Patent Publication Number: US-8991548-B2

Title: Acoustic diaphragm suspending

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of patent application Ser. No. 12/977,484 filed on Dec. 23, 2010. 
    
    
     BACKGROUND 
     This specification describes a suspension element (or “surround”) for an acoustic diaphragm for use in an acoustic driver or an acoustic passive radiator. 
     SUMMARY 
     In one aspect of the specification, a suspension element for mechanically coupling an acoustic diaphragm to a stationary element is characterized by a total compliance, and the total compliance comprises a shear compliance and a beam compliance and the beam compliance is not significantly larger than the shear compliance. The shear compliance may be greater than the beam compliance. The material of the suspension element may have a Young&#39;s modulus of about 0.031 MPa. The material of the suspension element may be silicone rubber. The silicone rubber may be treated with a softening agent. The material of the suspension element may be a polyurethane. The suspension element and the diaphragm may be components of a passive radiator. The suspension may include flanges for capturing the acoustic diaphragm. 
     In another aspect of the specification, a suspension element for mechanically coupling an acoustic diaphragm to a stationary element is characterized by a width and a thickness. The ratio of the width to the thickness is less than 2:1. ratio of the width to the thickness may be 1:1 or less The suspension element may include a material with a Young&#39;s modulus of about 0.031 MPa. The silicone rubber may be treated with a softening agent. The material of the suspension element may be a polyurethane. The suspension element and the acoustic diaphragm may be components of an acoustic passive radiator. The suspension element may include flanges to capture the acoustic diaphragm. 
     In another aspect of the specification, a suspension element for mechanically coupling an acoustic diaphragm to a stationary element includes a ring shaped structure characterized by a radial axis. In operation, the suspension element deforms in a direction perpendicular to the radial axis and in operation the radial axis remains substantially straight. The ring shaped structure may be characterized by a width measured along the radial axis and a thickness measured perpendicular to the radial axis. The width may be less than twice the thickness. The width may be less than the thickness. The suspension may be formed of silicone rubber. The suspension may be formed of a polyurethane. 
     Other features, objects, and advantages will become apparent from the following detailed description, when read in connection with the following drawing, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIGS. 1A ,  1 B, and  1 C are views of an acoustic assembly including a suspension element; 
         FIGS. 2A ,  2 B, and  2 D are diagrammatic illustrations of acoustic suspension elements; 
         FIG. 2C  is a force v. deflection curve for two acoustic suspension elements; 
         FIGS. 3A and 3B  are diagrammatic views of an acoustic assembly including a suspension element, for illustrating shear deformation; 
         FIG. 4  is a cross sectional view of a tool for manufacturing an acoustic suspension element; 
         FIG. 5  is a block diagram of a process for manufacturing an acoustic assembly including a suspension element; 
         FIG. 6A  show views of an actual acoustic assembly including an acoustic suspension element; 
         FIG. 6B  is a partial isometric view of an actual suspension element and of an acoustic assembly including a suspension element; 
         FIG. 7  shows diagrammatic views of an acoustic assembly including a suspension element; and 
         FIG. 8  is a force v. deflection curve for two acoustic suspension elements. 
     
    
    
