US2819772A - Wide range loudspeaker - Google Patents

Description

Jari. 14, 1958 s. BRYAN WIDE RANGE LOUDSPEAKER- Filed Dec. 14, 1954 INVENTOR BRYAN,

5KJMUEL ATTORNEY United States Patent WIDE RANGE LOUDSPEAKER Samuel Bryan, Silver Spring, Md., assignor to Laboratory ff Electronic Engineering, Inc., a corporation of Maryand Application December 14, 1954, Serial No. 475,112

4 Claims. (Cl. 181-31) This invention pertains to high fidelity loudspeakers, and more particularly to improvements in the speakerhorn combination by which speech, music or other signal are efiiciently coupled from the output of an amplifying system to the atmospheric load represented by an ordinary room or space.

It has been long recognized that a true horn is the most practical means for reproducing the power and dynamic qualities of the low or bass tones of recorded music, especially of full range music such as that produced by a symphony or an organ. The true horn speaker, as the term is here used, has a completely scaled back volume (behind the moving element or cone), and semi-horn devices which allow radiation from both sides of the cone; the true horn more nearly matches the mechanical impedance of the speaker cone or diaphragm to that of the air-load of the room or other space in which the signals are to be reproduced as audible sound.

When a direct radiator cone is employed as a speaker, its acoustic radiation resistance and low-frequency power handling ability decrease rapidly when its diameter falls below one-half of the wave length of the signals being reproduced. Thus, a 30 cycle organ note would require a cone diameter of over 17 feet. Such a radiator, for the low frequencies, is not practical, both from size (or bulk) and from cost considerations. By employing a horn as a pressure transformer to achieve a match between the speaker mechanical impedance and the airload, the diameter of the moving element or driver, as Well as the amplitude of movement required, can be reduced many times as compared with the direct radiation.

With the above considerations in mind, many efforts have been made to design a practical horn type speaker suitable for the high fidelity reproduction of speech and more especially music, in the limited space of the average living room or other chamber of modest size. These attempts have resulted either in monstrous enclosures or serious compromises with the fidelity desired to be attained.

It is accordingly a principal object of this invention to provide a speaker and enclosure in which the optimum reproduction characteristics of a true horn are substantially achieved, but in such a configuration that the enclosure size can be held within limits which are acceptable to the ordinary user, and whose construction is such that it can be manufactured at a comparatively moderate price due to savings both in materials and labor.

The general approach to a solution of this problem as exemplified in the present invention, is the use of a folded horn designed for installation at a trihedral corner of the normal room the diverging walls (and floor, where the speaker is installed at floor level) providing an approximate continuation of the horn provided by the invention. It is recognized that consideration has formerly been given to adoption of the folding principle, and also that it has been proposed to utilize a horn delivering energy into a room corner for propagation along a column defined in part by the diverging planes of two walls and the floor or ceiling. The present invention aims to improve the speaker enclosure itself, not only for better coaction with the corner geometry of a room, but also to utilize a minimum number of flat pieces of structural material whereby the size and cost of the speaker and its enclosure may be brought within tolerable limits.

Basically, the invention utilizes a single low-frequency speaker coupled to the novel horn formed by the enclosure of the invention, the nature of the construction being such that identical structural panels define not only the walls of the enclosure for this speaker, but also form in part the shaped duct or horn sections which surround the space or chamber occupied by that speaker. In addition, the invention utilizes auxiliary speakers which are adequate to complement the lower range of frequencies served by the horn, whereby to extend the entire operative spectrum of the unit into the middle and higher registers which can adequately be accomplished by such auxiliary speakers. However, it is a novel feature of the present invention to form and relate the panels defining the duct which is the horn itself, so as efficiently to provide within a neat and wieldy enclosure the necessary space for housing these auxiliary speakers.

With the above and other objects and advantages of the invention in mind, a preferred embodiment thereof will now be described in the following detailed specification, taken in connection with the appended drawings, in which:

Fig. 1 is a perspective view, with parts broken away, of a preferred form of the novel speaker-horn combination,

Fig. 2 is a vertical sectional view taken substantially on the line 2-2 of Fig. 1 and looking in the direction of the arrows, and

Fig. 3 is a horizontal sectional view taken on the line 3-3 of Fig. 2 of the drawings.

