Hi Bhundu, Nick
any sprung system will have a resonance point at which it moves most easily - energy will tend to get channeled into that frequency resulting in a peak in amplitude.
The mass/springing system of the tonearm, where the only spring is the suspension of the cartridge works in the same way.
Even when the resonant frequency is at the "ideal" frequency of 10Hz, what happens is anything near that frequency will tend to excite the sprung system, and magnify those frequencies.
Test records have tracks of low frequency sweeps - basically a tone at a set level with gradually dropping or increasing frequency usually starting at 3 or 5 Hz and rising to 20Hz or 25Hz.
If you record this track and plot the amplitude to frequency, you will note that it does not reflect a line (as it theoretically should) but shows a marked peak at the resonant frequency. Depending on the arm/cartridge this peak can be anywhere from 2-3db (ok) up to 8db+ (bad).
The peak is not a narrow one, and usually influences frequencies on either side of the maximum point to a lesser extent.
As the RF (resonant frequency) rises it can start to impact on the audible low frequency response of the system - a substantial boost in amplitude at 15Hz as an example can cause huge subwoofer/woofer pumping, sucking up amplifier current, causing intermodulation of the bass frequencies, and generally affecting the sound as a result. (amps being under pressure using up all its grunt moving the woofers ends up losing its sweetness in the high end...) This is one of the reasons golden age phono centric amps often have a subsonic filter!
As the RF drops it starts to approach the frequencies generated by things like footfalls, and other vibrations external to the TT.
If you watch an arm/cartridge at its resonant frequency it clearly starts to "wobble" - there is substantial physical movement.. as it rises the needle will get pulled slightly back, as it drops it will get pushed slightly forward, a form of wow that is seldom considered, but does affect all frequencies reproduced.
Additionally the movement generates vibrations in the entire system which in turn intermodulate with the signal, generating other spurious "noise" (Intermodulation Distortion).
Also when the RF is triggered, the cartridge movement can cause mistracking - with all of its associated problems, including increased wear and tear on record and stylus!
A well tuned arm/cartridge have the RF positioned where it does the least harm - by keeping it away from footfall/external vibration frequencies, and the resonant frequencies of the TT suspension (usually around 5Hz), keeping it away from the audio range (15Hz+) and if possible keeping its amplitude as small as possible.
Regardless of the final RF, the resonance remains an undesirable aspect of TT technology.
There is a way to control it, it is called damping. Damping can 1) reduce the amplitude of the RF, and 2) slightly shift the peak frequency.
The most common damping is fluid damping using a paddle system in an oil bath attached to the arm - it provides a slight resistance in a frequency sensitive way - the end result can be very effective. (the damping can be internal within the tonearm pivot column, external, it can be done with fluid or electro magnetic servo means...)
As an example, my JVC turntable has a damped arm - running a TK9e cartridge, I measured the resonant frequency at 5.9Hz - this being with the damping disabled.
Enabling the damping "widened" the peak somewhat - going from 5.8Hz to 6.5Hz, but dropped its peak from +4db down to +1db.
The stylus compliance is 31cu, the arm mass calculated out to 23.8g (including cartridge and fixings) - so high mass arm with high compliance cartridge - resulting in low RF (much too low).
With damping disabled - the mistracking was relatively obvious - and the system was very very sensitive to anyone walking around the house.
With damping enabled - the mistracking disappeared, and footfall sensitivity although still there, became much much reduced.
This is clearly NOT an ideal setup - I can make it useable with damping - but it will be better if I can lighten the mass by 4g or 5g with a lighter headshell.
And then I can still damp the resulting higher RF....
Some test records have an interesting track that can be used to identify the RF they run the low frequency sweep tone, at the same time as a steady audible tone...
When the arm hits the resonant frequency you clearly hear the tone "warble" - a result of the arm/cartridge movement.
A lesser and more subtle version of this happens when you play a record, and the arm is excited at its RF.
Here is a usefull calculator for working out Effective Tonearm Mass
http://www.luckydog.demon.co.uk/images/EMC.XLS
It requires weighing different components of the arm, but provides a relatively accurate figure for real life effective mass, which is useful in working these things out, as opposed to very vague light-med-heavy categories and manufacturer arm specs that can be out substantially based on what fixings, headshell and cartridge you are using at the time.
I strongly recommend damped tonearms regardless of the RF.... and for undamped tonearms, the Shure styli with the damping brush is a very effective alternative too...
Some of the top arms that are "undamped" have a touch of friction in the way the bearings are set up and lubricated that provides a bit of damping - this is one of the reasons some apparently less than ideal combinations work well!
(some people tweak their arms by using a slightly thicker lubricant to provide a touch of damping too...)
bye for now
David