Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R7/00—Diaphragms for electromechanical transducers; Cones
  • H04R7/02—Diaphragms for electromechanical transducers; Cones characterised by the construction

Definitions

• Loudspeakers are an integral part of sound reproduction systems.
• Conventional loudspeakers include electrostatic and magnetic loudspeakers.
• An electrostatic or condenser loudspeaker is fundamentally a huge capacitor and generally comprises a conducting diaphragm situated in a polarized field. Sound is produced when an audio signal is induced onto the diaphragm, the capacitors inner plate, which modulates the polarized voltage, causing the diaphragm (inner plate) to be both pushed and pulled between two outer, fixed immovable electrodes in accordance with the signal.
• the flexible, conductive diaphragm has generally consisted of a nonconducting material coated with thin conducting material having a high degree of resistance, e.g., a very thin plastic film with a conducting metallic coating.
• a magnetic or driving coil loudspeaker comprises a voice coil, diaphragm and suspension system.
• An electromagnetic motor transforms an audio signal into a vibrating diaphragm, the diaphragm being conically shaped to maximize mechanical stability.
• the vibrating surface of the diaphragm compresses air into motion (sound waves) which we recognize as sound.
• Conical diaphragms have been composed of various materials, including metal, cellulose paper, plastic, cloth, wool and expanded polystyrene. However, it was apparent that no material previously attempted was ideal nor did it provide the proper balance between mechanical stability and acoustic performance.
• sonic velocity of paper is meant the square root of the ratio of its Young's modulus to its density. Sonic velocity aids in analyzing the extent to which a conical diaphragm simulates ideal piston-like motion. Increasing the sonic velocity not only decreases frictional energy losses in the diaphragm produced from out of phase vibrational patterns and corresponding cancellation effects that result therefrom, but also increases high frequency response. Consequently, the sonic velocity of a diaphragm is a measure of the limits in the crispness of transient responses and range of linear frequencies the diaphragm may provide.
• Aluminum for example, provided a speaker diaphragm having a sonic velocity about twice that obtainable with cellulose paper, but the diaphragm required reinforcing joints to dampen circumferential vibrations associated with its inherent thinness. Anodising the aluminum diaphragm increased its maximum sonic velocity to about four times that obtainable with cellulose paper.
• An object of this invention is to provide a woofer diaphragm comprising a carbonaceous fibrous web impregnated by a resin, which diaphragm is capable of reproducing a wider frequency range than comparable paper woofer diaphragms.
• a further object is to provide an electrostatically driven speaker diaphragm comprising a conductive carbonaceous fibrous web impregnated by a resin.
• this invention relates to a diaphragm for use in an electroacoustical device, the diaphragm comprising a carbonaceous fibrous web impregnated by a resin.
• the diaphragm of this invention is characterized by having a high specific modulus and sonic velocity.
• modulus as used throughout the specification unless otherwise indicated, should be understood to equal the modulus of elasticity (Youngs modulus) divided by the basis weight of the material, ie its weight per unit area.
• impregnated should be understood to signify the result of impregnating and/or surface coating the carbonaceous fibrous web with a resin. It does not necessarily imply that all, or even a major proportion of the interstices or other voids of the carbonaceous fibrous web must be filled or covered with resin. However, it is preferred to have the resin adhering or clinging to a major portion of the fibers of the carbonaceous fibrous web.
• Individual filaments of carbon fiber which are useful in forming a diaphragm material are known to have a range of moduli extending from about 10 million psi to about 100 million psi and a maximum density of about 2 gm/cm.
• a carbonaceous fibrous web of this invention may be formed using these fibers and impregnated by a resin. It is possible for the diaphragm of this invention to reach a fiber loading of between about 30% to about carbon fibers by volume, the balance being a resin.
• diaphragms having a fiber loading between about 60% to about 70% carbon fibers by volume, the balance being a resin will be used in this invention.
• the carbonaceous fibrous web impregnated by a resin which is used as the diaphragm material of this invention may have a maximum density of about l.7 gm/cm and a maximum modulus of elasticity of about 30 million psi.
• a carbonaceous fibrous web impregnated by a resin useful in this invention should have a modulus of elasticity varying from about 3 million psi to about 30 million psi and a density varying between about 0.8 gm/cm and about 1.7 gm/cm respectively.
• the diaphragms of this invention should have sonic velocities that exceed the maximum sonic velocity of a high quality, resin impregnated, cellulose paper diaphragm and equal or exceeding even aluminum diaphragms.
• Loudspeaker diaphragms comprising a carbonaceous fibrous web impregnated by a resin are characterized by having sonic velocities between about 2.5 X 10 cm/sec and about 1 l X cm/sec, based on available test data, the examples reported hereinafter and theoretical expectations.
• a diaphragm comprising a carbonaceous fibrous web impregnated by a resin and having a sonic velocity greater than about 2.6 X 10 cm/sec and more preferably greater than about 4.6 X 10 cm/sec should be used in connection with this invention.
• the I sv sv wherein SV is the sonic velocity of the diaphragm of this invention and SV is the sonic velocity of an aluminum diaphragm.
