Abstract:
A compact high acoustic power loudspeaker system includes an enclosure in which at least one low frequency speaker device and a bass-reflex port are provided. The bass-reflex port is mounted on the enclosure in a semiflexible manner such that the port can vibrate to provide additional bass reinforcement as compared to prior art loudspeakers. The bass-reflex port broadens the frequency response of the port and is less subject to “one-note bass” as compared to prior art ported systems.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an autoaugmented bass-reflex port that provides increased sound pressure levels across a broader frequency range. The port is suspended in an aperture in a wall of a loudspeaker enclosure in a manner than the port can vibrate reducing air turbulence within the port with a comparative improvement in low frequency response. 
         [0003]    2. Brief Description of the Prior Art 
         [0004]    Loudspeaker systems capable of generating large power outputs have been developed using various configurations. When large output power is to be achieved with small enclosures, radiation from the back of the cones of small speakers often becomes a source of distortion; accordingly absorbent materials are employed to reduce this effect. The absorbent material, however, alters the frequency response of the speaker system, usually attenuating the response at low frequencies excessively. 
         [0005]    Techniques have been proposed to improve the low frequency response and efficiency of a small speaker system. As a result, vented loudspeaker systems have become popular. The two common techniques to realize vented loudspeaker systems are the ducted port and passive radiator. In one technique, such as described by Schott, a bass-reflex speaker enclosure is depicted with a ducted port to reduce low frequency distortion. The port in these prior systems acoustically loads the loudspeaker cone at low frequencies and reduces distortion. The output of the port can approach that of the cone in a tradeoff between bass power and extension. The effect of the bass-reflex port is based on the Helmholtz resonator principle, which is known per se, the frequency of the reproduced sound being dependent on the volume of the enclosure, the length and cross-section of the port and the velocity of sound. Small loudspeaker enclosures require a port of comparatively small cross-section. The acoustic mass of air in the enclosure vibrates at the Helmholtz frequency, or port-tuning frequency. However, in order to obtain the same sound pressure at or near the Helmholtz frequency in a small enclosure as in a large enclosure, it is necessary that, per unit of time, the same amount of air flows through the port. Consequently the flow velocity of the air in the port is comparatively high for a small enclosure. It has been observed that at high sound levels at or near the Helmholtz frequency, bass-reflex ports of constant cross-section experience sound distortion due to air turbulence. The port output decouples from the speaker at these levels and induces noise and acoustic losses, particularly for small port cross-sections. Often a simple increase in port cross-section solves these problems, but can produce others, such as inconveniently large enclosures and excessive midrange output. Instead of ports with larger cross-sections, flared ports can be used to decrease port nonlinearity with some success. Thus flared ports decrease port cross-section, reduce port turbulence and improve low frequency performance in modest enclosures. These complicated ports and embodiments can still suffer from diminished port output at high sound pressure levels. Means to increase coupling the port output with the speaker diminish the undesired organ pipe resonance previously indicated. 
         [0006]    Tuned baffles can be used inside ported loudspeaker enclosures to amend some of these deficiencies. These baffles include multiple reed fingers that act as an internal tuning port to control the resonance of the air inside the loudspeaker cabinet. The result is often a smoother, deeper bass response than prior art ported enclosures. 
         [0007]    Passive radiators have also been employed in a fluid-tight enclosure to improve the low frequency response of the speaker system and produce bass response similar to that of ported loudspeakers. Passive radiators eliminate noises at high levels, but lack the efficiency of ported systems. Small enclosures seldom accommodate passive radiators, which need a larger cross-section than the active bass radiator. It has been shown that augmenting the acoustic mass of air contained in a loudspeaker enclosure can produce bass quality in between that of loudspeaker ports or passive radiators. The acoustic lens utilizes multiple passive radiators and occasionally amplifies bass output. Previous methods nonetheless demand sophisticated construction methods. Typically the interior of the speaker enclosure is slightly damped because either type of bass loading fails to transform the back wave of the moving speaker cone into useful acoustic output with perfect efficiency. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    In view of the above, it is an object of the present invention to provide a compact loudspeaker system with smaller low frequency speaker devices that provides a bass response comparable to a larger box with larger low frequency speaker devices. More particularly, it is an object of the present invention to provide a bass-reflex port that improves the bass response and output of a loudspeaker system. It is another object to provide a bass-reflex port that reduces the half-power response point of low frequency speaker devices. It is also an object to provide a bass-reflex port that preserves or increases the bandpass while amplifying the port output. Other objects and features of the invention will be in part apparent and in part pointed out hereinafter. 
