Plinth Wood Choice

[losthorizon]

I notice that Rega has changed the construction of its plinths and they are much lighter with more resonance deadening properties, but the thought comes to mind that those of us that like to make our own plinths are spoilt for choice as to which material we should use, as wood seems to be a favourite, the question is what type of wood? it seems that solid construction is not ideal and layering is better at reducing unwanted resonance, but what about the properties of hardwood versus softwood, hardwood resonates very much more than softwood so surely a plinth constructed from layers of softwood should be what we are aiming at, maybe pine blocks glued with grain in differing directions and clamped then machine to the correct size would be a great idea, but I have never seen nor heard of such a plinth.
Can I have your thoughts please?

 

[rothwellaudio]

If you want light and stiff, spruce is usually the wood of choice. Spruce is used for musical instrument soundboards and was used in aircraft.
If you want dense and rigid, there lots to choose from, oak being an obvious choice.
Whether high mass or low mass is a good thing, well, that's a matter of debate. Personally, I'd choose high mass.

Btw, this guy has done a lot of research:
https://qualia.webs.com/plinthbuilding.htm
https://qualia.webs.com/plinths.htm
The engineering principles are sound, but they apply to a simple rectangular panel of one material. In practice, gluing things together and/or adding reinforcement struts etc. will change everything.

 

[losthorizon]

So according to the above theory, we need a stiff material or wood that is not conductive of resonance, and as a dense wood will resonate more than a less dense one, softwood should be ideal. as with cork, hence why cork is used as a sound deadening layer.

 

[doctor fuse]

I've been researching this topic as I prepare to make a plinth for a Lenco L75/S.

The current accepted best performance to cost design seems to be constrained layer damping, using regular plywood alternating with mdf.

Lots of discussions about this on audiogon (granite etc, if cost is no object) and lencoheaven.

 

[rothwellaudio]

The current accepted best performance to cost design seems to be constrained layer damping, using regular plywood alternating with mdf.

The pages linked-to above go into some detail about constrained layer damping and claim that layers of wood glued together do not constitute constrained layer damping because there needs to be a visco-elastic layer. However, if the glue is rubbery - such as silcone adhesive - the glue will act as a visco-elastic medium.
Anyway, there are thousands of opinions on various forums.

 

[rothwellaudio]

So according to the above theory, we need a stiff material or wood that is not conductive of resonance, and as a dense wood will resonate more than a less dense one, softwood should be ideal. as with cork, hence why cork is used as a sound deadening layer.

No, I don't think that's what it says at all.
Anyway, I'd take it as interesting background reading rather than "the one true way".

 

[losthorizon]

I've been researching this topic as I prepare to make a plinth for a Lenco L75/S.

The current accepted best performance to cost design seems to be constrained layer damping, using regular plywood alternating with mdf.

Lots of discussions about this on audiogon (granite etc, if cost is no object) and lencoheaven.

I can see where you are coming from with respect to the Lenco L75/s where the plinth is affectively a platform that the turntable sits in and is not part of the structure as it is with the Rega range and hence why the composition of the Rega plinth is so important to its sound with the motor, arm, and bearing all being part of the plinth.

 

[adnick]

There’s resonance charts for different wood species, and any Luthier could give you related advice….

However for plinths it’s fun to experiment, oak is an example of greatly differing characteristics within a single “family” of trees. One can pick up a piece of dried flat sawn red oak put one end in water blow on the other and blow bubbles

Given the characteristics of red oak some will say it’s not a good plinth choice, however I’d argue that one could incorporate a properly sawn red oak layer as a “barrier layer” within a layered plinth; taking advantage of those linear air pockets to transfer noise laterally. Moreover, if one likes the look of red oak it can certainly be used as what I call “show wood” or the outside of the plinth.

Spanish Cedar is also an interesting choice, have had really good results using it as arm board material, with measurable reduction in noise when directly compared to an aluminum equivalent.

Alternating grain layers in plywood is common practice in a layer plinth but I’ve found that introducing sound deadening between the layers a more effective practice. One can use a variety of materials for this purpose, cork, acoustic caulk, modeling clay, “Dynamat”, have even used expandable foam

Plinths can drastically change the audible presentation of a turntable, sometimes even in a good way



Have fun,
Andy

 

[losthorizon]

There’s resonance charts for different wood species, and any Luthier could give you related advice….

However for plinths it’s fun to experiment, oak is an example of greatly differing characteristics within a single “family” of trees. One can pick up a piece of dried flat sawn red oak put one end in water blow on the other and blow bubbles

Given the characteristics of red oak some will say it’s not a good plinth choice, however I’d argue that one could incorporate a properly sawn red oak layer as a “barrier layer” within a layered plinth; taking advantage of those linear air pockets to transfer noise laterally. Moreover, if one likes the look of red oak it can certainly be used as what I call “show wood” or the outside of the plinth.

Spanish Cedar is also an interesting choice, have had really good results using it as arm board material, with measurable reduction in noise when directly compared to an aluminum equivalent.