     DETAILED DESCRIPTION 
     Some of the processes may be described in block diagrams. The activities that are performed in each block may be performed by one element or by a plurality of elements, and may be separated in time. The elements that perform the activities of a block may be physically separated. One element may perform the activities of more than one block. 
       FIGS. 1A and 1B  show, respectively, a top plan view and a side plan view of an acoustic assembly  20  including an acoustic diaphragm  10  mechanically coupled along its circumference to a support structure  12  by a suspension element  14 . The suspension element permits vibration of the acoustic diaphragm  10  in the direction indicated by arrow  16 . 
     The acoustic diaphragm  10  can be planar, as shown, or may be cone shaped or some other shape. The acoustic diaphragm  10  may be circular as shown, or non-circular, for example an oval shape or a “racetrack” shape, or a shape not bounded by continuously curved line, such as a square. The suspension element  14  is characterized by radial axes such as radial axis  30  that lie in a plan perpendicular to the intended direction of motion indicated by arrow  16 . “Radial” does not limit the suspension to circular diaphragms. If the diaphragm is non-circular, “radial” is taken relative to the geometric center of the diaphragm, and extending through the diaphragm and the suspension element. The support structure can be the wall of an acoustic enclosure or may be the frame or “basket” of an acoustic driver. For purposes of this specification, the support structure is fixed and is therefore represented in  FIGS. 2A ,  2 B,  2 D, and  3  as a mechanical ground. The acoustic assembly  20  may be a passive radiator as shown, or may be an acoustic driver, in which case the acoustic assembly could include a linear motor, which could include a magnet structure and voice coil. The suspension element  14 , in this example a surround for a passive radiator, could instead be a surround for an acoustic driver, or could be a spider, depending on the requirements of the spider. 
       FIG. 1C  shows a partial cross-sectional view taken along line  1 C- 1 C of  FIG. 1A , from an elevated position in an oblique direction, as indicated by arrow  22  of  FIG. 1B . The ratio of the width w of the body (that is, excluding the width of the flanges  24 ) of the suspension element  14  to the thickness t (hereinafter the width to thickness ratio) of the suspension element is less than 2:1, in this example approximately 1:1. 
     The suspension element has at least three functions: (1) to permit pistonic motion in the directions indicated by arrow  16  while inhibiting non-pistonic motion; (2) to exert a restorative force to urge the diaphragm to a neutral position; and (3) to provide a pneumatic seal between the two sides of the acoustic diaphragm. “Pistonic” motion, as used herein, refers to rigid body motion in which all points of the diaphragm move in the same direction (typically axially) at the same rate. Non-pistonic rigid body motion in which some points of the diaphragm move in different directions or move in the same direction at different rates is referred to as “rocking” and adversely affects the efficiency of the acoustic assembly or results in less acoustic energy being radiated that when the diaphragm is operating pistonically, or both. Non-pistonic motion in a radial motion adversely affects the operation of the acoustic assembly, and in the case of an acoustic driver, can cause damage to elements of the acoustic driver. 
       FIGS. 2A and 2B  illustrate different configurations of suspension elements. In  FIG. 2A , the suspension element includes two compressible, stretchable sections,  14 A 1  and  14 A 2 . Motion indicated by arrow  22  compresses one section, in this example  14 A 1  and extends the other section, in this example,  14 A 2  as shown in  FIG. 2B , resulting in a restorative force in this example in the direction indicated by arrows  26 . 
     In a suspension element such as shown in  FIG. 2B , in which the width to thickness ratio is large, for example greater than 5:1, in this example, about 16:1 the suspension element exhibits predominantly beam-like deformation. In beam-like deformation, motion in the intended direction causes the suspension element to deform so that an axis  28  of the cross section of the suspension element  14 B becomes curved. The deformation of the beam causes strain, which places the beam partially in compression and partially in tension, which results in a restoring force which has an axial component, as indicated by arrow  26 . 
     A curve  25 A of Force (F) vs. deflection (δ) of  FIG. 2C  varies linearly over a range  27  of forces and deflections. To increase the range of linearity of force and deflection in the configuration of  FIG. 2B , the geometry of the suspension element  14  can be modified. For example, the width w can be increased. However, this is disadvantageous because it increases the overall diameter of the acoustic assembly. One method of providing a larger range of linearity without increasing the overall diameter as much as simply increasing the width of the suspension element is to modify the geometry of the suspension element, for example using a half roll surround  14 D shown in  FIG. 2D . Motion of the diaphragm causes the half roll to “unroll”, resulting in a curve  25 B with a larger range of linearity of force and deflection, for example range  29  of  FIG. 2C . 
     One problem of changing the geometry of suspension elements is that changing the geometry of the surround can in itself cause non-linearities. For example the slope of the Force vs. Deflection curve may be asymmetric so that the curve has a different slope or has a different range of deflection in which the suspension element behaves linearly depending in which direction the diaphragm is moving. 
     Other types of suspension elements that improve the symmetry of the Force vs. Deflection curve or increase the range of linearity include more complex geometries such as multiple rolls, and radial or circumferential ribs, for example, as described in U.S. Pat. No. 7,699,139 issued Apr. 20, 2010 to Subramaniam et al. 
