So what about the plinth?

[gusten]

Note that I meant to quote Gusten's post regarding rumble being due primarily to motor hum:

Are those tests taken under actual usage conditions?

Namely, are there speakers nearby being played at high SPL's?

Is there any foot traffic over the flooring?

Or are the tests merely being performed on a turntable that is effectively playing in a vacuum?

If so, then motor hum and platter bearing noise would be the only significant sources of rumble.

I´m guessing that in the conditions You mention the outcome would be unpredictable. Because of the vast variety of designs.
gusten

 

[SaSi]

Interesting information being accumulated.

So, I understand the physics behind high mass. Given a specific force (F) is applied to the plinth from the outside, then it will create a vibration to the plinth (m*a) and since F=m*a, the higher the mass of the plinth, the less the acceleration will be. And the less the acceleration is, the less the vibration amplitude will be. Correct?

Now, let's expand this into a more complicated system. Let's consider the two-piece plinth design of a suspended turntable. There we have a plinth (enclosure) that rests on the shelf and interfaces with the external environment. Then we have a suspended sub-chasis where the motor, platter and tonearm rest on. The excursion of this springed suspension can be large (like the Dual TTs where the excursion is in the order of cm) or very little (like a Technics SL-1300MkII) where the excursion is in the order of a few mm.

We do have two separate masses (m1 and m2) and a springed suspension between them. Considering that m1 <> m2, I am assuming that we have two different resonance frequencies there. And we do have the springed suspension interfacing them.

What laws of physics are in play there in such a system? The first law (F=m*a) obviously still applies, but since we have a complex system there has to be more to it now. Perhaps difference in resonance frequencies? How would the system behave if a given low frequency trigger would hit the base plinth? What would be transmitted via the springs to the subchasis?

 

[gusten]

As I see it it will only be one resonance depending on the upper mass and spring coeff, also damping plays a part. If the frequency applied to the plinth is coinciding with this res. freq. there will be a damped res. If the freq. is higher it will be filtered out by the springs. If the freq. is very low, the two masses will move simultaneously.
gusten

 

[DougMac]

And lastly, I don't have a problem with un-informed opinions. It's just when they're accompanied by closed-mindedness that I lose my patience...

Thanks for your very informative response.

Sorry I didn't respond sooner, but I've actually been in the shop making a new plinth for my Technics SP-15 out of acrylic! I chose it primarily for its aesthetic value, I'd seen one here at AK that I thought looked great. It will be used mostly to transcribe 78's.

I guess I'm lucky. My main turntable listening area is in my office in the daylight basement, sitting on an 8" poured slab. The turntables are at the opposite end of a 24' room. Just to be safe, when I'm transcribing, I usually use headphones or turn the volume down very low. Footfalls and acoustic feedback aren't much of a problem.

I think this could become a sticky. It's nice to have someone in the field generously share his expertise. AK at its best.

 

[tromba]

I've been out of town and am just now catching up with my reading on this thread. I have a pretty good understanding of resonance as it relates to my trumpet -- vibrating air columns on specific frequencies, etc. but not always the scientific background to use all the words in the correct manner. Relating what I know from trumpet into plinths is a new thing altogether.

What this thread has been covering is great.

 

[totem]

Great sticky potential!
Vibration theory mixed with material characteristics equals real world results.

One aspect I am unclear about is how the physical separation of the tonearm via its own pillar/column changes things versus the statement

Ideally, any plinth should rigidly locate both the arm and the platter bearing

 

[Muddywaters]

Interested observer...

may have some plinth building/modding coming up

WJG is there a simple rule of thumb regarding mass loading in the sense if one has a plinth of a specific material, say birch ply, doubling mass produces X attenuation? Is the effect of mass linear; ie doubling mass over and over produces a similar attenuation ratio? (not sure if I've phrased that appropriately) Perhaps it is material dependent? Some materials could possibly produce more attenuation than others and I realize the intrinsic damping and resonant characteristics of a material would be influential.

Any sources to review that aren't too technical? Are there simple ways for us to quantify changes/improvements to plinths we build/mod?

 

[waterjetguy]

Great sticky potential!
Vibration theory mixed with material characteristics equals real world results.

One aspect I am unclear about is how the physical separation of the tonearm via its own pillar/column changes things versus the statement

I was considering the tonearm as a unit, but of course it's actually 2 sub-units: the pillar and the arm. Essentially, I've assumed that the tonearm maker has done their job well, and that the arm is free to pivot up/down and side/side, but is rigidly fastened in every other direction. Obviously, any slop in the tonearm bearings (either direction) will cause tracking problems. Mechanical information (which then becomes electrical signal, then audible music) will be lost due to the arm's ability to be shaken back and forth by the stylus' movement. Since the arm is much heavier than the stylus/cantilever/magnet assy, it won't move much, but the information that we're trying to extract is so tiny that some of it will be obscured by that movement. Loss of detail, in other words. That's why the cart needs to be fastened rigidly to the headshell, which must fit tightly to the tonearm, whose bearings must have no slop.

