General question on Carbon fiber tonearms versus aluminum

@lewm I compared the CF headshell with my standard magnesium one and, while bass was better on the CF, it lost absolute detail—diminishing the color of brass, for example—and "dried" out the "air."

1+ @elliottbnewcombjr. Theoretically, a carbon are can be made stiffer, without the need for more damping materials and most importantly lighter.
Lighter, shorter tonearms with more compliant cartridges sound better. They have a much lower polar moment of inertia. This allows the arm to better follow low frequency undulations in the vinyl lowering distortion. My guess is this is why Michael Fremer prefers a shorter arm and why some tonearm manufacturers like Schroder go out of their way to reduce mass while trying to maintain a stiff arm. This is the reason behind his minimalist " head shell" design. As for which one sounds better? I have no idea. I will find out as I have a Schroder CB waiting for a turntable. I suspect they can both sound excellent given proper design.

This has been a very interesting and enlightening post.  Earlier when I made a comment about bicycle frames, a lot of what was mentioned in this post echo's what folks have found comparing frames made of steel, aluminum, and carbon fiber.   Granted, early fiber bicycles were not as good as steel or aluminum and had a tendency to crack and break, but over a ten year period, it is hard to beat fiber today for a  comfortable ride, yet still have a frame that climbs like aluminum, and is non fatiguing like steel.   With carbon, the vibrations that come up from the pavement dissipate (think damped) and don't tire your legs, arms, and butt like aluminum.  But climbing hills, the last thing you want is a soft frame as what little horsepower you develop goes into making the frame bend rather than to the rear wheel.   Try riding 100-120 miles in a day, climb 10 to 12,000 vertical feet, and do it in high altitude and this lesson will be branded in your brain.   That was my basic training ride back when I did extreme endurance  cycling.

This is pure speculation on my part, but it is possible that the live sound from aluminum, if indeed  it is from the arm itself, could be energy from the cartridge that is causing the arm to color the sound in a rather pleasing way.   The dead sound from fiber, if indeed it is from the arm itself, could be the lack of this coloration.  If you remember, Ray Dolby had the same problem selling his Dolby Noise Reduction to tape manufacturers.   That subtle lack of noise and distortion in the upper frequencies were considered a lack of "air" and consequently colored the sound in a bad way.  He had a real problem for a while until folks realized what he was selling was a lack of coloration and distortion.

Frankly, I don't know which technology is better.   The best sound I had was from my aluminum Grado arm and top tier Grado cartridges.   The carbon arm got the second rate cartridges.   Recently, I upgraded cartridges and now that carbon arm is producing the finest sound my stereo has ever produced.   The Grado setup sounds great and I would be very happy with it, indeed it is better than ever, but I do have that carbon setup right next to it and it beats Grado in a noticeable way.   

It is a tough call since I'd like to upgrade my arm to a modern, transcription length unit.

Clearly execution matters, but if we know what it takes to make a great tone arm, then this question is answerable. Carbon fiber simply has more strength and stiffness as aluminum at the same weight. If that’s what it takes, then yes. But I expect it’s not that cut and dried. There’s got to be cartridge-arm interplay, so I’ll bet it just depends on getting a match. Like the bicycle frame example. The most comfortable is steel. It flexes. Riding cross-country 100 miles a day day after day? I’ll take steel. Racing around an oval for 60 seconds? Probably carbon. Want an 11-inch tone arm that accommodates a “heavy” cartridge? Ask an engineer, but it’s great to have the options we do. The first time an aerospace engineer handed me an early carbon fiber rod and its titanium (or whatever metal it was ) counterpart, I was flabbergasted. And it was a rotor control rod on a helicopter! It goes, the helicopter goes (down). Yet the carbon rod weighed, I don’t know, ounces vs. pounds. It was that dramatic. Amazing stuff.

That subtle lack of noise and distortion in the upper frequencies were considered a lack of "air" and consequently colored the sound in a bad way.  

This isn't the same as acoustic ringing or damping. Dolby effectively filtered frequencies (-3 dB at 600 Hz/ -6 dB at 1.2 kHz/ -8 dB at 2.4 kHz/ -10 dB at 5 kHz) resulting in a removal of ambient information. Back in the 80s, we never used Dolby B or C as both sounded worse than the clear and airy unadulterated sound—albeit with tape noise.