• a carbonaceous fibrous web impregnated by a resin and useful in this invention may have a modulus of elasticity between about 4.4 X 10' dynes/cm and about 220 X 10 dynes/cm preferably between about X 10 dynes/cm and about 220 X 10 dynes/cm More significant a characterizing property of the diaphragm of this invention is its specific modulus of elasticity.
• the diaphragm of this invention preferably has a specific modulus of elasticity which may be defined by the formula wherein SM, is the specific modulus of elasticity of the diaphragm of this invention and SM is the specific modulus of elasticity of an aluminum diaphragm.
• the diaphragm of this invention moreover, possesses both a higher specific modulus and sonic velocity when compared to the finest cellulose paper.
• the diaphragm comprising a carbonaceous fibrous web impregnated by a resin may follow higher frequencies than a cellulose paper diaphragm of the same shape, size and weight.
• the specific modulus and sonic velocity of the diaphragm comprising a carbonaceous fibrous web impregnated by a resin is also comparable or superior to the finest aluminum diaphragms.
• the transient response of the diaphragm of this invention to be sharper than a comparable cellulose paper diaphragms due to its superior specific modulus.
• High modulus, high strength carbon fibers suitable for use in this invention may be prepared as described in US. Pat. Nos. 3,412,062; 3,503,708 and 3,529,934.
• the carbon fibers when cut to a size suitable for process may be employed in the instant invention.
• the carbonaceous fibrous web preferably employed in this invention may be prepared by processing carbon fibers by any method, either wet or dry, which effects the dispositiono f such fibers in intimately contacting relation in a fibrous body. Air laying operations such as carding and garnetting which effects a relatively oriented disposition of fibers into a paper sheet are suitable for this purpose.
• a carbonaceous fibrous web impregnated by a resin is the preferred fabric for use in this invention.
• a carbon fiber sheet or paper may be prepared by water laying short carbon fibers using well known paper making techniques.
• a carbonaceous fibrous web is also preferably prepared by collecting pitch fibers and forming them into a carbonaceous fibrous web.
• a non-woven carbonaceous fiber web may be resin impregnated, shaped by well known techniques and the resin cured, forming a shaped diaphragm.
• the fibers are first cut or chopped to a size suitable for processing, e'.'g., about %'inch in length; homogeneously intermixed with water and a suitable binder, such as starch or other well known binder, to form an aqueous slurry; then deposited from the slurry upon a substrate such as a flat sieve of fine mesh which retains the fibers to form a sheet.
• a suitable binder such as starch or other well known binder
• a wet sheet is generally formed either by running a dilute suspension of carbon fibers evenly onto the surface of a moving endless belt of wire cloth, through which excess water may be drained, or by running an endless belt of wire cloth through a suspension of carbon fibers.
• the Fourdrinier process part of the water drains off by gravity, a part is taken from the sheet by suction, and a part is removed by pressure; in the second case, a vacumm is maintained below the stock level in the cylinder in which the wire cloth is rotating and the sheet forms on the wire by suction much as does a cake on a vacuumfilter.
• most paper grades are formed by the'first process. In either case, the thickness of the sheet is controlled by the speed of travel of the machine, by the consistency (ratio of fiber to water) of the suspension, or by the amount of stock allowed to flow onto the machine.
• a loudspeaker diaphragm typical of this invention may be constructed with paper prepared from artificial or natural carbon fibers having a diameter between about 3 pm and about 15 um, and preferably between about 4 pm and about 8 p.rn.
• the diaphragm may have a modulus exceeding fibers may also be employed in about 3 million psi and preferably exceeding about million psi and more preferably exceeding about million psi.
• non-woven carbonized fiber web made from chopped or blow spun. staple fibersmaybe used as the precursor to the speaker diaphragm of this invention.
• the non-woven carbonaceous web may be impregnated with a suitable resin such as an epoxy, furane, phenolic, melamine or the like. Thereafter, the web may be physically processed in a conventional manner to form a shaped web which shape the diaphragm will have.
• the resin impregnated web may then be heated a predetermined time at a temperature sufficient to cure the resin and maximize the diaphragm stiffness. This heat treatment may be combined with the shaping step, if desired.
• a diaphragm typical of this invention may be formed by blowing carbonized staple fibers onto a preshaped screen.
• a vacuum is created behind the screen to assist collecting and binding the staple fibers together into a carbonaceous fibrous web.
• a binder dissolved in a solvent is preferably added to the staple fibers during this process.
• the binder used is a resin such as an epoxy, furane, phenolic, melamine or the like dissolved in such solvents as acetone, toluene, benzene, methyl ethylketone or the like.
• the diaphragm may be directly formed in the above manner without need for further processing, the diaphragm having the shape of the preshaped screen.
• Adding a binder not only joins the fibers together, but strengthens the carbonaceous fibrous web.
• the binder such as a carbonizable resin is added during the carbonaceous fibrous web making process or applied as a coating to a finished carbonaceous fibrous web, it should be heated to a temperature sufficient to cure the resin and maximize stiffness of the web.
