Abstract:
A vibration absorbing device for a speaker enclosure is tuned upon installation to the speaker enclosure to achieve maximum attenuation of vibrations in a selected frequency range. The vibration absorber, which includes a series of vibration damping plates separated by spacers, is positioned at varying degrees of overhang on a mounting plate on a panel of the enclosure until attenuation of the desired frequency band is maximized. Then the absorber is secured to the speaker enclosure with an appropriately sized permanent mounting plate.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This application is continuation-in-part of application Ser. No. 263,927, filed Jun. 21, 1994, now U.S. Pat. No. 5,583,324. The entire disclosure of that application, including the drawings, is incorporated herein by reference. 
    
    
     Applications Ser. Nos. 263,927, filed Jun. 21, 1994, now U.S. Pat. No. 5,583,324, and 193,299, filed Feb. 8, 1994, now U.S. Pat. No. 5,629,503, disclose vibration dampening devices particularly for use with speaker cabinets, including speakers mounted in walls. 
     This application describes an improvement over the disclosures of the two copending applications, an improvement whereby a vibration absorber according to the invention of either of those applications is tunable upon installation into a speaker cabinet or other speaker enclosure. 
     The stack of viscoelastic damping plates secured together with spacers preferably at one edge, is connected to the speaker panel with a tuning mounting plate between the damping unit&#39;s base or mounting plate and the speaker panel. As in the copending applications, the stack of mounting plates is secured together and to the unit&#39;s mounting plate at one edge. Upon installation on a panel of a speaker enclosure, the edge of the unit where all plates are secured together is cantilevered over the edge of the tuning, mounting plate. The degree of cantilever is adjusted during installation, to find the optimum position at which vibration is best absorbed. The vibration absorbing unit is then fixed in this position of optimum cantilever. 
     It is therefore among the objects of the invention to improve the vibration absorbing capability of vibration damping units such as described in applications Ser. Nos. 263,927, now U.S. Pat. No. 5,583,324, and 193,299now U.S. Pat. No. 5,629,503, with a simple and efficient procedure. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a vibration damping unit with which the procedure and apparatus of the invention may be used. 
     FIG. 2 is a similar perspective view, showing the unit of FIG. 1 positioned against a tuning mounting plate, for installation on a panel of a speaker enclosure. 
     FIGS. 3, 4 and 5 are graphs showing amplitudes of vibration versus frequency, in reference to the tuning method of the invention. 
     FIG. 6 is a perspective, exploded view showing a procedure for assembling a vibration damping unit to a speaker enclosure panel in accordance with the invention. 
     FIG. 6A is an elevation view illustrating a technique for optimally tuning the damping device for lower frequencies. 
     FIG. 6B is a view similar to FIG. 6A, showing permanent illustration of the damping unit at optimal damping position; 
     FIG. 7 is a table showing position dimensions for approximately optimal mounting of a particular model of vibration damping unit. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the drawings, FIG. 1 shows a vibration damping unit 10 in accordance with application Ser. No. 263,927, now U.S. Pat. No. 5,583,324, the disclosure of which is incorporated herein by reference. The damping unit 10 includes a series of vibration damping plates 10a, 10b, 10c, 10d and 10e (the number of plates can be greater or smaller), secured together and to a mounting plate 12 via spacers 11 and fasteners 13 passing through holes 14 at one edge of the assembly. Outer edges of the damping plates 10a-10e, generally identified by the reference number 16 in FIG. 1, are freely suspended, i.e., cantilevered, the plates and the mounting plate 12 being secured together only at the left edge 18 of the assembly as seen in the drawing. This construction is in accordance with an embodiment described in the copending application incorporated by reference. 
     FIG. 2 is similar to FIG. 1 but shows the damping unit 10 secured to a separate spacer mounting plate 20 which serves as a tuning mounting plate for the vibration damping assembly. The tuning mounting plate 20 serves as a spacer between the unit&#39;s mounting plate or base plate 12 and a panel of a speaker enclosure (not shown). As can be seen from the drawing, this effects a cantilever of the region of the unit 10 which is adjacent to the spacers 11, that is, the left edge 18 as seen in the drawing. A cantilever distance Y is variable according to how the unit 10 is mounted on the tuning spacer 20. This variable distance Y is free to resonate independently from the plates 10a, 10b, etc. which are also free to resonate relative to secured-together edge 18 of the damping unit. The intermediate tuning plate or spacer 20, which is preferably made of a one-fourth inch thick, high density material such as fiberboard, is bonded securely to the mounting plate 12, which may be accomplished using a solvent such as acetone. Acetone, by its chemical action, momentarily melts (liquifies) the surface of the plastic mounting plate 12 thereby causing the mounting plate 12 and the tuning mounting plate or spacer 20 to be permanently fused together. 
