thekong The "ideal resonance" of 8-12hz came about historically because it was below 20hz, i.e. well below the cut-off in most systems, and above 3-4hz. "3-4hz" is the typical resonant frequency of footfalls on a sprung timber floor. The resonance frequency and amplitude can have quite an impact on overall tracking ( whether you can hear it or not ) and sound quality. Here is a very good extract from the Shure white papers on resonance .. what happens at the resonance frequency? One important characteristic of resonance is that motions are magnified considerably, in this case, typically from 2 to 10 times.
In both situations, the output from resonance frequency signals in the groove will be increased from 6 to 20 dB. These numbers are just the dB equivalent of the magnification numbers previously mentioned. By itself, this may not be all bad, since this resonance peak determines the low-frequency response "limit" of the pickup and system, and a bit of boost here may not be unpleasant. This was certainly true fifteen years ago, when arm resonance frequencies of 30 to 50 Hz were common. However, with modern pickups and arms, these resonance frequencies are usually subsonic (below 20 Hz), so that reproduction by the loudspeakers may cause distortion. Additionally, preamp overload is most likely to occur at boosted low frequencies since the preamp clipping level is lowest here. Consequently, the arm resonance has lost whatever usefulness it once had and must now be regarded as a liability.
The most pernicious effect of the resonance is shown in Figures 1 and 2 by the "scrubbing" notion developed by the stylus in the groove. This causes program material to warble in pitch, just as if the turntable speed were fluctuating. In fact, the groove speed is changing (relative to the tip), because a fraction of the velocity of arm vibration is added to the groove velocity. (See Appendix I.) The effect is that about 1/3 of the arm vibration velocity is alternately added to and subtracted from the groove speed. For example, at arm resonance, total amplitudes of 1/32" are easily observed by eye. If the frequency is 8 Hz (typical for high compliance pickups and average arms), the resonance velocity will be about 2 cm/sec (see Appendix II). This velocity will produce a "scrubbing" velocity of 0.6 cm/sec along the groove axis. The groove speed at a 4.5 inch radius is about 40 cm/sec; so the frequency modulation will be about 0.6/40 = 1.5% and easily audible.
Another less obvious consequence of the arm resonance is that the stylus force is "used up" when the arm is vibrating. In the previous example, if the compliance of the pickup is assumed to be 20 x 10-6 cm/dyne, 2.0 grams of stylus force will be required to accommodate the arm vibration alone. This is larger than the usual stylus force, so mistracking is quite certain at the extremes of the vibration.
The above explanation demonstrates why the Eminent Technology ET2 utilises a patented decoupled counterweight which splits the horizontal and vertical resonant frequencies (the horizontal and vertical masses are different ). This decoupled counterweight system results in 2 resonant frequencies of substantially less amplitude, the benefits of which are superior tracking and less distortion ( along with the tangential tracking ). It also explains any the removal of the decoupling spring as recommended in this forum by richardkrebs on the ET thread is ludicrous and destroys the fabric of this arm. |
Dover, thank you for the information! My question is, if we can block all, or most, of the external vibration at that resonance frequency (6Hz in my previous example) from reaching the TT by a Vibraplane or somthing similar, would that still has any affect on the tracking ?
My way of thinking is that if there isn't any 6Hz vibration (as in my example) reaching the TT, then the arm/cartridge resonance will not be excited, and the tracking will not be affected. Is there anything wrong with my thinking? Thanks
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thekong -
I would think that reducing vibration to the TT may help to reduce some tonearm/cartridge resonance if there are external vibrations that are causing instability to the TT/arm itself. However the tonearm/cartridge resonance will still come into play when playing records.
Records have massive imperfections built in - eccentricity, surface variations, bumps, warps (micro), not to mention groove imperfections. All of these will impact stylus tracking and the arm/cartridge resonance will affect the stylus/cantilever response.
Bear also in mind that there are resonances in the audio band above the fundamental resonance of the arm/cartridge and therefore some low bass in the audible region will be impacted by the arm/cartridge resonance as well as tracking distortion.
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Dover, thank you for your detailed reply! |
Dear the kong: The " ideal " frequency resonance range of 8hz-12hz is for some gentlemans not the ideal range for example: G.Merril ( of Merrill TT and AR ones. ) had an explanation that the range should be 15hz-18hz and other took 8hz-10hz.
Anyway, when we make the calculations for our tonearm/cartridge combination that calculation gives us the theoretical resonance frequency that normally is out of reality because almost all tonearm manufacturers gives the tonearm effective mass figure with out taking in count the tonearm counterweight position with that cartridge and normally too we don't take in count the weight of the hardware to mount the cartridge but exist an additional factor to take in count and that's the real cartridge compliance, this is under playback and through the time. Those factors moves the real resonance frequency on our tonearm/cartridge combinations and your Vibraplane can't helps here.
That kind of TT damping and other kind of TT damping ways can't helps to additional ssues happening down the LP/stylus/TT:
- if we have active subwoofers in our system we know that in the LP recording comes very low bass frequency lower than 10hz. Example: some London/Decca old recordings made it/recorded in the Royal Opera house comes with intermitent very low bass non musical/no music content sounds that happens came from the London subway that pass under or nearest that Hall.
In some of the RCA reissues by Chesky recordings happens the same for the same reason.
But the subway sound goes lower and affect that cartridge/tonearm resonanse frequency that affect what we are hearing because as music information " distortions " generates harmonics too.
- exist more subsonics signals in the LPs that we can imagine and are affecting and degrading what we are listening.
- something that TT damping can't helps is the " distortions/vibrations " coming from the platter/bearing ( even in air bearing designs. ) because any body/platter in movement generate ( because that movement/dynamic mass. ) vibrations/micro-vibrations transmitted throught the platter spindle and platter surface to the LP and disturb the cartridge ridding.
all those and others are focus to exite that cartridge/tonearm resonance frequency during playback and the problem is that we can't see it ( almost all happens at microscopic levels. ) even we are unaware of its damage to the musical signal.
Obviously that we must work to damp perfectly at each of those system links and those distortions sources and one way to start is try to stay between that resonance frequency range.
We can't avoid those degrading musical signal sources but at least try to have it at minimum elsewhere.
The real " magic " ( if any ) for a better quality level audio system performance is exactly that: every kind and elsewhere distortions keep it at minimum but this is more easy to say it than to do it because the first issue is to identify in precisily way where are those distortions sources.
Yes, the analog experience is a lovely one but full of imperfections as no other music home reproduction medium.
Regards and enjoy the music,
R. |
