Transmissibility - Mechanical "Q"

[kaiserman]

In the automotive durability lab I work in, random vibration is used for durability/life testing and sinusoidal vibration is used for dynamic investigations - resonant frequency and transmissibility(mechanical Q).

My questions invove sinusoidal vibration.

At resonance, it is my understanding that the transmissibility (mechanical Q) is the ratio of (output acceleration to input acceleration).
Q1:How does this transmissibility at resonance (mechanical Q) relate to product stiffness and/or dampening? Any equations?

As in any lab, A to B comparisons are often made in an effort to help product engineering make decisions. Often it is not clear as to which compared scenario is better.
Q2:When comparisons are made, which is a better scenario - the sample with a higher resonant frequency or the sample with the lower mechanical Q? For example, would it be better to:
(1) Increase the resonant frequency of sample B by lets say 20% to that of sample A. -or-
(2) Maintain the same resonant frequency for both samples A & B but reduce the transmissibilti at resonance (mechanical Q) of sample B by lets say 20% to that of sample A.

Thanks!
Kaiserman

 

[IRstuff]

Your question has no general answer, since what is better is dictated by design requirements. As an example, passenger comfort requires lower natural frequency in the suspension system. This is what used to make Cadillacs ride like a cloud. It sucks for handling and other drivability requirements.

Conversely, a high-performance sports car would require a much stiffer suspension.

TTFN

 

[tomirvine]

Q = 1 / ( 2 * damping ratio )


Q is not directly related to stiffness.

Q is equal to the ( output / input ) at the system's natural frequency, assuming single-degree-of-freedom behavior.

Tom Irvine

http://www.vibrationdata.com