Natural reverberation occurs when walls reflect sounds back to you from walls and ceilings a short distance away, such as 30 to 100 feet. Cathedrals and auditoriums have shapes that produce this effect; complex, repeated reflections sustain the sound for a second or two. Classic electronic reverb effects use a amplifier circuit, a pair of small, loose coil springs and a second amplifier. The first amplifier takes an electronic audio signal, boosts its power and sends it through the spring as mechanical waves. The waves bounce back and forth along the spring and convert back to electrical signals at the other end. The second amplifier increases these signals to a standard audio line level, about one volt. Properly designed, the springs create a realistic reverb effect.
Echo is an effect similar to reverberation, but working on a longer time scale. Sound echoes take a quarter-second to a few seconds, whereas reverb takes a few hundredths of a second. In addition, echoes have a distinct, one-at-a-time quality; reverb is more diffuse and complex. Most echo effect circuits today use a digital memory scheme called a "bucket brigade." The circuit digitizes audio signals into a stream of computer data bytes. The memory circuit takes each byte in turn, and passes it through a series of thousands of storage cells. When the first data byte emerges from the last cell, it has been delayed by a time interval, such as a half-second. A control on the echo device varies the amount of time delay, producing long echoes, rapid ones and a variety of related effects.
Electronic filter circuits shape the tone of audio signals in different ways. The simplest filter, consisting of a resistor and capacitor, removes high or low frequencies from the signal, depending on the capacitor-resistor configuration. Active filter circuits use sets of resistors and capacitors in conjunction with amplifying elements like transistors. An active filter removes frequencies more aggressively than passive designs. An equalizer combines several filters in a row, letting you boost or cut bands of frequencies. For example, a five-band equalizer has separate controls for very low, low, middle, high and very high frequencies. Low frequencies affect bass sounds such as thunder and drums, middle frequencies cover the human voice and most instruments and high frequencies affect music’s brightness and clarity.
Phase shifting and flanging produce two similar kinds of audio effects. Generally, they are slow-moving jet-plane "swooshes" superimposed on music and vocals. In the 1960s, engineers recorded a song on two separate tape recorders. On playback, one tape would be slowed down slightly, and the mix of the two produces this classic effect. Today, engineers use digital delay circuits in the same manner as for echo, but having a delay of milliseconds. When you mix the original audio with the delayed version, it creates the swoosh effect.
A phase shifter or “phaser” imparts a similar effect. Its circuit consists of several filters, some of which emphasize a frequency, others reducing frequencies, so it has an overall up-and-down response shaped like the teeth of a comb. A special control increases and decreases the frequencies of all the filters as a set, creating a dramatic "churning" effect in the audio.