Speaker Enclosure

Abstract

An improved speaker enclosure provides a close approximation to a true exponentially expanded horn for a bass-responsive speaker in a relatively small-volume cabinet that is disposed at the corner of a room. The cabinet also includes a separate section for a mid-range responsive speaker which can be properly phased with respect to the acoustic waves at the output of the bass-responsive section.

Description

BACKGROUND OF THE INVENTION

It has long been known that proper acoustic matching between a bass-responsive speaker operating down to about 30 cycles per second and the volume of air in a given region or room usually requires an exponentially expanded horn which is approximately one-half wavelength or 16 feet long and which has an opening into the region that is approximately 8 feet in diameter. These requirements severely limit the practical use of such equipment for true low-frequency acoustic coupling to a typical size room. Also, where the low frequency and mid-range frequency speakers are closely disposed, proper phasing of the acoustic waves from the speakers is required in order to prevent objectionable acoustic-wave delays that give rise to undesirable echo effects in the range of signal frequencies for which both the low frequency and mid-range frequency speakers show comparable response.

Several designs of speaker enclosures are known in the prior art for overcoming these problems associated with the proper matching or loading of the acoustic impedance of a speaker with the volume of air in a given region or room. A 16-foot horn having a straight mean path is one design which has proven to be useful in coupling low frequency acoustic waves into theaters and large auditoriums. However, this horn design usually presents phasing or acoustic delay problems in connection with a mid-range speaker disposed near the opening of the horn. Also, this horn design is not practical for typical room-sized installations. In these installations, folded horns have been used in which the mean path for acoustic waves has little or no exponential expansion with length and is a tortuous or serpentine path through a series of chambers and baffles. Several variations of this horn design are described in the literature but most of them commonly include a mean path length for acoustic waves that is less than one-half wavelength at 30 cycles per second. This introduces a significant internal loss of acoustic wave energy at the lowest audible frequencies with the result that higher harmonics of the lowest audible frequency are radiated into a room at acoustic power levels that are higher for the harmonics than for the lowest audible frequency. Also, several folded horn designs known in the prior art attempt to introduce exponential expansion with length along the mean path by widening out the acoustic passage along its length, but only using side-to-side expansion or top-to-bottom expansion with length, and not both. Folded-horn speaker enclosures of these designs usually introduce large internal acoustic mismatches and losses with concomitant poor response down to the lowest audible frequencies and also usually introduce undesirable standing wave resonances between the parallel surfaces that form the acoustic passage.

SUMMARY OF THE INVENTION

In the speaker enclosure of the present invention, the mean acoustic path spirals outwardly in a vertical plane from a central speaker to an opening of the horn disposed at floor level below the speaker. The present design utilizes the floor and intersecting walls of a room (as at a corner) as a portion of the exponential expansion of the horn with length along the mean acoustic path. Also, the speaker enclosure and associated apparatus of the present invention obviates the usual phasing problems of mid-range and low-range responsive speakers that are disposed in the same enclosure. In the present apparatus, the horn opening for the mid-range responsive speaker is positioned above the horn opening for the low-frequency responsive speaker so that phasing between the two sections can be accomplished.

DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of the speaker enclosure of the present invention;

FIG. 2 is a sectional view of the enclosure of FIG. 1 along the vertical plane of symmetry;

FIGS. 3a, b and c are graphic presentations of the logarithmic expansion utilized by the present invention;

FIGS. 4a and b are graphic diagrams of the shape and location of the sections of acoustic passage according to the present invention; and

