Damping Platter With Liquid Latex

[Rap]

Energy coupling between different materials at a boundary is determined mostly by impedances of the materials, of course, especially the difference between them. In the case of say a lead/hard metal boundary, there should generally be quite a difference, on a par with metal/elastomer. However, damping is not just about reflection at boundaries, but absorbtion of energy through transmission loss a lossy material.

The "absorbtion" takes secondary importance after the impedance match. Energy that does not travel from the medium to be dampened into the dampening material is not dissipated in the dampening material if the acoustic impedance of the dampener is higher than that of air (which is the material dampening on the other side ) unless we are talking about dampening in a vacuum.

[dlaloum]

And
Ego is not a dirty word.....

[Rap]

Rap, the second moment of inertia (or area) concerned with bending, (and we are concerned with bending here) .

and you are overlooking how those "bending forces" travel from one medium to the other. Unless you know the energy transfer rate the measurements wont add up.

[Ldg]

Energy coupling between different materials at a boundary is determined mostly by impedances of the materials, of course, especially the difference between them. In the case of say a lead/hard metal boundary, there should generally be quite a difference, on a par with metal/elastomer. However, damping is not just about reflection at boundaries, but absorbtion of energy through transmission loss a lossy material.

The "absorbtion" takes secondary importance after the impedance match. Energy that does not travel from the medium to be dampened into the dampening material is not dissipated in the dampening material if the acoustic impedance of the dampener is higher than that of air (which is the material dampening on the other side ) unless we are talking about dampening in a vacuum.

Yes. Firstly one has to persuade vibration energy to cross the boundary into damping material, rather than reflect at the boundary. Secondly, the damping material has to be lossy and the vibration has to be conveyed by it. Those requirements can often be conflicted, because generally the impedance of a lossy material is significantly different from that of a hard metallic objective. All that can happen, if one gets it wrong, is the damping material acts as an inertia load. Changing, but not damping, vibration behaviour.

CS' 100mm square test did not take care to preserve moving mass, so it's pretty meaningless. Because it's a single frequency test, and the impedance of the whole object at that frequency is subject to change, and impedance is prone to change with moving mass in a complex way.

[cats squirrel]

Rap, I'm sure it doesn't. The impedance maths are usually done on systems using ultrasonic frequencies, and relies on them touching, but not coupled. For damping to work, the energy in the vibrating material has to be transferred to the damping material, and depends to a large extent on how they are connected together, they must bend together. This is the problem with vinyl discs (just sitting on mats).

If you consider a block of high density polyethylene resting on a block of Zircaloy, the maths shows a reflectance of 0.81, but I have just measured a value of 0.54 for the same materials, measured both ways with very good agreement between values. It would seem that at audio frequencies, the acoustic impedance, in this instance, is not the same as it is at ultrasonic ones, certainly the attenuation (per cm, eg) is not.

And if it were true that acoustic impedance is of primary importance, then why do things like damping sheets for cars work?

And a cork mat (with vanishingly small acoustic impedance) should stop all but 2% of the vibrations in the platter it is resting on from passing through.

[Rap]

Rap, I'm sure it doesn't. The impedance maths are usually done on systems using ultrasonic frequencies, and relies on them touching, but not coupled..

There is no difference in calculating ultrasonic or normal sonic. The formula is the same as they both deal with pressure waves.But what is the difference of coupling and touching? When is a material coupled to another and when is it touching? When is metal coupled to air and when is it just touching it? When is water touching air and when is it coupled?

Measuring with "particle board" does not really predict the dampening properties of plasticine with steel accurately and if using a single frequency to generate the signal then mass and self resonance frequency of the material becomes an issue. Impact is a better way as it generates all frequencies at the same time. But this is just nit picking on my part as I'm sure you can't be bothered with fast Fourier transform for the purpose of this experiment but the point I'm trying to make is that each material will have different dampening values depending on what other material it is supposed to dampen.

[cats squirrel]

Rap, all I can say is that the statement that it doesn't matter at which frequencies (ultrasonic or audible) is not borne out in practice, neither of my examples (damplast and cork) seem to observe the 'fact', and neither does the experiment I've just done on a Rega 250 tone arm. I vibrated the arm with the exciter at 600Hz (near to the arm resonance, well known fact!) and plonked the accelerometer on the tungsten counterweight, and measured the amplitude. I then measured the vibrations in just the stub, and a steel counterweight.

Now, the counterweights have two rubber o rings within them which stop most of the vibrations getting through. I calculate:

steel:rubber:tungsten should be 1% transmission

steel:rubber:steel should be 3% transmission

the results I obtained were Tungsten 45% transmission
and S/S 27% transmission.

Doesn't agree with 'theory'.

All my damping factor values are done by the impulse method and analysed by FFT.

[Rap]

And if it were true that acoustic impedance is of primary importance, then why do things like damping sheets for cars work?

Because the impedance match is better between metal and bitumen than metal and air.

And a cork mat (with vanishingly small acoustic impedance) should stop all but 2% of the vibrations in the platter it is resting on from passing through.

The amplitude is in relation to the mass! Cork has very little.

By the way I't large acoustical impedance as the lower the number the larger the impedance just like electrical impedance the less flow through the material the more impedance it has.

[Rap]

Now, the counterweights have two rubber o rings within them which stop most of the vibrations getting through. I calculate:

steel:rubber:tungsten should be 1% transmission

steel:rubber:steel should be 3% transmission

the results I obtained were Tungsten 45% transmission
and S/S 27% transmission.

Doesn't agree with 'theory'.

.

