How to measure tonearm effective mass

Most actually don't know what tonearm effective mass is. Tonearm effective mass can be expressed as the moment of inertia about the pivot divided by the effective length of the tonearm squared. Which makes it very hard to calculate mathematically as it is dependent on the distance of mass distribution from the pivot.

There is a simple practical method for determining the effecive mass of a tonearm and it is to put in a cartrige of known compliance and use a test record to determine the resonant frequency of the moving system, such as the HFN&RR Test Record and then you can work the math back from there to determine the exact effective mass of the arm.

Resonant interactions between arm and cartrige audibly swamp any and all contributions that are a result of the primary arm/cartridge resonance and are much less predictable. There is some common angst about primary resonance, but just being in the ball park is generally more than enough.

Oh and to work the maths backwards the resonant frequency is equal to 159 divided by the square root of the effective mass of the arm times the compliance of the cartridge. Even so, you will only come out approximately, because of variance in compliance with frequency and static vs. dynamic compliance to name just two of the reasons.

Google might make it easier if you find someone that explains it an easy way.[http://www.google.com/search?hl=&q=measure+tonearm+effective+mass&sourceid=navclient-ff&rlz=1B3MOZA_enUS325US326&ie=UTF-8][http://www.theanalogdept.com/cartridge___arm_matching.htm]

I couldn't resist the second link because of the stylus picture.