The Shocking Truth

Jun 1, 2001 12:00 PM, By Scott Wilkinson

           
 

Illustration: George Abe

The secrets of electricity revealed.

During the past 15 years, many fundamental music-technology concepts have been explained in “Square One” (originally titled “From the Top”). In 1997 EM technical editor Scott Wilkinson combined many of those columns into a comprehensive primer titled Anatomy of a Home Studio: How Everything Really Works, from Microphones to MIDI, published by EMBooks, an imprint of Artistpro.com (www.artistpro.com). Our readership has continued to grow, and new readers shouldn't be left behind. Rather than try to reinvent the wheel, we will periodically reprint excerpts from the book as “Square One Classics.” These articles will clarify the essential, unchanging concepts that make it possible to be an electronic musician.

Electricity can be mysterious to many people, even those who have worked with music technology for years. But to get the most from the tools of electronic music, you need to understand the fundamental concepts of electricity. For example, manufacturer specifications mean nothing without them. (Some specs mean nothing anyway, but that's another story.) How can you make an informed purchasing decision without understanding what the specifications mean?

For now, I'll concentrate on four basic properties of electricity: current, voltage, impedance, and power. Understanding these properties is essential if you want to know how electronic equipment works. It's also critical for comprehending other concepts, such as decibels (which I'll explain in the next column).

CURRENT AND VOLTAGE

Most audio signals consist of electrons flowing through a conductor, such as a copper wire. This flow of electrons is called a current. The amount of current is measured in units called amperes, or amps (abbreviated A), after French physicist Andre Ampere; it is represented by the letter I in electrical equations.

There are two types of current: direct and alternating. Direct current (DC) flows steadily in one direction through a conductor; alternating current (AC) changes direction in the conductor at various frequencies. Analog-audio signals are alternating currents with waveforms and frequencies that correspond to acoustic sounds. These audio signals are called analog because the current's waveform is analogous to the acoustic waveform it represents.

An electromotive force (EMF) causes current to flow. The name makes sense when you think about it: EMF is a force that makes electrons move. EMF is more commonly called voltage, which is measured in units called volts, after Italian physicist Alessandro Volta, and is abbreviated V. It is represented by the letters V or E in electrical equations. Voltage is produced in many different ways, such as chemical reactions in a battery.

For a helpful analogy, think of voltage as the height of a hill. Because there is a difference in height between the hill's top and bottom, a ball rolls down the hill under gravity's influence. When the ball is at the top of the hill, it has potential energy; that is, it has the potential to move down. As it rolls down the hill, the potential energy is converted into kinetic energy, the energy of motion.

So the hill's top and bottom are at different heights, and the ball moves from one to the other. The same is true for voltage and current. Any voltage source has two poles, and electrons flow from one to the other. There is a potential difference in voltage between these poles: the bigger the difference, the greater the potential for moving electrons. However, that potential can't be fulfilled until an electrical conductor connects the two poles. If you connect the poles in this way, you create a circuit — or closed loop — through which the current flows.

FIG. 1: If you connect a battery’s poles with a conductor, current flows from the negative to the positive pole. The amount of current ­depends on the conductor’s impedance.

One common voltage source is a battery, which has positive and negative poles. If you connect a conductor to these poles, electrons flow from the negative pole to the positive pole (see Fig. 1). If you've played with magnets, you know that opposite poles attract and similar poles repel each other. The same is true for electrons, which are negatively charged. They are repelled by a battery's negative pole and attracted to its positive pole.

AC/DC

Because a battery's voltage produces a direct current, its voltage is specified in units of VDC. If the poles of a voltage source alternate between positive and negative (as they do in a wall's power outlet, for example), the current changes direction periodically, and the voltage is specified in VAC.

FIG. 2: A voltmeter measures the DC or AC ­voltage between two points in an electrical ­circuit. Many voltmeters can also gauge DC ­resistance; such devices are called volt/ohm meters (VOMs).

Measuring the voltage from a battery or other DC source is easy. Returning to the hill analogy, the higher the hill, the more potential energy the ball has. The battery's voltage is analogous to the height of the hill: the more voltage, the more potential it has for moving electrons. To measure a battery's voltage, simply attach the two leads from a voltmeter to the poles and read the voltage (see Fig. 2).

Measuring alternating voltages is not so straightforward. You could simply measure the highest voltage level as it varies up and down, but what if the peak level changes from one cycle to the next, as it does at the output of most audio equipment? Taking the average of several peaks is better, but engineers have devised a more accurate way to measure alternating voltages: root mean square (RMS).

Here's how RMS voltage is calculated. (The process sounds complicated, but don't worry — you never have to do it. I'm describing it to explain what RMS means.) First, measure the instantaneous voltage value at many points during one complete cycle; that is similar to digital-audio recording. Then, square each voltage value (that is, multiply the value by itself). Next, calculate the average of those squared values and take the square root of that average. If the voltage variation takes the form of a sine wave with constant amplitude (as the voltage from a wall outlet does), the calculation becomes simpler: multiply the peak value by 0.707.

This is relatively complicated, but it yields a meaningful voltage value, even in the face of different peak levels over time. Fortunately, you don't need to worry about this process; anyone who wants to measure an alternating voltage can simply connect a voltmeter to the poles of the voltage source. The voltmeter does the squaring and averaging, giving you a readout in VRMS or VAC.