Alternating grain layers in plywood is common practice in a layer plinth but I’ve found that introducing sound deadening between the layers a more effective practice. One can use a variety of materials for this purpose, cork, acoustic caulk, modeling clay, “Dynamat”, have even used expandable foam

Plinths can drastically change the audible presentation of a turntable, sometimes even in a good way



Have fun,
Andy

Thanks, Andy, great talking to you.

 

[AvFan]

As @adnick notes solid lumber, even within the same species, can have drastically different properties making it tough to estimate performance. For that reason many plinth makers use manufactured materials such as plywood, mdf, acrylic, particle board, aluminum, etc for consistency and workability. Probably the gold standard of plinth materials is Panzerholz plywood with its exceptionally high damping factor. However, it is generally only available in Europe (maybe one North American supplier in Canada) and it is machined like metal given its resin content. Making it tougher to figure out plinth performance is the impact of the holes in the plinth and their shape. The overall shape of the plinth (e.g. square vs VPI's lack of a straight line) also has an impact on how standing waves propagate inside the plinth. Just to get an idea of how complex all this is take a look at this video of a Chladni plate.

There are couple of general approaches to address vibration in a non-suspended turntable plinth: 1) lots of mass and 2) engineered materials. These two methods regularly overlap but an Artisan Fidelity plinth is more like #1 and Clear Audio is more like #2. The site @rothwellaudio noted is good place to spend some time to get a feel for how different materials, including solid lumber, respond to vibration. Just note, that the higher the damping factor the better.

This can seem pretty overwhelming, so what can a DIYer do if they want to build a plinth? First, don't sweat that only an estimate of plinth performance can be predicted. Second, after doing some research choose the style, mass or engineered, that works for your DIY skills and given it a go. My only recommendation is to not use solid lumber as a core element of the design given its generally poor damping factors, variable properties (even in the same board!) and expansion potential that could impact the relationship between the platter and the tonearm.

 

[HPMguy]

There is a reason MDF is used for speaker cabinets over quality plywood other than cost -- MDF is "dead" in comparison. Just rap your knuckles on a piece of both and you will see what I mean.

Resonance and stiffness are related (obviously not in a linear fashion), so the stiffer a piece of wood is, the more resonant it is. Spruce is light and very stiff, so it's a perfect wood for sound boards like the top of a guitar where you WANT it to resonate and produce sound. Not a good choice where you want to damp resonance.

Mahogany is a good choice, as it's light and fairly stiff but not like oak. Sadly it's not available in wide pieces (yet, should be coming along in a decade or so from plantations in Nicaragua). Maple is probably another good choice because it's dense and not a resonant as oak (I think). Walnut is always a good choice for a variety of reasons, ease of manufacture is one, along with great color. Probably more resonant than maple, but I don't know that for sure.

Tulip Poplar sapwood is a good choice I think, because it's harder than pine and easier to work, but soft enough to be fairly non-resonant and it's available in knot and figure free wood. Heartwood is significantly harder and may not work as well (the green part). "whitewood", which is often sweet gum (Liquidamber sp) is also light and should be fairly non-resonant and is fine grained. Both will probably require veneer, but I don't see that as a drawback.

If you are serious about damping vibrations though, you should ether make a double box with a layer of cork glued between them, or make the plinth of your wood of choice laminated with cork in a couple layers. No promises, but it should be less resonant due to the sound absorbing nature of the cork.

However, in the long run I suspect if you use dense wood and isolate the rotating assembly on good rubber bushings it probably doesn't matter all that much. Isolating the whole table from speaker vibrations and isolating the motor from the plinth will do you much more good that messing about with the resonance characteristics of the plinth.

 

[adnick]

Thanks, Andy, great talking to you.

Enjoy talking about plinth building much more than actually doing it but I’ve built a lot of plinths over the years….hate working in plywood, it’s a personal thing


Edit: Here’s a documented, layered inside a box, inside a box approach, use it only for gleaning the positive results (-;

https://audiokarma.org/forums/index.php?threads/russco-studio-pro-plinth-build.966246/

 

[tnsilver]

Personally, I'd choose high mass

Me too. A turntable is not a guitar, a musical instrument or a resonance box and we generally don't want any plinth related characteristics added to the output. We want damping! We want environmental resonances and stray frequencies absorbed and dissipated - not augmenting the audio signal. You know what damps and dissipates resonances? Mass! "Dense" just means more material per the same unit of volume - so yeah, dense, massive hard wood would be my choice for a plinth too.

 

[rothwellaudio]

However, in the long run I suspect if you use dense wood and isolate the rotating assembly on good rubber bushings it probably doesn't matter all that much. Isolating the whole table from speaker vibrations and isolating the motor from the plinth will do you much more good that messing about with the resonance characteristics of the plinth.

I agree, except I prefer to see a proper suspension rather than just some rubbery stuff.