As shown in the drawings, the main or outside wall members of the speaker enclosure comprise a vertical front panel 10, a pair of parallel partial side panels 12 and 14 extending rearwardly from each side of the front panel for a portion of the depth of the enclosure, and perpendicular to the front panel 10, and rearwardly converging additional side panels 16 and 18. These converging side panels 16 and 18, in their lower portions, actually extend to a vertical line of intersection or apex line 20, but their respective upper portions 22, 24 terminate as at edges 26, 28 to define therebetween an acoustic mouth or outlet from the interior of the enclosure. The upper edges of panels 10, 12, 14 and portions 22 and 24 lie preferably in a common horizontal plane, and are connected by a top panel 30 whose front edge may be coplanar with the upper edge of the front panel, or may of course overhang if desired.

When installed in a corner position, the perpendicular relationship between front panel 10 and partial side panels 12 and 14 produces the result that these panels, together with top 30, are the only walls of the enclosure which are visible from normal positions within the room. Hence, the effect of a rectangular cabinet or enclosure is achieved, even though the general configuration of the cabinet is in fact designed to utilize the corner space rearwardly of the side panels 12 and 14.

All of the panels described, as well as others to be mentioned below, are made of suitable rigid materials of adequate thickness and stiffness to minimize transmitted sound vibrations through their substance. A good grade of plywood, preferably of or A; inch thickness, is recommended for the outer panels so far described, and the same may also be used for the interior partitions. The joints between panels are preferably well made so as to be tight and secure, both glue and fastening screws being preferred for maximum integrity and tightness. Where desirable, corner blocks may also be used to connect the meeting panels.

Within the main outer enclosure is disposed a chamber 32 to receive the main or low-frequency driver unit 34, which may be an ordinary dynamic loudspeaker of good quality having a cone or diaphragm 36. The speaker is secured face down over an aperture 38 in the floor panel 40 of this chamber, said panel having rearwardly con verging edges 42 and 44 which correspond to the convergence of the lower portions of side panels 16 and 18, and being secured thereto to form (with other parts to be described) the substantially air-tight enclosure or chamber 32 of which the speaker diaphragm forms a part of one wall. The forward wall 46 of this chamber is nearly but not quite parallel to the front wall panel 10 of the enclosure; specifically, wall 46 diverges upwardly from the plane of wall 10, so as to define therewith a channel 48 of rectangular horizontal cross-section, the smaller side of the rectangle increasing somewhat from bottom to top of such channel. The inclination of panel 46 depends upon the dimensions of the enclosure, and in the example shown in the drawings (which are practically in correct scale) may be degrees from vertical, for an enclosure having a total height of 36 inches and a front-panel width of 25 inches. The volume of chamber 32 is chosen so that its mechanical capacitance and the mass of speaker 34 has a frequency of resonance well within the pass band of the bass horn.

The remaining major Wall surface enclosing the chamber 32 is an inclined rear panel 50 whose rearward portion is of triangular profile to conform to its intersections with the converging side panels 16 and 18. Since chamber 32 also extends into the space between the parallel side panels 12 and 14, a part of it forward of the triangular portion has parallel edges 52 and 54. The forward edge of inclined panel 50 is connected with the front chamber wall 46 by an inclined narrow panel or batten 56, to close the complete chamber 32.

Space for one, two or more auxiliary higher frequency reproducers is provided by an inclined panel 58 extending upwardly from front wall to a vertical partition 60 depending from the top 30 to meet the same. Such reproducers, two in number in the example shown, are indicated at 62 and 64, one being a middle range speaker and the other a high frequency speaker or tweeter. Either of these may be of the horn type or a direct radiator, the selection being immaterial so far as concerns the novelty of the present combination of horn with speaker 34. What is significant with respect to the disposition of these auxiliaries is the fact that they are contained within an enclosure whose walls 58 and 60 form also walls of the horn structure cooperating with speaker 34.

Since the present reproducer is intended to be mounted in a trihedral corner defined by room walls and floor (or ceiling), it is essential that the effective speaker mouth be in communication with the corner. For this reason, and to prevent accidental complete or partial closure of the room corner by location of the enclosure too close to the walls, the rear edges of the top panel 30 overhang the sides 12, 14, 16 and 18, as indicated, and the apex 20, by several inches.

