Abstract:
A loudspeaker enclosure includes a rectangular cabinet having an acoustical port mounted in one of the vertical walls. A loudspeaker is installed in a bottom wall of the cabinet facing away from an interior volume of the cabinet. A highly resistive acoustic foam is positioned in front of the loudspeaker to restrict the direct radiation of the loudspeaker, so that the usable acoustic radiation emanates from the acoustical port.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of The Invention  
           [0002]    The subject invention relates to loudspeaker enclosures, and more particularly, to vented loudspeaker enclosures.  
           [0003]    2. Description of The Related Art  
           [0004]    It is difficult to provide a high quality loudspeaker reproduction of the lowest audio frequencies without resorting to large bulky enclosures.  
           [0005]    One solution to this problem is the vented enclosure. U.S. Pat. No. 3,037,081 to Carlsson discloses such an enclosure in the form of an acoustic resonator in which the diaphragm of the loudspeaker forms a part of the wall of the resonator. The enclosure includes at least one port.  
           [0006]    While such an enclosure has, at low frequencies, a smooth frequency response curve, good transient response and low nonlinear distortion, the low frequency performance still tends to be lacking.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the invention is to provide a loudspeaker enclosure which has small size and exhibits good low frequency performance.  
           [0008]    A further object of the invention is to extend the low frequency response of a vented enclosure by at least one octave.  
           [0009]    The above objects are achieved in a loudspeaker enclosure comprising a cabinet for containing a loudspeaker mounted in a wall of said cabinet, said loudspeaker facing away from an internal volume of said cabinet; a port mounted in a further wall of said cabinet; and acoustic resistive foam material positioned in front of said loudspeaker for limiting the amount of direct radiation from the loudspeaker thereby causing most of the usable acoustic radiation to emanate from said port.  
           [0010]    Applicant has found that below the tuned frequency of a reflex enclosure, the driver and port radiation are in phase opposition which tends to cancel the usable acoustic output. By limiting the direct speaker radiation, the port radiation can extend the low frequency response by at least one octave. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    With the above and additional objects and advantages in mind as will hereinafter occur, the invention will be described with reference to the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is an elevational view, partly in section, of a loudspeaker enclosure incorporating the subject invention;  
         [0013]    [0013]FIG. 2 shows an equivalent circuit diagram of the loudspeaker enclosure of the subject invention;  
         [0014]    [0014]FIG. 3 shows response curves of the loudspeaker enclosure of the subject invention in which the amount of direct radiation is varied; and  
         [0015]    [0015]FIG. 4 shows response curves of the loudspeaker enclosure of the subject invention in which the loudspeaker enclosure is installed in a secondary enclosure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    [0016]FIG. 1 shows an elevational view, partly in section, of a loudspeaker enclosure. The enclosure includes a rectangular cabinet  10  having at least one vertical side wall  12 . An acoustic port  14  is mounted in this side wall  12 . A bottom wall  16  of the cabinet  10  has a loudspeaker  18  mounted therein, where the loudspeaker  18  is facing away from the interior volume of the cabinet  10 . The area directly in front of the loudspeaker  18  is filled with a highly resistive acoustic foam material  20 . Arranged as such, the loudspeaker enclosure of the subject invention exhibits at least an octave extension of the low frequency response, for example, from 60 Hz to 30 Hz.  
         [0017]    In a practical embodiment of the loudspeaker enclosure, the cabinet is a cube having 13 inch sides. This yields an internal volume of the cabinet of approximately 1 cu. ft. The port has a length of 2.65 inches and an area of 12.57 sq. in. The loudspeaker has a nominal impedance of 7 ohms and a diameter of 8 in.  
         [0018]    [0018]FIG. 2 shows an equivalent circuit diagram of the loudspeaker enclosure. An AC voltage source, simulating an input to the loudspeaker, is connected to a resistor R 1  and an inductor L 1 . The parallel combination of an inductor L 2 , a capacitor C 1 , a resistor R 2 , a second capacitor C 2  and a series combination of a capacitor C 3  and a resistor R 3  connect the inductor L 1  to ground. A series arrangement of an inductor L 3  and a resistor R 4  are connected to the inductor L 1 , and the resistor R 4  is connected to ground via the parallel combination of a capacitor C 4 , a resistor R 5 , and a series combination of a capacitor C 5  and a resistor R 6 . The junction between capacitor C 3  and resistor R 3  is connected through a resistor R 7  to the inverting input of operational amplifier A 1 , a resistor R 8  connecting the non-inverting input to ground, and a resistor R 9  connecting the inverting input to the output of operational amplifier A 1 . A resistor R 10  connects the junction between capacitor C 5  and resistor R 6  to the output of the loudspeaker, while a resistor R 11  connects the output of operational amplifier A 1  to the loudspeaker output. The operational amplifier A 1  serves to create the necessary phase reversal to simulate the difference between the direct radiation of the loudspeaker, applied to the inverting input of operational amplifier A 1 , and the rear radiation of the loudspeaker, applied to the output through resistor R 10 .  
         [0019]    The value of these components are as follows:  
                                                                     VALUE                                        RESISTORS                   R1   7.5   Ω           R2   47.43   Ω           R3   0.1   Ω           R4   0.51   Ω           R5   984.6   Ω           R6   0.67   Ω           R7   4.732   KΩ           R8, R9, R11   10   KΩ           R10   2.041   KΩ           CAPACITORS           C1   79.82   μF           C2, C3   77.54   μF           C4   354.39   μF           C5   124.67   μF           INDUCTORS           L1   1   MH           L2   32.85   MH           L3   11.63   MH                      
 
         [0020]    [0020]FIG. 3 shows response curves of the loudspeaker enclosure of the subject invention, in which the amount of direct driver radiation is varied. It should be noted that when the driver is allowed to freely radiate, while the high frequency response (over 100 Hz) is satisfactory, the low frequency response (under approx. 70 Hz) drops off rapidly. When the driver radiation is limited, while the high frequency response is somewhat restricted, the low frequency response is significantly enhanced. While one may suspect that by completely blocking the driver&#39;s direct radiation should even more enhance the performance, the response curves show that while the low frequency response may by somewhat enhanced, the high frequency response suffers significantly.  
         [0021]    The loudspeaker enclosure of the subject invention may be used within a larger cabinet, as would be the case when the loudspeaker enclosure is used in, for example, a projection television receiver. In that case, the port should vent to the outside of the cabinet of the television receiver.  
         [0022]    [0022]FIG. 4 shows response curves when the loudspeaker enclosure of the subject invention is placed within a secondary cabinet of varying sizes.  
         [0023]    Numerous alterations and modifications of the structure herein disclosed will present themselves to those skilled in the art. However, it is to be understood that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.