Please correct me if I am wrong! As far as I understand, there is nothing special about this so call ideal resonance range of 8 - 12 Hz. It is just a general range to stay away from the lowest audio frequency (20Hz) and structural borne vibration (around 3 -6Hz), so these frequencies would not excite the arm/cartridge resonance.
So, if I have an arm/cartridge combo that resonate at 6Hz, but I have something like a Vibraplane, which block 90% of the vibration at 6Hz from reaching my TT, why should I worry as there is none / minimal energy at this frequency to excite my arm/cartridge. How would this affect the system's performance?
As far as I understand, other vibrations generated from within the system, such as warped or off centered LP, happen in much lower frenquency than 6Hz.
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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! |
