Spider DSA User’s Manual
360
◼ Mass, M
◼ Spring, K
◼ Damper, C
The natural frequency, F
n
, and the critical damping factor, ξ , characterize a SDOF
system, where:
For a light damping ratio where is less than or equal to 0.05, the peak value of
the frequency response occurs in the immediate vicinity of f
n
and is given by the
following equation, where Qis the quality factor:
Any transient waveform can be presented as an SRS, but the relationship is not
unique; many different transient waveforms can produce the same SRS. The SRS
does not contain all of the information about the transient waveform from which
it was created because it only tracks the peak instantaneous accelerations.
Different damping ratios produce different SRS’s for the same shock waveform.
Zero damping will produce a maximum response while high damping will
produce a flatter SRS. The damping ratio is related to the "quality factor", Q,
which can also be thought of as transmissibility in the case of sinusoidal vibration.
A damping ratio of 5% (ξ=0.05) results in a Q of 10. An SRS plot is incomplete if it
doesn't specify the document the damping factor (or Q).
Frequency Spacing of SRS Bins
An SRS consists of multiple bins distributed evenly in the logarithmic frequency
scale. The frequency distribution can be defined by two numbers: a reference
frequency and the desired fractional octave spacing, such as 1/1, 1/3 or 1/6. (An
octave is a doubling of frequency.) For example, frequencies of 250 Hz and 500
Hz are one octave apart, as are frequencies of 1 kHz and 2 kHz.
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