         [0009]    In accordance with the invention, a compact loudspeaker system is described. At least one low frequency speaker is mounted inside a sealed enclosure. A bass-reflex port is located inside the enclosure relative to the low frequency speaker to provide generally uniform acoustic loading. 
         [0010]    The bass-reflex port is mounted on the enclosure in a somewhat flexible manner that provides additional bass reinforcement compared to prior art loudspeakers and reduces the half-power response point. Frequency response curve show that these improvements are obtained because the bass-reflex port allows the back wave from the moving cone of the bass speaker to exit the enclosure more efficiently through the port. This provides more useful acoustic energy from the back wave and amplified the port output. The bandpass, defined as the −3 dB points in the port response, of a conventional port and the present loudspeaker system may differ, with the loudspeaker system of the present invention having a broader bandpass. In summary, the loudspeaker system of the present invention has greater bass extension than a conventional port in the same size enclosure. 
         [0011]    Other features of the invention include the provision of a loudspeaker system which will furnish a more closely linear response from low to medium frequencies dramatically reducing resonance peaks; improving bass transient response; reducing unnatural coloration in low frequencies by attenuating cabinet resonant output by directing the pressure build-up energy to be expended in useful sound radiation instead of producing spurious “box” resonances. Power handling is also improved. The present invention provides a loudspeaker system capable of generating a high volume output with a relatively flat response and good efficiencies through the low frequencies. 
         [0012]    The invention summarized above comprises the constructions hereinafter described, the scope of the invention being indicated by the subjoined claims. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0013]    In the accompanying drawings, in which one of various possible embodiments of the invention is illustrated, corresponding reference characters refer to corresponding parts throughout the several views of the drawings in which: 
           [0014]      FIG. 1  is a cross-sectional view of a loudspeaker system including an enclosure, a low frequency speaker device and a bass-reflex port in accordance with the present invention; 
           [0015]      FIG. 2  is an exploded perspective view of the loudspeaker system; 
           [0016]      FIG. 3  is a front elevation on an enlarged scale of the bass-reflex port mounted in a wall of the enclosure; 
           [0017]      FIG. 4  is a cross-sectional view on an enlarged scale taken along the line  4 - 4  in  FIG. 1 ; and, 
           [0018]      FIG. 5  is an exploded perspective view of the bass-reflex port detached from the enclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring to the drawings more particularly by reference character, reference numeral  10  refers to loudspeaker system including an enclosure  12  which accommodates a low frequency speaker device  14  and a bass-reflex port  16  which is autoaugmented as described below. 
         [0020]    Enclosure  12  comprises an acoustical cavity and, as illustrated in  FIG. 3 , is typically formed as a wooden cabinet with front  18  and rear walls  20  interconnected with sidewalls  22  and top  24  and bottom  26  walls. Woods frequently used for enclosure  12  include plywood, particle board, medium density fiberboard (MDF) and laminated wood board. Among the many types of materials available, MDF, plywood or particle board are presently the best choice in terms of price, processability and acoustic characteristics. While enclosure  12  is illustrated as a rectangular or parallelpiped structure, it will be understood that enclosure  12  may have other shapes, e.g., tetrahedral, spherical or compound, which may be dictated more by aesthetics than function. Enclosure  12  may also be formed of other materials including metal, plastic or the like so long as it is air-tight and the wall in which low frequency speaker device  14  is mounted is comparatively rigid and vibration-free. 
         [0021]    Front wall  18  has a plurality of speaker openings to receive low and midrange speakers and tweeters but for clarity of illustration only opening  28  for the low frequency speaker device  14  is shown in  FIG. 2 . The term low frequency speaker device  14  as used herein includes speakers resonant below about 200 Hz and thus includes sub-woofers, woofers and in some instances midrange speakers. The speakers, including low frequency speaker device  14 , can be mounted to either the inner or outer surfaces of front wall  18  with appropriate screws, glue or other fasteners, with gaskets  30  applied as necessary. A speaker grill  32  is shown in  FIGS. 2 and 3  for aesthetics. As illustrated grill  32  does not cover bass-reflex port  16 , an optional arrangement. An assembly for crossover  34  is also depicted. 
         [0022]    Bass-reflex port  16  includes a tubular body  36  open at one end to the inside volume of enclosure  12  and at an opposite end to the listening space. Tubular body  36  may be formed of suitable materials such as wood, plastic, rubber, cardboard or the like. Suitable woods for tubular body  36  include all those materials mentioned above for enclosure  12 . Plastics such as polyvinylchloride may be preferred for cost and mechanical properties. While tubular body  36  is illustrated cylindrical and unperforated, it will be understood that tubular body  36  may have flared ends or a constricted mid-section as well as other shapes in cross-section such as star-shaped, oval and the like and be perforated. 
