6. Arm resonance in low frequency range
Illustration shows spring in suspension from overhang and
weight slung from it. When pulled down and then
released, spring weight combination will reciprocate
longitudinally at a certain frequency. This is generally
called spring mass oscillation system. Oscillation of two
units of mass at a common fixed frequency is called
resonance. This frequency ('f') Is mathematically
expressed
Where 'c' is system compliance in cm/dyne and 'm' stands
for mass of system.
Similar phenomenon is observed in arm-cartridge
assembly as it reproduces sound from record disk.
Graphically, cartridge compliance may be likened to
spring, and arm pivot to weight.
This is what is called arm resonance in low frequency
area, and there are as many resonance sources as there
are pivoting gadgets in tone arm.
At such a frequency of resonance arm becomes very
sensitive to vibration and, even when frequencies of 'flat'
characteristics are reproduced from record, arm
resonance causes larger output than true to record or
'peak' phases (See diagram 3). At frequencies higher than
arm resonance, arm does not so vibrate at all, and
cantilever oscillation alone translates into output. But at
frequencies lower than that, cantilever and arm oscillate
together and fail to generate electricity. This is why arm
resonance frequency 'fo' is called low sound reproduction
margin.
With conventional type tone arms vertical and horizontal pivots are placed in close proximity. This causes
these two resonance sources to vibrate at the same frequency, as if vibrating as a unit. Peaks then become
larger and so much harder to damp.