 

[Bigerik]

I was considering the tonearm as a unit, but of course it's actually 2 sub-units: the pillar and the arm. Essentially, I've assumed that the tonearm maker has done their job well, and that the arm is free to pivot up/down and side/side, but is rigidly fastened in every other direction. Obviously, any slop in the tonearm bearings (either direction) will cause tracking problems. Mechanical information (which then becomes electrical signal, then audible music) will be lost due to the arm's ability to be shaken back and forth by the stylus' movement. Since the arm is much heavier than the stylus/cantilever/magnet assy, it won't move much, but the information that we're trying to extract is so tiny that some of it will be obscured by that movement. Loss of detail, in other words. That's why the cart needs to be fastened rigidly to the headshell, which must fit tightly to the tonearm, whose bearings must have no slop.

I'm guessing a lot of the move towards fixed head shells is due to what you are discussing. If I understand it correctly, there are few worse places to put a joint then just behind the cartridge.

 

[SaSi]

I'm guessing a lot of the move towards fixed head shells is due to what you are discussing. If I understand it correctly, there are few worse places to put a joint then just behind the cartridge.

Actually, if there's truth behind the brochures descriptions, some low mass tonearms discarded the SME mount on the tonearm to reduce weight away from the pivot point as it increases effective mass and also to increase rigidity.

Some makes have a replaceable wand that mounts halfway through it's length (see Thorens designs) or close to the pivot point (Technics EPA500 IIRC).

 

[Bigerik]

Actually, if there's truth behind the brochures descriptions, some low mass tonearms discarded the SME mount on the tonearm to reduce weight away from the pivot point as it increases effective mass and also to increase rigidity.

Some makes have a replaceable wand that mounts halfway through it's length (see Thorens designs) or close to the pivot point (Technics EPA500 IIRC).

VPI still does replaceable wands too, IIRC. Always seemed to me to make more sense then having the headshell pop off. There is, comparatively, a lot of weight in the headshell/cartridge area.

 

[totem]

With respect to having the tonearm mounted apart from the physical plinth
wouldn't the isolation from the drive/motor only reduce any unwanted vibration etc?

I would think if going the independent mounting route, maintaining the precise distance
with no movement is then the prime concern.

What if any other arm and motor interactions come into play when considering this option other than the additional mount for the outboard arm pillar and its sonic characteristics.

 

[waterjetguy]

With respect to having the tonearm mounted apart from the physical plinth
wouldn't the isolation from the drive/motor only reduce any unwanted vibration etc?

I would think if going the independent mounting route, maintaining the precise distance
with no movement is then the prime concern.

What if any other arm and motor interactions come into play when considering this option other than the additional mount for the outboard arm pillar and its sonic characteristics.

It makes sense to have the two critical components of the system, the tonearm and the platter mounted on some sort of sub-chassis. You can isolate them pretty effectively from both external and motor generated vibration that way. Then the platter bearing is the only remaining possible source of vibration. (Assuming that the tonearm bearings aren't so awful that they're noisy).

So, what does platter bearing noise do to the situation? If the bearing creates vibration, then we know by Newton's Third Law, (for every force, there is an equal and opposite force) , that is transmitted to both the platter and the bearing support structure. Since the platter is being vibrated directly by the bearing we can't isolate the platter from it, so using sort of damping is our only real chance to keep that from reaching the stylus. A 2 -piece platter which has damping material between the parts could help, as could a rubber mat.

Since there is likely SOME vibration being generated by the bearing, mounting the tonearm on a separate, isolated platform could be a good idea. And yes, it needs to be rigidly and accurately aligned to the platter so that the cartridge alignment stays constant. That means that can't use an isolation technique that has much compliance. That is, it can't allow much movement. The small rubber grommets on the mounting screws of the old SME arms are a good example of that type of isolation. You could also use a layer of rubber bonded to the underside of the armboard, but keep in mind that is the type of system that usually requires a lot of development to get tuned to the point that it becomes effective.

A simple way to check for rumble transmitted through the arm board/tonearm pillar might be to rest the stylus on a stable support that is OFF the 'table while the platter is spinning, and monitor the output on an oscilloscope. Way cheaper than an instrument grade accelerometer and the supporting electronics. If the tonearm swing is not limited, that could be pretty simple to arrange. Then you could check the effect of your tonearm board mods without the influence of platter-transmitted vibration.

Regarding the motor vs arm pillar interaction, isolating the motor from the plinth in the first place would be way to go. Massive motor rubber mounts works well. Then, isolate the platter/tonearm from the plinth, AKA a suspended turntable , like an Empire 598 or Ariston RD40. And then add some isolation for the arm board if platter bearing noise is an issue. And, voila, you've got the internally generated turntable vibration under control. To a degree...

It's worth pointing out at this point that is is nearly impossible to COMPLETELY eliminate vibration once it is present. Passive techniques, (compliant mountings, damping) can only reduce vibration, not eliminate it. But, you can achieve very significant reductions, on the order of -120 dB in the best cases.
With that in mind, minimizing the amount of vibration generated in the first place is the best strategy. So a really quiet motor and platter bearing would be the first priority, if you have any control over the design.