It will be seen from the above that sound energy issuing from the front (herein, the downwardly facing side) of the speaker cone 36 passes to a chamber 66 which merges at the lower front of the enclosure cabinet with the vertical horn section or channel 48. The desired taper of the horn section starting just below the center of speaker 34 and passing about this corner is obtained by inserted solid plugs (which may be a properly shaped single plug) indicated at 68, 70 and 72. These plugs may also conveniently be formed of plywood or other solid material. Plug section 68 provides the proper taper into the vertical horn section 48, plug 70 performs the same function along the bottom of the section 48, and corner plug section 72 effects the proper taper through the right angle transition itself. Transition out of the vertical section or channel 48 is effected by the slanted panel 58 defining the enclosure for the auxiliary speakers 62 and 64. It will be noted that the effective horn cross-sectional area increases quite smoothly and steadily from the point of driving (by speaker 34) until it merges with the room trihedral. From Fig. 1, it might be supposed that the section defined by the converging upper portions 22 and 24 of side panels 16 and 18 represents a decreasing area. However, Fig. 2 makes it clear that the area actually increases in this region also, due to the downwardly inclined panel 50, which causes the area to increase rapidly enough to offset the convergence of the panels 22 and 24.

Omitted for clarity from Fig. 1, but shown in Figs. 2 and 3, is a vertical central brace or strut 74 extending between the inner surface of the front wall panel 10 and the outer surface of the forward wall 46 of the speaker enclosure. This strut is of negligible cross-sectional area, but provides desirable stiffening for the front wall 10 as well as for the large wall 46 of the chamber 32. The other major wall panel 50 of the chamber is of course smaller in area than wall 46, but it also may be stiffened and damped in any conventional manner. In Fig. 2, it is shown as stiffened by having secured thereto a rather large and massive condenser 76 forming a part of the electrical network connecting the three reproducers shown.

It will be seen from the above description that the present invention provides a true horn which is folded to secure an effective length, within the compass of the enclosure, much greater than could be obtained merely by successive reversals of the main horn axis direction. Of course, the effective length within the compass of the enclosure is not the true length of the horn when installed (as it is intended and designed to be installed), as merely the first part of a horn system utilizing the room trihedral as its final component. The advantage achieved by the invention is however not merely one of compactness, because it also permits a closer approximation to a desired flaring constant and shape parameter, and a smoother transition, at all points at which danger of reflection and acoustical discontinuity would otherwise exist. In this connection, it should be realized that a true horn, in the mathematical sense, is not only free of geometrical discontinuities along its length, but follows some mathematical law which relates its cross-section at any point to the axial distance of that point from the driver position. This is necessary in the theoretical case to accomplish the fundamental purpose of the device, which is to act as an impedance transformer between the acoustical energy of high pressure and low velocity, produced by the driver, to acoustical energy at low pressure and high velocity where it impinges upon the ear of the listener.

The difliculty with the design and fabrication of a practical horn lies not in the lack of adequate mathematical description of the optimum shape, but in the attainment of a structure which can be fabricated of commercial materials and still represent a sufiiciently satisfactory compromise with the mathematical requirements. Itis, moreover, most important that the deviations from the theoretical shape be least in regions of high acoustic pressure; e. g., near the driver. The present invention provides special treatment in this region by the employment of space-filling plugs which permit a close approach to theory Within outer boundaries themselves dictated by the necessity for a compact cabinet of attractive format. The invention is well adapted as (and the disclosed embodiment is based upon) a horn having generally catenary mathematical configuration so far as its lengthwise boundaries are concerned, the cross-section at any axial point being generally rectangular. However, with modifications that will be obvious to those skilled in the art, other known or desired horn relationships, including the exponential, can be adapted to the con structional principles disclosed herein. A concise treatment of acoustic horn design is included in chapter of the Radiotron Designers Handbook, 4th edition, published by the Wireless Press in 1952, to which reference may also be made for citation of more extended treatises on the subject.

A characteristic feature of the present invention is the use of a minimum number of different structural panels or pieces, which feature is best realized when it is considered that there is no essential piece (neglecting the optional strut 74) which does not represent an acoustic boundary for some part of the horn channel. Moreover, in most instances, each panel represents two acoustic boundaries. Thus, considering the elements inthe general direction of propagation along the horn, the panel 40 supporting the driver is a wall of the closed chamber 32 and also a wall of the entrance region 66 of the horn. Wall 46 of chamber 32 also defines one side of the vertical horn section 48. Wall 58 defines both the chamber for the auxiliaries and a boundary of the transition section at the top of channel 48. Wall 50 of chamber 32 additionally defines a boundary of the mouth region between wings 22 and 24.

It may appear at first sight that all of the exterior panels cooperate only with one significant acoustical space. The side panels 16 and 18, however, in cooperation with the room walls (indicated in Fig. 3 by numeral 78) define the divided channel from which the sound energy finally issues toward the listener, and side panels 12 and 14 efiect a continuation of this region.