         [0023]    The dimensions of tubular body  36  are selected to furnish suitable bass loading for low frequency speaker device  14 . Numerous software programs and tables are available in the loudspeaker literature using the Thiele-Small parameters of the bass speaker. Enclosure  12  may also be selectively damped to modulate loading. 
         [0024]    As illustrated in the drawings, an aperture  38  is provided in front wall  18  for mounting bass-reflex port  16 . It will be understood, however, that bass-reflex port  16  may be mounted in any other wall of enclosure  12 . Aperture  38  is slightly oversized such that tubular body  36  does not make hard contact with enclosure  12  when mounted therein. For example as shown in  FIGS. 3 and 4 , a spacing  40  of about ¼ inch may be provided around tubular body  36  in aperture  38 . It will be understood that this measurement is illustrative for a tubular body  38  with an OD of 2 inches and is not limiting. 
         [0025]    Tubular body  36  is suspended in aperture  38  by a flange  42 . Flange  42  is larger than aperture  38  and includes an opening  44  in registry with the inner wall of tubular body  36 . Tubular body  36  may be attached to flange  42  with fasteners, glue or may be fused or molded as an integral unit. Alternatively, flange  42  may include a collar into which tubular body  36  slips and is frictionally engaged. Flange  42  may be formed of the same classes of materials suitable for tubular body  36 . 
         [0026]    Flange  42  and tubular body  36  are mounted in an air-tight manner to front wall  18  with a dampening pad  46  interposed there between. Dampening pad  46  may be formed of a damping material. Such materials include elastomers such as rubber or plastics such as some polyurethanes. Sponge, profiled polymers (e.g., eggcrate foam), felt or sandwich materials like Dynamat which is felt embedded with lead with an adhesive backing may also be useful. Flange  42 , dampening pad  46  and front wall  18  may be drilled for receipt of screws  48 , rivets, Hurricane Nuts or other suitable fasteners. A coarse screen (not shown) that does not unduly restrict air flow may be provided over bass-reflex port  16  for aesthetics and to prevent the insertion of a foreign object. 
         [0027]    In use, tubular body  36  is free to vibrate in aperture  38  while dampening pad  46  suppresses the transfer of vibrations from bass-reflex port  16  to enclosure  12 . The result is that loudspeaker system  10  produces higher quality bass from a smaller enclosure  12  than was possible with the prior art. Other advantages include: Bass reflex port  16  is smaller and more cost-effective than an acoustic lens, which requires multiple passive radiators and significantly more intricate cabinet work. Bass-reflex port  16  is also more cost-effective and easier to tune reliably than base loading with a passive radiator. Bass-reflex port  16  is preferable over small enclosures with small drivers that use flared ports to load the front wave of the loudspeaker because those loudspeaker systems require elaborate and expensive cabinetry and extensive tuning. Bass-reflex port  16  is also preferable over small enclosures with flared ports because those enclosures can suffer port decoupling at high outputs and can be unwieldy because the port projects outward from the enclosure. 
         [0028]    Like reed-loaded ported designs, bass-reflex port  16  broadens the frequency response of the external port and improves the transient response of a loudspeaker. On the other hand, bass-reflex port  16  is less expensive to build than reed-loaded ported designs which require intricate cabinetry and have high tuning demands. In addition, although reed-loaded ported design require relatively small enclosures and diminish the one-note quality found in the bass response of many ported designs, reed-loaded ported designs fail to provide sufficient bass amplification unlike bass-reflex port  16 . 
       Example 
       [0029]    In one specific example, a Vifa 8″ woofer, #MG22WO09-08, was mounted in enclosure  12  having outer dimensions 22″(H)×12″(W)×9.5″(D). A polyvinylchloride tubular body  36  having an outside diameter of 2″ and a length of 5.3″ was mounted in aperture  38  as described above. It is estimated that bass-reflex port  16  reduced the half-power response point of low frequency speaker device  14  from 44 Hz as measured in an anechoic chamber. Improvements were obtained because the bass-reflex port  16  amplified (i.e., autoaugmented) the port output. Moreover, the tuning frequencies of a conventional and the autoaugmented port remained the same. Consequently, the bandpass, defined as the −3 dB points in port response, of the subject bass-reflex port  16  was broader than those of a conventional port. 
         [0030]    In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.