Skeletal vs Plinth style turntables


I am pondering a new plinth design and am considering the virtues of making a skeletal or closed plinth design. The motor unit is direct drive. I know that as a direct drive it inherently has very low vibration as opposed to an idler deck (please do not outcry Garrard and Lenco onwners coz I have one of those too) but simple facts are facts belt drive motors spin at 250rpm, Lencos around 1500 rpm, DD 33 or 45 rpm. That being the case that must surely be a factor in this issue. What are your thoughts. BTW I like closed designs as they prevent the gathering of dust.
parrotbee
Halcro
I must be dreaming.......

let us know if you hear the song - I Got U BABE when u
wake up .....

Lewm

(call it "arm pod drag").

LOL - Good one Lewm. Learn something new every day.

Kind of sounds like a new dance to me ? Ok everybody
off your asses and onto the dance floor, Lets all do the
arm pod drag ....

You shuffle across the floor with the front leg leading
(closest to the TT)).... the back leg dragging...
Chris.

I re calculated the train thing.
Assuming the following
Make it a 100gm sticky snow ball traveling directly towards the train at 100 KPH, to make things easier.
Make the train, 1000 tonnes, also traveling at 100 KPH
The snow ball hits and sticks to the front of the train.

The train will slow to 99.99998 KPH, more or less. I accept that I may have an extra or too many 9's

Conservation of momentum.

You did ask!

Re the pivoted arm/speed question. It makes sense, since the arm is moving forward relative to the radial line for some of its travel across the record.

"The song, I got U BABE".
My dreams, shall we say, are usually more exciting in content.

Cheers.
Poem. By Henry Gibson

Was a man named Henry, poor fellow
His tonearm pods were made of jello
They slipped around like on ice
Watching them twirl was quite nice
But his music still sounded like it was supposed

Peace,
Regards, all: I'm quite impressed by the depth of experience reflected in the comments above. Except that Gibson fellow. He's a hack and prone to gross exaggeration.

Viridian suggests that the removal of the tonearm from the plinth tends to remove plinth based excitations from the tonearm.

Atmasphere proposes precise coupling of the platter bearings and tonearm base, LewM seems to agree.

Richardkrebs writes: "A skeletal design makes this goal more difficult to achieve since we are introducing the support structure, shelf, platform, into the equation. It effectively becomes the plinth. Related to this is the minimization of joints and material changes in the platter-arm loop."

Ct0517 (hi, Chris) shares this experience "An exercise of Function over Form for me. The tonearm attached to the POD is a leading example of this.
If for some reason one "hears" a mismatch in resonances, the ET2 (tonearm) provides resonances tuning capability."

Halcro: "I simply use my ears and listen to actual examples BEFORE I blindly theorise." Henry implements a tweaked wall mounted shelf and considers it a plinth. There has not yet been proof offered of unwanted movement in the spindle/pivot dimension but neither should one deny the possibility.

These things are known for certain:
The greater the mass, the lower the vibrational frequency. Beams (tonearms) with a lesser mass vibrate at a higher frequency.

Vibration takes place when a system oscillates under the action of disturbance. A system will vibrate at one or more of its natural frequencies as determined by its mass and stiffness distribution.

Friction, viscous damping and mass acting as a vibrational sink influence the decay and amplitude of vibration. Damping has very little effect on eigen frequency of a system but is of some importance in limiting the amplitude of resonance.

These resonances are influenced by the different boundary conditions of a tonearm.

Our concerns are complicated if a tonearm is viewed as a semi-fixed beam with a "lumped" mass (cartridge) at the end. Boundary (reflected) resonances are observed at edges, shoulders and at the junctions of differing materials. Boundary resonances have been measured at levels greater than the original disturbance.

We are now confronted by Young's modulus, the Eigen frequency of the system, the mass of the tonearm and the interaction of boundary resonances, all of which are capable of interacting and consequently generating additional resonances.

Voltaire wrote "Best is the enemy of better". As observed in the past, not all roads lead to Rome but many will. Neither is there a single avenue to the great city.

It would seem the first choice is that of tonearm, the remainder is a matter of tweaking for best outcome.

As Henry said, much more directly, "I simply use my ears and listen to actual examples BEFORE I blindly theorize."

Peace,