Which tonearm with a Soundsmith The Voice?

22 cu is the same as an Ortofon 2M. There are plenty to choose from that are less than 12g. Even a little more than 12g should be fine if your set-up isn't prone to feedback.

An arm with mass lower than recommended (Black Widow) isn't a good idea. If the effective mass is too low, the resonant frequency goes above the recommended range and can cause intermodulation distortion. This would be especially true in an arm w/o fluid damping.

Origin Live Silver or Zephyr should be good (12g), and also a Moerch with a yellow arm tube. Some people rave about the Audiomods arm from UK, sold direct. There are plenty of excellent used and vintage arms. You have to hunt.

Regards,

There is no ideal mass. Those calculators are only an estimate anyway, results may vary. You could buy a mediocre arm and have an "ideal" resonance of 10Hz, and it would be outperformed by a higher quality arm where the resonance is outside of the recommended range.

What did Soundsmith say, Schroeder? If that's not doable, you should get as close to that quality as possible within your budget. A resonant frequency of 7.4Hz is not a problem.
Regards,