Abstract:
This invention is an audio frequency loudspeaker device with characteristics of broad frequency response, high efficiency, and simplicity. These qualities are achieved by the novel combination of a resonant pipe and an audio transducer dipole.

Description:
BACKGROUND OF THE INVENTION 
     1. Field 
     The invention is a device for the efficient propagation of high fidelity sound from an audio frequency electrical signal. 
     2. Prior Art 
     For comparison purposes, one may refer to other devices in the same field. First is a device shown in U.S. Pat. No. 5,073,945. It is a dipole type device built into a sealed box and uses an electrical crossover. Second is a device shown in U.S. Pat. No. 4,268,719. It is also a dipole type device within a closed housing, but with no crossover network. Third is a device described in U.S. Pat. No. 3,729,061. It could be shown to be a folded resonant pipe with one transducer located concentrically at one open end. Number four is represented in U.S. Pat. No. 5,111,905. It is a folded closed end resonant pipe with one transducer mounted tangent to the approximate centerline of the pipe. Fifth is shown in U.S. Pat. No. 5,105,905 and is essentially a resonant pipe with a concentrically mounted transducer and reflectors mounted at each end to redirect the propagated sound. Finally, number six is shown U.S. Pat. No. 4,064,966. It could be shown to be a closed dipole type arrangement concentrically installed on the end of a folded resonant pipe. 
     SUMMARY OF THE INVENTION 
     My invention is a loudspeaker device which incorporates an open acoustic dipole arranged to be both tangent and concentric to a folded resonant pipe. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In illustration of my invention, I have provided drawings listed as FIGS. 1 through 7. They are all representative of the preferred embodiment. 
     FIG. 1 shows a plane front view. 
     FIG. 2 is a plane side view of either side. 
     FIGS. 3 and 4 are plane top and bottom views. 
     FIG. 5 is an isometric view showing front, side and top. 
     FIG. 6 is a vertical cross-section of the view shown in FIG. 2. 
     FIG. 7 is an isometric of the uninstalled internal divider or baffle. There is no back view included because it is featureless. 
    
    
     In all figures, letter &#34;a&#34; refers to transducers. In all figures, letter &#34;b&#34; refers to the tongue of the baffle. In all figures, &#34;c&#34; refers to the baffle assembly. In all figures, &#34;d&#34; refers to the enclosure. In FIGS. 6 and 7, letter &#34;e&#34; refers to deflectors. In FIG. 6, there are also numbers 1 and 2 which show the installed transducers. 
     DETAILED DESCRIPTION 
     The loudspeaker device of my invention is a combination of two elements. These are audio electric transducers and an enclosure box or resonant pipe. 
     The transducers are full range type, and used in pairs. They are electrically connected either in series or parallel and the connections are in phase. The transducers, as can be seen at &#34;a1&#34; and &#34;a2&#34; of FIG. 6, are installed back to back and concentrically to each other. This arrangement comprises an open acoustic dipole. 
     An open acoustic dipole, by itself, has no useful output. In this lies the importance of the box or pipe. The box is constructed of any suitable high density material and appropriate method of assembly. There is an internal divider or baffle which functionally causes the box to become a folded resonant pipe. The round openings in the box and baffle allow the installation of transducers. The divided rectangular opening in the front becomes both open ends of the resonant pipe. 
     The pipe in other embodiments could have round or oval or other cross section, in which case the invention might not have the same external appearance. It is necessary, however, to be folded as in this embodiment. There are two reasons: First, the folds cause the correct number of phase shifts, and second, the dipole must be installed in a fold. The correct number of folds allow all the propagated sound to be additive. The dipole when so mounted, will propagate energy both internally and externally to the resonant pipe. 
     Specific dimensions of this embodiment are not given because of the many combinations possible. I will, however, describe the aspects of dimensional limitations. There are two kinds of factors involved. First are those which affect acoustic performance indirectly because of the dimensions of the transducers. Second are those caused by the nature of acoustic vibrations. 
     Three aspects of transducer dimensions are considered. First, frame width restricts the minimum width of the enclosure. Second, frame height is a consideration in the space where the transducers &#34;a1&#34; and &#34;a2&#34; of FIG. 6 are back to back. Third is the piston or cone area. It is necessary that piston area nearly match the cross sectional area of the pipe. In the space mentioned above where transducers are back to back, both transducers&#39; cone areas are additive. Also noteworthy, transducers are usually considered to have a smaller piston area from the back. The remaining pipe length is driven by the front side of only transducer &#34;a2&#34;, therefore, those dimensions can be more easily matched to the cone area. 
     The first limitation on pipe dimensions due to acoustic limit is the need for correct proportion of pipe length on either side of the point of energy injection. Practice has shown that odd number ratios are required to either add or remove energy in a waveguide. Waveguide being analogous to resonant pipe, therefore indicates use of proportions one to three. This is closely achieved in this box enclosure embodiment by locating the rectangular hole in the front, where the ends of the pipe terminate, at a distance half way between top and bottom. The internal height of the box is equal to one half of the quarter wave of the primary resonance frequency. The prototype of this embodiment was constructed four feet tall and is therefore theoretically capable of sound reproduction down to thirty two cycles. 
     The internal baffle, &#34;c&#34; in FIG. 6 and 7, is constructed so that it has the necessary features to make a box become a folded pipe. There is a hole to mount the transducer frame near as possible to the top. The bottom portion is laminated of the same or similar material in order to increase in thickness. The extra thickness is required in order to maintain a consistent cross-section in that portion of the pipe. The bottom of the baffle is made short of the bottom of the box. The reason is to provide a passage for continuation of the pipe from back to front. There is also a tongue attached to be vertically centered in the bottom front opening. The tongue has the purpose of causing the final 90 degree turns needed to give the total 360 degree phase shift. Each fold or turn of 90 degrees throughout the entire pipe length constitutes a 90 degree phase shift of the sound, and is required in all embodiments. 
     Finally, deflectors are installed in this embodiment but may not be needed in other embodiments. Their purpose is to prevent reflection within the pipe and help make pipe dimension more uniform. 
     The internal height of the box is equal to one half of the quarter wave of the primary resonant frequency. The prototype of this embodiment was constructed four feet tall and therefore is theoretically capable of sound reproduction down to thirty two cycles. One half of the dipole, that is, transducer number &#34;a1&#34;, partially radiates directly into the listening space. The upper limit of frequency response is thereby determined. Because high frequencies are dissipated within a low frequency pipe, the internal transducer &#34;a2&#34;, supplies mostly low frequency sound. Four inch diameter commercially available transducers were chosen because the upper response is up to twenty thousand cycles. 
     The net benefit of the back to back transducer combination is due to the dynamic dampening of this dipole arrangement which occurs in the space between them. This allows efficiently propagated sound energy at frequencies below the transducer&#39;s rated normal lower limits. Resonant amplification at these low frequencies is caused by the pipe, this allows overall frequency response of the invention to include nearly the entire spectrum of human hearing.