1. B. Erlandsson, H. Håkansson, A. Ivarsson, P. Nilsson, and B. Salén, Scand. Audiol. (to be published).
2. J. T. Broch,Acoustic Noise Measurements(Bruel and Kjaer, Copenhagen, 1973), pp. 43–44.
3. G. B. Clayton,Operational Amplifiers(Butterworth, London, 1971). pp. 59–64.
4. See Ref. 3, pp. 145–149.