for sale.
Theory
Sound, electricity, and sound
Time now to establish a clear idea of how sound is transformed into electricity and back into sound again.
Suppose someone bangs a gong with a stick. The flat metal face of the gong vibrates in and out, creating sound waves. A sound wave is a peak of higher air pressure, followed by a trough of lower air pressure.
The wavelength of the sound is the distance (usually ranging from meters to millimeters) between one peak of pressure and the next peak.
The frequency of the sound is the number of waves per second, usually expressed as hertz.
Suppose we put a very sensitive little membrane of thin plastic in the path of the pressure waves. The plastic will flutter in response to the waves, like a leaf fluttering in the wind. Suppose we attach a tiny coil of very thin wire to the back of the membrane so that it moves with the membrane, and let’s position a stationary magnet inside the coil of wire. This configuration is like a tiny, ultra-sensitive loudspeaker, except that instead of electricity producing sound, it is configured so that sound produces electricity. Sound pressure waves make the membrane move to and fro along the axis of the magnet, and the magnetic field creates a fluctuating voltage in the wire.
This is known as a moving-coil microphone. There are other ways to build a microphone, but this is the configuration that is easiest to understand. Of course, the voltage that it generates is very small, but we can amplify it using a transistor, or a series of transistors. Then we can feed the output through the coil around the neck of a loudspeaker, and the loudspeaker will recreate the pressure waves in the air. Figures 5-29 through 5-32 illustrate this sequence.
Figure 5-29. Step 1 in the process of converting sound to electricity, and back again. When the hammer hits the gong, the face of the gong vibrates, creating pressure waves that travel through the air.
Theory
Sound, electricity, and sound (continued)
Somewhere along the way, we may want to record the sound and then replay it. But the principle remains the same. The hard part is designing the microphone, the amplifier, and the loudspeaker so that they reproduce the waveforms accurately at each step. It’s a significant challenge, which is why accurate sound reproduction can be elusive.
Time now to think about what happens inside the wire when it generates a magnetic field. Obviously, some of the power in the wire is being transformed into magnetic force. But just what exactly is going on?
Figure 5-30. Step 2: the pressure waves penetrate the perforated shell of a microphone and cause a diaphragm to vibrate in sympathy. The diaphragm has a coil attached to it. When the coil vibrates to and fro, a magnet at its center induces alternating current.
Figure 5-31. Step 3: the tiny signals from the microphone pass through an amplifier, which enlarges their amplitude while retaining their frequency and the shape of their waveform.
Figure 5-32. Step 4: the amplified electrical signal is passed through a coil around the neck of a loudspeaker cone. The magnetic field induced by the current causes the cone to vibrate, reproducing the original sound.
Experiment 28: Making a Coil React
A capacitor will absorb some
DC current until it is fully charged, at which point it blocks the flow. There’s another phenomenon that I haven’t mentioned so far, which is the exact opposite of capacitance. It’s known as self-inductance, and you find it in any coil of wire. Initially it blocks DC current (it reacts against it), but then its opposition gradually disappears. Here are a few definitions:
Resistance
Constrains current flow and drops voltage.
Capacitance
Allows current to flow initially and then blocks it. This behavior is properly known as capacitive reactance.
Self-Inductance
Blocks the flow of current initially and then allows it. This is also often referred to as inductive reactance. In fact, you may find the term “reactance” used as if it means the same thing, but since self-inductance is the correct term, I’ll be using it here.
In this experiment, you’ll see self-inductance in action.
You will need:
LEDs, low-current type. Quantity: 2.
Spool of hookup wire, 26-gauge, 100 feet. Quantity: 1.
Resistor, 220Ω, rated 1/4 watt or higher. Quantity: 1.
Capacitor, electrolytic, 2,000 μF or larger. Quantity: 1.
SPST tactile switch. Quantity: 1.
Procedure
Take a look at the schematic in Figure 5-33. At first it may not make much sense. The curly symbol is a coil of wire—nothing more than that. So apparently the voltage will pass through the 220Ω resistor, and then through the coil, ignoring the two LEDs, because the coil obviously has a much lower resistance than either of them (and one of them is upside-down anyway).
Figure 5-33. In this demonstration of self-inductance, D1 and D2 are light-emitting diodes. When the switch is closed, D1 flashes briefly because the coil obstructs the initial flow of electricity. When the switch is opened, D2 flashes as the collapsing magnetic field induced by the coil releases another short burst of current.
Is that what will happen? Let’s find out. The coil can be a spool of 100 feet of 26-gauge (or smaller) hookup wire, although the spool of magnet wire listed in Experiment 25 will work better, if you have that. Once again, you will need access to both ends of the wire, and if the inner end is inaccessible, you’ll need to rewind the coil, leaving the end sticking out.
Now that you have a coil, you can hook it up on your breadboard as shown in Figure 5-34, where the green circle is a tactile switch and the two circular red objects are LEDs. Make sure that you use low-current LEDs (otherwise, you may not see anything) and make sure that one of them is negative-side-up, positive-side-down and the other is positive-side-up, negative-side-down. Also, the 220Ω resistor should be rated at 1/4 watt or higher, if possible (see the following caution).
Hot Resistors
You’ll be passing about 50mA through the 220Ω resistor, while the current is flowing. At 12 volts, this works out at 0.6 watts. If you use a 1/8-watt resistor, you will be overloading it, and it will get quite hot and may burn out. If you use a 1/4-watt resistor, it will still get hot, but is unlikely to burn out, as long as you don’t press the button for more than a second or two.
