Patent Publication Number: US-8116512-B2

Title: Planar speaker driver

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application No. 60/825,690 entitled “Planar Speaker Driver” filed on Sep. 14, 2006, the content of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Art 
     The present invention generally relates to acoustic devices, and more specifically to a planar speaker driver. 
     2. Description of the Related Art 
     Planar (planar-magnetic, ribbon, thin film drivers) drivers have always been praised for exceptional sound quality associated with their unique acoustic attributes. This invention describes a wide-band planar transducer with high sensitivity, extended lower frequency operating band, higher power handling and low distortion. 
       FIG. 1  illustrates a cross-section showing a basic construction of a typical planar-magnetic transducer. A common type of such transducer incorporates a diaphragm  1  with areas of multiple electrical conductors  2 . A diaphragm  1  is clamped in a frame  5  and is positioned between two rows of magnet bars  3 . Magnets are sequentially located on top and bottom metal plates  4  with spaced areas  7  between the magnets. Holes  6  in metal plates  4  correspond to the spaced areas  7  between magnets  3 , acoustically connecting diaphragm  1  with outside media. 
     Magnets  3  are magnetized in a direction perpendicular to metal plate  4  so that a magnet from one side of a diaphragm and the opposite magnet from the other side of diaphragm are facing diaphragm and each other with the same magnetic poles (S or N). Each adjacent magnet bar that is located on the same side of the diaphragm has the opposite direction of magnetization, thus each following magnet faces the diaphragm with the opposite magnetic pole, following the sequence N,S,N,S,N and so on. Magnetic field created by the magnet arrangement has the magnetic flux vector B in a plane of the diaphragm across the lines of conductors. 
     When an electrical signal is applied to the diaphragm, the current that flows through conductors interacts with the magnetic field and resulting electromotive force makes the diaphragm vibrate in the direction perpendicular its plane. Vibrating, the diaphragm  1  radiates sound waves that emanate through the openings  7  between magnets  3  and holes  6  in metal plates  4  in both directions from the diaphragm  1 . Different acoustical loading conditions may be applied to the design such as using a metal plate  4  with variations in the holes  6  (e.g., slots, or solid regions) or attaching an enclosure form one side of a transducer. 
     The use of rear earth magnetic materials such as NdFeB (Neodymium) that has become the magnet material of choice in transducers recent years, allows significant reduction of size and efficiency improvement of transducer designs. As a result such designs can provide very high quality sound with minimal front to back space required, thus allowing building of “flat” panel planar loudspeakers for many critical applications. 
     Among performance limitations traditionally associated with planar drivers are limited low frequency extension and limited dynamic range at those frequencies. Both of these issues are mostly related to two aspects of driver design and operation: maximum diaphragm excursion capability and vibration behavior of the diaphragm within the operating range. 
     In order to extend effective frequency range of such design in a region of lower frequencies, a transducer has to have significant radiating area. However, a larger diaphragm has much less vibration control and generates significant modal vibrations due to insufficient mechanical losses in diaphragm substrate, usually plastic film. These pronounced vibrations at diaphragm resonance frequencies lead to response irregularities and parasitic noises at lower frequencies that are very often encountered in planar transducers. 
     Many designs use coating of the diaphragm with dampening materials and/or corrugation over the whole diaphragm area. Both of these methods have negative effects. A coating leads to higher mass and efficiency losses. Corrugation of the entire diaphragm increases the effective thickness of the diaphragm where active conductions are located and thus limits maximum excursion of the diaphragm. Additionally the corrugation of diaphragm in the area of active conductors that are made of very thin metal foil can introduce internal stresses in the conductor and/or in the bond between polymer film and the foil conductor. Under high thermal and mechanical stress due to vibrations the internal stresses can then lead to premature de-lamination or cracks in the conductors. 
     SUMMARY OF THE INVENTION 
     A planar transducer with extended low frequency operational band and high efficiency is disclosed. In one aspect, the planar transducer comprises a diaphragm having a corrugated peripheral region disposed between an edge of the diaphragm that is secured between the plates, and the operative area (a planar central region) of the conductive portions of the diaphragm within the magnetic gap of the magnets. This aspect increases the internal dampening of the diaphragm, and provides for extended lower frequency response, and an overall smoother frequency response due to reduced parasitic diaphragm noise and buzz. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a cross-sectional view of a conventional planar-magnetic transducer. 
         FIG. 2  is a perspective and cut-away view of a planar driver in accordance with an embodiment of the present invention. 
         FIG. 3  is a detailed view of a cut-away of a planar driver in accordance with an embodiment of the present invention. 
         FIG. 4  is a cross-sectional view of a planar driver in accordance with an embodiment of the present invention. 
         FIG. 5  is view of a diaphragm without corrugation for use in a planar magnetic driver in accordance with an embodiment of the present invention. 
         FIG. 6  is a view of a diaphragm with a corrugated peripheral region for use in a planar magnetic driver in accordance with an embodiment of the present invention. 
     
