7 Feb
2014
7 Feb
'14
3 p.m.
On Fri, 7 Feb 2014, Fons Adriaensen wrote:
The cantilever of which the needle is the end has an
elastic
mount in the cartridge. Together with the mass of the arm
and cartridge this forms a spring + mass system, which has
a resonance frequency.
Above that frequency the arm will not follow the groove,
the needle moves relative to the cartridge, and this
produces the signal. Below the resonance frequency the
whole arm will start to follow the modulation of the groove.
On a well-designed system, the resonance frequency should
be below the audio range, but still high enough to enable
the arm to follow any warping of the disk.
If the resonance frequency is in the audio range, then
a strong groove modulation below or at that frequency can
make the needle jump out. But this means bad design, or
the wrong combination of arm and cartridge.
Thanks again, Fons. I can always count on you for an absolutely clear
explanation of a complex phenomenon.
An off-center hole in the record at 45RPM would induce a .75hz noise in
the signal. (I don't think any stereo recordings were made in the 78RPM
format) A severely warped record might give you three or four waves
per revolution, so 6hz max? If the player were designed for audible
signals at 20hz and above, it would seem that would be a wide enough
target for mechanical resonance of the tone arm. But engineering always
demands compromise, and the need to keep the stylus pressure low, and the
length of the tone arm short enough to fit in living-room consoles, may
force a higher tonearm resonance. I'm not a mechanical engineer, but just
from observation of instruments such as organ pipes, Marimba keys, and
grand piano bass strings, I imagine achieving a resonance <20hz in a
physical structure of ~30cm length would be a non-trivial exercise.
--
Rick Green
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