Analog audio signals are alternating current (AC) electrical voltages, meaning they constantly reverse polarity from positive to negative. The rate of this oscillation determines the frequency of the signal, which can vary from less than 20 times a second (20Hz) to more than 22,000 (22kHz), the range of human hearing. The amplitude of a signal's peaks determine its volume when amplified, and together these two variables can create complex signals capable of accurately replicating sounds.
Theory
Unlike direct current (DC), which flows steadily in one direction, alternating currents are constantly changing from positive to negative polarity. Most AC voltages, like the 60Hz utility power common in American homes, alternate at a fixed rate (60 times per second).
In AC audio signals, however, this frequency is modulated to match the pitch of the sound being reproduced, just like sound waves alternate between high and low pressure. Though it's easiest to imagine a simple sine wave oscillating smoothly, most audio signals are far more complex, containing multiple frequencies and overtones.
But frequency is only half of the equation. The amplitude of the positive and negative peaks is variable as well, and this property determines the level or volume (when amplified) of the signal, measured in decibels. It's these two constantly changing values—frequency and amplitude—that allow analog systems to recreate complex sounds as electrical signals with astonishing accuracy, and to convert them back again so we can hear them through speakers and headphones.
Transducers
Audio signals can originate in a number of ways: They can be electrically generated from the start such as in a synthesizer, they can be generated based on digital information by a digital audio converter, or (in many cases) they are converted from another type of input by a transducer. A transducer is any device that converts one form of energy to another. Different types of transducers are found in many musical devices:
- Microphones convert acoustical energy from sound waves.
- Guitar pickups convert magnetic energy from vibrating metal strings.
- Piezoelectric pickups convert mechanical vibrations from resonating instruments.
- Speaker cones convert audio signals back into acoustical energy, the opposite of microphones.
Signal Processing
Once a sound is converted into an electrical signal, it can be manipulated in infinite ways. High or low frequencies can be filtered out. Certain frequencies can be emphasized or minimized with an equalizer. Harmonic overtones can be added by distortion.
Amplitude can be controlled with a compressor, and other musical effects can be added like reverb, chorus, and delay. The overall level of the signal can be attenuated with a fader or boosted with an amplifier, and multiple signals can be combined with a mixer.