A spectrum analyser for the range 100 c/s to 100,000 c/s

The specification is given of an analyser to measure the spectral distribution of the noise emanating from an air jet, and several methods of performing this analysis are discussed. The development of a practical instrument is described which uses a variable frequency selective amplifier with a Wien bridge as the frequency-dependent feedback network, and some details of its performance are given. Errors due to the short time available for analysis are derived and some of the less familiar aspects of the circuits used, such as a symmetrical-to-asymmetrical convertor, are analysed.