Another characteristic feature of the invention is involved with the fact that the essential shape of the region defined by a room trihedral and a vertical front cabinet surface symmetrically disposed therein, is that of a triangular right prism, using the term right prism in its descriptive geometry sense; that is, in the sense of the figure generated by moving a triangle parallel to itself along a line perpendicular to its plane. This is the general outer shape of the enclosure of this invention, neglecting the use of the partial side panels 12 and 14. According to the invention, the interior closed chamber 32 is geometrically similar to this outer configuration, being in fact a truncated triangular prism whose truncation plane is the inclined panel 50. Since the interior shape of. chamber 32 is largely immaterial except insofar as it conditions the positions of surrounding parts, this truncation permits the provision of the expanding mouth area of the horn by the single panel 50. In this sense, there is only a single region within the enclosure which is solid or acoustically filled, to wit the region of the small-volume plugs 68 to 72. Hence, the total volume of the enclosure is utilized to the extent of over 99% in obtaining the desired horn properties.

Considering the design as above, that is, as a prism within a prism, it is also seen that the inner prism cannot be centered with respect to the outer, if a horn defined solely by their boundaries is to have a desired taper in sectional area. Hence, the disposition of the inner prism nearer the apex of the room trihedral than is the center of volume of the outer prism or enclosure, also accomplishes two ends. It permits the horn taper desired, and permits common outer panels (22 and 24) for both prisms. The saving in spatial volume and construction material due to this relationship is apparent from Fig. 1.

While the invention has been described herein in connection with a single embodiment chosen by way of preferred construction and example, it is to be understood that the principles of the invention can be applied to various specifically difierent forms of the invention, and the invention is considered as including all such changes and modifications as fall within the scope of the appended claims.

What is claimed is:

1. A loudspeaker horn comprising a front panel and rearwardly converging side panels arranged to form an outer enclosure of generally prismatic form having a substantially triangular horizontal section, an inner enclosure in the form of a truncated triangular prism, the inner enclosure being decentered with respect to the outer enclosure to define therebetween a horn channel of tapered efiective cross-sectional area, occupying substantially the entire enclosed volume of the outer enclosure less that occupied by the inner enclosure; said channel communicating at one end with said inner enclosure and at its other end opening rearwardly away from the front of said outer enclosure.

2. A loudspeaker comprising a cabinet formed of outer walls defining rectangularly related front, top and side walls, converging panels extending from said side walls to correspond to walls of a room corner, inner panels defining a closed chamber whose center is below and rearward of the center of said cabinet to define a hornlike channel, open at one end only, between the cabinet walls and the inner panels, one of said inner panels forming an upper closure panel for the chamber, the open end of said channel being defined between the proximate edges of the upper portions of said converging panels and between said top wall and the upper closure panel of said closed chamber.

3. A loudspeaker comprising a cabinet formed of outer walls defining a front, top and side walls, the latter having converging portions to correspond to walls of a room corner, inner panels defining a closed chamber whose center is below and rearward of the center of said cabinet to define a horn-like channel, open at one end only, between the cabinet walls and the inner panels, a sound reproducer mounted in said chamber to deliver sound energy into said channel at its closed end, a second closed inner chamber formed in a corner of the outer enclosure between the front and top walls by a panel inclined to the axis of the channel, said front wall being apertured into said second chamber, and at least one auxiliary sound reproducer mounted in said second chamber to deliver sound energy through an aperture of the front wall.

4. A loudspeaker horn comprising an outer enclosure of generally prismatic form having a substantially triangular horizontal section, an inner enclosure in the form of a truncated triangular prism, the inner enclosure being decentered with respect to the outer enclosure to define therebetween a horn channel of tapered efiective crosssectional area, said channel communicating at one end with said inner enclosure and at its other end opening rearwardly away from the front of said outer enclosure, the truncation plane of said inner enclosure being located in one wall of the mouth of the channel.

References Cited in the file of this patent UNITED STATES PATENTS 1,865,550 Barrows et al. July 5, 1932 2,160,166 Pausin May 30, 1939 2,310,243 Klipsch Feb. 9, 1943 2,373,692 Klipsch Apr. 17, 1945 2,604,182 Massa July 22, 1952 FOREIGN PATENTS 225,340 Great Britain Dec. 4, 1924 OTHER REFERENCES Pub., advertising circular, The Lee Catenoid," Lab. of Electronic Engg, Inc., 413 L Street N. W.

Pub., Radio & Television News, Dec. 1954, page 46, Fig. 1B is pertinent.

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