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The level of an audio signal would seem to be the most basic of measurements, and yet widespread misunderstanding and disagreement about programme levels has become arguably the biggest single obstacle to high quality sound reproduction. Live sound covers an enormous range of levels, but this is not something that can be demonstrated with a conventional are recommended). The Sound Level Meter is also useless for properly assessing Noise levels, since the commonly used A-weighting is based on equal-loudness contours for pure tones, and is not valid for the random noise. The Subjective Loudness of noise is best measured using a noise-meter to the ITU-R 468 Noise Weighting standard. The chart below shows, on this basis, the real range of live music, and then the level capabilities of various stages in the audio chain, from Microphone to Loudspeaker . ANALYSING PROGRAMME LEVELS The above chart is based on the assumption that what goes in should come out; true high-fidelity, and so an ‘Alignment Level’ corresponding to 100dB SPL has been assumed throughout. Any lower level would imply severe clipping at the first stage; the master recording. With this assumption, it can be seen that top quality microphones do not present a problem; most will handle 130dB SPL without severe distortion, and some manage 140dB SPL. The master recording process, using current 24-bit techniques, can be seen to offer around 99dB of ‘true dynamic range’ (based on the 468 noise measurement); which is identical to the dynamic range of a good studio microphone, though it should be noted that very few recordings will use just one microphone, and so the noise on most recordings is likely to be the sum of several microphones after mixing, and probably at least 6dB worse than shown. Allowing 24dB of headroom, the highest peaks will be clipped, indicating the desirability of using a soft-limiter at the microphone input to attenuate the very highest levels. There is little point trying to record the highest levels faithfully, since they will not be reproducible on any known currently available loudspeaker, even if they were passed through the rest of the chain! A modern 24-bit master recording aligned to 100dB SPL is therefore capable of handling amost anything, without any need for the recording level to be adjusted to suit the programme level, although some increase in level for quiet material might make the recording more robust (less dependant on the lowest noise levels being consistently maintained on playback). After this, things get more difficult, with compact disc or other 16-bit formats requiring some degree of compression of the highest levels, to fit within the 18dB of headroom. This turns out to be irrelevant though, because a typical ‘hi-end’ loudspeaker is not capable of reproducing anything above 105dB SPL. Even very expensive professional monitors only manage 110 or 115dB SPL, and then only at 3m listening distance! Attempting to play a master recording through any known loudspeakers at original recorded SPL is therefore doomed to result in clipping of peaks and considerable distortion! SEE ALSO
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