GB2482655A

GB2482655A - Evacuated sealed tonearm

Info

Publication number: GB2482655A
Application number: GB201009500A
Authority: GB
Inventors: Craig Milnes
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-06-07
Filing date: 2010-06-07
Publication date: 2012-02-15
Also published as: GB201009500D0

Abstract

A sealed gas impervious tonearm tube for a gramophone record player. The tone arm being made from precisely aligned carbon fibre exploiting a pressure differential caused by the introduction of a vacuum to the interior in order to pre-stress and damp the structure without any additional mass. The tone arm may be provided with an extraction valve 4 for further extraction if the vacuum becomes depleted. Additional shielding materials may be deployed to eliminate interference from radio waves. The tone arm is hollow and can be of carbon fiber orientated by the geometry of the tube to provide a helix. The arm may be a carbon fibre / epoxy resin tube based on a hyperbolic curve.

Description

I

Title of the Invention

Complex form, super low mass, vacuum damped, carbon fibre tonearm.

Field of the Invention

This invention relates to a tonearm for the purpose of transcribing the data stored in the groove of a vinyl record.

Background of the Invention

This invention relates to tone arms for gramophone record players. A tone arm assembly of a record player is commonly constructed of a pivoted beam with the cartridge and its stylus at one end and a counter weight at the opposite end of the beam. so as not to affect the function of the stylus mechanism the mass of the tonearm should be as near to zero as possible.

Inertia in the system will impair the function of the stylus I cartridge mechanism. Conflicting with the need for low mass however is the need for stiffness as any resonance in the tonearm will interact with the compliance of the cartridge suspension and will affect the quality of transcription also. Finally t is important that any solution provide good damping of the structure that will exhibit resonance at some point but that this will also add mass so should be executed with due consideration.

For the Cartridge I Stylus to function at optimum levels the transcription of the data stored in the micro grooves of a vinyl record requires that the pivoted structure known as a tonearm should trace the groove precisely and for this reason, in theory at least, longer tonearms (12 inches) are superior due to the lower levels of tracing error as they approximate more the straight line of the cutter that produced the original data in the master from which the vinyl is pressed. Unfortunately, in all prior art the additional mass required to achieve this length of tonearm often negates any benefits gained by the superior geometry as the dynamics of the resulting system are severely compromised.

In summary then, the determining factors or main criteria behind the success of any tonearm tube but especially longer length tubes of 12 inches or 305mm is dependent upon the following variables, choice of material, the form or geometry of the material, the overall length and its ability to control and recover from the resonant energy injected into it by the cartridge.

These variables are guided by the impossible goals of infinite stiffness, zero mass and total damping.

It is the realisation of a system that is not as severely compromised as prior art that is the subject of this invention and that in the case of a longer tonearms in particular, is significantly superior in performance according to the above criteria.

Object of the Invention The object of this invention is to overcome the limitations that have been noted above in all prior art. The invention sees a high modulus carbon fibre I epoxy resin, tube based upon a hyperbolic curve that is sealed so as to allow a vacuum to be maintained that will pre-stress and damp the helix of carbon fibre that will function at one end of the tube as a high performance conductor of resonant energy but then transform seamlessly, through trillions of fibres orientated in a helix, with no discontinuity into a high impedance damping material at the large diameter, opposite end of the tube. The resulting solution is superior in performance because it provides the cartridge with the best possible geometry in order to perform its task of transcribing the groove modulations but without the compromises and recognised draw backs of prior art all of which are compromised by one or more of the following critical and measureable concerns: excessive mass, insufficient stiffness, or ineffective damping of the materials that become excited by the energy transmitted by the cartridge into the tube.

Summary of the Invention

According to the present invention the preferred design should be comprised of a sealed tube formed to a large extent but not exclusively from a woven carbon fibre braid that is consolidated epoxy resin. The lower the mass the easier it is to damp the materials so the preferred option is for higher modulus fibres (120 GPa). Higher modulus fibres also provide greater disparity between themselves and the resin that is used to link the fibres which is important for reasons that are explained in more detail below. How the fibres are orientated in relation to the main axis of the tube is important the fibres should also run the full length of the tube with no discontinuity in a helix in order to deliver superior damping and high torsional stiffness. Approximating to a hyperbolic curve the gradual change in diameter will invoke change in the fibre orientation so as to provide the highest impedance and damping due to fibres running almost perpendicular or 90 degrees to the length axis of the tonearm tube. In complete contrast at the opposite, small diameter end of the tube, fibre direction is completely different with an angle of between 2 and 12 to the main axis of the tube. It is at this point that the cartridge that measures the groove of the vinyl record is attached, so fibre orientation is optimised to provide the highest velocity transfer function for the resonant energy from the stylus. At the tip, the smallest diameter will provide for the lowest mass but in comparison to the large diameter a much higher concentration of fibres resulting in very high modulus longitudinally that opposes the first bending mode that commonly occurs in tubes at this point.

