Audio system measurements are used to quantify audio system performance. These measurements are made for several purposes. Designers take measurements to specify the performance of a piece of equipment. Maintenance engineers make them to ensure equipment is still working to specification, or to ensure that the cumulative defects of an audio path are within limits considered acceptable. Audio system measurements often accommodate psychoacoustic principles to measure the system in a way that relates to human hearing.
Subjectivity and frequency weighting Subjectively valid methods came to prominence in consumer audio in the UK and Europe in the 1970s, when the introduction of compact cassette tape, dbx and Dolby noise reduction techniques revealed the unsatisfactory nature of many basic engineering measurements. The specification of weighted CCIR-468 quasi-peak noise, and weighted quasi-peak wow and flutter became particularly widely used and attempts were made to find more valid methods for distortion measurement.
Measurements based on psychoacoustics, such as the measurement of noise, often use a weighting filter. It is well established that human hearing is more sensitive to some frequencies than others, as demonstrated by equal-loudness contours, but it is not well appreciated that these contours vary depending on the type of sound. The measured curves for pure tones, for instance, are different from those for random noise. The ear also responds less well to short bursts, below 100 to 200 ms, than to continuous soundsMoore, Brian C. J., An Introduction to the Psychology of Hearing, 2004, 5th ed. p137, Elsevier Press such that a quasi-peak detector has been found to give the most representative results when noise contains click or bursts, as is often the case for noise in digital systems.BBC Research Report EL17, The Assessment of Noise in Audio Frequency Circuits, 1968. For these reasons, a set of subjectively valid measurement techniques have been devised and incorporated into BS, IEC, EBU and ITU standards. These methods of audio quality measurement are used by broadcast engineers throughout most of the world, as well as by some audio professionals, though the older A-weighting standard for continuous tones is still commonly used by others.
No single measurement can assess audio quality. Instead, engineers use a series of measurements to analyze various types of degradation that can reduce fidelity. Thus, when testing an analogue tape machine, it is necessary to test for wow and flutter and tape speed variations over longer periods, as well as for distortion and noise. When testing a digital system, testing for speed variations is normally considered unnecessary because of the accuracy of clocks in digital circuitry, but testing for aliasing and timing jitter is often desirable, as these have caused audible degradation in many systems.
Once subjectively valid methods have been shown to correlate well with listening tests over a wide range of conditions, then such methods are generally adopted as preferred. Standard engineering methods are not always sufficient when comparing like with like. One CD player, for example, might have higher measured noise than another CD player when measured with an RMS method, or even an A-weighted RMS method, yet sound quieter and measure lower when 468-weighting is used. This could be because it has more noise at high frequencies, or even at frequencies beyond , both of which are less important since human ears are less sensitive to them (see Noise shaping). This effect is how Dolby B works and why it was introduced. Cassette noise, which was predominantly high frequency and unavoidable given the small size and speed of the recorded track, could be made subjectively much less important. The noise sounded quieter, but failed to measure much better unless 468-weighting was used rather than A-weighting.
Measurable performance
Analog electrical Frequency response This measurement tells you over what frequency range output level for an audio component will remain reasonably constant (either within a specified decibel range or with reference to amplitude at 1 kHz). Preamplifiers may also contain equalizers for example to play LPs requiring RIAA frequency response correction, in which case the specification may describe how closely the response matches the standard. On the other hand, frequency range is a term sometimes used of loudspeakers and other transducers to indicate the frequencies that are usable, without necessarily specifying a decibel range. Power bandwidth is also related to frequency responseindicating the range of frequencies usable at high power. A component having a flat frequency response will reproduce content at all frequencies across the specified frequency range at the same intensity. The frequency range often specified for audio components is between to 20 kHz, which broadly reflects the human hearing range. Well-designed solid-state amplifiers and CD players may have a frequency response that varies by only 0.2 dB between 20 Hz to 20 kHz.Metzler, Bob, "Audio Measurement Handbook" , Second edition for PDF. Page 86 and 138. Audio Precision, USA. Retrieved 9 March 2008. Loudspeakers tend to have considerably less flat frequency responses than this.
