Cartridge Loading and Compliance Laws

Lewn,
To use your truck/sports car analogy however, a truck would be less likely to be knocked off its straight path than a sports car hitting the same bump. I dont know that the analogy works because the tonearm is not moving, and therefore has no momentum inertia of its own, only its fixed inertia as an impediment to motion. But since I dont want the tonearm to move relative to the stylus, why wouldnt that be better. Dont we want to keep the headshell/cartridge/stylus relationship fixed except for those movement in the stylus that correspond to the vinyl groove. Why would we want the tonearm to move? And if it did at the same rate as the stylus, which of course it can't, wouldnt that result in no sound at all. Isnt it the movement of the stylus and coil assembly relative to a fixed magnet what produces the sound. And if that fixed magnet moved the same as the stylus/coil, no sound would be reproduced.

Picture the stylus like a wheel on a car. The stylus has micromotion as it tracks the groove on a record and that motion is absorbed by the suspension. Just like the wheel on a car moves over small bumps in the road but the car remains fixed. In both cases in regards to the micromotion and inertia of the stylus and the wheel, they are very small compared to the mass of the tonearm or mass of the vehicle. So that micromotion causes little or no motion in their relatively massive counterparts. But when the stylus moves over a warp in the record, for example, now the entire tonearm must move in response to that warp. Consider that a macro-motion. In this case, if the tonearm has too much interia, the tonearm raises up to ride over the warp but takes too long to come back down resulting in a skip. Similar sitation in a car- a dip in the road or bump in the road causes the car to move up/down, but if the car has too much inertia, then it leaves the road surface. In both cases, a car or a tonearm, the spring rate and the effective mass affect how they respond to those macro-motions.

Manitunc, You wrote, "To use your truck/sports car analogy however, a truck would be less likely to be knocked off its straight path than a sports car hitting the same bump. I dont know that the analogy works because the tonearm is not moving, and therefore has no momentum inertia of its own, only its fixed inertia as an impediment to motion."

I don't really see what you are getting at. The tonearm most certainly does move, in the lateral plane it has to move in order for the stylus to trace the groove, in the vertical plane we don't want it to move (up and down), but it will to a degree that is directly dependent upon the compliance of the cartridge (the springiness of the cantilever) and the mass of the whole ensemble of the tonearm/cartridge. Obviously, the less vertical motion of the tonearm wand, the better.

As an aside, "momentum" and "moment of inertia" are two different things. Don't know what "momentum inertia" is. Finally, for both a truck and a sports car, the correct shock absorber damping will result in the least reaction to a "bump". (I don't really like my own sports car/truck analogy so much, either.)

Tony,
I accept your analogy with respect to record warps. I was more referring to the stylus/arm relationship in the normal condition of tracing a flat record groove.

Lewn,
What I meant is that the tonearm is not moving in relation to itself, while a car or truck is moving, thereby creating what I referred to as momentum inertia. You know, an object in motion tends to stay in motion. Knocking a moving vehicle off its line requires a force that increases with the speed of the vehicle. You dont have that issue with a tonearm, which is relatively fixed as compared to theh speed of the record groove. Sure, it has to travel across the record, and up and down over warps, but not at anything close to the speed of the wiggles in the groove, which at the outer groove is traveling around 1.74 ft per second. A 20 minute record requires the arm to move horizontally about .001375 ft/sec assuming a 4" playing surface. Hardly a meaningful comparison. That makes me believe that holding the cartridge steady and just allowing the stylus to move is the more accurate method. Again, I am not addressing warps. I do see, however, that once the arm does move, a heavier arm will tend to overshoot and be slower to react and return to the neutral position.

Doesnt the Townshend fluid damper trough at the headshell end essentially create a condition that the stylus would see as a more massive tonearm. I can tell you from experience that the Townshend system works very well, and cleans up the bass tremendously as compared to the same cartridge/tonearm without the damping trough.

Dear Manitunc, My point is that you cannot make the general statement that a heavy tonearm is "better" than a light one, or vice versa, because there are other factors involved and for sure there are conditions within which either proposition is the correct one. And the model of a cartridge held static in space is a bad one to start from, I still say.

"An object in motion tends to stay in motion". Yes, that is inertia.
"Momentum" is a quantity applicable to a body in motion. Momentum is equal to mass X velocity. Inertia also says that a body at rest will tend to stay at rest. A body at rest has no momentum.