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This paper describes the design, construction, and performance of a magnetic-recording drum time-delay system. The device, which has been developed for use in such acoustic studies as noise localization, reverberation analysis and others amenable to correlation techniques, utilizes conventional proportional recording. Two channels yield a relative time delay between their outputs which may be varied from minus 15 milliseconds to plus 190 milliseconds with an accuracy of Â±0.2 per cent using the instrument dial. Principal attention in the paper will be paid to the following: (1) Obtaining a uniform layer of magnetic material by spraying with a dispersion of iron oxide; (2) analyzing the effect of spacing between the magnetic material and the head upon the processes of recording and reproducing; (3) developing a mechanical driving system as free from flutter as possible.