Thekong.
The answer to your question regarding frequencies impacted by horizontal Fr is shown in the attached graph.
From memory your Horiziontal Fr was around 5hz with the A-90. So substitute 5 for 1 on the x axis and scale up from there. At 15 Hz (3) the rise in response is almost zero. This is what BT was talking about when he mentions 3xFr.
It could be that the perceived improvement in bass performance of subsequent Rockport arms has more to do with the wand, gooseneck and counterweight arm stiffness, rather than total weight.
Different bearings make virtually no difference to this graph since it is the mass that the cartridge has to push sideways that counts.
The flavour of different arm bearings, however, is another matter entirely.
The various curves are showing different values of Q (damping) Note the level of damping makes virtually no difference at frequencies of 3x Fr and above.
This does not mean that we can ignore the amplitude at resonance, since it is shaking the arm and this has an impact in the audio spectrum. FM and AM modulation.
Bruce reduces this amplitude by decoupling the counterweight. It is an elegant, brilliant solution. Others damp the resonance as I have done. The oil trough, way less elegant, is also a very effective way of doing this.
Both methods reduce FM and AM artefacts.
http://s1173.photobucket.com/user/CT-993/media/ResonanceGraph_zpsdd78e0f4.png.html?sort=2&o=7
What this means, as Spock15 says, at 3xFr and above the arm appears to be solid to the cartridge. We want the cantilever to move not the arm.
Now look at frequencies below Fr. At say 0.25 Fr we get transmissibility of 1. What this means is that, with the compliance of the cartridge used, the whole arm moves sideways. The cantilever does not deflect. This is important for eccentric record issues. In other words stay above a horizontal Fr of 3hz. (4x 0.75hz) ) 0.75 Hz being the frequency seen with an eccentric record ay 45rpm.
Putting this another way. BT uses at 30cu cartridge in his manual to calculate horizontal Fr. The arm weight he uses is 30gm plus 7gm for the cartridge. We have to assume that he is ok with this combination and that he is not worried about cantilever deflection on eccentric records. When using a 10cu cartridge we can increase the horizontal mass of the arm to 111 gm and have the same peace of mind about cantilever deflection with eccentric records. The cartridge is 3x stiffer so we can push around 3x the weight. It is that simple. Forces on the record groove wall are another thing. We use a stiff cartridge this is the price we pay. Don't play heavily eccentric records.
Ha_ha_he_man.
What I hear with a heavy linear arm is this... The music takes on mass. Individual notes are solid. This is not to be confused with "heavy" in the derogatory sense. A live struck triangle has this mass. One can imagine walking up to the sound of it and holding it. It would have mass and a textured surface. This is very hard to reproduce and I do not hear this effect with light linear arms when carrying low compliance cartridges.
The answer to your question regarding frequencies impacted by horizontal Fr is shown in the attached graph.
From memory your Horiziontal Fr was around 5hz with the A-90. So substitute 5 for 1 on the x axis and scale up from there. At 15 Hz (3) the rise in response is almost zero. This is what BT was talking about when he mentions 3xFr.
It could be that the perceived improvement in bass performance of subsequent Rockport arms has more to do with the wand, gooseneck and counterweight arm stiffness, rather than total weight.
Different bearings make virtually no difference to this graph since it is the mass that the cartridge has to push sideways that counts.
The flavour of different arm bearings, however, is another matter entirely.
The various curves are showing different values of Q (damping) Note the level of damping makes virtually no difference at frequencies of 3x Fr and above.
This does not mean that we can ignore the amplitude at resonance, since it is shaking the arm and this has an impact in the audio spectrum. FM and AM modulation.
Bruce reduces this amplitude by decoupling the counterweight. It is an elegant, brilliant solution. Others damp the resonance as I have done. The oil trough, way less elegant, is also a very effective way of doing this.
Both methods reduce FM and AM artefacts.
http://s1173.photobucket.com/user/CT-993/media/ResonanceGraph_zpsdd78e0f4.png.html?sort=2&o=7
What this means, as Spock15 says, at 3xFr and above the arm appears to be solid to the cartridge. We want the cantilever to move not the arm.
Now look at frequencies below Fr. At say 0.25 Fr we get transmissibility of 1. What this means is that, with the compliance of the cartridge used, the whole arm moves sideways. The cantilever does not deflect. This is important for eccentric record issues. In other words stay above a horizontal Fr of 3hz. (4x 0.75hz) ) 0.75 Hz being the frequency seen with an eccentric record ay 45rpm.
Putting this another way. BT uses at 30cu cartridge in his manual to calculate horizontal Fr. The arm weight he uses is 30gm plus 7gm for the cartridge. We have to assume that he is ok with this combination and that he is not worried about cantilever deflection on eccentric records. When using a 10cu cartridge we can increase the horizontal mass of the arm to 111 gm and have the same peace of mind about cantilever deflection with eccentric records. The cartridge is 3x stiffer so we can push around 3x the weight. It is that simple. Forces on the record groove wall are another thing. We use a stiff cartridge this is the price we pay. Don't play heavily eccentric records.
Ha_ha_he_man.
What I hear with a heavy linear arm is this... The music takes on mass. Individual notes are solid. This is not to be confused with "heavy" in the derogatory sense. A live struck triangle has this mass. One can imagine walking up to the sound of it and holding it. It would have mass and a textured surface. This is very hard to reproduce and I do not hear this effect with light linear arms when carrying low compliance cartridges.