    
    
     The figures depict various embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
     DETAILED DESCRIPTION 
       FIGS. 2-6  depict various views of embodiments of a planar driver showing various features of the present invention.  FIG. 2  illustrates a perspective and cut-away view of planar driver in accordance with the present invention.  FIG. 3  illustrates the cutaway portion of  FIG. 2  in further detail, with force arrows showing the directions of magnetic force. Shown here are top and bottom plates  10  with attached magnets  12  on the interior facing surfaces of the plates, with diaphragm  14  disposed therebetween along the edges  24  of the plates  10 . Holes  16  are disposed in the plates  10 . The diaphragm  14  comprises a foil film  18  and conductors  20  (e.g., aluminium strips). A terminal  22  is coupled to the conductors  20  to provide electrical communication to a signal. 
     Generally, a clamped diaphragm does not vibrate as a piston. At lower frequencies especially at the fundamental resonance the amplitude of vibrations are much larger in the middle of diaphragm than at the periphery near clamped edges. As shown in  FIG. 3 , magnets  12  with opposing direction of magnetization generate a repelling force. The repelling force acts on the plates  10  and pushes them away from each other. In accordance with the present invention, the magnets  12  provide a repelling force that pushes the plates  10  away from each other, so that the plates are maintained under biased tension, each plate being in a slightly curved (buckled) or arcuate shape, producing a small rise in the middle of the plates  10  relative to the sides of the plates next to the edges  24 .  FIG. 4  shows the cross-section of the planar driver across the magnets. Broken lines  42  show the rise in the middle of the plates  10  relative to the edges. The repulsion effect makes magnetic gap between opposing magnets  12  along the medial axis of the plates  10  larger than the magnetic gap between the opposing magnets  12  near the edges  24 . The diaphragm  14  produces its lowest frequencies at its largest excursion. Light shaded lines  40  in  FIG. 4  show the contour of diaphragm  14  at maximum excursion at fundamental resonance. The maximum possible excursion is limited by magnetic gap geometry, and hence the larger the gap, the lower the fundamental frequency of the driver, and the greater its low frequency extension. 
     According to one embodiment of the present invention, the plates  10  are made of a sheet metal that has a thickness dimensioned so that under a given repelling force, and for a designed width of the plates, depending on magnets grade and size, the magnetic repulsion of the magnets  12  effect is sufficient to push the plates away from each other within the medial portion thereof, and which thus produces a larger magnetic gap in around middle of the diaphragm. For example, if Hg is the height of magnetic gap between magnets  12  at the outer edges of the driver, then preferably a gap about 1.5-2 Hg is achieved in the middle of the plates due to the flexing of the plates. With a driver size of about 10″×5″ (outer dimension) and N35H Neodymium magnet cross-area size of about 4×4 mm, a 1008 CRS steel plate may be used with thickness of about 1.5 m to 2 mm to achieve desired separation under magnetic repulsion. This allows the diaphragm  14  a larger excursion than with convention flat plates  4  and higher maximum SPL output by about 3-6 dB. At the same time the efficiency can be largely retained and construction would use thinner stamped plates without necessity to use expensive cast parts or very thick metal with special arrangements. The plates, while preferably formed from metal, can as well be formed from other relatively dense but flexible materials, including plastics and composites, so that the thickness of the plates given their width, allows for bending in response to the opposing magnetic forces of the magnets  12 . 
     One benefit of the plates being buckled under the tension relates to structural vibrations of the plates. In a conventional driver as shown in  FIG. 1 , when the plates  10  are made of sheet metal and clamped at the edges, they are prone to vibration. At resonances these vibrations can be quite significant producing noise, buzz and making magnets vibrate in relationship to each other. The movement of the magnets  12  in turn acts to modulate magnetic field in the gap and hence affect the reproduced signal, thereby introducing distortion. In accordance with the present invention, placing the plates  10  under the mechanical stress so as to cause them to bend into a curved, arcuate shape greatly reduces their ability to resonate freely. The use of magnetic repulsion effect between the opposing magnets  12  is one way to achieve the desired degree of tension and curvature. This not only eliminates buzz and noise but significantly stabilizes magnetic gap geometry and reduces modulation effects, improving overall sound quality of the driver. 
     Another aspect of the present invention relates to the construction of the diaphragm  14 . Generally, when planar driver operates, power from amplifier is dissipated in the driver and heats the diaphragm. Typically planar diaphragm is very light and as such heats up very quickly. Different coefficients of thermal expansion of the diaphragm layers, consisting of polymer substrate and metal foil, result in generation of tensile stresses in the plane of the diaphragm. Those thermal stresses, when over-imposed on mechanical stresses due to diaphragm vibrations, produce such phenomena as wrinkling and buckling. There are several negative consequences of these phenomena:
         diaphragm looses mechanical stability in those buckled zones. Parts of the membrane began vibrating chaotically, generating parasitic vibrations that manifest themselves as wide band buzz. The buzz can be spectrally located above and below the tones that generated it. This buzz is very objectionable and should be eliminated. Many planar drivers suffer from this effect.   wrinkling and buckling, if not controlled, lead to de-lamination of conductors from the film and/or developing cracks in the conductors in the areas of maximum stress.   deformations in the diaphragm may lead to conductors touching the magnets and consequent short circuit that would immediately lead to driver failure. For the same reason, a wrinkled diaphragm will have very limited excursion capability before it hits magnets. Therefore if wrinkles and buckled areas are developed in the operating area of the diaphragm between the magnets, this would severely limit transducer&#39;s output capability at low frequency were amplitude of diaphragm&#39;s vibration is maximum.       