FIG. 5 is a top view of the enclosure of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the perspective view of FIG. 1, there is shown the speaker enclosure 9 of the present invention disposed in the corner of a room. The enclosure 9 includes three vertically oriented principal sections, namely, the opening 11 of the low frequency horn at the base immediately adjacent the floor 13, the low frequency speaker enclosure section 15 disposed above the horn opening 11 and the top section 17 which houses the mid-range and high frequency responsive speakers. Each of these principal sections are acoustically isolated from each other by horizontally disposed rigid baffles 19 and 21. The front of central section 15 is enclosed to prevent acoustic energy from radiating outwardly into the room from the back side of the low frequency responsive speaker 23 (shown in broken-line view for clarity). This speaker 23 is so mounted in the central section 15 of the enclosure 9 that acoustic energy emanating from the front of the speaker is radiated in a direction that is generally upwardly and rearwardly within the enclosure 9. The mean path 29 of acoustic waves from the speaker 23 then diverts downwardly near the back corner of the enclosure and then diverts forwardly out through the lower opening 11 of the enclosure 9, as shown in FIG. 2. The mean path for acoustic waves thus spirals smoothly in a vertical plane from the speaker 23 to the lower opening 11 of the enclosure 9 without foldbacks or serpentine transitions. More importantly, the cross-sectional area of the acoustic passage from the speaker 23 to the lower opening 11 is substantially exponentially expanded in two orthogonal directions with length along the mean path. This more closely approximates the true form of an exponential horn (e.g. a tuba) for improved acoustic coupling characteristics and concomitant efficiency of the over-all design.

In FIG. 2, the enclosure 9 is shown sectioned along a vertical plane that is aligned with the bisector 24 of the intersecting angle of the rear walls 25, 27 of the enclosure 9 (typically, the rear walls are commonly disposed at right angles but it should be understood that other intersection angles may also be used). The enclosure 9 is substantially symmetrical about this vertical bisector plane and the present design of folded horn may thus be viewed conveniently from this plane. As was previously mentioned, a half wavelength horn which expands exponentially with length (approximately doubling the rate of expansion every 24 inches) is the ideal horn for properly coupling the lowest audible frequencies from a speaker to the volume of air in a given region or room. Accordingly, the acoustic passage in the present enclosure design has an approximate exponential expansion in two orthogonally lateral directions (within a few percent error) over a mean path length to the opening 11 of the horn of approximately 4 feet. The acoustic passage of the enclosure 9 may be viewed at regular intervals of, say, 24 inches along the mean path 29 for acoustic waves. Viewed in this way, it should be noted that the corner volume occupied by the present enclosure 9 is utilized to a substantially optimum degree to achieve an approximate bilaterally exponential expanded horn. Specifically, the speaker 23, say, of 15 inch circular design is properly back loaded by the confining chamber 31 at the front of the enclosure 9. The volume in cubic feet of the confining chamber is computed in a conventional manner to properly load speaker 23 on its rear side. Also, the speaker 23 is matched to the horn at its front radiating face by partially blocking the upper and lower portions of the speaker face using an inlet port 33 to the horn that is circular at the sides with upper and lower chords that define an oblong inlet port 33, as discussed in detail hereinafter. From this port, the acoustic passage is defined by upper and lower walls 35 and 37 and by the sidewalls 39, 41, as shown in FIG. 1. Thus, the horn inlet port 33 is approximately 8 inches by 15 inches, or less, and expands laterally with length along mean path 29 beyond location 46 to the first transition region that includes corner baffle 43. There, the mean path 29 for acoustic waves in the passage is diverted downward as the lateral dimensions expand and also undergoes a transition, say, at location 47 from the rectangular passage near inlet port 33 and at location 46 to a polygonal shape including the walls 25 and 27 of the enclosure 9 and the corner baffle 43, as shown in the top sectional view of FIG. 3 and in FIG. 4b. Further along the mean path 29 at location 49 the acoustic passage is triangular as formed by the rear baffle board 45 and the walls 25 and 27 of the enclosure 9. The cross-sectional area of the horn thus expands about orthogonally lateral axes at each location along the mean path 29. This passage again becomes polygonal in shape at the lower transition region including the lower corner baffle 51. Thus, at location 53, the rear baffle board 45, the walls 25 and 27 of the enclosure 9 and the lower corner baffle 51 divert the mean path 29 outwardly toward the opening 11 of the horn. Similarly, the acoustic passage retains polygonal shape of larger cross-sectional area further along the mean path at location 55 where the horizontal baffle 19, the walls 25 and 27 and the corner baffle 51 define the passage. The acoustic passage widens out with length along the mean path 29 toward the opening 11 which is defined by the base 57, the horizontal baffle 19 and the walls 25 and 27 and which thus approximates the curved shape of a wave front emanating from the horn. These aforementioned sections and transitions along the mean path are shown in FIGS. 4aand b. The vertical walls, horizontal baffles and transition baffles are all rigidly interengaged in the angular relationships shown in FIG. 1 using such suitable fastening means as glue and screws. The planar surfaces that form the acoustic passage and surroundings are thus inherently braced against undesirable mechanical resonances.