     One drawback of the suspension elements described above is that, even with complex geometries and structure such as ribs, the suspension elements may be wider than desired. For example, if high excursion is required from a transducer with a transducer with a relatively small diaphragm, the area of the suspension element may approach or even exceed the area of the radiating surface. Wide surrounds are also especially disadvantageous if it is desired to place an acoustic driver or passive radiator in a physically small device, particularly if a large displacement in required. Stated differently, the maximum excursion over which suspension has a linear force deflection curve depends on the width of the suspension element and the geometry of the suspension element. Additionally, the suspension material may have a non-linear stress-strain curve (non-constant Young&#39;s modulus of elasticity), which also can define the range of excursion over which the suspension behaves linearly. Typically, the maximum excursion of diaphragm mechanically coupled by a suspension element as described above is no more than about 0.6 times (measured from neutral position) the width of the suspension element for a half roll surround operating in the linear region of a force/deflection curve. 
     Another drawback of relatively wide suspension is that they may be prone to deformation from internal enclosure pressures. For example, if a diaphragm mounted in a closed enclosure, particularly a small enclosure, moves inward, the pressure inside the enclosure increases, causing an outward force to be exerted on the suspension over its area. If the width is relatively large, for example, five times or more the thickness, the stiffness of the suspension may not be adequate to resist deforming outwardly, for example by bowing outwardly, which reduces acoustic output. Similarly, outward movement of the diaphragm results in a reduction of pressure inside the enclosure, resulting in an inward force on the suspension, resulting in inward deformation of the suspension. Since the direction of the deformation is opposite to the direction of movement of the diaphragm, the deformation can result in a reduction of the acoustic output of the device. 
       FIG. 3A  illustrates a configuration for the suspension element that provides the same excursion with significantly narrower width (or provides more maximum excursion with the same width) than the suspension elements of  FIGS. 2A ,  2 B, and  2 D, and in addition is less prone to deformation due to internal enclosure pressures. In the configuration of  FIG. 3A , the suspension  14  is a ring shaped mass of compliant material with a width to thickness ratio less than 2:1, in this example about 1:1. In the suspension element of  FIG. 3A , shear deformation is a significant component of the total deformation. In shear deformation, motion in the intended direction (indicated by arrow  23 ) causes the suspension element to deform so that the two suspension element surfaces  34 ,  36  that are parallel to the intended direction of motion when the diaphragm is in a neutral position, and substantially all cross-sectional planes that are parallel to surfaces  34  and  36  remain substantially parallel to each other and to the intended direction of motion, but are displaced relative to each other in the intended direction of motion. The axis  30  of the suspension element remains straight over most of its length, but becomes non-perpendicular with surfaces  34 ,  36 . With shear deformation, a restorative force (indicated by arrow  37 ) opposing the motion of the diaphragm  10  is exerted in a direction substantially parallel to the moving surfaces  34 ,  36 . The suspension element  14  may have flanges  24  to capture the diaphragm  10 , to increase the surface area of attachment between the suspension element  14  and the diaphragm  10  and between the suspension element  14  and the support structure (depicted here as a mechanical ground), and to eliminate the high stresses that would otherwise occur at the top and bottom edges of the suspension element  14  where it connects to the diaphragm in  FIG. 3 . 
     Chemical bonding, or some method of maintaining the connection between the diaphragm and the suspension element may be desirable. 
       FIG. 3B  shows a cross-section of an actual implementation of a suspension element according to  FIG. 3A  in an undeformed state and a finite element analysis (FEA) simulation of the actual implementation in a deformed state. Subsequent testing on a suspension according to  FIGS. 3A and 3B  confirms that the actual suspension element behaves substantially as predicted by the FEA simulation. 
     As stated above, a suspension elements according to  FIG. 3A  can increase the maximum excursion an acoustic element can provide for a given suspension element width. Alternatively, suspension elements according to  FIG. 3A  can decrease the width requirement of the suspension element for a given maximum excursion. This advantage is very significant particularly if the space in which the acoustic assembly is limited. If the space is limited, a narrower suspension permits more radiating surface. 
     In an actual suspension element, the application of the force F to the diaphragm  10  causes both beam deformation and shear deformation to occur in the suspension element, which results in a deflection  6 . The amount of deflection is δ=FC total  where C total  is the total compliance of the suspension element. The total compliance C total  has two components, the beam compliance C beam  and the shear compliance C shear , so that δ=F(C beam +C shear ). C total , C beam , and C shear  are substantially constant over the linear portion of the Force v. Deflection curve. The beam compliance is 
               C   beam     =       0.75   ⁢     w   3         Elt   3             
where w is the width as defined in  FIG. 1C , t is the thickness as defined in  FIG. 1C , l is the length of the circumferential axis of the suspension element, and E is Young&#39;s modulus, and the material is assumed to be incompressible and the width w is assumed to be much less than the outer diameter of the surround. The shear compliance
 
