FR2522912A1 - Poly:norbornene support membranes for low density speaker diaphragms - filled and oil extended for optimum mechanical properties - Google Patents

Abstract

Elastomeric membranes for suspending a stiff electroacoustic transducer (loudspeaker diaphragm) has for the main constituent of the elastomer a polybicyclo-(2.2.1)heptene-2, otherwise known as 'polynorbornene'. Suitable for supporting cone, dome or trough diaphragms made of pressed pulp, polypropylene, or, esp. polymethyl pentene having a relative density of about 0.83. The diaphragm may be driven by a moving coil or by a piezo electric exciter. The polynorbornene-based rubber pref. is compounded with finely divided fillers, such as carbon black, and a high viscosity aromatic or naphtenic oil, to produce a final relative density of between 0.8 and 1.25, a mechanical wave transmission rate of between 300 and 600 m/s and a mechanical loss angle (tan S) between 0.3 and 1.4. Offers a better performance, though a combination of higher mobility, low mass and acoustic properties, than supports of expanded polyurethane, coated papers or fabrics, butyl rubber or plasticized PVC, for supporting a light stiff diaphragm responding to low frequency sound, i.e. 40 Hz-3kHz.

Description

PERIPHERAL ELASTOMER SUSPENSION OF THE MEMBRANE OF A TRANSDUCER
ELECTRO-ACOUSTIC
The present invention relates to electro-acoustic transducer membranes in which the membrane is driven by an electromechanical transducer such as that of a speaker of the voice coil type or of the piezoelectric type.

It relates to the nature of the material used to make the part of the loudspeaker, commonly called an external suspension or edge, which connects the periphery of the membrane to the chassis of the loudspeaker. As such, one of the aspects of the present invention consists of an improvement to the object of FR-A-2,471,113 in the name of the current plaintiff, which object relates to the nature of the material constituting the membrane. of the electro-acoustic transducer. According to this patent document, a loudspeaker includes a thermoformed membrane based on polymethyl-pentene, which product is known by the registered trademark "TPX" and manufactured by MITSUI PETROCHEMICAL INDUSTRIES Ltd (JP). The loudspeaker has many qualities hitherto not encountered in so-called traditional loudspeakers. Among these qualities, we can cite the following
- a very high efficiency providing a very high sound level for a given applied electrical power; this results from the very lightness of the membrane material, the density of which is around 0.83, and from its very great speed of the vibratory waves propagating in the membrane material and responsible for the sound emission, which speed is around 2000 m / s.

- a high mechanical resistance as a function of the temperature, which makes the loudspeaker usable in cases of climatic severity such as those encountered in the passenger compartment of motor vehicles
- a great neutrality of sound reproduction which is the consequence of the low value of the mechanical overvoltage factor due to the excellent internal damping of the material, which reduces the amplitude of deformation of the natural modes of the membrane
- dragging of the loudspeaker reduced in large proportions.

 It should be noted that the use of a polymethyl-pentene membrane has made it possible to solve the problems usually posed when designing a traditional speaker with a paper membrane. These problems amount to reconciling the antinomies on the one hand, between the quality of sound reproduction and the energy efficiency, and on the other hand, between the energy efficiency and the directivity of the acoustic response in space.

The excellent qualities linked to the use of polymethyl-penthene to constitute a loudspeaker membrane are unfortunately reduced when it has to be included in a loudspeaker specialized in the reproduction of bass and midrange, that is - pull covering the frequency range between 40 Hz and 3 KHz. Indeed, to radiate low frequencies, the membrane is constrained X large axial displacements which would not be authorized due to the stiffness, if the suspension was made in the same material based on polymethylpentane. The choice for the material of the suspension is therefore reduced to traditional materials such as
- polyurethane foam;
- coated fabric;
- treated corrugated paper;
- butyl elastomer;
- Plasticized PVC.

 But none of these materials has the required qualities-to be associated with a membrane based on polpmethyl-pentene, neither with regard to the speed of propagation of vibrational disturbances, nor with regard to the internal damping in the material .