Although the fibre content decreases with diameter, the stiffness is in fact higher, by virtue of the larger diameter and the addition of low mass, closed cell structural foam, crosslinking bulkheads which when bonded to the carbon fibre join the walls together and close the structure. These low mass additions delay the onset of bell modes in the walls of the tube structure. Having successfully created a gas impervious structure a tube is used to introduce a vacuum to the interior of the structure. Once evacuated the structure is sealed to maintain the vacuum. The structure is now damped across its whole surface due to atmospheric pressure differentials typically in the region of 101.325 kPa or 14.7 lbf/sq or 4.45 Newtons being the typical pressure available at sea level. The vacuum pre-stresses the carbon fibre and by so doing elevates its first natural frequency, it also acts as a damping mechanism that is totally consistent on the whole surface of the structure. Significantly, both these important benefits are realised without any added mass.

As alluded to above, it has been found that a key factor in the performance of the carbon fibre when constructed according to the description above, relates to the velocity of sound transfer along the length of the fibre. Carbon fibre and Zylon are second only to Carbon nano tubes in terms of there ability to conduct resonant energy at high speeds. In fact the resonant energy transfer function along the fibre axis of high modulus carbon fibre is approximately 3000 rn/s.

Compare this to almost every metal where the maximum possible will be around 800 m/s and there is clearly a significant difference. Conversely, the energy transfer function perpendicular to the fibre is very poor. Like the speed of sound along the fibre, the characteristic of damping within carbon fibre is directly related to the modulus of elasticity and density. When structural borne resonant energy attempts to travel from one medium with impedance zi to another with impedance z2. the first medium being carbon fibre, the second medium being epoxy resin, transmitted energy goes to zero if there is a large mismatch between zi and z2. (After Fletcher and Rossing, 1991) It follows that by carefully orientating the fibres to the requirements of the tonearm a structure can be optimised in such a way as to provide high resonant energy conductivity adjacent to the cartridge whilst moving increasingly to being a high impedance material further along the length of the tube. To this end the form and geometry of the authors structure, plays a key role in fully exploiting the properties of the braided fibre. It has been found that the ability of the material to rapidly transfer energy impacts directly upon the behaviour of the tonearm when it does become excited by resonances. It has been found that by transmitting energy rapidly from the cartridge and into regions of the tonearm where the fibres impose more impedance to the energy, that the energy is more rapidly dissipated. The outcome is a structure that settles or stops resonating more quickly.

Advantages of the Invention The invention exploits known advantages pertaining to geometry to achieve near optimum use of the carbon fibre in a tonearm. The invention exploits the known anisoptropic properties of carbon fibre to provide a single structure embued both with conductivity and high impedance in terms of structural borne energy. The atmospheric pressure differential exerted on the whole surface of the tonearm is sufficient to provide very high levels of damping with no added mass. The energy that is subjected to the tonearm by the cartridge stylus mechanism is dealt with firstly by conducting it at the highest speed away from the cartridge and then increasingly dissipating that energy by virtue of the change in fibre alignment, avoiding the need for additional mass damping. Being pre-stressed by the vacuum pressure differential, the low mass, high modulus material is damped at the frequencies where it will begin to resonate first of all. The net effect of these advantages translates directly in to better performance with a system that will be better able to transcribe the data in the grooves of the vinyl and produce less wear. As a result of the lower overall mass of the tonearm and its dynamic capabilities the dynamic functions of the cartridge and stylus assembly will be less burdened and so will be able to carry out their own respective functions more effectively.

Preferred or Optional Features The preferred design would use a braided carbon fibre. Although other methods of manufacture are possible this is the most elegant way of determining the exact alignment of the fibres. The preferred fibre would be a high modulus carbon fibre although alternative fibres can be used as can mixed fibres. Zylon for example has similar properties to high modulus carbon fibre but is adversely affected by ultraviolet light.

The preferred design might also exploit Carbon nano tubes to further enhance the stiffness of the epoxy resin elevating the stiffness of the consolidated matrix, although a reduction in damping would occur if it is used throughout the structure. Ideally if carbon nano tubes are used, they should be confined to the small diameter and up to and no further than half way down the length of the tube.