                 C   shear     =       2   ⁢     (     1   +   v     )     ⁢   w     Elt       ,         
where v is Poisson&#39;s ratio. If the suspension is assumed to be compressible, v=0.5,
 
               C   shear     =       2   ⁢     (     1   +   v     )     ⁢   w     Elt           
becomes
 
               C   shear     =         3   ⁢   w     Elt     .           
Young&#39;s modulus E and Poisson&#39;s ratio v are properties of the material from which the suspension element is made. The deflection can then be expressed as
 
               δ   =     F   ⁡     (       (       0.75   ⁢     w   3         Elt   3       )     +     (     3   Elt     )       )         ,         
which in terms of the width to thickness ratio
 
             w   t         
is
 
             δ   =       F   ⁡     (         (     0.75   El     )     ⁢       (     w   t     )     3       +     (       3   El     ⁢     (     w   t     )       )       )       .           
For purposes of analysis, the suspension element may be approximated as a ring with a width w, thickness t, and a depth l which is taken to be
 
     
       
         
           
             
               
                 inner_diameter 
                 + 
                 outer_diameter 
               
               2 
             
             . 
           
         
       
     
     For a suspension element made of a material such as ECOFLEX® 0010 supersoft silicone rubber available from Smooth-On Inc. of Easton, Pennsylania, USA, url www.smooth-on.com with a 
             w   t         
values of 5 or greater, the beam component
 
               (     0.75   El     )     ⁢       (     w   t     )     3           
of the compliance is significantly larger than (about 6× or greater) the shear component
 
                 2   ⁢     (     1   +   v     )       El     ⁢     (     w   t     )           
and the shear compliance is an insubstantial component of the total compliance. For a suspension element made of ECOFLEX® 0010 supersoft silicone rubber, which has a Young&#39;s modulus of about 0.031 MPa and a
 
             w   t         
value or 2, the beam component
 
               (     1   El     )     ⁢       (     w   t     )     3           
of the compliance is not significantly larger (about 1× or less) than the shear component
 
               3   El     ⁢     (     w   t     )           
and the shear component is a significant component of the total compliance. For suspension elements with
 
             w   t         
values between 5 and 2, the shear component can be characterized as a transitioning from an insubstantial to a substantial component of the total compliance.
 
