Pierre Lurné's Corner

The Naked Truth (TNT) about Air Bearing Tone Arms

[Italian version here]

Author: Pierre Lurné - TNT France
Published: October, 2020

Traditional or Air Bearing, the tangential system proves to have two other advantages over the pivoting kind:

  1. The first one comes from the fact that all constitutive parts can be perfectly aligned along the main longitudinal axis, including the stylus, the cartridge body and even the general Center of Gravity. In terms of “Old Good Physics” this is of the first importance because all the forces applied to the arm-cartridge combo are correctly and symmetrically distributed and remain at their minimum number and values.

  2. Secondly, there is no skating effect on any tangential arm, by definition, which could compromise the above.

In comparison, on any pivoting arm, the cartridge is always mounted at an offset angle to reduce the tracking angle error. With a few extremely rare exceptions, the stylus usually ends displaced on the left side of the axis and the general design deviates from a pure straight line, particularly with the S and J tube shapes. Finally the general Centre of Gravity is anywhere and nobody cares about it, though its position is a fundamental parameter. All this creates new forces, adds bad Moments of Torsion, increases friction in the bearing, etc. The system becomes more complex, ceases to be NEUTRAL and gets a sonic SIGNATURE, ceases to be as simple as possible, ceases to be a PURE MASS and adherence to the laws of Physics is lost.

Unfortunately, a tangential arm suffers from too much mechanical complexity, detrimental to the vibration behaviour, and due to the presence of the necessary additional parts (control device, etc.) Consequently a part of the above advantage is lost. The biggest strength of the pivoting arms is their unmatched SIMPLICITY. The ideal would be a tangential as simple as a pivoting! (This is actually being studied and could become the subject of a future article).

For the moment and in short, there are generally two kinds of tangential arms available in the market today: 1) The traditional one (normal arm mounted on a carriage); 2) The Air bearing one (including arms which substitute the air bearing by a 2 wheel + 1 rail device). If both of them could be improved further, like anything else, the theoretical basic design of the first one agrees with Physics. This is not true for the Air Bearing kind which is badly affected by several physical flaws.

The major error concerns the Mass Distribution and therefore the Dynamic Properties: clearly an Air Bearing arm behaves like any other arm in the vertical plane only. The head moves up and down, the tube less and the bearing stays about still. In this plane, the Effective Mass is usual, deals correctly with the cartridge compliance and the resonance frequency is kept under control.

On the other hand, the “entirety” of an Air Bearing arm moves on the horizontal plane. The Effective Mass increases a lot, up to 10 times and even more (!), as a result that another frequency of resonance is created. Things are already complicated enough with one single general resonance so close to the troubles of record eccentricity and warp that adding a second one is an open door to tracking problems. The new resonance rings very low in the worse region and far from the well known safe area (9 to 12 Hz approx). More, two working planes are affected because of the 45x45 engraving. Manufacturers consent and EVEN call the attention of the user in their manual! To keep the arm mass low enough, light cartridges and the choice of the smaller counterweight are strongly recommended.

Furthermore a single glance at any Air Bearing arm shows that every effort has been done to lighten the product: the counterweight is always small and consequently away from the bearing, never a removable head-shell, definitely too heavy, and a very reduced and light general arm structure. Usually arms are much heavier, though in the limit imposed by the main frequency of resonance (No confusion between Mass and Effective Mass!).

This is the second drawback of the Air Bearing arm: they must be light to stay relatively out from the tracking problems and also for the bearing air film to work properly. This lack of Mass is obviously detrimental to good vibrations control and damping which are already limited by the air film in the bearing because a good part of these bad vibes can hardly travel through it and be evacuated and dispersed away.

More inherent flaws cannot be forgotten:

Things can even become really worse and totally “anti-mechanic”. For example, when the arm wand is on one side of the air bearing tube and the counterweight assembly on the other like on the Eminent Technology item. On the opposite the Air Tangent arm is a good example of a good Air Bearing design, but still nothing but an Air Bearing arm.

- Caution 1: all this above does not mean that Air Bearing arms are bad. Indeed their musical qualities are alright because Analog is “good boy” and always works. If not...after all, even those cheap turntables from the supermarket do the job!... They just would sound much better if “Old Good Physics” would have been scrupulously respected. Probably the reason why some says that the Air Bearing system is just good enough to make cheap tangential tone arms.

- Caution 2: why so little things are so important for music reproduction? “All That Moves Makes Noise”. Virtually everything that moves generates vibrations and therefore noises. A turntable is no exception. The drive motor creates vibrations, the spinning platter and bearing, the suspensions, the record tracking itself which even can be heard (called the “needle talk”), etc. Vibrations expand very quickly and reflect everywhere into limited volumes. They establish standing waves and excite all possible resonances ready to ring. Happily they decay and die very quickly, but this natural phenomenom is a continous flow fed by the music itself going on playing. Those parasites constantly blur the music and are hardly controlled. Compared to the so fragile, delicate and minute musical details engraved in the groove, bad vibes are gigantic pests. The size of a -50 dB musical signal, for example, ranges about one millionth of mm! So small and so fragile, it still can be tracked, heard... and very easily distorted.

The next article will be entitled: Platter Design.

DISCLAIMER. TNT-Audio is a 100% independent magazine that neither accepts advertising from companies nor requires readers to register or pay for subscriptions. After publication of reviews, the authors do not retain samples other than on long-term loan for further evaluation or comparison with later-received gear. Hence, all contents are written free of any “editorial” or “advertising” influence, and all reviews in this publication, positive or negative, reflect the independent opinions of their respective authors. TNT-Audio will publish all manufacturer responses, subject to the reviewer's right to reply in turn.

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© 2020 Pierre Lurné - pierre.lurne@tnt-audio.com - www.tnt-audio.com