Frequency & Damping Tab

This tab contains Global and Local curve fitting methods for estimating modal frequency & damping.

Frequency & Damping Tab Showing Frequency & Damping Estimates for Six Modes.

Global Polynomial Method

The Global Polynomial method estimates one global value of frequency & damping for each mode.

Local Polynomial Method

The Local Polynomial method estimates one local value of frequency & damping for each mode and for each FRF.

Extra Numerator Polynomial Terms

The residual effects of out-of-band modes are compensated for by using additional numerator polynomial terms with the Polynomial method.

Both the Frequency & Damping and Residue curve fitting methods use the extra polynomial terms to account for the residual effects of out-of-band modes.
The number of extra numerator polynomial terms can be user-specified between 0 & 10.
Four (4) extra terms is usually sufficient for most cases.

Vertical Frequency Lines

Each frequency estimate in the modal parameters spreadsheet is displayed as a blue vertical line on the Mode Indicator graph.

Horizontal Damping Lines

Each modal damping estimate is displayed as a horizontal line crossing the vertical frequency line at the top of the Mode Indicator graph.

The length of the Horizontal Line is 2s. s is called the half power point damping (in Hz), the 3 dB point damping, or the damping decay constant.
2s is approximately equal to the width of the resonance peak at 70.7 % of the FRF peak magnitude, or half of the peak magnitude squared.
2s is the width at the half power point of a resonance peak.

Zoomed Display Showing Half Power Point Damping Lines.