It should be noted that further along the mean path 29 beyond the enclosure 9, say, at location 59, the floor and walls of a room at a corner thereof, together with the front walls of the enclosure 9 define the acoustic passage which continues to expand approximately exponentially with length. Also, for a distance of about 8 feet from the present enclosure, unobstructed walls and floor of the room may constitute the exponentially expanding boundaries of the acoustic passage of the horn associated with speaker 23 which thus permits production of fundamental acoustic waves down to about 31 Hertz. The present horn design contained within the lower portions of the enclosure 9 which is approximately 3 feet high thus provides acoustic coupling of the speaker 23 to the volume of air within the room which is approximately 75 percent as efficient as the coupling that is possible using the ideal 16-foot long exponential horn having an 8-foot diameter opening. Where it is not essential that acoustic waves be produced down to the lowest design frequency of the present horn, the enclosure may be disposed in a room away from a corner and, since its acoustic passage is substantially complete within itself, may still provide comparatively efficient coupling down to about 35 to 40 Hertz.

The mathematical relationships between the various shapes and dimensions of sections of the acoustic passage are determined by the exponential rate of expansion of the cross-sectional area with length along the passage from the inlet port 33 for a selected low frequency note. Thus, from certain design considerations, a low frequency note of, say, 30.8 Hertz (i.e. B below low C) may be selected as the design standard for the exponential flare of the acoustic passage. It can be shown that the cross-sectional area of the acoustic passage must double approximately every 24 inches to provide the desired exponential flare for operation above cutoff at this design standard frequency of approximately 31 Hertz. Referring to FIG. 3a, there is shown a graphic presentation of the increments of cross-sectional area alone, the length of an exponential horn. In particular, FIG. 3ashows the ideal expansion of the cross-sectional area at 24-inch increments along the mean length of an exponential horn over approximately one-quarter wavelength of an acoustic wave in air at approximately the design standard frequency of 31 Hertz. Thus, FIG. 3agraphically illustrates the orthogonal two-axis expansion of cross-sectional area of an ideal acoustic passage over area limits that may be broader than the actual areas involved in the present invention.

In practice, a speaker or driver unit is selected which has a natural resonant frequency at about the design standard frequency (i.e. about 31 Hertz) and such a unit 23 of commercially available variety may be found to have approximately 15-inch diameter face. Thus, by backloading the speaker or driver unit 23 in the enclosure 31, as shown in FIG. 2, and by decreasing the inlet part area 33 immediately adjacent the face of the driver unit to an area of approximately 8 inches by 15 inches, the acoustic impedance of the driver unit 23 may be properly matched to the horn mouth with a natural resonance frequency down to or below the design standard frequency. The area of the inlet port 33 thus becomes the initial area from which the exponential flare of the associated acoustic passage is established, as shown in FIGS. 3b and c . At a location along the length of the ideal exponential horn represented by FIG. 3a, there is an area of approximately 120 square inches (i.e. about 11 inches square) and, within a 24-inch increment of length from there, another area that is twice the initial area (i.e. about 15 1/2 inches square, or about 240 square inches), and so on at each 24-inch increment of length thereafter.