     In the assembly shown in FIG. 2, the tuning mounting plate or spacer 20 may be assigned a specific length by varying the dimension shown as X in FIG. 2, thereby providing the assembly of FIG. 2 with the cantilever portion described in length by the dimension Y. Varying of the dimension Y therefore assigns to the structure a mass-compliance product factor which causes the structure to resonate at a specific frequency residing in the lower range of the audible sound spectrum. 
     It has been found by means of accelerometer measurements that the structure of FIG. 2 is able to attenuate enclosure panel vibrations to a far greater degree in the lower frequencies of the audible sound spectrum by inclusion of the tuning mounting plate or spacer 20 between the damping unit and the panel. It has also been found by these accelerometer measurements that the structure of FIG. 2, like the structure of FIG. 1, is able to attenuate enclosure panel vibrations to a far greater degree in the higher frequencies by excluding the intermediate spacer member or tuning mounting plate 20. 
     FIG. 3 shows the frequency spectrum from 80 Hz to 1000 Hz for a sweep signal applied to a laboratory test panel. FIG. 4 compares the reference spectrum of FIG. 3 with one embodiment of FIG. 2 attached to the test panel, wherein the tuning mounting plate 20 was cut to give the dimension X a length of 2 and 5/8 inches, for a particular vibration damping unit. It can be seen in FIG. 4 that the peak panel resonance at 108 Hz was attenuated by approximately 28 decibels. 
     FIG. 5 compares the reference spectrum of FIG. 3 with a situation wherein a damping unit such as shown in FIG. 1 was attached to the test panel, without the spacer or tuning mounting plate 20. FIG. 5 shows that the peak panel resonance at 440 Hz was attenuated by approximately 37 decibels. 
     Thus, optimum vibration attenuation in speaker enclosures can be attained by using at least two of the damping assembles 10 of the invention, one secured with the damping mounting plate or spacer 20 and one secured without. 
     It will be seen by those skilled in the art to which this invention relates that the structure of FIG. 2 is comprised of spacers 11 causing the portion of the structure so constructed by the use of spacers to form a monolithic mass whose natural resonance frequency is governed by the cantilevered distance Y. Decreasing the distance Y will raise the natural resonance frequency of the monolithic mass while increasing the distance Y lower the natural resonance frequency of the monolithic mass formed by spacers 11. A procedure for optimizing the assembly shown in FIG. 2, i.e., selecting the correct dimensions X and Y, is described with reference to FIGS. 6, 6A and 6B. The installer, using standard accelerometer measurement techniques, finds the frequency and location on the outside of a speaker enclosure where the peak panel resonances occur. The installer then refers to the tuning chart of FIG. 7, which is given here as an example for a particular model of vibration damping unit 10. A value is selected for X that will tune the absorber to the frequency of maximum panel resonance. 
     As a preliminary step in installation and to assure optimum tuning, the user can first secure the damping unit 10 and tuning mounting plate 20 to the panel using a temporary device such as self-adhesive sheet magnets 22, 24. A protective backing (not shown) is peeled away from the self-adhesive sheet magnets and sheet magnets of this type are applied to the speaker panel and to the bottom of the mounting plate 12 of the damping unit 10. This is schematically indicated in FIG. 6A. The relative positions of the damping unit 10, including the sheet magnet 22 secured to its bottom, and the sheet magnet 24 secured to the speaker panel, are shifted to the chart-selected values of X and Y. Tuning is checked using the accelerometer. The damping unit 10 is moved to vary the value of X until the optimum value has been found at which the resonance peak has been reduced to the lowest level. Then, without moving the absorber 10, the distance X is measured. This value found for X is the dimension of the tuning mounting plate or spacer plate 20 which should be used. The tuning mounting plate 20 is cut to the dimension X and the vibration damping unit 10 is then secured as shown in FIG. 2B and FIG. 6, in a permanent installation. 
     The above described preferred embodiment is intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to this preferred embodiment will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.