 The use of the abovementioned traditional materials to constitute an external suspension of a membrane based on polymethyl-penQne results in notable accidents in the amplitude-frequency transfer function, the acoustic radiation of the suspension being not negligible compared to to that the membrane. The consequences are manifold and lead to a deterioration in the characteristics of direction, harmonic distortion and mainly of the pulse response and dragging.

The object of the present invention is therefore to select from the materials which can be transformed - flowable or conformable - easily by any process into an external membrane suspension, those which are commercially available at a relatively low price and whose main characteristics range from the following directions - density as small as possible - vibratory speed as close as pssiie 6 ceUe dl pcilynéb- slope ne - internal damping :: tang 6 close to 1
To this end, a peripheral elastomeric suspension of the membrane of an electro-acoustic transducer is characterized in that the elastomer has as its principal constituent a polybicyclo-D2.2.10 heptene-2.

 Polybicyclo- 2.2. heptene-2 are also known as "polynorbornenes" and are marketed by C.D.F. CHEMISTRY (FR) under the registered trademark "NORSOREX". A polynorbornene is a polymer of bycyclo 2.2.1 heptene-2 called "norbornene". Among the numerous known elastomeric compositions having a polynorbornene as the main macromolecular constituent, the Applicant has selected such elastomeric compositions whose main mechanical characteristics make it possible to reduce to a large extent the drawbacks of known suspensions.

These features are:
- density from 0.8 to 1.25
- speed of vibratory waves propagating in the material of the suspension between 300 and 600 m / s;
- internal damping value leading to a tangent of the mechanical loss angle, generally indicated by tang 6, between 0.3 and 1.4.

Among the known elastomer compositions based on polynorbornene, those having the above mechanical characteristics are such that a compound with a high glass transition point of between 0 and 150 Celsius has been incorporated in a relatively high proportion, for example from 100 to more 200 parts by weight per 100 parts of polynorbornene, and finely divided charges. Preferably, the compound with a high glass transition point is a hydrocarbon or a mixture of hydrocarbons such as predominantly aromatic and / or naphthenic oils with high viscosity. The fillers are preferably carbon black such as black Vulcan 3 (HAF black; in English "High Abrasion Furnace") in a very high proportion, for example 150 to 300 parts by weight per 100 parts of polynorbornene. also be from silica, kaolin, chalk, calcium carbonate, etc. The elastomer composition
may also include an ozone protection agent.

These different formulations of the elastomeric composition are only intended to achieve significant damping in the temperature range of use of a loudspeaker, that is to say between + 15 and + 300 Celsius.

Other advantages and characteristics of the present invention will appear more clearly on reading the following description of several embodiments of the invention with reference to the corresponding appended drawings, in which
- Fig. 1 shows three waveforms delivered by an audio-frequency generator to excite a loudspeaker;
- Fig. 2 shows three waveforms emitted by a loudspeaker according to the invention in response to the excitations by the three waveforms of the generator shown in FIG. 1 respectively
- Fig. 3 shows three waveforms emitted by a known loudspeaker in response to excitations by the three generator waveforms shown in FIG. 1 respectively ;;
- Fig. 4 shows three waveforms emitted by another known speaker in response to excitations by the three generator waveforms shown in FIG. 1 respectively;
- Fig. 5 is a sectional view of a conical diaphragm speaker and suspensions according to the invention;
- Fig. 6 is a sectional view of a dome diaphragm speaker and suspension according to the invention; and
- Fig. 7 is a sectional view of a dihedral diaphragm speaker C and suspensions according to the invention.

By way of example, measurement results relating to a loudspeaker according to the invention of the conical membrane type, of the kind described below with reference to FIG. 5, are presented below. The membrane and the suspensions of this loudspeaker are made of materials according to the invention. The membrane is made of polymethyl-pentene. The external membrane suspensions are made of an elastomeric composition based on a polynorbornene and have the following mechanical characteristics
- hardness: 50 to 70 according to the SHORE standard;
- density: 1.12;
- vibratory speed: 410 m / s;
- internal damping: tang 6 0.40 for a temperature range from + 15 to + 30 Gisius and a frequency range from 100 Hz to 10 KHz.

