Vibrationdata 1 Power Spectral Density Function PSD Unit 11

Vibrationdata 2 PSD Introduction A Fourier transform by itself is a poor format for representing random vibration because the Fourier magnitude depends on the number of spectral lines, as shown in previous units The power spectral density function, which can be calculated from a Fourier transform, overcomes this limitation Note that the power spectral density function represents the magnitude, but it discards the phase angle The magnitude is typically represented as G 2 /Hz The G is actually GRMS

Vibrationdata 3 Sample PSD Test Specification

Vibrationdata 4 Calculate Final Breakpoint G^2/Hz Number of octaves between two frequencies Number of octaves from 350 to 2000 Hz = 2.51 The level change from 350 to 2000 Hz = -3 dB/oct x 2.51 oct = dB For G^2/Hz calculations: The final breakpoint is: (2000 Hz, G^2/Hz)

Vibrationdata 5 Vibrationdata > Miscellaneous > dB Calculations for log-log Plots > Separate Frequencies

Vibrationdata 6 Overall Level Calculation Note that the PSD specification is in log-log format. Divide the PSD into segments. The equation for each segment is The starting coordinate is (f 1, y 1 )

Vibrationdata 7 Overall Level Calculation (cont) The exponent n is a real number which represents the slope. The slope between two coordinates The area a 1 under segment 1 is

Vibrationdata 8 Overall Level Calculation (cont) There are two cases depending on the exponent n.

Vibrationdata 9 Overall Level Calculation (cont) Finally, substitute the individual area values in the summation formula. The overall level L is the “square-root-of-the-sum-of-the-squares.” where m is the total number of segments

Vibrationdata 10 dB Formulas dB difference between two levels If A & B are in units of G 2 /Hz, If C & D are in units of G or GRMS,

Vibrationdata 11 dB Formula Examples  Add 6 dB to a PSD The overall GRMS level doubles The G^2/Hz values quadruple  Subtract 6 dB from a PSD The overall GRMS level decreases by one-half The G^2/Hz values decrease by one-fourth

Vibrationdata 12 vibrationdata > Overall RMS Input File: navmat_spec.psd

Vibrationdata 13 dB Formula Examples  Add 6 dB to a PSD The overall GRMS level doubles The G^2/Hz values quadruple  Subtract 6 dB from a PSD The overall GRMS level decreases by one-half The G^2/Hz values decrease by one-fourth

Vibrationdata 14 PSD Calculation Method 3 The textbook double-sided power spectral density function X PSD (f) is The Fourier transform X(f)  has a dimension of [amplitude-time]  is double-sided

Vibrationdata 15 PSD Calculation Method 3, Alternate Let be the one-sided power spectral density function. The Fourier transform G(f)  has a dimension of [amplitude]  is one-sided ( must also convert from peak to rms by dividing by  2 ) Δf (f)*GG(f) 0Δf lim (f) PSD X   ˆ

Vibrationdata 16 Recall Sampling Formula The total period of the signal is T = N  t where N is number of samples in the time function and in the Fourier transform T is the record length of the time function  t is the time sample separation

Vibrationdata 17 More Sampling Formulas Consider a sine wave with a frequency such that one period is equal to the record length. This frequency is thus the smallest sine wave frequency which can be resolved. This frequency  f is the inverse of the record length.  f = 1/T This frequency is also the frequency increment for the Fourier transform. The  f value is fixed for Fourier transform calculations. A wider  f may be used for PSD calculations, however, by dividing the data into shorter segments

Vibrationdata 18 Statistical Degrees of Freedom The  f value is linked to the number of degrees of freedom The reliability of the power spectral density data is proportional to the degrees of freedom The greater the  f, the greater the reliability

Vibrationdata 19 Statistical Degrees of Freedom (Continued) The statistical degree of freedom parameter is defined follows: dof = 2BT where dof is the number of statistical degrees of freedom B is the bandwidth of an ideal rectangular filter Note that the bandwidth B equals  f, assuming an ideal rectangular filter The BT product is unity, which is equal to 2 statistical degrees of freedom from the definition in equation

Vibrationdata 20 Trade-offs Again, a given time history has 2 statistical degrees of freedom The breakthrough is that a given time history record can be subdivided into small records, each yielding 2 degrees of freedom The total degrees of freedom value is then equal to twice the number of individual records The penalty, however, is that the frequency resolution widens as the record is subdivided Narrow peaks could thus become smeared as the resolution is widened

Vibrationdata 21 Example: 4096 samples taken over 16 seconds, rectangular filter. Number of Records NR Number of Time Samples per Record Period of Each Record T i (sec) Frequency Resolution B i =1/T i (Hz) dof per Record =2B i T I Total dof

Vibrationdata 22 Summary Break time history into individual segment to increase degrees-of-freedom Apply Hanning Window to individual time segments to prevent leakage error But Hanning Window has trade-off of reducing degrees-of-freedom because it removes data Thus, overlap segments Nearly 90% of the degrees-of-freedom are recovered with a 50% overlap

Vibrationdata 23 Original Sequence Segments, Hanning Window, Non-overlapped

Vibrationdata 24 Original Sequence Segments, Hanning Window, 50% Overlap