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In Reply to: RE: Arm mass categories? posted by M3 lover on June 06, 2017 at 14:05:36
My source is basically common sense. The lowest mass arm that I know of is the Infinity Black Widow with effective mass of 3-grams. Another low-mass tonearm that everyone accepts to be low-mass is the SME III with effective mass of 5-grams.
The SME V, the Technics tonearm and Rega tonearms are all accepted to be medium mass tonearms. These have effective mass in the range of 10 to 12-grams. Therefore, the division between low-mass and medium-mass is somewhere in-between 5-grams and 10-grams.
I think most people would consider a tonearm with effective mass of 20-grams to be a high-mass tonearm. Therefore, the division between medium mass and high mass must be somewhere in-between 12-grams and 20-grams. I chose 15-grams.
However, regardless of how you choose the divisions, a tonearm like the FR 54 with effective mass of 16-grams would never be considered a light arm or low-mass tonearm when 12-grams is clearly defined as medium-mass.
> Common headshells can weigh at least 9-10 grams which must have an affect.
I'm not sure what you're getting at. The headshell is always included in the effective mass of the tonearm. For example, the weight of the Technics headshell is 7.0-grams without cartridge wires and between 7.5 and 8.0-grams with cartridge wires, depending on the specific cartridge wires being used. I just weighed a Technics headshell and several different types of cartridge wires.
The effective mass of a headshell is actually slightly less than its total mass because effective mass is the headshell's moment-of-inertia about the tonearm pivot divided by the square of the tonearm's effective length. Therefore, a 7.5-gram headshell might have effective mass of only 5.75-grams, depending on its mass distribution. Consequently, a Technics tonearm might have effective mass without its headshell of about 6.25-grams.
Do you know the weight of the FR 54 headshell? Whatever it weighs, the tonearm's effective mass of 16-grams includes the headshell.
If a common headshell weighs 9-10 grams that would be a major part of a medium mass arm range of 9-13 grams. Of course I understand there are many lighter headshells.
The whole subject of tonearm mass is more complicated than what I remember from HS physics. :^(
"The piano ain't got no wrong notes." Thelonious Monk
The effective mass of any tonearm is simply its moment-of-inertia about its pivot point divided by the square of its effective length. The difficult part is determining a tonearm's moment-of-inertia because its mass distribution is so nonuniform. The only accurate way to determine its moment-of-inertia is to measure it.
The advantage of effective mass is that it allows us to compare the inertia of different length tonearms directly whereas their moments-of-inertia are not directly comparable. Two different length tonearms having the same effective mass will react identically with cartridges of the same mass and compliance; however, their moments-of-inertia will be different. The longer tonearm will always have a larger moment-of-inertia than the shorter tonearm even though their arm/cartridge resonance frequencies will be identical when using the same cartridge.
Well, heck, if it's that damn heavy, I'm gonna throw it in the trash.
I think it's light, and that's what counts. ;) Yeah, I know there's weightless arms out there for a premium price. But I picked up this gently-used TT/arm combo back in 1977/78 for something like $50, and I'm still happy with it. All's I know is that, if I look at it cross-eyed, it'll skip a groove, and if I don't have a decent grip on the fingerlift thingy, it'll bounce right out of my hand and go skipping across the record and I'll have to get my spare stylus.
Still, a half-ounce headshell?!
Anyway, a moment ago I had some inertia. Now I'm in a stationary orbit. (That's a physics joke. Made it up all by myself.)
Edit: Btw, I have a digital scale. It says I weigh 178. Fortunately, the digital scale at my doctor's office says I weigh 174. I'm going to the post office today, and might take my headshell with me.
In or out of orbit, we all have inertia. It's the Law.
Not exactly sure how fast I'm going, but when I'm sleeping or watching "the news", I feel like I'm stationary.
Maybe I should investigate further.
Watching astronauts float around in the ISS, you'd never think that they're going 18,000 miles an hour.
'Course, you'd also never think that the Russians are their only way to get home (with a valid passport).
you and I are traveling at around 600 to 800 mph all the time. Speaking for myself, I'm exhausted.
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