The Korf Blog

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19 December 2017
Armtube Prototypes — Summary of Findings
In April, we started an experiment with the tonearm arm tubes. 8 months and 10 prototypes later, it's time to take a break. We've learned a lot — but, more importantly, we found unexplored areas still waiting for our attention.

With this blog post, we end our armtubes experiment for now. Here's the table summarizing the key findings and measurements. Ideally, I would've added Young's modulus, but at the moment I don't have a reliable way to measure it.

What did we learn?
Broadly, these 11 points can be divided into 2 groups: 1 to 6 cover "Rigidity vs Damping", and 7 to 11 are about "Resonances in other tonearm parts"
    Rigidity is important. A lot more important than damping. Rigidity can be estimated by amplitude of arm tube resonances.
    Added viscous damping is lethal to sound quality.
    Rigidity and damping look more or less the same on vibrometric charts. Thus, it is impossible to predict arm's sonic behaviour by looking at the vibrometric charts only.
    Sonically, inherent damping of fibrous materials like carbon fibre and wood seems to be harmless compared to added viscous damping. Perhaps even slightly beneficial.
    Coating wood measurably increases its rigidity and improves the sonic result.
    Harmonics are a lot more sonically important than the main vibrational mode.
    Counterweight design matters a lot. Deficiencies in its design are very visible on vibrometry charts.
    Bearing design matters a lot. Difference in design and bearing quality is very visible on the chart. Counterweight and bearing design probably account for most of the audible differences between tonearms with similar headshells and arm tubes.
    Ancillaries like antiskating levers, lateral balances etc do not contribute to vibration profile of an arm and probably do not affect its sonics.
    Quality of mechanical coupling between various tonearm parts is important and visible on the charts.
    Headshell resonances matter a lot, but are too close to accelerometer's own resonance frequency (~11kHz) to measure reliably
    Our whole accepted thinking about tonearms might be wrong
    Why did damping have such a detrimental effect on the sound? I think that our whole accepted thinking about tonearms might be wrong.

    There is a chance that the role of the tonearm is not to be the inert steady platform for the cartridge. More likely, it is a transmission line routing vibrations generated during playback away from the cartridge to some place where they can be safely dissipated. The farther away this dissipation point is from the stylus, the better. Damping interferes with the transmission function.

    This leaves open the question of why CF and wood armtubes, with their high inherent damping, sound good. I have no answer to it at this time.
    One very funny finding is the relationship between perceived sound quality and area under the vibrometry chart. There is a fairly significant correlation (PCC) — 0.61. What's so funny? It's positive. The larger the area under chart, the more peaks, valleys, jiggles and jaggles there are, the better the arm sounds. It doesn't feel quite right, does it?

    Ok, ok, I must admit that if we exclude the awful-sounding CLD prototypes from the calculation, the correlation basically disappears. One more example of theoretically significant measurement having no relation to practice.

    What shall we do next? Oh, our calendar for 2018 is already full! But this is a topic for the next blog post. Stay tuned!
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