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Tyvärr jobbar jag enormt mycket nu......det är ju GIK som ska ha dom (skäms lite).

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Den bygger ursprungligen på Anders/Bubbel idé.

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PostPosted: 26 Feb 2010 12:34 Post subject: Reply with quote
nope, mdf has no damping properties, another urban myth. The property that describes the damping property of a material is the damping factor, Greek letter eta (n). I have measured n for lots of materials. Anything above 0.07 is said to be damping, anything above 0.1 is a good damper. Here are the results of some of the materials I have measured:

mdf 0.0174
Russian 7 ply 0.040
chipboard 0.101
slate 0.0172

panzerholz 0.387

just for the record (sorry Very Happy ) here is a scan of an mdf panel, struck percussively, measured with an accelerometer, upper trace z axis (out of the panel) and x axis (along the panel)


© Cat's Squirrel 2010

The axis along the top is in seconds, you can see mdf rings for over half a second, when struck.

The reason that mdf is used as a platter material is that it is slightly better than rigid metal (Al, n = 0.0043) and the reason it is used for loudspeakers cabinets is because it is very cheap and can be machined, not for its sonic qualities, which we all have to suffer. Sad

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Joined: 16 Dec 2008
Posts: 77
Location: East London, UK

PostPosted: Sat Feb 27, 2010 10:33 pm Post subject: Reply with quote
I think there are a few misconceptions here. Resonances are vibrations, but not all vibrations are resonances.

The aim we have, as audiophiles, is to reproduce the original signal, from whatever medium, in as faithful way as possible. Hence high fidelity.

This means introducing as little distortion of the original waveform, as presented by the loudspeaker/headphones, distortion here means any change from the original waveform.

If the system uses analogue techniques, such as using a vinyl record as a storage medium for analogue waveforms, then it will be open to distortion by other analogue phenomena, including vibrations not associated with the original waveforms. Such vibrations are those associated with every part of a record deck.

For every vibration there could be an affect. If the vibration is a resonance, the affect will include an amplification. Many resonances will contribute to something similar to a transfer function, where an original waveform is transformed into another. The transformed waveform is that fed to the next stage in the chain.

If the resonances are large in amplitude, they will have a large affect on the original waveform; so reducing the vibrations of a record deck cannot be a bad thing. Allowing vibrations will produce a transfer function which will change the output waveform, which cannot be a good thing.



Nearly anything we make plinths from will vibrate, we as audiophiles (well, some of us) want to suppress these vibrations, and we use three main principles to do this, stiffness, mass and damping. For any plinth-shaped construction, its stiffness, mass and damping will all determine the transfer function.

For a material to be useful for plinth building it must be stiff (but not too stiff, as this reduces the critical frequency) have enough mass and have intrinsic damping. If it doesn't have enough damping, more can be added, but its efficacy will be dependant on other things.

Of the things I have tested, only a few things have suggested themselves as being suitable, including bamboo laminates, aluminium composites, panzerholz and filled polyester resins, the latter being the best.

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fromans4 wrote:


Please let me know if my thought process is fundimentally wrong. That being said, does anyone have any suggestions outside of the materials already suggested, which are clearly out of my budget and probably beyond my skill level to work with. I think I have decent wood working skills and most common carpentry tools available to me. I am not familiar with working with synthetic materials but I am not apposed to trying.


Again, the most efficient way to do this is using constrained layer construction using a visco-elastic damping core. Look at my graph in my last post above. About 20+dB of attenuation averaged, across the bandwidth, averaged. That is roughly 10x reduction in amplitude! You can effect the same range of vibration reduction, here is just one example: Glue together two layers of 3/4" MDF. Use a router and route in 4 or 5 cutouts, 13/16" wide, 15/16" deep, and as long as the depth of the table plynth, except a 1/2" shorter. Place a 3/4" square steel rod or squre tube in each cut out, using epoxy to secure it. (Clean the steel first with sandpaper and degreaser or the external oxidized layer will have a poor adhesion) Strategize/plan the location for the bars, to leave blank the areas where you will install main bering, arm base and motor(if mounted on plynth). Use Bondo or other high quality filler and fill in the slight indents left by the metal bars glued into their cutouts. You want to cover these over to be flat/flush; sanding down the filler. Now, repeat this and make another one of these 2 x 3/4" MDF pieces with the steel in it. Use 1/4" of Dynamat or Peel N' Seal in the middle of both of these assemblies you have made, cutting out clearance for parts, once again. Use a strong adhesive like red label DAP contact cement. Use filler to fill in the gap on sided that the visco-elastic layer left. Apply wood veneer of choice. You will now have a 3 1/4" thick plynth that has extraordinary low vibration amplitude response compared to if you just used wood, MDF or anthing else of this thickness as a mono structure. The two ultra-rigid structures, with a visco-elastic damping material bonded between them, cause the visco-elastic layer to move it's molecules against each other with maximum potential, in a shearing-type action, thus eliminating a large amount of motion energy by converting it to heat.

-Chris

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fromans4 wrote:
I have received more information then I ever thought I would and I will consider everything I have read in this thread in designing and building my plinth. I understand the need to keep resonance to a minimum and to deaden whatever resonance that can't be eliminated by design and materials by dampening, but with the cost of materials like panzerholz being out of my range, can other materials be recommended? I am well aware of the inherant resonance problems of most hard woods as I have some experience with speaker design. It seems to me there are several factors that come to play in the materials that I can use in the building of my plinth. 1. The cost of the material. 2. Can it be acquired. 3. How easy is it to work with. 4. It's inherent resonance properties. 5. It's mass. The final choice would have to be a balance of these qualities, unless someone knows of a substance or combination of materials that would satisfy all of these requirements.

Please let me know if my thought process is fundimentally wrong. That being said, does anyone have any suggestions outside of the materials already suggested, which are clearly out of my budget and probably beyond my skill level to work with. I think I have decent wood working skills and most common carpentry tools available to me. I am not familiar with working with synthetic materials but I am not apposed to trying.


I am sure you have it correct. It has been shown how important stiffness, mass and intrinsic damping is to building an effective structure. Your other contraints are real, and befalls us all.

If you like wood, and are confident, then I would stick with that. I recommend you look for wood with the properties discussed, use wood with intrinsic damping, why allow it to vibrate and then find a cure?

I would say chipboard (particle board) is about a minimum, as regards damping. If you can source bamboo, I would recommend it, about twice the damping factor of chipboard, 5 times better than ply, and over 10x better than mdf.

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Jag kan förstå att man provar material som är nya när man letar efter det optimala för hifi men jag tror samtidigt att man glömmer bort århundraden av erfarenhet på de olika material som finns där ute jag är nämligen övertygad att där finns något som är bättre än det övriga men glöms bort bara för att det kommer material i en tjusigare förpackning.

Jag läser väldigt mycket mellan raderna det finns för många självskrivna experter där ute på nätet som påstår att det skall vara på ett vis och oftast finns det gamla hifi påstående som ligger bakom utan att det finns en sanning bakom. Men sedan finns det en väldigt stor kunskap som många gärna delar med sig av det gäller bara att hitta den det är svårt bland allt snack.

anders

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Att tänka på är att man bör prata om materialets frekvenser och inte dämpegenskaper vaför det kan jag inte riktigt svara på men de senare känns inte rätt.

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Vilka krafter har jag.
Hur kan jag utnyttja dom för att få den önskade funktionen.