The preferred design would exploit the structural benefits to be gained by the form or geometrical benefits of large diameters within structural design. By increasing the diameter of the tube as it approaches the bearing adhering to an approximation of a hyperbolic curve huge increases in stiffness can be achieved. It should be noted that the form has to avoid collisions with other parts of the system so the form only approximates to a hyperbolic curve.

The preferred design would incorporate into the hollow structure towards and as close as possible to the pivot either a tube to allow a vacuum pump to be attached so that a vacuum might readily be introduced into the internal cavity of the structure to achieve the pressure differential that will provide the pre-stressed damped structure. The system would be detached once the vacuum had been realized and terminated in such a way as to maintain the vacuum indefinitely.

The preferred design would be a comprised of a matrix that exhibited low levels of off gassing when subjected to a vacuum. Before the final vacuum is introduced the tube would be conditioned by a number of pre vacuum states that would minimise the effects of future off-gassing. Calculations show that even a significant drop in vacuum still provides high levels of pressure differential.

The preferred design would incorporate a number of low mass transverse bulkheads within the hollow tube that would link the skins of the structure and provide the facility for the dressing of internal wires. These would typically be manufactured from a closed cell structural foam with a specification that includes a high resistance to compression.

The preferred design might also exploit mu metal or other shielding incorporated into the matrix to provide additional shielding of the conductors contained within the structure, to eliminate the polluting effect of other wave energies such as radiowaves or microwaves.

Brief Description of the Drawing

Drawing I of 3 shows the key elements of the tonearm design including the location of the vacuum valve.

Drawing 2 of 2 shows the horizontal axis of the tonearm and the pivot point.

Drawing 3 of 3 shows a schematic view of the tonearm tube to indicate the changing angle of the braided fibres relative to the main axis of the tonearrn tube as it alters according to the diameter of the tube.

Detailed Description of the Drawing

Drawing I I 3 A tonearm comprised of a location point to mount a cartridge I stylus at point I at the small diameter of the tube 2. The tube approximates a hyperbolic curve as closely as possible without interfering with adjacent structures required in the function of the record player. The largest diameter of the tube 3 is as close to the pivot point as possible whilst providing an evacuation point 4 to enable repeat evacuations of the air from the interior of the gas impervious tube.

Drawing 2 I 3 A tonearm indicating the location of the pivot 1.2 and the main axis of the tube structure 1.1 Drawing 3 I 3 A tonearm viewed from the top in schematic form to show the main axis of the tube 2.4 and the change in angle of the braided fibres that are used to create a helix of carbon fibre along the tube begining at the smallest diameter 2.1 where the warp and weft of the fibres have an included angle of approximately 22 degrees to each other and 11 degrees to the main axis of the tube. At this point the damping of the fibres is relatively low and energy is conducted rapidly away from the cartridge. As the fibres move down the tube the increasing diameter invokes a relative change in the angle of the fibres of the braid and at the mid point of the tonearm 2.2 the fibres will be nominally at 38 degrees to the main axis of the tonearm. At this region the fibres will be providing the best possible physical properties in order to resist vertical, lateral and torsional bending modes whilst also increasing the damping function of any energy attempting to cross the fibre boundaries. As the fibres move from the mid point the diameter increases and as a result the fibres change until at the largest diameter 2.4 it can be seen that the fibres are almost at 90 degrees where they function as a high impedance material to any energy attempting to move along the axis of the tonearm tube.

Claims

CLAIMS1. A low mass, high modulus gas impervious, sealed tonearm tube structure that benefits from high levels of zero mass damping of the materials from which the tube is made as a direct result of a high atmospheric pressure differential brought about by evacuating the tube.
2. A structure as claimed in Claim 1, and any claim appendant thereto, wherein the casing is provided with an extraction valve whereby further evacuation may be effected should any vacuum depletion over a period of time be experienced.
3. A structure as claimed in Claim 1, wherein, with a tone arm is manufactured from a braided carbon fibre.
4. A structure as claimed in Claim 1, wherein, the tone arm which is hollow and of carbon fibre construction has its fibre orientated by the geometry of the tube so as to provide a helix of carbon that changes orientation from being almost in line with the longitudinal axis of the tonearm tube to being almost perpendicular to the axis at the pivoted end of the tonearm.
5. A structure as claimed in any preceding claim, wherein additional shielding materials are also deployed around, and/or in association with the audio structure, to eliminate the polluting effect of other wave energies such as radiowaves or microwaves.
6. An audio structure substantially as herein described with reference to the accompanying drawings.