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In Reply to: Re: Glass platter trial posted by HenryH on December 17, 2005 at 03:20:38:
I don't think it would hurt the Lp's in the least bit to use a glass platter directly under them. Glass can be kept extremely clean unlike other surfaces and is super smooth so there is really nothing to harm the grooves as the surface will be supported all the way across in the most level fashion. As far as sound goes, all bets are off, that would depend on the table being used, the cartridge being used, and the system they are used in. It would not be my first choice, instinctively. Using a mat as you say would soften up the sound some, but perhaps there is too much acrylic deadening going on with his table already and the glass improves transient response.
-Bill
Follow Ups:
Acrylic deadening??? Acrylic focusing maybe. I've not found acrylic to deaden sound at all. I hear more detail and a sharper focus with acrylic platters/mats. Glass platters ring. Acrylic platters do not or at least do to a far lesser degree.
Henry
Not at all what I hear. Acrylic can be a decent platform but most tables that use it to any extent sound dull to me. I have heard some exceptions from Avid (not all acrylic, but a constrained layer) and Basis.
Glass by itself can ring, but it is easily buffered by the use of various mats. Aluminum and steel are by far the hardest materials to keep from ringing. Many companies use lead or rubber inserts and add-ons to combat this. Glass is much more ideal in that it has the mass desired and is obviously very flat and is inexpensive to machine as you only need to do the edges. The stiffness of glass is simply top class and it transmits sound very quickly. That can either be used to the designers advantage or of course it could work against it. It is never as difficult to tame as metal as it transmits energy so quickly that it will not store it but for the shortest instants. Much of the ringing is eliminated also by simply using a thicker billet. Acrylic is always dead no matter it's thickness as it is simply not hard enough to transmit the energy effectively and so many listeners hear this blurring or softening effect. Some like it and some don't. It can be a good isolation material and help create a nice, black background, so it's absorbtive properties work both ways. It is best used on heavier tables.
-Bill
I understand the coupling aspect of acrylic but it always seems to diminish PRAT vs other platter materials.Henry may have a really good match, and system matching is critical in all aspects of vinyl reproduction.
The best platter I have heard Bill was a sandwich of Acrylic lead and aluminum. I wish I could remember the table. It sounded mighty fine.
It's my understanding that some of the HW-19s (MK IVs?) and TNTs used a platter of acrylic, lead and stainless steel.
Henry
The Avid Diva uses glass, MDF, and stainless steel. I thought it was acrylic at first but it is glass. It also has a brass bearing housing so a lot of materials there.
-Bill
Yes, but acrylic is much closer, resonant property-wise, to vinyl and therefore is more effective in helping to drain away stylus induced resonance in the LP, particularly if the LP is clamped to the acrylic platter/mat. I find just the opposite in regards to smearing, i.e., acrylic is far better than glass/metal platters in this regard. For instance, when I switched the glass platter on my Planar 3 to an acrylic platter, a veil was lifted from the music, particularly the upper midrange/lower treble region.
Henry
I don't think being of a similar material has anything to do with it, but I will gladly stand corrected if someone can back-up the claim with compelling evidence. From I understand and from what the engineers of both the high performance table manufacturers that we represent understand, glass is superior as it is harder, not more similar. The resonant frequency determines how fast the energy is relesed or how little energy is stored, I believe or perhaps it is the other way around. Either way, cause or effect aside, the glass is definately better suited for that job as it has both the higher frequency and lower storage. That doesn't mean that you have to prefer its sound. That is just the physics of of from what I have gathered. Vinyl and acrylic are both lossy. In an extreme example, rubber would be even more lossy and I don't think anyone would want a platter made of a soft rubber material. Look on the bright side of it; only one of us is wrong! ;-)
-Bill
Sorry to rain on your parade but you are both wrong.Similarity of material is important because the tranmission of vibration beteen two different materials is governed by their acoustic impedance which in ordinary conditions approximates to the product of the density and the propagation velocity. The coupling coefficient is 1 - ((Z1-Z2)/(Z1+Z2)) 2 . Z vor vinyl is about 3500, for acrylic is about 2700 and for glass is about 14000. The coupling between acrylic and vinyl is about 98% whereas between glass and vinyl it's about 64%. Put another way, 2% of the energy is reflected back from one interface while 36% is reflected from the other, so acrylic is about 20 times better than glass in this respect.