     Suspension elements including various combinations of geometries, dimensions, and material parameters, (for example, Young&#39;s modulus, Poisson&#39;s ratio, shear modulus) can be simulated using finite element analysis (FEA) software to determine if the suspension elements have the desired performance parameters, for example free air resonance, tuning frequency, maximum excursion, frequency range of operation and damping) and that maximum stress and strain limits are not exceeded. 
     Empirical testing under the actual operating conditions of the combinations of geometries, dimensions, materials, required compliance, and required performance parameters may be advisable, for a number of reasons: some of the parameters may not be specified by the manufacturer; the parameters specified by the manufacturer may have been measured under conditions different than the conditions under which the suspension element is required to operate (for example the suspension element operates in a cyclic manner while the parameters may have been measured statically); or some of the assumptions made by the FEA program may not be valid for the actual operation of the suspension element. 
     The material from which the suspension element is made can be modified to provide additional features. For example, if the diaphragm of an acoustic element with a suspension made of silicone rubber has non-pistonic modes, for example rocking modes, at a frequency within the range of operation of the acoustic element, the loss factor of the silicone rubber can be modified by adding a softening agent to increase the damping factor (tan delta) of the silicone rubber. 
     Unlike suspension elements with insubstantial shear compliance, the maximum excursion of a suspension with substantial shear compliance is not limited to less than the width of the suspension; in some implementations, the maximum excursions can be up to four times the width of the suspension before tearing of the suspension. 
       FIGS. 4 and 5  show, respectively, a diagrammatic cross-section of an apparatus for forming the acoustic assembly of  FIG. 3 , and a method for forming the acoustic assembly of  FIG. 3 . The apparatus of  FIG. 4  includes two sections  40 A and  40 B of a mold  40  for insert molding. Positioned inside the mold  40  are the acoustic diaphragm  10  and if desired, a portion of the support structure  12 . A locating dowel or pin  44  may assist in positioning the acoustic diaphragm in the mold. Injection channel  46  provides a passageway through which the material of the suspension element ( 14  of  FIGS. 1 and 3 ) can be injected into the suspension element cavity  48 . The apparatus of  FIGS. 4 and 5  may have other features and elements not shown, for example air vent channels, not shown in these views. 
     In the process of  FIG. 5 , at optional block  50 , the frame and/or the diaphragm are primed. At block  52 , the frame and diaphragm are inserted into the mold  40  of  FIG. 4 . After the two portions  40 A and  40 B of the mold  40  are closed, at block  54 , the uncured suspension material is injected into the suspension element cavity  48  of  FIG. 4  through the injection channel  46 . Preferably, the suspension element cavity  48  is dimensioned and configured so that the suspension element material flows over the edge of the acoustic diaphragm to form the flanges which capture the acoustic diaphragm. At block  56 , the suspension material is cured. At block  60 , the mold is opened and the acoustic assembly is removed. 
     The priming at optional block  50  enhances the chemical bonding of the suspension element to the acoustic diaphragm or the frame, or both. An example of an appropriate primer for a silicone rubber suspension element, a polycarbonate acoustic diaphragm, and a polycarbonate frame is MOMENTIVE™ SS4155 silicone primer, currently available from Momentive Materials Inc. of Albany, N.Y., USA, www.momentive.com. For some suspension element materials, for example polyurethanes, priming may not be as advantageous. Chemical bonding may provide better results than friction alone, or to mechanical devices such as clamps 
       FIGS. 6A and 6B  show an actual implementation of a suspension element of  FIG. 3 . The elements of  FIGS. 6A and 6B  correspond to like numbered elements of previous figures. The suspension element  14  of  FIG. 3  is intended for use as a surround of a passive radiator. The suspension element  14  is a ring shaped mass made of Ecoflex silicone rubber with a Young&#39;s modulus of 0.031 MPa and has been softened with a softening agent so that the tan delta is 0.58. 
       FIG. 7  shows a cross section of the suspension element of  FIGS. 6A and 6B , with a diaphragm. For comparison purposes, the suspension element  14  is overlaid on a half roll suspension  14 ′ with the same.  FIG. 7  also shows how the suspension elements  14  and  14 ′ could be mounted to the acoustic diaphragm and to the support structure  12 . 
       FIG. 8  shows force/deflection curves according to a finite element analysis simulation of the suspensions of  FIGS. 6A and 6B  using materials with a constant Young&#39;s modulus of elasticity (linear stress-strain curve). The force/deflection curve for shear suspension  14  (curve  72 ) remains substantially linear for at least ±6 mm (=1.2×the width of the surround), while the force/deflection curve for the half roll suspension (curve 74 ) becomes substantially non-linear at about ±3 mm (=0.6× the width of the surround). The curves of  FIG. 8  are a finite element analysis simulation. 
     Numerous uses of and departures from the specific apparatus and techniques disclosed herein may be made without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features disclosed herein and limited only by the spirit and scope of the appended claims.