Don’t run the circuit without the coil of wire; you’ll be trying to pass more than 50mA through the LEDs.
Figure 5-34. The breadboarded version of the schematic in Figure 5-33 shows a simple way to set it up for a quick demo. The green button is a tactile switch. The two red LEDs should be placed so that the polarity of one is opposite to the polarity of the other.
When you press the button, one LED should flash briefly. When you release the button, the other LED should flash.
What’s happening here? The coil possesses self-inductance, which means that it reacts against any sudden change in the flow of electricity. First it fights it, and during that brief moment, it blocks most of the current. Consequently, the current looks for an alternative path and flows through D1, the lefthand LED in the schematic. (D2 doesn’t respond, because it can pass current only in the opposite direction.)
Meanwhile, the voltage pressure overcomes the coil’s self-inductance. When the self-inductance disappears, the resistance of the coil is no more than 10 ohms—so now the electricity flows mostly through the coil, and because the LED receives so little, it goes dark.
When you disconnect the power, the coil reacts again. It fights any sudden changes. After the flow of electricity stops, the coil stubbornly sustains it for a moment, because as the magnetic field collapses, it is turned back into electricity. This residual flow of current depletes itself through D2, the LED on the right.
In other words, the coil stores some energy in its magnetic field. This is similar to the way a capacitor stores energy between two metal plates, except that the coil blocks the current initially and then lets it build up, whereas the capacitor sucks up current initially, and then blocks it.
The more turns of wire you have in your coil, the more self-inductance the coil will have, causing your LEDs to flash more brightly.
Here’s one last variation on this experiment to test your understanding of electrical fundamentals. Remove the 220Ω resistor, and substitute a 1K resistor (to protect your LED from sustained current). Remove the coil, and substitute a very large capacitor—ideally, about 4,700 μF. (Be careful to get its polarity the right way around.) What will you see when you press the button? Note that you will have to hold it down for a couple of seconds to get a result. And what will you see when you release the button? Remember: the behavior of capacitance is opposite to the behavior of self-inductance.
Theory
Alternating current concepts
Here’s a simple thought experiment. Suppose you set up a 555 timer to send a stream of pulses through a coil. This is a primitive form of alternating current.
We might imagine that the self-inductance of the coil will interfere with the stream of pulses, depending how long each pulse is, and how much inductance the coil has. If the pulses are too short, the self-inductance of the coil will tend to block them. Maybe if we can time the pulses exactly right, they’ll synchronize with the time constant of the coil. In this way, we can “tune” a coil to allow a “frequency” to pass through it.
What happens if we substitute a capacitor? If the pulses are too long, compared with the time constant of the capacitor, it will tend to block them, because it will have enough time to become fully charged. But if the pulses are shorter, the capacitor can charge and discharge in rhythm with the pulses, and will seem to allow them through.
I don’t have space in this book to get deeply into alternating current. It’s a vast and complicated field where electricity behaves in strange and wonderful ways, and the mathematics that describe it can become quite challenging, involving differential equations and imaginary numbers. However, we can easily demonstrate the audio filtering effects of a loudspeaker and a coil.
Experiment 29: Filtering Frequencies
In this experiment, you’ll see how self-inductance and capacitance can be used to filter audio frequencies. You’re going to build a crossover network: a simple circuit that sends low frequencies to one place and high frequencies to another.
You will need:
Loudspeaker, 8Ω, 5 inches in diameter. Quantity: 1. Figure 5-35 shows a typical example.
Audio amplifier, STMicroelectronics TEA2025B or similar. Quantity: 1. See Figure 5-36.
Figure 5-35. To hear the effects of audio filters using coils and capacitors, you’ll need a loudspeaker capable of reproducing lower frequencies. This 5-inch model is the minimum required.
Figure 5-36. This single chip contains a stereo amplifier capable of delivering a total of 5 watts into an 8Ω speaker when the two channels are combined.
Figure 5-37. A nonpolarized electrolytic capacitor, also known as a bipolar capacitor, looks just like an electrolytic capacito
r, except that it will have “NP” or “BP” printed on it.
Nonpolarized electrolytic capacitors (also known as bipolar). 47 μF. Quantity: 2. A sample is shown in Figure 5-37. They should have “NP” or “BP” printed on them to indicate “nonpolarized” or “bipolar.”
Nonpolarized electrolytic capacitors (also known as bipolar). 100 μF. Quantity: 5. (Because you’ll be working with audio signals that alternate between positive and negative, you can’t use the usual polarized electrolytic capacitors. If you want to avoid the trouble and expense of ordering nonpolarized capacitors, you can substitute two regular electrolytics in series, facing in opposite directions, with their negative sides joined in the middle. Just remember that when you put capacitors in series, their total capacitance is half that of each individual component. Therefore, you would need two 220 μF electrolytics in series to create 110 μF of capacitance. See Figure 5-38.)
Figure 5-38. You can make a nonpolarized electrolytic capacitor by putting two regular electrolytics in series. (In fact, that’s what you’d find if you opened a real nonpolarized capacitor.) The symbol at the bottom is roughly equivalent to the pair of symbols at the top; bear in mind that two capacitors in series have a total capacitance that is half that of each of them.
Potentiometer, with audio taper if possible. 100K. Quantity: 1.
Coil, for crossover network. Quantity: 1. You can search a source such as eBay for keywords “crossover” and “coil,” but if you can’t find one at a reasonable price, you can make do with a spool of 100 feet of 20-gauge hookup wire.
Make: Electronics Page 33
