Effective mass is not the same thing as the mass of the tonearm. It seems people are confusing these two terms. My SAE tonearm, for example has an effective mass at the headshell of 9.5g- according to my owners manual. The entire tonearm and counterweight certainly weighs more than 9.5g. The tonearm manufacturer has simplified a more complicated inertia calculation into an easy to apply value- effective mass. Now, I can add the mass of my cartridge to the effective mass of my tonearm and then apply the compliance of my cartridge to find the Resonance Frequency, Fn of my Tonearm/Cartridge system using: Fn=sqrt(k/m) where K=compliance (N/m) and m=mass (kg). Ideal Fn is around 10Hz. I recently lived this issue when I mounted my new Denon cartridge to my SME tonearm. The SME is a relatively low mass tonearm. The Denon cartridge has a mass of 8.5g and a compliance of 7.5X10-3N/m. This put my system Fn at about 16 Hz. (Insufficient mass) This was too high. I was fine until I tried to play a record that had a slight wave to it. The tonearm would dance across the record. I could see the cantilever move upward and kick the tonearm into the air. Searching AudiogoN, I found a place in England that sells a 4g mass (Aluminum) with threaded holes specifically made for the Denon cartridge. The added 4g mass lowers my Fn to just over 10Hz. I applied the mass to my cartridge and now it plays that same record perfectly. I also knew the added mass would work because my now dead Koetsu cartridge had a mass of 14g. I also found this 4g Al plate improved the sound of the Denon. The bass is better and imaging more focused. Not bad for a $19 tweak w/shipping. I have always had heavier (14g range) cartridges in the past and so never encountered this issue before. I could see how these kinds of problems could be frustrating to someone not trained in Engineering.
