What is absolute and relative damping?

[506818]

Hi,

I was wondering what absolute and relative damping mean? I am looking into vibration testing methods - impact hammer testing can measure the absolute damping while shaker testing measures relative damping. Can anyone help with their definitions and possible application?

 

[GregLocock]

Since the results of a shaker test and an impact test should be identical that seems a curious distinction.

Cheers

Greg Locock


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[506818]

I have been told that a limitation of shaker testing is that it can only measure relative damping and not absolute. Just wondering if anyone had come across this previously.

 

[electricpete]

I haven't heard the terms relative and absolute damping.
Do you have a definition for them?


=====================================
(2B)+(2B)' ?

 

[electricpete]

Do you have a definition for them?

Never mind... just re-read the thread title ;-)

=====================================
(2B)+(2B)' ?

 

[IRstuff]

I think that's a distinction without a basis.

A hammer test is absolute in the sense that the object is "fixed" at some point, or surface, and that "fixed" point experiences no disturbances, while the struck point experiences the hammer. A shaker table disturbs both points, and everything is relative to the control (formerly fixed) point. BUT, because the vibrating point is normalized to the control point, the system response is "absolute," since the results are as if the control point were "fixed"

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[GregLocock]

In a properly performed modal test the system is unaffected by everything downstream of the force gauge. So long as the force gauge measures all the force applied to the system, the shaker can be freely suspended, floating on a mgnetic field, bolted to the ground, stuck on the face of a hammer or any other arrangement. The devil is in the details, the forece gauge can only measure the axial force, therefore the shaker must be arranged so as to provide an axial force only, typically using a stinger.

Cheers

Greg Locock


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[hacksaw]

this is mostly for the responders...

relative damping is defined as a percentage of critical damping...with critical damping implicitly defined as that damping parameter that results in a critically damped response.

Yes, it all assumes a linear response regime.

Absolute damping is a coefficient that is related to a force that opposes motion. It can have various units depending on how you define it and can be dimensionless or dimensioned.

Relative damping is most commonly referred to as it is more easily identified and used.

 

[GregLocock]

OK, thanks for that. The distinction made in the OP is still wrong.

Cheers

Greg Locock


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