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  #46  
Old 16-02-17, 12:35 PM
guydarryl guydarryl is offline
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Quote:
Originally Posted by Helen Bach View Post
The idea is predicated on the assumption that it is important to reduce the amount of unwanted vibrations to a minimum. In the case of a turntable, this means reduction of the vibrations in the mechanicals of the tt, and any aerial and/or seismic intrusion. Any plinth (worthy of the name) must be able to cope with all these vibrations, reducing them to a level where they are not a problem.

As energy is said to be unable to be created or destroyed, the only only course of action is to turn vibrational energy into heat energy, by dissipation, or as we have come to call it, damping. Damping in this context can be defined as the exponential decay of vibrational energy, and although there are several ways of damping, frictional damping seems to be used the most.

So we use frictional damping to reduce the energy of the vibrations as quickly as possible, because that gives us the best chance of ensuring a 'clean signal', one that helps us with the fidelity of reproduction.

It can be shown that higher damping factor (DF) values are of the most benefit, but in reality, a value of 0.4 is a good one to aim for, as above this figure, there are diminishing returns. However, most materials used for audio structures (which need damping) fall far short of this 0.4 DF value, and most are less that 0.1. As an example, the OP mentioned Panzerholz, which has a DF of 0.6 - 0.9. By stark comparison, Corian has a DF of 0.044.
This means that a structure made of Corian would not be able to cope with vibrations at all well, and would allow these vibrations to continue to be a problem for some time (slate takes over a second to dissipate a single pulse, with a DF of 0.017, same as mdf !) whereas Panzerholz would dissipate the energy very quickly, not allowing those amplitudes to build, which is what happens with low DF materials.

HTH
Refreshing to read a post that makes scientific sense to me. thanks
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  #47  
Old 16-02-17, 12:50 PM
JemHayward JemHayward is offline
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If we want a structure to transmit vibrations, rather than dissipate them as heat, would we want a high damping factor, or a low damping factor, or, is damping factor just irrelevant in this context?
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  #48  
Old 16-02-17, 12:53 PM
guydarryl guydarryl is offline
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I would have thought that if you want transmission then keep it simple and no damping factor!
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  #49  
Old 16-02-17, 01:19 PM
Helen Bach Helen Bach is offline
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what do you mean by 'transmit vibrations'? from where to where, and why?

No damping factor? Every solid material has a damping factor.
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  #50  
Old 16-02-17, 02:20 PM
guydarryl guydarryl is offline
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I guess that Jem was asking from the point of view that rather than trying to "damp" vibrations within a structure (such as a turntable plinth), would it be more sensible to try and transmit/conduct/carry the vibrations to an alternative location - "earth"?

My (tongue in cheek) response is in that case you would want no "damping factor" in the material.

Often in the case of trying to find a solution to a problem it is best to try and take the theoretical solution to an extreme.
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  #51  
Old 16-02-17, 05:01 PM
Helen Bach Helen Bach is offline
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I think, then, I would ask what the 'mechanical earth' was.

Vibrations (in the structures we are discussing, at audio frequencies) can be thought of as tiny up and down movements of a panel (often called a plate in physics). These displacements cover virtually all of the panel, depending on the resonance frequencies of the modes. Internal damping of the panel material will convert the vibrations into heat by friction (although other damping will probably be present). The greater the damping, the shorter the time taken for the vibrations to diminish.

If the panel is connected to a mechanical earth, depending on the relative masses, either the earth will vibrate, or the vibrations will not be 'earthed', and the panel continues to vibrate. If the vibrations are transferred to the 'earth', then that must have damping properties, or it will continue to vibrate, and transfer these vibrations back to the panel. I can't see that this is a good thing. Either way, the panel will still vibrate a lot.

For a simple approach (all for that) just choose materials with very good intrinsic damping, and please, no-one mention damping sucks the life out of their music.
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  #52  
Old 17-02-17, 07:17 AM
nicetone nicetone is offline
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Helen - great reading in your posts here and on Suffolk Tony's thread. Much food for thought. Thanks, and more please!
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  #53  
Old 17-02-17, 07:46 AM
Helen Bach Helen Bach is offline
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thank you, kind sir.
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  #54  
Old 17-02-17, 08:12 AM
JemHayward JemHayward is offline
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Quote:
Originally Posted by Helen Bach View Post
what do you mean by 'transmit vibrations'? from where to where, and why?

No damping factor? Every solid material has a damping factor.
I was thinking about the mechanical job of connecting two things together so they move in unison, e.g. parts of car suspension, and maybe even a subchassis of a turntable (if we think that is desirable) - or at a smaller scale, from the stylus to the coils in a cartridge. Is there an advantage / disadvantage to high/low DF in these sort of structures.
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  #55  
Old 17-02-17, 08:37 AM
Helen Bach Helen Bach is offline
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as I understand it, a car's suspension is built to be as stiff as possible, with with some riggle room in the form of bushes. The springs give the compliance, whereas the so-called 'shock absorbers' are the damping. There is no earth, unless it is floating!

In the pickup cartridge, one must aim to conduct the vibrations of the stylus to the other end of the cantilever as accurately as possible, but damp it (significantly) with an elastomer. Damping values around 0.2 to 0.6 I have measured.

I think the essence of these designs (and many others, including loudspeakers) is that damping is used to obtain the response we desire. It seems almost essential in most situations.
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  #56  
Old 17-02-17, 08:53 AM
JemHayward JemHayward is offline
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Quote:
Originally Posted by Helen Bach View Post
as I understand it, a car's suspension is built to be as stiff as possible, with with some riggle room in the form of bushes. The springs give the compliance, whereas the so-called 'shock absorbers' are the damping. There is no earth, unless it is floating!

In the pickup cartridge, one must aim to conduct the vibrations of the stylus to the other end of the cantilever as accurately as possible, but damp it (significantly) with an elastomer. Damping values around 0.2 to 0.6 I have measured.

I think the essence of these designs (and many others, including loudspeakers) is that damping is used to obtain the response we desire. It seems almost essential in most situations.
But is damping desirable in the elements that we want to be stiff, e.g. the cantilever, or the suspension arm itself?
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  #57  
Old 17-02-17, 09:02 AM
Julf Julf is offline
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Quote:
Originally Posted by Helen Bach View Post
as I understand it, a car's suspension is built to be as stiff as possible, with with some riggle room in the form of bushes. The springs give the compliance, whereas the so-called 'shock absorbers' are the damping.
That is true for a modern car, but you clearly haven't driven in a Morgan...
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  #58  
Old 17-02-17, 11:01 AM
PigletsDad PigletsDad is offline
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Quote:
Originally Posted by JemHayward View Post
But is damping desirable in the elements that we want to be stiff, e.g. the cantilever, or the suspension arm itself?
Yes. At a resonant frequency, the effective stiffness falls to roughly the static stiffness divided by the Q of the resonance. Damping, which reduces the Q, maintains the stiffness across frequency.
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  #59  
Old 17-02-17, 02:25 PM
Helen Bach Helen Bach is offline
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Quote:
Originally Posted by Julf View Post
That is true for a modern car, but you clearly haven't driven in a Morgan...
no, but I have driven a Triumph (passed my test (first time) in one!) which also has leaf springs, which is what I think you are alluding to, but both had/have shocks?
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  #60  
Old 17-02-17, 03:20 PM
ChrisPa ChrisPa is offline
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There are 2 other very important systems in a car's suspension - tyres, and upholstery

The tyres are probably at least as significant as the suspension. They are springs and damping all in one. They have different lateral and vertical behaviour. There are different behaviours from the sidewall/air system and from the tread/road surface system
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