widman said:
In structural mechanics it is common to represent the Youngs modulus as having a small imaginary part (i.e. it is complex with real and imaginary parts). In the equations the imaginary part turns out to be energy disipating - the real part is not, its just a spring. This is called structural damping. The approach is not precisely correct, but far more accurate than just assuming no damping. The main point here is that there is nothing to tie the damping part (imaginary) to the spring part (real) of Young's Modulus. They are completely independent.
Its not backwards, it depends on the variables used. The modulus is defined as real and it defines the stiffness. Stiffness does not disipate energy. Hence it has to be the imaginary part of the modulus that dissipates energy. The electronics situation is the odd one since it uses voltage and current as the conjugate variables. If charge and current (first dirivative) are used, analogous to displacement and velocity (first dirivative), then it would be the imaginary part that is dissipative.
I guess I'm still trying to understand damping in the real world.
What does it mean when a material has good damping? If you have a sound wave propagating in a speaker cabinet, I would expect the wave to be dampened more effectively if the speed of sound in the cabinet material is much less than the speed of sound in air. Isn't that why some high-end systems are lined with lead?
Are you saying the energy dissipation isn't related to the slowing of the sound wave in the cabinet material?
pete
What does it mean when a material has good damping? If you have a sound wave propagating in a speaker cabinet, I would expect the wave to be dampened more effectively if the speed of sound in the cabinet material is much less than the speed of sound in air. Isn't that why some high-end systems are lined with lead?
Are you saying the energy dissipation isn't related to the slowing of the sound wave in the cabinet material?
pete
widman said:
I'm no expert on the subject of material damping, but my engineering background tells me that damping is a coefficient that describes how quickly the amplitude of sound waves is lowered, or reduction of amplitude as a function of time. I'm pretty sure it doesn't have anything to do with reduction of sound velocity.
runethechamp said:
wikipedia agrees with you:
damping
Damping is any effect, either deliberately engendered or inherent to a system, that tends to reduce the amplitude of oscillations of an oscillatory system.
Read this, very informative, link and see that Dr. Geddes was (mostly) right. However, stiffness does equal the Young's modulus.
widman said:
In the real world, when you deflect structure A and structure B (of the same proportions but different material) the same amount and release it, the better damped material will return to the static condition more quickly.
If the less well damped material is underdamped, it will vibrate for a while and return to static more slowly.
If the less well damped material is overdamped, it will slooowly return to the static position and also return to static more slowly!
Hence more damping is not always better. But in the context of loudspeaker cabinets it is always better, because you won't find overdamped speaker cabinets.
As pointed out above, E is not pertinent to considerations of damping.
gedlee said:
You were completely right in talking about damping. However, Young's modulus and stiffness are not the same. Just like conductance and conductivity are not the same.
Stiffness has units of m/N while modulus has units of pressure. Stiffness is a material parameter, and modulus is a material property.
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