In the present enclosure, the initial area at the inlet port 33 is about 120 square inches in approximately rectangular form of about 8 inches by 15 inches. And at section A--A which is about 24 inches away from the inlet port 33 along the mean path 29, as shown in FIG. 2, the cross-sectional area is about 240 square inches in trapezoidal form, as shown in FIG. 3c. Further, at section X.sub.1 --X.sub.1, X.sub.2 --X.sub.2 (i.e. the curved mouth of the horn) which is about 24 inches away from the plane of section A--A along the mean path 29, the cross-sectional area is about 480 square inches in rectangular form as shown in FIG. 3c. Thus, the acoustic passage within the present enenclosure provides substantially true two-axis exponential expansion over approximately one-eighth wavelength at about the design standard frequency of 31 Hertz. This exponential expansion is also maintained at the intermediate intervals, as shown in FIGS. 4a and b, to assure a reasonably smooth expansion over the mean path length 29 of the acoustic passage. This one-eighth wavelength of the acoustic passage at the design standard frequency supports fundamental acoustic waves at the design frequency for high-efficiency acoustic coupling to the volume of air in a room. However, the unobstructed portion of a room corner (i.e. walls and floor in the region about 8 feet in front of the present enclosure) contributes to the formation of a substantially true exponentially-expanded horn of one-third wavelength for acoustic waves at the design standard frequency of 31 Hertz. The present enclosure and the resulting acoustic horn thus provide a relatively compact design which is conveniently adaptable to a room as an unobtrusive piece of furniture and which provides approximately 75 percent over-all coupling efficiency at the design value of low frequency acoustic waves. Also, the continuously and reasonably smoothly expanding horn of the present invention substantially eliminates plane-parallel surfaces (except, perhaps, at the mouth where the acoustic wave energy is considerably less concentrated) which can support undesirable lateral acoustic reflections or standing waves.

Referring again to the perspective view of the enclosure 9, as shown in FIG. 1, the upper section 17 is arranged to couple acoustic waves of mid-range frequencies to the volume of air in a room using a horn design which includes exponential flaring in only one lateral direction with length, the other lateral dimension remaining constant as the plane-parallel spacing between the horizontal baffle 21 and the top 18. Ideally, the rate of expansion of the horn with length for operation at its lowest frequency of 300 Hertz should double approximately every 21/2 inches over a half wavelength path of approximately 8/10ths of 1 foot, or about 10 inches. In the enclosure of the present design, this rate of expansion with length is accomplished, as shown in the top view of FIG. 5, by forming the sidewalls 62, 64 of the acoustic passage in approximately three planar sections from the inlet port 61 at the front of mid-range speaker 63. The back side of this speaker is also completely enclosed to form a backloading chamber 65. The mid-range horn thus formed is believed to be of conventional design but has its opening 67 so arranged with respect to the opening 11 of the bass-responsive horn in accordance with the present invention that proper phasing of the acoustic waves from each such horn can be attained at frequencies for which the acoustic outputs of the two horns is comparable and this eliminates objectionable acoustic delays or echo effects at the cross-over frequencies of the low and mid-range speakers. More importantly, however, within selected efficiency limits the ratio of the size of the mouth of this mid-range horn to the size of the mouth of the bass horn must be approximately 2:3, plus or minus about 25 percent, in accordance with the present invention to ensure that the speaker system within the present enclosure provides properly balanced or substantially flat frequency response over the frequency range covered by the associated speakers 23 and 63.

The cross-over frequency for the low and mid-range speakers 23 and 63, respectively, is determined by the cross-over and phasing network 68 connected as shown in FIG. 2. This cross-over frequency may be set in practice to be about 300 Hertz as required by the characteristics of the low and mid-range speakers 23 and 63 and the phasing of the two speakers is determined by the relative connections of these speaker to their respective pairs of signal conductors 71, 73. By "phasing" is meant the movement (i.e. forward or rearward) of the speaker surface in response to the instantaneously-applied signal. Thus, for the mean length 29 of the low frequency horn of the present invention designed about approximately 30- 32 Hertz, and for the mean length of the mid frequency horn 17 designed about approximately 300 Hertz. The phasing of the speakers 23 and 63 to avoid objectionable acoustic delays at about the cross-over frequency should be that the speaker surfaces move in the same direction for a given instantaneous signal applied to both speakers.