 

[krautmaster]

Here are a couple more ideas that can be bought in the US (unlike Panzerholz) to chew over:

Richlite
http://www.richlite.com/

PaperStone
http://www.paperstoneproducts.com/

 

[Selmerdave]

Do the resonant properties of woods like pine, maple etc play a different role when mixed with denser woods, CLD-style or does it just reduce damping? Ie: Baltic birch layers alternated with pine.

 

[adnick]

Do the resonant properties of woods like pine, maple etc play a different role when mixed with denser woods, CLD-style or does it just reduce damping? Ie: Baltic birch layers alternated with pine.

The answer is yes, but as @AvFan pointed out determining what wood, it’s characteristics and how to apply would require knowledge/skill and if I was doing it, a lot of luck

My approach is to build some type of model to test my premise, probably not for beginners, there’s lots of proven approaches/examples one can follow….

VR
Andy

 

[AvFan]

Do the resonant properties of woods like pine, maple etc play a different role when mixed with denser woods, CLD-style or does it just reduce damping? Ie: Baltic birch layers alternated with pine.

I think the whole idea is to maximize damping and discourage resonance. The only desirable vibration is the stylus in the groove and any others may adversely impact the sound. It is better to layer materials with significantly different damping factors that can create greater damping than the individual layers. For my SP-10 MKII plinth I layered 1/2" MDF - 1/8" Aluminum - 1/2" MDF held together with a very strong 3M double sided tape that acts as the viscoelastic layers in a CLD arrangement. The plinth is on Sorbothane hemispheres and then the TT is setting on another isolation shelf made with same MDF/AL/MDF sandwich that rests on Sorbothane pucks. This arrangement provides a way to convert vibration to heat in the viscoelastic layers and damp external vibrations before they even reach the plinth or those within the plinth due to the vibrations from the cartridge and tonearm.

There is an opportunity to use solid wood in plinths, much like tonearm tubes, and that is arm boards. I have experimented using Sitka Spruce that has an internal speed of sound along the grain that is very close to that of aluminum (approx 6000m/s). The interesting thing is Spruce emphasizes even order harmonics that are generally considered more pleasing than odd order harmonics. Compare the sound of a Taylor acoustic guitar with a Spruce top and an oboe (odd harmonics). The arm board grain is orientated vertically so tonearm vibrations quickly enter the CLD plinth.

 

[Selmerdave]

I think the whole idea is to maximize damping and discourage resonance. The only desirable vibration is the stylus in the groove and any others may adversely impact the sound. It is better to layer materials with significantly different damping factors that can create greater damping than the individual layers. For my SP-10 MKII plinth I layered 1/2" MDF - 1/8" Aluminum - 1/2" MDF held together with a very strong 3M double sided tape that acts as the viscoelastic layers in a CLD arrangement. The plinth is on Sorbothane hemispheres and then the TT is setting on another isolation shelf made with same MDF/AL/MDF sandwich that rests on Sorbothane pucks. This arrangement provides a way to convert vibration to heat in the viscoelastic layers and damp external vibrations before they even reach the plinth or those within the plinth due to the vibrations from the cartridge and tonearm.

There is an opportunity to use solid wood in plinths, much like tonearm tubes, and that is arm boards. I have experimented using Sitka Spruce that has an internal speed of sound along the grain that is very close to that of aluminum (approx 6000m/s). The interesting thing is Spruce emphasizes even order harmonics that are generally considered more pleasing than odd order harmonics. Compare the sound of a Taylor acoustic guitar with a Spruce top and an oboe (odd harmonics). The arm board grain is orientated vertically so tonearm vibrations quickly enter the CLD plinth.

Thanks, I guess a better way of phrasing my question would be, would the Birch/pine combination I suggested benefit from the mixed damping factors compared to, say, only Baltic birch which would have greater mass?

 

[AvFan]

Thanks, I guess a better way of phrasing my question would be, would the Birch/pine combination I suggested benefit from the mixed damping factors compared to, say, only Baltic birch?

Baltic birch (Russian 7 ply) has a damping factor of 0.040 while Pine is 0.064 and I can't imagine solid Birch is much different. Even solid oak has a higher damping factor of 0.102. There is not enough info on a birch/pine layering to know if those two would combine for a better damping factor. I wish I'd used particle board vs MDF with aluminum for my plinth because particle board has higher damping factor than MDF. I believe the theory is based upon refraction between dissimilar layers whose speed of sound is dramatically different. That is why the center of my layering is aluminum. Wood to wood, even two different woods, just doesn't have a big enough delta between their speeds of sound to have a big benefit IMO.

From a woodworking perspective layering solid wood is a recipe for problems due to wood movement from season to season. Plywood and other fabricated panels work best since their dimensions change very little with different temps and humidity. I was concerned about relative movement between the platter and tonearm due to the MDF changing dimension so I sealed those layers with varnish and painted them before assembly. I also have the SP-10 and tonearm board bolted directly to the center aluminum plate to minimize any relative movement.