Patent Application
20060231327
1. Technical Field of the Invention
This invention relates generally to loudspeaker enclosures, and more specifically to location of damping material in loudspeaker enclosures.
2. Background Art
Acoustic transducers are known to cause vibration, flexure, expansion, contraction, and bending modes in the loudspeaker cabinets to which they are coupled. These effects can be directly caused by the physical coupling of the oscillating transducer to a panel of the cabinet—as the motor rapidly and powerfully extends and withdraws the diaphragm assembly, the non-moving transducer components and the cabinet structures to which they are coupled under go an equal-and-opposite reaction type of mechanically induced movement. Furthermore, the oscillation of the diaphragm assembly causes pressurization and rarefaction of the air volume within the cabinet, especially in a sealed cabinet. Low frequency vibrations can cause gross flexure of the cabinet panels, and even the higher frequency vibrations can cause partial flexure or higher order flexure of the panels.
It is desirable to minimize these vibrations, flexures, etc., as they can interfere with ideal operation of the loudspeaker. They can cause output loss, reducing acoustic output above and below flexure resonance. At a panel's resonant frequency and its harmonics, modes of destructive interference between the enclosure and the transducer cancel some amount of acoustic output of the transducer, and modes of constructive interference add and create higher output spikes in the acoustic output.
Various damping materials have been added to loudspeaker cabinets in attempts to reduce such vibration, expansion, and flexing. Some manufacturers have simply laminated a damping material layer onto the interior surfaces of their cabinets; this is known as extensional damping. Others have sandwiched or laminated damping materials between two or more layers of the cabinet panels or walls; this is known as constrained layer damping. Extensional damping and constrained layer damping are designed to absorb vibrations in the panel structures themselves, and are somewhat in contrast to the practice of placing acoustical batting against the panels to absorb vibrations in the enclosed air itself.
Damping materials function by converting the kinetic energy of the moving panels into heat. Previous configurations have not been especially effective in doing so. Very little compression and expansion of the damping material is induced by the vibration, and very little shear is applied to the damping material because of the geometries of the cabinet panels. When a panel flexes, the extensional or constrained damping layer coupled to it undergoes a very small degree of compression or expansion caused by the change in its curvature. It is very inefficient, geometrically, because the induced shear, compression, and expansion are nearly perpendicular to the direction of the panel motion.
Internal bracing is often added to loudspeaker cabinets, to reduce expansion and flexure of the cabinets. Internal braces can divide the cabinet's enclosed air into two or more separate, isolated volumes, if desired. Or, if the internal braces are small enough (meaning that they do not extend completely over the cross-sectional area of the air volume) or are provided with holes, the enclosed air remains a single effective air volume. Internal bracing stiffens the cabinet, shifting the panels' resonance to higher frequencies, but does not change the amount of damping of the enclosure. It changes the frequency but not the amplitude of the vibrational resonance.
What is needed is an improved loudspeaker cabinet with both improved damping and improved structural rigidity.
The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only.
The cabinet includes one or more shearing braces 30. In the embodiment shown, a first shearing brace 30-1 is coupled between the left and right side panels, and a second shearing brace 30-2 is coupled between the front and rear panels. The first internal brace includes a first brace member 30-1a which is coupled to the left panel, a second brace member 30-1b which is coupled to the right panel, and a layer of damping material 30-1d which is sandwiched between the first and second brace members. The first and second brace members overlap each other, but neither is coupled to the exterior panel to which the other is coupled. The damping layer is preferably, but not necessarily, affixed to both brace members, such as by adhesive. In some embodiments, the damping material may itself be an adhesive, rather than e.g. a dry layer of material separately adhered to the brace panels. Examples of suitable damping adhesives may include polyvinyl acetate, Armstrong flooring adhesive, E6000, North Creek soft glue, bookbinding glue, and other flexible glues.
Preferably, but not necessarily, the two brace members are coupled to opposing exterior panels. As shown, the brace members may simply be constructed from the same panel material as the exterior panels.
Because the two halves of the internal brace are coupled to two different exterior panels, when either exterior panel flexes or otherwise moves toward or away from the other exterior panel, the internal brace's damping layer is subjected to shearing forces which are almost exactly parallel to the surface of the damping layer. The more normal to this movement the brace can be placed, the more efficient the shearing movement will be.
Another significant improvement results from the fact that the entire body of the damping is subjected to shear. Furthermore, the entire body of the damping material can be subjected to shear-induced displacement which is approximately equal to the maximum distance moved by the respective walls (assuming careful placement of the brace). By way of contrast, the constrained layer damping in the prior art undergoes very little shear, even at the middle of the panels, and essentially zero near the edges.
The internal brace members are coupled to their respective exterior panels by any suitable means. For example, the panels may be held in grooves 32 cut into the internal surfaces of the exterior panels, and may be affixed with adhesive and/or screws (not shown).