 For three excitation wave trains at 850 Hz, 2 kHz and 3 kHz shown in Fig. 1, the three respective sound responses - relative amplitude as a function of time in milliseconds - of the aforementioned loudspeaker according to the invention are shown in FIG. 2.

The three audible responses in Fig. 2 are to be compared with the three audible responses shown in Figs. 3 and 4 which come from
- for Fig. 3, of a conventional loudspeaker having a membrane in pressed cellulose pulp and suspensions in polyurethane foam,
- for Fig. 4 of a conventional loudspeaker having a membrane which is thermoformed of polystyrene known under the registered trademark "BEXTRENE" and enauite on both sides with the same layer of polyvinyl acetate of the genus "PLASTIFEX" (registered trademark), and suspensions in plasticized PVC.

 These two conventional loudspeakers have dimensions and arrangement of their components identical to those of the aforementioned loudspeaker according to the invention and have also been excited by the same three wave trains shown in FIG. 1.

 As expected by theory, the examination of the properties of a loudspeaker according to the invention equipped with a polymethyl-pentene membrane associated with external suspensions based on polynorbornene, clearly shows that the defects indicated in the preamble descriptive have almost disappeared.

 The extreme speed of reaction of the material which is equal to 410 m / s, that is to say more than six times that of butyl rubber equal to 60 m / s, is combined with the low density to contribute to reduce the response time of the peripheral suspension, which thus rejects outside the passband of the loudspeaker the peripheral accident of the transfer function always present for conventional loudspeakers. A suspension in the loudspeaker according to the invention now perfectly follows the movements imposed on it by the membrane, with maximum speed, thus making the suspension participate correctly in the acoustic radiation. In addition, the very significant internal damping of the suspens ion leads to a total overvoltage coefficient of the combination formed by the membrane and the suspension of the order of 2, which improves the dynamic behavior of the membrane in its peripheral region while prohibiting its own modes of oscillation.

 The amplitude-frequency response of a loudspeaker including a combination of membrane and suspension according to the invention is perfectly uniform, free from parasitic resonances.

 The directivity characteristic of the loudspeaker is greatly improved and comes from the control of the periphery of the membrane by the damping properties of the polynorbornene suspensions.

 But the main advantage of the joint use of polymethylpentene for the associated membrane and polynorbornene for suspensions lies in the extraordinary characteristic of impulse response close to the theoretical ideal.

This advantage becomes clear when comparing FIG. 2 with
Freezes. 3 and 4; the impulse responses of FIG. 2 for the loudspeaker according to the invention are almost the reproduction of the excitation wave trains of FIG. 1, which is not the case for the responses shown in Figs. 3 and 4.

 When we know that music, speech or noise, in short, sound and musical events are mainly made up of transient regimes, we better measure what brings the use of materials based on polynorbornàne for or the external and polymethyl-pentene-based suspensions for the membrane of a loudspeaker.

 One or more peripheral suspensions based on polynorbornene can moreover be advantageously associated with a membrane, the material of which has as its principal constituent a polypropylene or a chemical composition containing polypropylene and polyethylene of means; density or pressed or dried cellulose fiber.

 Referring now to Fige. 5 to 7, note that some.

loudspeaker components, such as the connections for supplying the coils, have been omitted from the figures to clarify the drawings and that certain components shown are not on the same scale as that of other components. This last remark is particularly true for the dimensions of the air gaps and the relative dimensions of members at their connection point.

 We recognize the Pig. I, a speaker specialized in your reproduction of low and medium frequencies. It has a membrane 1 which is frustoconical with a straight generator? as illustrated, or with a generator following a conic such as a hyperbola or parabola, or even with an exponential generator, or even any. The membrane 1 is made of a material having as a principal constituent of polymebyl-pentene or of polypropylene, or of a conventional material such as pulp of pressed or dried cellulose. In the following, “material of membrane according to 1 invention” will denote a material as defined above.