If you overload the mass of the system being measured you just measure it's mechanical frequency. One can't dampen an earthquake by standing on a carpet of bitumen

Were both counterweights of equal weight? Also for Rega 600hz where is the counter weight and what is the weight of the cartridge? Did you use the same and at what amplitude did you generate the signal?

[Ldg]

Doesn't agree with 'theory'.

That's because both your calculations and measurement methodology are seriously flawed, CS. Not to mention interpretation of the results ! There's a lot of incorrect physics in your posts, CS, and plain wrong interpretations. Rap has a good understanding and grip of the issues.

[cats squirrel]

so, Rap, by that reasoning, a metal layer would be even better than the bitumen, because it has less of an impedance mismatch, which is what we are talking about, not impedance per se. Doesn't work for me. And if frequency is not important, why is it called 'impedance', surely 'resistance' would suffice?

I believe it is possible to calculate the damping factor of a two (or more) layer system, comprising substrate and damping layer, if all the parameters are known. But it relies on coupling, which I see as 'moving as one'. Just placing one thing on another doesn't guarantee coupling.

I think you mean density, mass would just be proportional to the size! given a material.

But how do you know I overloaded the mass of the system?(whatever that means). Both counterweights were 100g, no cartridge (why is a cartridge needed?) All three readings were taken with the same set up at the same frequency, at the same amplitude signal from the sig gen.

Luckdog, would you care to explain where you think I have been going wrong? I have no issues with what Rap has said with regard to ultrasonic frequencies, but it doesn't stand up to close scrutiny at audio frequencies, as far as I can tell and from my experiences. Again, I will state again that we are concerned with bending waves, not transverse and longitudinal waves.

[Ldg]

Luckdog, would you care to explain where you think I have been going wrong? I have no issues with what Rap has said with regard to ultrasonic frequencies, but it doesn't stand up to close scrutiny at audio frequencies, as far as I can tell and from my experiences. Again, I will state again that we are concerned with bending waves, not transverse and longitudinal waves.

Rap is absolutely correct that the physics/maths involved is identical, whether audio or ultrasonic. Physics of vibration encompasses wave propagation of many and various types. Except the mexican wave They all apply, and are derived from the same basic physical principles and maths. Your physics continues to be way off the mark, CS.

Where you find discrepancies between your own results and established figures/theory on this topic, you can confidently bet it is your calculations, methods and interpretation that are incorrect, CS. It's a very complicated topic, and you've got a long way to go before getting a grip with your tests, calculations, and interpretations, CS. Bluntly.

I made my position about further help plain, CS. If you care to 'put right' your own position on commercial aims in this matter, that would remove what i still see as an obstacle. Until then, the best treatment I am aware of is "Vibrations and Waves in Physics" Iain G Main ISBN 0 521 27846 5 which covers much of what you are likely to need to correct a lot of what's wrong.

HTH !

[Ldg]

I suppose one acid question is whether cutting lathe platters are damped and inert ? I have no idea, BTW !

[Rap]

so, Rap, by that reasoning, a metal layer would be even better than the bitumen, because it has less of an impedance mismatch, which is what we are talking about, not impedance per se. Doesn't work for me. And if frequency is not important, why is it called 'impedance', surely 'resistance' would suffice?

In the instance of a metal then lead would work better and all the maths say so. It's not used in a car for weight reasons, apart from bitumen being cheaper and easier to bond and probably a million other things. And since when have cars been made from the best material available?

A wave is a wave and the maths for dealing with it is the same weather it is large or small, or do you change the way you add and subtract depending on the size of the numbers involved?

It is called "impedance" because it denotes both Resistance and Reactance and is the correct scientific term.

It behaves and is calculated the same way as "wave impedance" and I hope your not asking someone with your first language as his second language to explain the reasoning behind the choice of impedance over say resistance or even why it wasn't called flowerpot? Impedance is the scientific term for it so I see no problem with sticking to that instead of following one of your rhetorical red herrings.

I believe it is possible to calculate the damping factor of a two (or more) layer system, comprising substrate and damping layer, if all the parameters are known. But it relies on coupling, which I see as 'moving as one'. Just placing one thing on another doesn't guarantee coupling.

You are the one who started making distinctions about coupling and touching Bryan. I just pointed out that those are often the same thing and the distinction you made is not a scientific term so not much use in a formula.
[/quote]

I think you mean density, mass would just be proportional to the size! given a material.

No I think I mean mass.
A pound of feathers weighs the same as a pound of lead and the same force is needed to move them both. But ad to the mass and more force is needed to move it. Be that vibration or a push.

But how do you know I overloaded the mass of the system?(whatever that means). Both counterweights were 100g, no cartridge (why is a cartridge needed?) All three readings were taken with the same set up at the same frequency, at the same amplitude signal from the sig gen.

I know because your measurement data is erratic and doesn't correlate to the underlying physics of Acoustical impedance.

If you put more energy into the system than it can dampen it starts to oscillate. The system behaves in the same way as if it is underdamped for it's mass.
As someone on wiki puts it so elegantly:
" Underdamped (0 < ζ < 1): The system oscillates (at reduced frequency compared to the undamped case) with the amplitude gradually decreasing to zero."



So change in resonance frequency with it's harmonic dips and peaks will affect the readings depending on time of when the reading is taken apart from any thing else.

Now I suggest you revise and refute the principals behind this in stead of reverting to rhetorical word twisting about cars and cork and what not. Bring some science to the table instead of hunches and do look up the words impedance, dampening, under-dampening and Resistance and Reactance they are not long words and I really shouldn't have to explain to a native English speaker why they are used in the context that they are, I humbly suggest you take it up with your Queen rather than me.