How to measure tonearm effective mass

For some reason the first of my two posts hasn't turned up - I've had no end of problems with this forum lately. It referred to a post on another forum, maybe that's part of it.

Anyway, in it I posted a link to an improvement of my method by "markse", which involves using a rod of known inertia balanced at its midpoint in the place of the weights I used to calibrate the system. One end of the rod is attached to the speaker cone and the shift in resonance is plotted the normal way. This removes the inaccuracies generated by pushing the speaker's suspension off centre.

It does complicate the calculation somewhat, but a shorthand method is available - if the rod is uniform and thin in porportion to its weight, the "equivalent mass" seen by the speaker is 1/12th of its actual mass. Thus if you wanted a claibration range of say 10 to 20 g equivalent mass, choose rods around 120g and 240 g. Convert the frequency to a period (eg by inversion) and the result is a linear correlation of period shift to equivalent mass.

Mark Kelly

Dgarretson,

where you say "This set-up separates observations related to changing effective mass from observations related to changes in wand composition or length.", there is a further complication.

One of the things affected by wand composition and length is the presence and amplitude of arm resonances. If you happen to clamp the sliding weight at a vibrational antinode you will substantially alter the resonant character of the arm tube. This shows that there is some interaction between the two sets of parameters.

I've been playing with an arm with deliberately split vertical and horizontal effective masses (an idea I stole from the Dynavector DV50X series) and am investigating making the vertical effective mass adjustable. My first method was a simple sliding weight on the arm tube and I came across the resonance interaction I described.

Next up is a method of tuning the counterweight(s) so that the VTF and effective mass are separately tuneable. This gives me a problem if a very light cartridge is used, reducing the distance between the centre of mass of the counterweight and the pivot and reducing the possible adjustment range for effective mass, so I may have to combine the two.

Mark Kelly

Correction to above post:

Where I said "convert the frequency to a period (eg by inversion) and the result is a linear correlation of period shift to equivalnet mass"

I should have said "convert the frequency to a period and square the result. The result is a linear correlation of this quantity and equivalent mass."

My apologies for the error.

Mark Kelly

Lewm,

any arm that has split counter weights can have its effective mass adjusted. Triplanar, Talea, etc.

Mark, My tonearm has the front and rear counterweights riding on separate threaded rods going into the pivot and independent of the arm tube. Thus adjustment of counterweights does not affect the secondary resonance characteristic associated with wand composition or length. Also, its linear design allows separate manipulation of horizontal and vertical mass. Thus far I've found that the vertical mass is the more critical adjustment. Up to two-thirds of the assembly's total moving mass resides in the counterweights-- allowing a wide enough range of adjustment to characterize the tonearm as "universal."

Dan_ed, Indeed a split rear counterweight allows some adjustment. The question is whether the range of adjustment is sufficiently wide for a particular cartridge. In addition, with such an approach it is not possible to separately adjust horizontal and vertical mass; the adjustment of one affects the other.

I would add that with any long pivot arm, the rear counterweight(s) is in the final analysis the slave of the wand/cartridge lever. With a linear wand it is possible to unpack and play with each variable separately.