 The small base of the membrane 1 is terminated by a cylindrical section 10, of short length, which is suitably glued to the front of the cylindrical support or mandrel 20 of the voice coil 21 of the electro-mechanical motor 2 of the loudspeaker. This bonding does not particularly require a special glue. The rear part of the support 29 supports the turns of the coil 21 which are housed in the middle of the air gap 22 of the magnetic circuit of the motor 2. The magnetic circuit consists of a pole piece before discold 23 which rests on a permanent annular magnet 24 placed on a rear magnetized plate 25. The core 26 of the magnetic circuit is coaxial with the pole piece 23 and the magnet 24 and is partly internal to the support 20 of the voice coil 21.

 Above the magnetic circuit, the sectional frame or bowl 3 of the loudspeaker rests coaxially on the pole piece 23. Two or more end and intermediate suspensions can connect the membrane 1 to the frame 3 to center the latter. The material of the suspensions has as main constituent of polynorbornene. The external 11 and internal 12 suspensions are only shown in FIG. 5. The external suspension 11 connects the periphery of the large upper front base of the membrane 1 to the large base of the chassis 3. The internal suspension 12 connects the small rear cylindrical section 10 of the membrane 1 to the small base of the chassis 3. These last connections are made by gluing.

 Significantly above the small base of the membrane 1, a dust cover dome 13 closes the front opening of the motor 2, that is to say the front of the support 20 of the coil 21. This dust cover dome 13 may also be made of a material having the main constituent of polynorbornene.

 Fig. 6 shows a speaker specialized in the reproduction of medium and high frequencies. The membrane 4 of this loudspeaker is made of a “membrane material according to the invention”. It is in the shape of a hemispherical dome. In general, this dome of revolution can be of rectilinear, circular, conical or any similar generator. The rear portion of the membrane 4 is terminated by a small cylinder 40 which is glued around the front of the support 50 of the voice coil 51 of the motor 5 of the top gun. In a similar manner to the motor 2 described above, the motor 5 according to FIG. 6 also includes a pole piece before discold 53 surrounding the coil 51, an annular permanent magnet 54 resting on a rear magnetized plate 55 and a core 56 which is coaxial with the magnet 54 and which forms with the pole piece 53 an air gap 52.

 Above the motor 5, the perforated front part of the chassis 6 of the speaker rests on the pole piece 53 by means of an annular spacer 60. The membrane 4 is centered above the spacer 60 by means of an annular suspension 41 which is a material having as its main constituent polynorbornene. The suspension 41 has its internal peripheral edge which is glued to the cylindrical rear section 40 of the membrane 4, and its external peripheral edge which is braced and clamped between the frame 6 and the spacer 60. Under the membrane 4 and above the core 56 is provided with an acoustic felt 43, also in the form of a dome.

In addition, it should be noted that the suspensions of the loudspeakers of the
Figs. 5 and 6 can be configured as a half-torus whose concavity is turned either towards the interior as for the suspension there (Fig. 5) or towards the outside of the loudspeaker. They can also be substantially S-shaped as for suspension 41 (Fig. 6) or V.

 Lg Fig. 7 shows a loudspeaker having a membrane 7 which is in the form of a dihedral and of a "membrane material according to the invention". This loudspeaker operates in a very wide frequency band covering, in general, the entire audible frequency band.

 The two front ends 70 of the two rectangular or square flat surfaces forming the dihedral of the membrane 7 are rigidly fixed to a rectangular or square front frame 90 of the chassis of the loudspeaker. More generally, the open dihedral front part of the membrane 7 can be developed along rectilinear, exponential generatrices, similar to a conical portion, or else any. In order not to overload Fig. 7, the chassis has only been shown in part.