The damping layer may extend beyond the floating ends of the brace members, as shown, or it may be trimmed even with them, or it may terminate prior to them. The amount of overlap between the first and second brace panels may be determined according to the various other parameters of the cabinet, such as the thickness and rigidity of the exterior panels, the volume of the enclosed air space, the power and size of the transducer, the panel resonant frequency which needs to be controlled, and the flexibility etc. of the damping material.
In various embodiments, the cabinet may include different combinations of braces. There may be only a single brace coupling a first pair of exterior panels, or there may be two braces coupling two pairs of exterior panels, or there may be three braces coupling three pairs of exterior panels.
In some embodiments, there may be two or more braces coupling the same pair of exterior panels. In some embodiments, there may even be braces coupling non-opposing panels, in addition to or in lieu of braces coupling opposing panels. The braces may be oriented in a variety of different manners.
The tubes and the damping layer may be perforated with one or more holes, such that the internal volume of the inner tube is not subtracted from the effective air volume of the cabinet. The outer tube may be formed with a split, or may be formed as two semi-cylindrical halves, for ease of assembly.
The internal diameter and the length of the port tube will be selected according to the tuning needs of the cabinet at hand, and these will dictate the placement of the internal brace 30-11. Note that the internal brace 30-11 could be coupled to the outer tube 110 at any position along the length of the outer tube, and is not necessarily located at the extreme end of the tube, as is shown. Note further that the internal brace could be angled, to scatter or reduce standing waves.
Similarly, the second perpendicular shearing brace includes a damping layer 122-2d which is coupled to a second brace member 30-12b of the first shearing brace, and to a second brace member 30-13b of the second shearing brace. It further includes a brace member 122-2a which is coupled to the top exterior panel (not shown).
Optionally, a third brace member 146 is coupled to the front panel, a fourth brace member 148 is coupled to the right panel (not shown), and a second damping layer 154 is affixed between their overlapping sections.
Optionally, a third damping layer 152 is affixed between overlapping sections of the second and third brace members. If the third brace member were to be below the fourth brace member, the third damping layer would be affixed between overlapping sections of the second and fourth brace members; in this configuration, no damping layer would be directly between brace members coupled to opposing exterior panels.
Optionally, a second damping layer 170 is laminated between the opposite sides of the panel brace members and a second floating brace member 172.
Optionally, the floating brace members may be provided with a plurality of holes 174, and the panel brace members may be provided with one or more holes 178 aligned with the holes through the floating brace members, with corresponding holes 176 through the damping layer. Bolts 180 can be inserted through these holes, with nuts 182 and washers 184, 186, and then tightened. The holes 178 through the panel brace members should be made sufficiently large that the bolts free-float within them as the panel brace members vibrate and move, to avoid mechanically coupling the panel brace members to each other or to the floating brace members other than indirectly via the damping material. The tension on the bolts will affect the tuning of the floating brace system; in general, the tighter the floating braces are pinched onto the damping material, the higher the system's damping frequency will be.
In some embodiments, the dots may be of a uniform size and pattern. In other embodiments, the dots may be of different sizes, and/or they may be in an irregular pattern or randomly placed.
In other embodiments, there may be multiple groups of stripes extending in different directions, some longitudinal, some lateral, some diagonal, and so forth. It is not necessary that the stripes be strictly linear; in some embodiments, they may be curved, spiral, or randomly shaped.
A support structure 240 extends across the inner dimension of the tubular body to support a first shear brace member 242. A second shear brace member is coupled to the rear panel, and a damping layer 246 is laminated between the first and second shear brace members. In one embodiment, the support structure can be fashioned as a thermal chimney, such as is disclosed in co-pending application Ser. No. 10/768,197 entitled “Thermal Chimney Equipped Audio Speaker Cabinet” filed Jan. 30, 2004, sharing an inventor with and commonly assigned with the present application.
Alternatively, the support structure can be omitted and the first shear brace member may be coupled to the back of the transducer motor or basket.
Armed with the teachings of this disclosure, the skilled cabinet designer will be able to select brace configurations, sizes, locations, angles, connections, materials, etc. according to the demands of the application at hand. The designer may, for example, wish to perform finite element analysis on the cabinet, to identify harmonic modes, rotational shear, panel flexure, and the like, and thereby determine where to place selected shear braces.
For ease of illustration, the invention has been shown in the figures in conventional rectilinear box configurations, but the invention may be practiced in a variety of cabinets having other shapes.
When one component is said to be “adjacent” another component, it should not be interpreted to mean that there is absolutely nothing between the two components, only that they are in the order indicated.
The various features illustrated in the figures may be combined in many ways, and should not be interpreted as though limited to the specific embodiments in which they were explained and shown.
Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.
The term “panel” is not limited to flat, planar members. The terms “layer” and “laminated” are not limited to continuous sheets, but are intended to also cover e.g. a series of disjoint pieces such as the dots of
