Technical Jargon Explained

Frequency response

The signal should be passed at least over the audible range (usually quoted as 20 Hz to 20 kHz) with no significant peaks or troughs. The human ear can discern differences in level of about 3 dB, so peaks and troughs must be less than this. Modern equipment is capable of less than ±1 dB variation over the quoted frequency range. Rapid variations over a small frequency range (ripple), or very steep roll-offs are considered undesirable as they can correspond to resonances associated with energy storage which produce delayed echoes and hence colouration on the sound.

For high fidelity, this is usually expected to be < 1% for electronic devices; mechanical elements such as loudspeakers usually have higher levels. Nowadays very low distortion is easy to achieve in electronics with use of negative feedback, but the use of heavy feedback in this manner has been the topic of much controversy among audiophiles — for more on this see electronic amplifier. Loudspeakers typically produce more distortion than electronics, and 1–5% distortion is not unheard of at moderately loud listening levels. Human ears are less sensitive to distortion in the bass frequencies, and levels are usually expected to be under 10% at loud playback. Distortion which creates only even-order harmonics for a sine wave input is sometimes considered less bothersome than odd-order distortion.

A genuine measurement quotes the maximum sine wave power output per channel, which by convention is considered the most meaningful measure of power available on music signals, though it should be noted that real, non-clipping music has a high peak-to-average ratio, and is usually of a lower level. The common figure of PMPO (peak music power out) is meaningless and generally used in disingenuous marketing literature. The correct measurement is average power, usually estimated from an RMS measurement of voltage or current, which gives it the common, but incorrect, designation of "RMS power".
Power specifications require the load impedance to be specified, and in some cases two figures will be given (for instance, a power amplifier for loudspeakers will be typically measured at 4 and 8 ohms). Any amplifier will drive more current to a lower impedance load. For example, it will deliver more power into a 4-ohm load, as compared to 8-ohm, but it must not be assumed that it is capable of sustaining the extra current unless it is specified as able to.

A measurement of the range of signal levels the device is capable of.

Dynamic range refers to the ratio of maximum to mimimum loudness in a given piece of music or programme, and this measurement quantifies the maximum dynamic range an audio system can carry. This is the ratio (usually expressed in dB) between the noise floor of the device with no signal and the maximum signal (usually a sine wave) that can be output without distortion.

Signal-to-noise ratio (SNR), however, is the ratio between the noise floor and an arbitrary reference level or alignment level. In "professional" recording equipment, this reference level is usually +4 dBu (IEC 60268-17), though sometimes 0 dBu (UK and Europe - EBU standard Alignment level). 'Test level', 'measurement level' and 'line-up level' mean different things, often leading to confusion. In "consumer" equipment no standard exists, though -10 dBV and -6 dBu are common.
Different media exhibit different amounts of noise and headroom. Though the values of course vary widely between units, typical analogue cassette might give 60 dB, a CD almost 100. Most modern amplifiers have >110 dB dynamic range, which approaches that of the human ear; 160 dB.