Patent Publication Number: US-6338395-B1

Title: Kapok loudspeaker enclosure damping material

Description:
FIELD OF THE INVENTION 
     The present invention relates to a loudspeaker enclosure material suitable for enhancing the performance of a loudspeaker and more particularly, the present invention relates to the use of a specific natural material for reducing internal resonance or standing waves as well as loudspeaker resonance. 
     BACKGROUND OF THE INVENTION 
     It has been well documented that various acoustic absorbing materials provide use in damping internal distortion within a speaker enclosure as well as enhance the sound quality and efficiency of the speaker system. In instances where standing waves are not damped, the internal reflections produce strong sonic colorations and thus sound quality. A further advantage associated with the use of damping material is that corporation of such material within the enclosure decreases the enclosure frequency resonance and thus produces more sonically desirable bass. 
     Damping materials which have proved to be suitable for achieving acoustic damping include glasswool, DACRON™, acoustic foam and other synthetic materials. The problem with these materials is that many of them are fairly expensive and can thus add to the manufacturing costs of the loudspeaker unit. 
     It would be desirable if an acoustic damping material were provided that was inexpensive, achieve the desirable results of damping internal vibration, enhancing bass and enhancing overall performance of the speaker were available. The present invention satisfies this need and provides such a material. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an acoustic damping material which does not suffer from the limitations of the known art. 
     A further object of one embodiment of the present invention is to provide a sound attenuating material suitable for use in a loudspeaker enclosure, comprising kapok material, the kapok for reducing enclosure resonance and driver resonance. 
     The use of the kapok material is an alternative material that comes from the seed of kapok trees  “Ceiba pentandra” . This material is a natural fibrous material and is relatively inexpensive. The kapok material also has a very low density and thus is ideal for acoustic damping. By selection of the appropriate amount of kapok, the result is lower resonance within the enclosure. It has been found that by making use of the kapok that the resonance is greatly reduced relative to other materials. In addition, the kapok is essentially uninfluenced by the heat generated within the enclosure as the speaker drivers are used. This is in marked contrast to synthetic materials, particularly acoustic foam, which inherently would experience property changes due to thermal degradation. 
     A further object of one embodiment of the present invention is to provide a loudspeaker enclosure having reduced internal resonance, comprising; 
     a loudspeaker enclosure having a plurality of walls forming an enclosure; 
     a loudspeaker driver mounted to one of walls; and 
     kapok material positioned within the enclosure for reducing internal frequency resonance of the enclosure and resonance of loudspeaker. 
     The kapok has proven particularly successful in closed box systems, vented box systems, passive radiators, transmission line systems and other sub-woofer boxes, such as bandpass enclosures. Excellent results have also been obtained by making use of kapok in television loudspeaker driver enclosures as well as single driver enclosures and multiple driver enclosures. 
     In accordance with a further object of the present invention, there is provided a method of reducing internal resonance in a loudspeaker enclosure, comprising: 
     providing a loudspeaker enclosure having a plurality of walls and at least one loudspeaker driver mounted therein; 
     providing kapok material; and 
     positioning the kapok material within the enclosure, whereby internal resonance of the enclosure is reduced relative to an enclosure absent the kapok. 
     Generally speaking, the positioning of the kapok will be in opposition to the wall mounting the loudspeaker driver. This will result in the reduction of standing waves and other intermodular distortions. 
     Having thus described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphically representation of an impedance curve comparison between an enclosure lacking damping material and an enclosure including various forms of damping material; 
     FIG. 2 is a graphical representation of the SPL curve comparison between a box without damping material and that which includes the damping material of the present invention as well as other damping materials for comparison; and 
     FIG. 3 is a cross-section of a typical loudspeaker with the kapok material positioned therein. 
    
    
     Similar numerals in the figures denote similar elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, a common enclosure volume was employed for all curves represented in FIG.  1  and accordingly, internal volume does not, therefore, present itself as a variable for the determination of the properties. Curve  5  represents the output generated for an enclosure containing no fill material. Curve  1  represents the data generated for the kapok lining in accordance with the present invention. Curve  2  represents the data for a glasswool filling. Curve  3  represents the data for a foam filling. Curve  4  represents the data generated for a Dacron™ containing enclosure and curve  5  represents the output generated for an enclosure containing no fill material. As is evident, the kapok lining generates the lowest box frequency (f H ) and the lowest driver resonance (f L ). This also represents the lowest system frequency with the driver and enclosure working together, generally represented by (f M ). This data is generally illustrated in Table 3 which demonstrates impedance as a function of frequency. 
     With respect to the data set forth in FIG. 1, the amount of material used to generate the data in FIG. 1 varied from the following amounts: 35 g to 70 g at 5.8 liter box volume. The amount of filler should be selected to achieve the lowest enclosure resonance and optimum speaker efficiency. 
     Turning now to FIG. 2, as is evident, the use of the kapok material produced the highest sound pressure level (SPL) at various frequencies. This is tabulated in Table 1 which demonstrates the frequency and sound pressure level for the various materials. Table 2 sets forth the difference between sound pressure levels for various materials. 
     It has been found that the use of the kapok material can be useful to reduce enclosure size and optimize speaker efficiency. It has been found that for the purposes of the data generated in FIGS. 1 and 2 that the preferred density of kapok may be within the range of 2.2 grams per liter and 20.5 grams per liter. In the event that the kapok is mixed with an additional material such as rag cloth, the density can then be within the range of 2.0 grams per liter to about 16.3 grams per liter. In the instance where the kapok is mixed with acoustic foam, the density may be in the range of 1.3 grams per liter to 11.8 grams per liter and with acoustic foam and DACRON™, the density may range from 1.1 grams per liter to about 10.3 grams per liter. 
     Referring to FIG. 3, a loudspeaker box  20 , having a loudspeaker driver  22 , is filled with kapok material  24  with porous material  26  used to pack it. This is particularly true for a posted box, i.e. a vented box system and transmission line system. Kapok can be used together with other damping materials such as thin foam or DACRON™ positioned at the back side of the loudspeaker, etc. 
     It will be appreciated by those skilled in the art that the material may be used in any form of loudspeaker enclosure as set forth herein previously. 
     Although embodiments of the invention have been described above, it is not limited thereto and it will be apparent to those skilled in the art that numerous modifications form part of the present invention insofar as they do not depart from the spirit, nature and scope of the claimed and described invention. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 SPL COMPARISON BETWEEN KAPOK, FIBERGLASS, 
               