     Referring now to the exemplary embodiment of  FIGS. 5-6 , in order to prevent above described effects the planar driver  14  includes a corrugated region  30  comprising a plurality of corrugations along the periphery of the diaphragm  14  between the conductive elements  18  and the edge  32 .  FIG. 5  shows the diaphragm  14  prior to corrugation, and  FIG. 6  shows the diaphragm after corrugation. In one embodiment, the corrugation is done in a form of ridges and valleys that are transverse to the direction of current carrying conductors. Also, the corrugated region is placed outside of the magnet gap, which allows keeping the geometrical thickness minimal and excursion maximal for the region of diaphragm that is acoustically active. The corrugation is preferably deep with peak to peak distance between indentations on the order of at least 1-2 mm (relative to a diaphragm thickness about 1 mil). The lateral distance between the peaks is about 2 mm as well. In one embodiment, in order to eliminate breaking of metal foil during deep draw corrugation, the metal strips at the periphery are made with a plurality of discrete regions  37 , with small gaps  34  between them that reduce stress and allow relatively deep corrugation for thin foil/polymer laminate. The corrugated region can be formed by hot press forming, so as to allow appropriate expansion of the diaphragm material in the corrugated region  30 , while maintaining the overall dimensions. Deep corrugation allows maximum stability of the diaphragm, most effective dampening of resonance and significant absorption and reduction of tensile stress associated with diaphragm heating. 
     The accordion-like corrugation provide significant elasticity in the direction of conductors greatly helps to reduce diaphragm buckling and wrinkling due to heat stress by absorbing those stresses. Another benefit of using such corrugation is that it provides lower fundamental resonance of the diaphragm Fs and as such lower operating frequency, thereby further extending the low frequency response. The resonance Fs depends on the longest dimension of the diaphragm, its degree of tensioning, material properties etc. Providing greater flexibility along the longest dimension thus allows lower Fs with other factors being equal. 
     Yet another benefit of the above corrugation is greatly improved dampening without the need to corrugate the whole area of the diaphragm. Thin stretched membranes as mechanical bodies have very negligible bending stiffness and constructional dampening. In many cases materials used in planar driver diaphragms (polymer film and aluminum foil) have rather low internal dampening. Thus, it is desirable to introduce additional dampening in the diaphragm. This dampening if possible should be of a constructional nature using diaphragm material itself without adding any coatings that greatly increase diaphragm mass. One of the most effective constructional dampening is corrugation. Deep corrugation according to the present invention allows very effective dampening of diaphragm resonances without introducing the problem associated with the use additional dampening materials.