Unfortunately this just relocates the transmission problem to where the acrylic interfaces with some other material (such as the bearing) where the difference is likely to be even greater. Acrylic / stainless has a coupling coefficient of about 20% so 80 percent of the energy is reflected back into the platter where it will then re-couple with the vinyl.
No free lunch.
It applies only to those that use those materials directly and most users will have a mat between surfaces. In the acrylic example, it is often the primary surface so that would be a good analysis there and is exactly what I described as "blurring" as the energy is stored and then released. The designers that I am speaking of don't want that energy hanging around.
As far as the OP's question goes you may be exactly right as I believe that he was speaking of placing an Lp directly on glass, but he can clear that point up. As far as what I am saying, I don't believe that you understood me. I was speaking of only a glass platter and a felt mat combination Vs a direct acrylic coupling as Henry was trying to illustrate the difference between a stock Rega and one with an aftermarket acrylic platter. What my centention is is that the felt offers the ability to both dampen or change the impedance of the coupling, plus the glass then dissipates the energy very quickly into the light chassis which also dissipates it. Although I only touched on the mass and rigidity of the plinth coming into play, Rega also advises that it works in reverse to isolate the Lp as vibration that manages to get into the plinth through the feet will also dissipate more rapidly. I think that some interaction both ways is unavoidable and so the question becomes how we treat it. A heavier plinth based on sheer mass and viscous dampening will hold more of the energy longer when coupled to a heavy acrylic platter and it may just then reduce the level of energy so not as much is left to return regardless of the time smearing. That would indicate two entirely different design approaches, however adding an acrylic platter to the light table is a hybrid and I understand it to be a folly of principle. Now that doesn't mean that everyone will dislike the results as i hear both sides to that. most favor the glass/felt combination from my count, but some like softer materials for some reason (I hear it as softening the sound). Maybe that better explains my understanding of the matter. You seem like a man of all talents (physics major maybe) so I'll accept your ideas as readily as those of the designers in terms of theory and of foundation, while also being aware that the designers have tried several approaches before settling on their specific choices and so have empirical evidence in terms of actual listening trials and perhaps vibration analysis.
-Bill
Mark,
But then there is a time factor, plus the mass or thickness of the acrylic platter. I guess I'm wondering how much resonance an acrylic platter can absorb or hold and the time factor before it bounces back into the vinyl. I'd much rather have the resonance going into a 1-2" thick acrylic platter than staying in the vinyl raising merry Ned with stylus/cantilever.
Henry
The models assume elastic materials on both sides of the interface. In the real world there is some loss, and the standing waves (what you call resonance) increase until the loss balances the input. Expanding the model to include this viscous loss makes the whole thing much more complex.Basically viscous loss is velocity dependent and, for a given displacement, velocity is proportional to frequency so the viscous loss increases with frequency. That's all very well and good but this frequency dependent lossiness also reduces the ability of the material to transmit vibration at the higher frequencies therefore the platter is acting as a non-linear filter. This partialy explains the common observation that application of lots of damping material makes for mushy and indistinct sound.
Mark, with trying to model the interactions of materials being "much more complex" is platter and mat research mostly trial and error testing of various materials and thicknesses? Not necessarily a bad approach; some of mankinds (and certainly vinyl addicts!) most significant discoveries have been found that way.And, like solid state vs tubes vs hybrids, where there probably isn't a "best" solution since people have different tastes in sound, are there generalizations about the type of sound you get depending on the platter and mat (and even cartridge) combinations?
Henry, I'm still getting things figured out with the whole system so haven't tried platter/mat combinations yet. But I'm paying attention to your discussions (and research!) on this topic.
Hmmm. That explains some things. I'm using an Iron Audio acrylic platter mat atop the acrylic platter on my MMF-7. The mat is about 2-3 mm thick with what Iron Audio calls "acoustic coupling compound" on the underside. It feels kinda like Sorbothane, but is only tacky, not sticky, to the touch. It is only about 1 mm thick.
Henry
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