Make: Electronics by Charles Platt (classic books to read TXT) 📗

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