the same extent that our ears do; it should have the correct dynamics. The proper measurement of noise therefore requires the use of a specified method, with defined measurement bandwidth and weighting curve, and rectifier dynamics. The two main methods defined by current standards are A-weighting and ITU-R 468 (formerly known as CCIR weighting). A-weighting A-weighting uses a weighting curve based on equal-loudness contours that describe our hearing sensitivity to pure tones, but it turns out that the assumption that such contours would be valid for noise components was wrong. While the A-weighting curve peaks by about 2 dB around 2 kHz, it turns out that our sensitivity to noise peaks by some 12.2 dB at 6 kHz. ITU-R 468 weighting When measurements started to be used in reviews of consumer equipment in the late 1960s it became apparent that they did not always correlate with what was heard. In particular, the introduction of Dolby B noise reduction on cassette recorders was found to make them sound a full 10.2 dB less noisy, yet they did not measure 10.2 dB better. Various new methods were then devised, including one which used a harsher weighting filter and a quasi-peak rectifier, defined as part of the German DIN2 45500 Hi-Fi standard. This standard, no longer in use, attempted to lay down minimum performance requirements in all areas for High Fidelity reproduction. 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 panel of listeners, using a wide variety of different types of noise. BBC Research Department Report EL-17 formed the basis of what became known as CCIR recommendation 468, which specified both a new weighting curve and a quasi-peak rectifier. This became the standard of choice for broadcasters worldwide, and it was also adopted by Dolby, for measurements on its noise-reduction systems which were rapidly becoming the standard in cinema sound, as well as in recording studios and the home. Though they represent what we truly hear, ITU-R 468 noise weighting gives figures that are typically some 112 dB worse than A-weighted, a fact that brought resistance from marketing departments reluctant to put worse specifications on their equipment than the public had been