Compliance is a term used to describe the suspension of the cantilever in a cartridge, specifically, how easy (high) or hard (low) it is to deflect, relatively speaking. Basically the same concept as how softly or stiffly a car suspension is sprung. Just as in the car example, the appropriate deflection rating will depend on the mass of the suspended body - in this case, the combined mass of the cartridge and tonearm. The higher the mass, the lower the appropriate compliance rating to match it, or for a low-mass 'arm/cart combo, a high-compliance cart is used. The main idea is that the groove modulations cut into the record should be vibrating the cantilever/stylus, not 'wagging' the 'arm/cart, and that this needs to happen while still maintaining agile tracking and minimum surface wear. Roughly stated, the history of compliance in carts has moved from early days of massive 'arms/carts featuring low compliances, to a middle period of greatly lowered masses that required high-compliance carts, to the situation today where most 'arm and cart masses ratings - and consequently cartridge compliance ratings - are considered 'medium' (though this is not universally true, or even precisely defined).
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"Z" did a very nice job of summing things up. Hats off to him for contributing a very concise and useful post.
As a general rule, most MC ( moving coil ) cartridges work best with arms of medium to high mass and most MM ( moving magnet ) or MI ( moving iron ) prefer low to medium mass. Both moving magnet and moving iron tend to share a lot of common traits ( tracking abilities, mass, compliance, output levels, etc.. ), so they are generaly lumped together into one category. Picking an arm that is somewhere in the middle ( medium mass ) would offer the most versatility in terms of cartridge selection but could negate offerings from the outer fringes of either type.
Personally, i have two Sota Star Sapphire's. One is equipped with an ET II arm and the other with a Clearaudio TQ-1. As you may know, both of these are linear tracking arms but vary pretty drastically in terms of the mass that the cartridge sees / has to deal with. My brother has a Sota Sapphire that is equipped with a pivoted arm. It is an Audio Dynamics Corporation ( aka "ADC" ) ALT-1 tonearm, which is a very low mass aluminum design that is pivoted and straight ( not "S" or "J" shaped ). This works out fine as he is running an MM cartridge that requires a very low mass arm.
Either way, think of the cartridge / arm / table as working together in unison. Assembling a bunch of "good quality" components that won't necessarily work well together is an easy mistake to make. As such, be careful with whom you work with when it comes to selecting arms / cartridges. I've gotten some REALLY bad suggestions from what were supposed to be "professional" shops that specialized in vinyl. If you can find a dealer that you feel comfortable with ( whether brick and mortar or on the web ), it might not be a bad idea to ask for opinions here before lying down the cash for a combo that they suggest. Biases aside, we'll do our best to try and help you out as to whether it would be a good match. Sean
"effective mass" is an arm manufacturer's spec. - just ask them.
Then read this link, & browse around all the other analog features on this site for a better understanding:
Thanks for the link, Bob. I'm glad I learned something about this before I purchase a second cartridge for my mmf-7. I have been able to find that the Project 9 arm is 11 grams and thus a medium mass arm. I have not been able to find the compliance rating for the shure v15vxmr that is currently mounted on the arm. Does anyone know what the compliance rating is for this cartridge?