I need a little help with springs

[pure sound]

One of my decks is an SP10 mounted in a 60mm thick piece of slate. That stands on another similarly thick piece of slate. All of that weighs around 65 Kg. It stands on 4 springs which compress from about 5 inches to about 2.5" The natural bounce frequecy of this arrangement is around 2 Hz. It all stands on a Sound Org type table on a wooden floor. You can jump up and down next to it with no effect on the stylus in the groove. I'd say it was earthquake proof. The only issue was that I had to coat the springs themselves to suppress higher frequecy resonance. It works very well. I think the biggest deal in a Garrard is to deal with the vibration from the motor first and prevent it from reaching where the arm mounts. Then concern yourself with external vibration.

If you know the weight to be supported and the compression desired when loaded most spring suppliers give the information needed to choose the appropriate items. Compression by half seems a decent rule of thumb.

[YNWaN]

Are those valve springs you are using Guy?

[walpurgis]

Anybody planning to use four springs to support a TT has some experimentation to do.

Assuming a spring is to be used at each corner, unless you add mass here and there to balance the complete TT base & motor assembly, it just won't sit level. The other way to keep things level is to move the spring positions around, ensuring the heaviest area has the most support.

Springs need damping otherwise they just keep bouncing and wobbling, an easy way to achieve this is to use neoprene 'O' rings in tension alongside the springs, the bottom end attached to a hook at the bottom board and the top end similarly attached by hook to the underside of the plinth board, Neoprene has high internal damping and will help keep the springs under control.

[sonddek]

I sit my Garrards on (double) yellow dot squash balls. Springs resonate, whereas yellow dot squash balls are critically damped. In fact, I doubt the ability of hefty metal springs to isolate from higher audible frequencies. Sound travels well in a metal bar, even round bends.

At the mongers you can buy, for pennies, black rubber plumbing seals about the diameter of a squash ball. I sit the squash balls in these so that they don't roll. Tuning a squash ball suspension is simply a question of deciding on the right number of balls. It is an extremely effective method of isolation and I can knock the support surface quite hard when the stylus is sitting on a stationary record without hearing anything come through the speakers, even at high volumes. Even elastic bands would be better than springs IMO.

The deck can be levelled by moving the squash balls around under it. Periodically I turn the balls so that their longest dimension is vertical and long term deformation is reduced.

[YNWaN]

But squash balls aren't critically damped , they are over damped. The resonant frequency will not be a few hertz it will be higher hundreds.

[pure sound]

No, bigger diameter than any valve springs I've seen.

Im not really that enamoured of rubber, squash balls or sorbothane. I prefer a more effective higher Q isolation. 65 Kg decoupled at an Fs of 2 Hz takes alot of exciting. More than this ever encounters in typical use.

[YNWaN]

I agree.

[Wakefield Turntables]

Certainly lots to think about, I'm just chasing a few ideas at the moment. My valve system is undergoing some upgrades and I thought I'd address the TT whilst the amps and speakers are having some work done.

[sonddek]

But squash balls aren't critically damped , they are over damped. The resonant frequency will not be a few hertz it will be higher hundreds.

If I displace the deck I would estimate that it wobbles at around 3 - 5 Hz, and the movement is only visible for about one cycle. That is the primary resonant frequency of the suspension system. As regards the higher frequencies, it would certainly be interesting to record cartridge output while the DAC plays a frequency sweep. My guess is that chunky metal springs might do a surprisingly good job of transmitting higher frequency sound into the plinth. Real springs are far from ideal. Only experimentation will give real answers, but my squash ball set up kicks the feedback problem into the long grass in practical terms, because as I said, I get SFA signal when exciting the support.

[YNWaN]

Does it move in a pistonic motion at 5 cycles per second? Wobbling is not the same thing at all (it may provide lateral isolation at 5Hz but not vertically. You may guess regarding springs but I don't have to and your guess is incorrect.

You can achieve low frequency isolation by mass loading racket balls but these have a significantly greater ratio of trapped air to rubber and are much more compliant.