The Four Parameters
Noise is the background hiss you hear when you raise the volume on a hi-fi receiver or microphone preamp. You can usually hear it clearly during quiet passages when playing cassette tapes. A close relative is dynamic range, which defines the span in decibels (dB) between the residual background hiss and the loudest level available short of gross distortion.
CDs and DVDs have a very large dynamic range, so if you hear noise while playing a CD, it’s from the original master analog tape, it was added as a by-product during production, or it was present in the room and picked up by the microphones when the recording was made.
Subsets of noise are AC power-related hum and buzz, vinyl record clicks and pops, between-station radio noises, electronic crackling, tape modulation noise, left-right channel bleed-through (cross-talk), doors and windows that rattle and buzz when playing music loudly, and the triboelectric cable effect. Tape modulation noise is specific to analog tape recorders, so you’re unlikely to hear it outside of a recording studio.
Modulation noise comes and goes with the music, so it is usually drowned out by the music itself. You can often hear it on recordings that are not bright sounding, such as a bass solo, as each note is accompanied by a “pfft” sound that disappears between the notes. The triboelectric effect is sometimes called “handling noise” because it happens when handling poor-quality cables. The sound is similar to the rumble you get when handling a microphone. This defect is rare today, thanks to the higher-quality insulation materials used by wire manufacturers.
Frequency response describes how uniformly an audio device responds to various frequencies. Errors are heard as too much or too little bass, midrange, or treble. For most people, the audible range extends from about 20 Hz at the low end to slightly less than 20 KHz at the high end. Some youngsters can hear higher than 20 KHz, though many senior citizens cannot hear much past 12 KHz.
Some audiophiles believe it’s important for audio equipment to pass frequencies far beyond 20 KHz, but in truth there’s no need to reproduce ultrasonic content because nobody will hear it or be affected by it. Subsets of frequency response are physical microphonics (mechanical resonance), electronic ringing and oscillation, and acoustic resonance. Resonance and ringing will be covered in more detail later in this and other chapters.
Distortion is a layman’s word for the more technical term nonlinearity, and it adds new frequency components that were not present in the original source. In an audio device, non-linearity occurs when a circuit amplifies some voltages more or less than others, as shown in Figure 2.1. This nonlinearity can result in a flattening of waveform peaks, as at the left, or a level shift near the point where signal voltages pass from plus to minus through zero, as at the right. Wave peak compression occurs when electrical circuits and loudspeaker drivers are pushed to levels near their maximum limits.
Figure 2.1: Two types of nonlinearity: peak compression at the top and/or bottom of a wave (left), and crossover distortion that affects electrical signals as they pass through zero volts (right). (click to enlarge)
Some circuits compress the tops and bottoms equally, which yields mainly odd-numbered harmonics—3rd, 5th, 7th, and so forth—while other circuit types flatten the top more than the bottom, or vice versa. Distortion that’s not symmetrical creates both odd and even harmonics—2nd, 3rd, 4th, 5th, 6th, and so on. Crossover distortion (shown in Figure 2.1) is also common, and it’s specific to certain power amplifier designs. Note that some people consider any change to an audio signal as a type of distortion, including frequency response errors and phase shift. My own preference is to reserve the term “distortion” only when nonlinearity creates new frequencies not present in the original.
When music passes through a device that adds distortion, new frequencies are created that may or may not be pleasing to hear. The design goal for most audio equipment is that all distortion be so low in level that it can’t be heard. However, some recording engineers and audiophiles like the sound of certain types of distortion, such as that added by vinyl records, transformers, or tube-based electronics, and there’s nothing wrong with that. My own preference is for gear to be audibly transparent, and I’ll explain my reasons shortly.