Accession Number : ADP006602

Title :   Accurate Frequency Analysis of Measured Time-Dependent Signals Over Short Intervals,

Corporate Author : ARMY ENGINEER WATERWAYS EXPERIMENT STATION VICKSBURG MS ENVIRONMENTAL LAB

Personal Author(s) : Olson, Reo ; Cress, Daniel H.

Report Date : MAR 1992

Pagination or Media Count : 13

Abstract : The resolution of the frequency utilizing the Fourier Transform on a time-dependent signal is generally considered to be restricted to the inverse of the length of the time interval. This suggests that a one second long signal will permit a frequency resolution of 1 Hz. A much more precise determination of the frequency is possible by a careful review of the phase when the source has an unknown narrow-band frequency. For purposes of the analysis presented herein, narrow-band frequency implies a bandwidth less than the normally interpreted frequency resolution. The phase of the cross-spectral density of successive time intervals indicates the difference between the Fast Fourier Transform (FFT) frequency resolution and the frequency of the input signal. Since this calculation is a trivial step after the calculation of the FFT, this method can be easily implemented on real-time systems using existing hardware for the FFT. This method has been applied to the acoustic data obtained from a helicopter. The improved analysis of the Doppler shift of the frequency for the moving aircraft permitted a good estimate of the velocity of the approaching helicopter and its range at the closest point of approach using one microphone on the ground.

Descriptors :   *ACOUSTIC SIGNALS, *FREQUENCY DOMAIN, ACOUSTIC DATA, ACOUSTICS, AIRCRAFT, APPROACH, BANDWIDTH, DENSITY, DETERMINATION, ESTIMATES, FREQUENCY, HELICOPTERS, INPUT, INTERVALS, LENGTH, MICROPHONES, PHASE, REAL TIME, RESOLUTION, SIGNALS, TIME, TIME INTERVALS, VELOCITY, FOURIER TRANSFORMATION, TIME DEPENDENCE.

Subject Categories : Acoustics

Distribution Statement : APPROVED FOR PUBLIC RELEASE