Audio noise measurement

Microphones, amplifiers and recording systems all add some electronic noise to the signals passing through them, generally described as hum, buzz or hiss.

Shielded cables help to prevent this, and on professional equipment where longer interconnections are common, balanced signal connections (most often with XLR or phone connectors) are usually employed.

Attempts to measure noise in audio equipment as RMS voltage, using a simple level meter or voltmeter, do not produce useful results; a special noise-measuring instrument is required.

Thirdly, the rectifier or detector that is used to convert the varying alternating noise signal into a steady positive representation of level should take time to respond fully to brief peaks to the same extent that our ears do; it should have the correct dynamics.

The introduction of FM radio, which also generates predominantly high-frequency hiss, also showed up the unsatisfactory nature of A-weighting, and the BBC Research Department undertook a research project to determine which of several weighting filter and rectifier characteristics gave results that were most in line with the judgment of a panel of listeners, using a wide variety of different types of noise.

Dolby tried to get around this by introducing a version of their own called CCIR-Dolby which incorporated a 62 dB shift into the result (and a cheaper average reading rectifier), but this only confused matters, and was very much disapproved of by the CCIR.

It is the only way to measure noise that allows fair comparisons; and yet the flawed A-weighting has made a comeback in the consumer field recently, for the simple reason that it gives the lower figures that are considered more impressive by marketing departments.

[neutrality is disputed][dubious – discuss] Audio equipment specifications tend to include the terms signal-to-noise ratio and dynamic range, both of which have multiple definitions, sometimes treated as synonyms.

In 1999, Steven Harris & Clif Sanchez Cirrus Logic published a white paper titled "Personal Computer Audio Quality Measurements" stating: Dynamic Range is the ratio of the full scale signal level to the RMS noise floor[when defined as?