 

[Andyman]

Sounds like a LOT of the design principles embodied in my AR....

 

[tromba]

I was having a PM with waterjetguy, and thought a response would be best in the forum....

From a PM:

An object's resonant frequency (resonance) is that frequency at which it will vibrate if struck (like a tuning fork). It will tend to vibrate at that frequency if excited by an external force, but it won't ring on its own. The stiffer the structure, the higher the resonant frequency will be, and the heavier it is the lower it will be.

That brought to mind one of the heaviest "musical" objects I've ever dealt with. VERY heavy but high frequency - I've used a piece of railroad track (probably12-16 inches) that was very heavy but produced a fairly high frequency when struck - similar to an anvil. Its stiffness was more significant that it's mass?

So as impractical as it is, a steel plinth that's very heavy wouldn't be useful because it can resonate fairly easily. Thus it's not just mass and weight but material as well. (since this is an email and not the thread, it's hard to go back and see if this is covered.)

Another conclusion I see is that since I know I can resonate objects from tones played on my trumpet, that if a plinth does have any easily resonated frequencies, turntables should be kept away from speakers. (I've recorded records to my computer and upon close listening can hear me practicing in the background - the cartridge functioning as a microphone)

You can design a structure to tailor its resonant , frequency (also known as natural frequency) to suit. That is a very tricky bit of engineering for all but the most simple structures, and requires finite element analysis (FEA) to accomplish. (Which can take many hours to run a single simulation, even on a very fast computer).

Hope I've clarified more than confused....

Absolutely.....

 

[totem]

Not to take the plinth and its interactions to far off track, we have touched
on how various materials react differently to vibration and how mass
or lack of will affect dampening.

In addition, the physical separation of the tonearm from the plinth is
another option to consider in the big picture.

Stepping back ever further, is taking the motor/drive and removing
the plinth completely, leaving a sort of skeletal structure.

The plinth less thing has been done for some time now, but in the
context of discussion what affect might be expected, assuming measurements and testing were done?

Just throwing things up in the air and seeing where they land.

 

[waterjetguy]

I was having a PM with waterjetguy, and thought a response would be best in the forum....

From a PM:
That brought to mind one of the heaviest "musical" objects I've ever dealt with. VERY heavy but high frequency - I've used a piece of railroad track (probably12-16 inches) that was very heavy but produced a fairly high frequency when struck - similar to an anvil. Its stiffness was more significant that it's mass?

So as impractical as it is, a steel plinth that's very heavy wouldn't be useful because it can resonate fairly easily. Thus it's not just mass and weight but material as well. (since this is an email and not the thread, it's hard to go back and see if this is covered.)

Another conclusion I see is that since I know I can resonate objects from tones played on my trumpet, that if a plinth does have any easily resonated frequencies, turntables should be kept away from speakers. (I've recorded records to my computer and upon close listening can hear me practicing in the background - the cartridge functioning as a microphone)



Absolutely.....

in the case of the railroad tie, yes the stiffness is dominating the mass.

A solid steel plinth might work just fine, since its natural frequency would likely be very high. That means that it would only be excited into resonance by structure-borne vibration at or near that frequency. It would be fairly easy to isolate it so as to filter that narrow band of vibration. (I don't see airborne vibration as a problem, due to the mass of the steel plinth. It would take a hell of a lot of SPL to drive it.)

 

[waterjetguy]

Sounds like a LOT of the design principles embodied in my AR....

Exactly. Once you get a handle on these concepts, you can easily see how various TT designers have gone about controlling vibration and providing stability. And you can also spot less useful, gimmicky stuff as well.

In design engineering, we're always looking for the "elegant solution", the one that gets the most done in the simplest way. Once you know what needs to be controlled in a TT design, you can close in on that elegance, because you won't waste design effort on things that don't matter. (If you don't know anything, then you tend to assume that EVERYTHING is critical, just to be safe)

 

[marc mc]

Exactly. Once you get a handle on these concepts, you can easily see how various TT designers have gone about controlling vibration and providing stability. And you can also spot less useful, gimmicky stuff as well.

In design engineering, we're always looking for the "elegant solution", the one that gets the most done in the simplest way. Once you know what needs to be controlled in a TT design, you can close in on that elegance, because you won't waste design effort on things that don't matter. (If you don't know anything, then you tend to assume that EVERYTHING is critical, just to be safe)

Others have already mentioned it but......

Massive plinths and massive torque IMO is the simple elegant solution. I just finished my second reference L75 because my first one made my well designed, sub and non sub chassied belt drives unlistenable on my other main system.

So much science and tech has gone into TT designs and sadly a large portion went into chasing the wrong rabbit. The Swiss simply said "let's make it like a watch that keeps perfect time ". We plunk it into 80lbs plinths and it's game over. I have listened to the monster tables since day one and these things give me absolute shivers.:thmbsp:

Marc mc

 

[theophile]

If no-one else has posted this link, I will:

http://qualia.webs.com/apps/forums/show/4460677-materials