Spider DSA User’s Manual
187
The frequency spacing of the resonance frequencies is logarithmic, much like the
1/3 octave filters used in acoustical analysis. That is, it is a type of proportional
bandwidth analysis where the half-power bandwidth of each SDOF system
increases in proportion to its resonance frequency. The resolution of an SRS is
defined by the number of simulated SDOFs included in the desired analysis span.
The percent damping of all the SDOFs is selectable (although most tests specify
5% damping).
The extreme motion of each mathematically simulated SDOF mass is monitored
by several peak detectors. The extreme positive and negative accelerations are
retained during the duration of the input pulse and after it. Maximum and
minimum values captured during the pulse’s duration are termed Primary
extremes. Those found after the pulse has returned to zero are termed Residual
extremes. Specific tests will prescribe whether positive, negative or extreme
absolute values captured should be displayed. They will further specify Primary,
Residual or combined (maxi-max) data be plotted.
Multi-Resolution Spectrum
Many characteristics of mechanical systems are better described logarithmically
in the frequency domain. Uniform frequency resolution that FFT offers is not
ideal because the resolution that is enough in the high-frequency range may not
be enough in the low frequency.
EDM provides the multi-resolution feature that applies the selected resolution in
the high-frequency range and 8 times of the resolution in the low-frequency range.
The cutoff frequency, which divides the low and high-frequency range, is
calculated by the software. A few adjacent frequencies can also be selected by the
user to avoid system resonance or anti-resonance.
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