SAE AIR817-1967 PDF

Scope: "It is often desirable to have a ""narrow band analysis'' at a particular instant during a transient signal@ in order that a more complete picture may be drawn regarding the discrete tone content of the signal at that particular instant. For example@ this data is useful and sometimes necessary when analyzing the noise from an aircraft flyover in cases where the customary 1/3 octave analysis of the complete flyover has not yielded sufficiently detailed information. The standard method of ana.lyzing tape recorded signals of only a few seconds duration is tbe ""loop method. "" The magnetic tape is cut and spliced to form an endless loop@ and the loop is replayed with the aid of a tensioning device (see Fig. 1). Diff1culty arises with transient signals when the length of tape required to make a loop covers a time over which there is a considerable variation. The principle of the system described herein is that of expanding the time-base of the signal to a point at which there ls a length of tape sufficient to make a loop across which tbe signal is sensibly constant. Although the melbod is one which bas been used specifically for the analysis of aircraft flyover noise@ it could equally well apply to other transient signals"

SAE AIR817-1967 history