Total harmonic distortion (THD) Music material contains distinct tones, and some types of distortion involve spurious tones at double or triple the frequencies of those tones. Such harmonically related distortion is called harmonic distortion. For high fidelity, this is usually expected to be <1% for electronic devices; mechanical elements such as loudspeakers usually have inescapable higher levels. Low distortion is relatively easy to achieve in electronics with use of negative feedback, but the use of high levels of feedback in this manner has been the topic of controversy among audiophiles. Essentially, all loudspeakers 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 low frequencies, and levels are usually expected to be under 10% at loud playback. Distortion that creates only even-order harmonics for a sine wave input is sometimes considered less intrusive than odd-order distortion.
Output power Output power for amplifiers is ideally measured and quoted as maximum root mean square (RMS) power output per channel, at a specified distortion level at a particular load, which, by convention and government regulation, is considered the most meaningful measure of power available on music signals. Power specifications require the load impedance to be specified, and in some cases, two figures will be given (for instance, the output power of a power amplifier for loudspeakers will be typically measured at 4 and 8 ohms).
Intermodulation distortion (IMD) Distortion that is not harmonically related to the signal being amplified is intermodulation distortion. It is a measure of the level of spurious signals resulting from unwanted combination of different frequency input signals. This effect results from non-linearities in the system. Sufficiently high levels of negative feedback can reduce this effect in an amplifier. Intermodulation in loudspeaker drivers is, as with harmonic distortion, almost always larger than in most electronics. IMD increases with cone excursion. Reducing a driver's bandwidth directly reduces IMD. This is achieved by splitting the desired frequency range into separate bands with an audio crossover and employing separate drivers for each band of frequencies. Intermodulation distortion in multi-driver loudspeaker systems can be greatly reduced with the use of active crossover, though it significantly increases system cost and complexity.
Noise The level of unwanted noise generated by the system itself, or by interference from external sources added to the signal. Hum usually refers to noise only at power line frequencies (as opposed to e.g. broadband white noise), which is introduced through induction of power line signals into the inputs of gain stages, from inadequately regulated power supplies, or poor grounding of components.
Crosstalk The introduction of noise (from another signal channel) caused by ground currents, stray inductance or capacitance between components or lines. Crosstalk reduces, sometimes noticeably, separation between channels (e.g., in a stereo system). A crosstalk measurement yields a figure in decibels relative to a nominal level of signal in the path receiving interference. Crosstalk is normally only a problem in equipment that processes multiple audio channels in the same chassis.
Common-mode rejection ratio (CMRR) In balanced audio systems, there are equal and opposite signals in inputs, and any interference imposed on both leads will be subtracted, canceling out that interference. CMRR is a measure of a system's ability to ignore such interference and especially hum at its input. It is generally only significant with long lines on an input, or when some kinds of ground loop problems exist. Unbalanced inputs do not have common mode resistance; induced noise on their inputs appears directly as noise or hum.
Dynamic range and Signal-to-noise ratio (SNR) SNR is the ratio between the maximum level a component can accommodate and the noise level it produces. Input noise is not counted in this measurement. Dynamic range refers to the ratio of maximum to minimum loudness in a given signal source (e.g., music or programme material), and this measurement also quantifies the maximum dynamic range an audio system can carry. The ratio is expressed in dB between the noise floor of the device and the maximum signal (usually a sine wave) that can be output at a specified (low) distortion level. Since the early 1990s, it has been recommended by several authorities, including the Audio Engineering Society that measurements of dynamic range be made with an audio signal present. This avoids questionable measurements based on the use of blank media or muting circuits. 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 per IEC 60268-17, though sometimes 0 dBu in UK and Europe, the EBU standard Alignment level. Test level, measurement level and line-up level mean different things, often leading to confusion. In consumer equipment, no official standard exists, though −10 dBV and −6 dBu are common. Different media characteristically exhibit different amounts of noise and headroom. Though the values vary widely between units, a typical analogue cassette might give 60 dB, a vinyl record around 70 dB, a CD almost 100 dB. Most modern quality amplifiers have greater than 110 dB dynamic range, which approaches that of the human ear, usually taken as around 130 dB.
Phase distortion, Group delay, and Phase delay A perfect audio component will maintain the phase coherency of a signal over the full range of frequencies. Phase distortion can be extremely difficult to reduce or eliminate. The human ear is largely insensitive to phase distortion, though it is exquisitely sensitive to relative phase relationships within heard sounds. The complex nature of our sensitivity to phase errors, coupled with the lack of a convenient test that delivers an easily understood quality rating, is the reason that it is not a part of conventional audio specifications. Multi-driver loudspeaker systems may have complex phase distortions, caused or corrected by crossovers, driver placement, and the phase behaviour of the specific driver.
Transient response