• Carbonizable resins which stiffen the carbonaceous fibrous web as described hereinabove include phenolic, epoxy, furane and the like.
• a diaphragm typical of this invention may be varied in shape. For example, it may be conical in shape or conically shaped near the apex of the cone. Furthermore, by any one of several conventional physical or machining methods such as a molding process and the like, a diaphragm typical of this invention may be provided with circumferencial, annular or sinusoidal shaped annular corrugations and the like.
• a diaphragm typical of this invention may be uniform in thickness or graduated, if desired at one end.
• a thickness of between about 0.005 inch and about 0.025 inch is employed for a diaphragm used in magnetic loudspeakers.
• a diaphragm as described in this invention may be used in any one of several magnetic loudspeakers commonly employed.
• the magnetic loudspeaker may employ multiple small conical diaphragms or conical diaphragms in sealed enclosures or conical diaphragms at the end of tubes.
• improved magnetic loudspeakers of various shapes, dimensions and designs having superior tone qualities and frequency ranges can be produced employing diaphragms typical of this invention.
• a diaphragm typical of this invention may be used to reproduce the entire audible range, i.e., from about 20 to about 20,000 HZ (cycles per second it is preferable, in certain instances, to employ a combination of several speakers in a sound reproduc- 6 tion system each accuratelyreproducing only limited bands within the audible frequency range.
• a diaphragm in accordance with this invention may be used in a woofer loudspeaker which generally reproduces the bass range of audible frequencies.
• an electrostatic speaker dia phrag typical of this invention comprises a carbonaceous fibrous web impregnated by a resin.
• the results of the examples described hereinbelow illustrate the electric conductivity of the carbonaceous fibrous web impregnated by a resin. Being electrically conductive, the carbonaceous fibrous web impregnated by a resin not only fulfills the conductivity requirement of an electrostatic speaker diaphragm but also eliminates the need for applying a surface coating to make the electrostatic speaker diaphragm conductive as is now done.
• the material employed for the electrostatic speaker diaphragm may be formed of a resin impregnated fibrous web and range in thickness from about 0.005 inch to about 0.01 inch. Preferably, a thickness of about 0.007 inch is employed.
• An electrostatic speaker diaphragm manufactured according to the teachings set forth herein can compare favorably with the linearity of frequency response curves of electrostatic speaker diaphragms consisting of conventional materials having comparable types of construction and capabilities.
• EXAMPLE Four epoxy resin impregnated sheets having a fiber loading of approximately 55% and four phenolic resin impregnated sheets having a fiber loading of approximately 60% were prepared for testing purposes. At the outset, two pieces each of four different carbon fiber mats were obtained. Two of the mats were blow spun, staple fiber mats, Grades VM0032 and VM0033 (commercially available from Union Carbide Corporation, Carbon Products Division, New York, N.Y.). The third was a VFB paper which is basically a low twist, rayon base, carbon yarn which has been carbonized at 700C., chopped into staple fibers and made into a paper using the Fourdrinier process. The fourth was a carbon fiber mat having 16 inch staples, produced from melt spinning of pitch and subsequentheat treatment at 1400C. In addition a resin impregnated cellulose paper was obtained from a 6 X 9 inch Utah RC 69 D speaker diaphragm (commercially available from Utah Electronics, Huntington, 1nd.).
• each sample was approximately 0.01 inch thick and held still /z hour at 100C.
• the temperature was raised to C. and held there for 2 hours, and then was raised to C. and held there for 2 additional hours.
• the samples were cooled under pressure and removed from the press at 120C.
• the samsamples were broken with guage lengths of 1 and 4 ples were then molded in a steel mold using silicone inches at a cross head speed of 0.05 inches per minute release paper to separate plies at a pressure of 1000 psi and tensile strength, modulus of elasticity and sonic and temperature of 125C. for minutes.
• the samples 15 velocity were calculated from head speed values and were then removed from the press. chart rates.
• a loudspeaker diaphragm said diaphragm consisting essentially of a carbonaceous fibrous web of carbon fibers and a resin, said carbon fibers representing between about and about 70% by volume of said diaphragm, the balance being said resin, said diaphragm having a Young's modulus between about 3 million psi and about 30 million psi and a density between about 0.8 gm/cm and about 1.7 gm/cm so as to provide a sonic velocity for said diaphragm between about 2.5 X 10 cm/sec and about ll X 10 cm/sec.
• a diaphragm as defined in claim 1 having a thickness between about 0.005 inches and about 0.025 inches.
• a diaphragm as defined in claim 11 having a thickness between about 0.005 inches and about 0.010 inches.

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• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Diaphragms For Electromechanical Transducers (AREA)
• Electrostatic, Electromagnetic, Magneto- Strictive, And Variable-Resistance Transducers (AREA)

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Citations (6)

• 1973
  • 1973-09-21 US US399319A patent/US3930130A/en not_active Expired - Lifetime
• 1974
  • 1974-08-21 CA CA207,489A patent/CA1009156A/en not_active Expired
  • 1974-09-20 GB GB41022/74A patent/GB1487942A/en not_active Expired
  • 1974-09-20 JP JP10866874A patent/JPS557755B2/ja not_active Expired

Patent Citations (6)

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