Audio system measurements

The designer of a system for sound reproduction needs to be able to measure the system's performance in a number of areas. While this is not a measure of quality but of quantity, it is a fact that certain quantifiable measurements must be of a certain order to assure quality. That said, it is possible to design systems that sound terrible nevertheless - in other words, a quality system will attain certain measurements, but the existence of those measurements does not guarantee quality.

Measurable performance

frequency response - should at least cover the audible range, usually quoted as 20Hz to 20kHz, 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 ą1 dB variation over the quoted frequency range.
total harmonic distortion (THD). For high fidelity, this is usually expected to be < 1%, usually significantly less. Nowadays very low distortion is easy to achieve 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.
Output power. A genuine measurement quotes the root mean square power output per channel, which is a true value of the power output. Other measurements such as PMPO are meaningless and generally used in disingenuous marketing literature. The quoted power is usually the steady state power into the rated load of the system.
intermodulation distortion is a measure of the spurious signals resulting from unwanted multiplication of different input signals. This effect is contributed by non-linearities in the system. Again, heavy negative feedback can tame this effect, but many believe it is better to design to minimise it arising in the first place.
noise is the level of unwanted noise generated by the system itself, or by interference from external sources.
dynamic range. This gives the signal-to-noise ratio at the system's maximum output. Different media exhibit different orders here - analogue cassette might give 60dB, a CD almost 100. Nowadays most amplifiers have >110 dB dynamic range, which approaches that of the human ear, 160dB.
phase distortion. A good system will maintain the phase coherency of a signal over the full range of frequencies.Phase distortion can be extremely difficult to reduce and eliminate.
transient distortion. A system may have low distortion for a steady-state signal, but distort sudden transients. This is often due to a lack of power delivery fast enough to supply the system during the transient. Related measurements are slew rate and rise time. Transient distortion can be hard to measure. Many otherwise good power amplifier designs have been let down by having an inadequately responsive power supply. Most typical loudspeakers generate significant amounts of transient distortion, though some exotic designs are less prone to this (e.g. electrostatic loudspeakers and plasma arc loudspeakers.
damping factor. A higher number is better. This is a measure of how well a power amplifier can control the reactive load of a loudspeaker. The amplifier must be able to damp out resonances caused by the mechanical inertia of the moving parts of the speaker. Essentially this involves ensuring that the output impedance of the amplifier is as close to zero as it can be made. It is also significantly affected by the cables used to connect the speakers to the amplifier.
wow and flutter. This pertains to the drive mechanism of analogue media, such as vinyl records and magnetic tape. "Wow" is slow speed variations, caused by longer term drift of the drive motor speed, whereas "flutter" is faster speed variations, usually caused by mechanical defects such as out-of-roundness of the capstan of a tape transport mechanism, a lower number is better.
rumble - this is the measure of the low frequency noise contributed by the turntable of an analogue playback system, a lower number is better.

Note that digital systems do not suffer from any of these mechanical effects, even though they have moving parts, since the data is buffered by a memory, and is clocked out by a very precise crystal oscillator, which totally isolates the mechanics of the transport, however digitizing adds quantization noise (random data) and is usually extremely small and negligible.