               
                 DACRON ™, ACOUSTIC FOAM AND NO FILL 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 ACOU- 
                   
               
               
                 FRE- 
                   
                 FIBER- 
                 DA- 
                 STICAL 
                   
               
               
                 QUENCY 
                 NO FILL 
                 GLASS 
                 CRON ™ 
                 FOAM 
                 Kapok 
               
               
                   
               
               
                 60 Hz 
                 70.98 dB 
                 73.95 dB 
                 75.72 dB 
                 76.71 dB 
                 78.57 dB 
               
               
                 65 Hz 
                 71.39 dB 
                 77.92 dB 
                 78.45 dB 
                 79.46 dB 
                 80.81 dB 
               
               
                 70 Hz 
                 73.71 dB 
                 81.30 dB 
                 79.72 dB 
                 80.65 dB 
                 81.72 dB 
               
               
                 80 Hz 
                 84.74 dB 
                 84.89 dB 
                 84.33 dB 
                 84.83 dB 
                 84.91 dB 
               
               
                 90 Hz 
                 87.22 dB 
                 86.53 dB 
                 86.06 dB 
                 86.44 dB 
                 86.24 dB 
               
               
                 100 Hz  
                 88.02 dB 
                 87.36 dB 
                 86.86 dB 
                 87.26 dB 
                 86.96 dB 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 SPL DIFFERENCE BETWEEN KAPOK, FIBERGLASS, 
               
               
                 DACRON ™, ACOUSTIC FOAM AND NO FILL 
               
            
           
           
               
               
               
               
               
            
               
                 FREQUENCY 
                 KG 
                 KA 
                 KD 
                 KN 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 60 Hz 
                 4.62 
                 dB 
                 1.86 
                 dB 
                 2.85 dB 
                 7.59 
                 dB 
               
               
                 65 Hz 
                 2.89 
                 dB 
                 1.35 
                 dB 
                 2.36 dB 
                 9.42 
                 dB 
               
               
                 70 Hz 
                 0.42 
                 dB 
                 1.07 
                 dB 
                 2.00 dB 
                 8.01 
                 dB 
               
               
                 80 Hz 
                 0.02 
                 dB 
                 0.08 
                 dB 
                 0.58 dB 
                 0.17 
                 dB 
               
               
                 90 Hz 
                 −0.29 
                 dB 
                 −0.20 
                 dB 
                 0.18 dB 
                 −0.98 
                 dB 
               
               
                 100 Hz  
                 −0.40 
                 dB 
                 −0.30 
                 dB 
                 0.10 dB 
                 −1.06 
                 dB 
               
               
                   
               
               
                 NOTE:  
               
               
                 KG = The SPL difference between kapok and fiberglass  
               
               
                 KA = The SPL difference between kapok and acoustic foam  
               
               
                 KD = The SPL difference between kapok and DACRON ™  
               
               
                 KN = The SPL difference between kapok and no fill  
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 FREQUENCIES FOR DIFFERENT MATERIALS 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 DAMPING 
                   
                   
                   
                   
               
               
                   
                 MATERlAL 
                 FL 
                 FH 
                 FM 
                 Ro 
               
               
                   
                   
               
               
                   
                 No Fill 
                 39.6 Hz 
                 92.7 Hz 
                 70.4 Hz 
                 4.2 Ohm 
               
               
                   
                 Dacron ™ 
                 35.8 Hz 
                 90.5 Hz 
                 60.6 Hz 
                 5.3 Ohm 
               
               
                   
                 Acoustic Foam 
                 37.2 Hz 
                 90.5 Hz 
                 60.6 Hz 
                 4.9 Ohm 
               
               
                   
                 Fiberglass 
                 36.7 Hz 
                 90.5 Hz 
                 62.2 Hz 
                 4.9 Ohm 
               
               
                   
                 Kapok 
                 34.9 Hz 
                 84.9 Hz 
                 54.8 Hz 
                 4.9 Ohm