1,163,193. Measuring noise levels. NORWAY, CENTRAL INSTITUTE FOR INDUSTRIAL RESEARCH. 20 Jan., 1967 [22 Jan., 1966], No. 3071/67. Heading H4D. In an apparatus for automatically indicating noise level in desired units, e.g. Phons, Loudness Index or PNdB (perceived noise), noise signals from a microphone are applied to a bank of octave filters 2 having contiguous pass bands, the R.M.S. values of the filter output voltages are determined by circuits 3 and modified by function generators 4 producing outputs which are not-linear functions of their inputs, the function generator outputs are multiplied by a fixed fraction (e.g. 0.3) in units 6, the largest signal from the function generators is selected by unit 5 and multiplied at 7 by a second fraction which is unity minus the first fraction, and the outputs from units 6 and 7 are passed to a summing amplifier 8 whose output may be connected to a recorder 10 through a further non-linear function generator 9. An amplitude compressor may be included in each R.M.S. value determining circuit 3, so that the apparatus can cover a 50 dB dynamic range.