 A planar element 71, shown in rectangular form, extends rearward the arrent 72 of the dihedral of the membrane 7 and is made of a "membrane material according to the invention". The rear plane element 71 is coplanar with the vertical plane of symmetry X'X of the dihedral and is symmetrically embedded in the air gap 82 of the motor 8 of the loudspeaker, here symmetrical structure. The rear edge 73 of the planar element 71 is connected to a rear upright 91 of the chassis by means of a wavy suspension 74 which is substantially coplanar with the plane X'X. The suspension 74 is made of a material having the main constituent of polynorbornene.

 According to another variant, two suspensions of material having the main constituent of polynorbornene can be provided to connect the two ends 70 of the dihedral front part of the membrane 7 to the front frame 90, and the rear edge 73 can be fixed rigidly and directly to the rear post 91.

 The motor 8 of the loudspeaker shown in FIG. 7 comprises a movable assembly consisting of a flat coil 81, the wire of which is spirally rectangular on the faces of the rear planar element 71 of the membrane 7.

The conductive strips 810 forming the long sides of the turns of the coil 81 are the only generators of an induction in the magnetic circuit of the motor 8. They are perpendicular to the direction of the small displacements of the membrane 7 and centered in the two magnetized air gaps 82 opposite the motor 8. The ends of the internal and external turns of the coil 81 are welded to two supply wires 811. The conductive strips 812 forming the short sides of the turns of the coil 81 are parallel to the movement of the coil between the air gaps 82 and simply serve as a current return.

The fixed part of motor 8 - half of which is shown in the
Fig. 7- comprises for example two parallelepiped magnets 84 which are arranged symmetrically on either side of the plane X'X and which are braced by two pairs of flat pole pieces 83. The large conductive tapes 810 of the coil 81 are inserted centrally in the two air gaps 82 formed by the four pole pieces 83. The whole of the fixed part of the motor 8 is fixed inside a double rectangular central frame 92 of the chassis of the loudspeaker.

 Although the invention has been described with reference to preferred embodiments of loudspeakers, membranes made of "membrane material according to the invention" and suspensions made of material having as main constituent polynorbornene for other types of loudspeakers speakers belong to the scope of the invention defined by the appended claims. Thus, the invention also relates to loudspeakers of the piezoelectric excitation type.

Claims (10)

Claims.  1 - Peripheral suspension (11, 12; 41; 74) in elastomer of the membrane (1; 4; 7) of an electro-acoustic transducer, characterized in that the elastomer has for principal constituent a polybicyclo- [2.2. 1] heptene-2 (polynorbornene).  2 - Suspension according to claim 1, characterized in that the material of the suspension (11, 12; 41; 74) has a density of 0.8 to 1.25, a speed of vibratory waves propagating in the material between 300 and 600 m / s, and an internal damping value leading to a tangent of the angle of mechanical loss (tang ô) between 0.3 and 1.4.  3 - Suspension according to claim 1 or 2, characterized in that the material of the suspension (11, 12; 41; 74) contains in particular a compound with a high glass transition point such as a predominantly aromatic oil and / or High viscosity naphthenic and finely divided fillers such as carbon black.  4 - Suspension according to one of claims 1 to 3, characterized in that it is shaped as a half-torus (11) whose concavity is turned towards the inside or the outside of the transducer.  5 - Suspension according to one of claims 1 to 3, characterized in that it is substantially shaped S (41) or V.  6 - Suspension according to one of claims 1 to 5, characterized in that the membrane to which the suspension is connected ion is based on polymethyl-pentene.  7 - Suspension according to one of claims 1 to 5, characterized in that the membrane to which the suspension is connected is based on polypropylene.  8 - Suspension according to one of claims 1 to 5, characterized in that the membrane to which the suspension is connected is in pressed or dried cellulose pulp.  9 - Loudspeaker of the voice coil type, characterized in that it comprises a peripheral membrane suspension in accordance with one of claims 1 to 8.  10 - Loudspeaker of the piezoelectric excitation type, characterized in that it comprises a peripheral membrane suspension according to one of claims 1 to 8.