The present invention may also include a high frequency speaker or speakers 76 arranged at the mouth of the mid-range horn 17, as shown in FIGS. 2 and 5. These speakers 76 may be driven by the crossover and phasing network 68 in proper phase and frequency relationship to the mid-range speaker 63. Since the human ear is relatively insensitive to acoustic phase differences at high frequencies, the relative phasing of the mid-range and high frequency speakers 63 and 76 at the cross-over frequency of these speakers is less critical than the phasing of the mid-range and low frequency speakers 63 and 23, as discussed above. In practice, however, the cross-over frequency for speakers 63 and 76 may be set at about 2,000 Hertz with both speakers' surfaces phased to move in the same direction on instantaneous signal applied to both speakers 63 and 76.

In general, the cross-over frequencies are selected in consideration of the fact that a speaker mounted in a horn usually cannot uniformly produce acoustic waves over more than three octaves because of limitations of the horn. The cross-over points in this enclosure are selected to allow each of its three sections to reproduce about three octaves for maximum clarity and presence. Also, this selection of cross-over frequencies eliminates harmonic distortion due to the inability of exponential horns to produce more than three octaves of sound above their lowest cutoff frequency without introducing harmonic distortion. The exterior of the enclosure may be styled and finished as desired with porous grill cloth covering the entire front face of the enclosure.

Therefore, the compact speaker enclosure of the present invention when properly positioned in a room corner provides an acoustic horn having substantially true exponential expansion over approximately 1/3 wavelength at about the lowest audible frequency. Also, the present invention obviates objectionable acoustic delays by providing proper phasing of low and mid-range speakers in relation to acoustic propagation delays through the exponentially-expanded horn.

Claims

I claim:

1. Acoustic apparatus comprising:

an enclosure having a front face and containing an acoustic channel therein having an inlet and an outlet for the passage therethrough of acoustic waves from the inlet to the outlet thereof, the channel having a mean acoustic path from the inlet to the outlet and having a cross section which increases with length along the mean acoustic path from inlet to outlet;

said channel being substantially symmetrically disposed about a vertical plane and being oriented with the outlet thereof disposed in the front face of the enclosure near the base thereof and with the inlet disposed above the outlet;

said mean acoustic path includes a vertically oriented portion which is disposed intermediate the inlet and outlet and which is located rearwardly of the front face of the enclosure; and

a bounded chamber within said enclosure which is disposed above the outlet and which communicates substantially only with said inlet of the channel in a boundary of said chamber that is disposed rearwardly of the front face of said enclosure, said inlet of the channel being disposed intermediate said front face and the rearmost portion of said enclosure and being aligned along an inlet axis which is directed generally rearwardly.

2. Acoustic apparatus as in claim 1 wherein the mean acoustic path of said channel spirals in said vertical plane approximately one half revolution from inlet to outlet, and the cross-sectional area of said channel disposed substantially normal to the mean acoustic path increases substantially exponentially with length along said mean acoustic path.

3. Acoustic apparatus as in claim 1 comprising an electroacoustic transducer for producing acoustic waves in response to electrical signals applied thereto, said transducer being disposed within said bounded chamber at the inlet to said channel for producing in response to applied electrical signals acoustic waves which emanate from the enclosure substantially only through said channel.

4. Acoustic apparatus as in claim 3 wherein:

said electroacoustic transducer is responsive to electrical signals applied thereto over a selected range of signal frequencies;

said enclosure includes an additional acoustic channel positioned above the chamber and inlet, said additional acoustic channel having a cross-sectional area which increases substantially exponentially with length along the mean path from the inlet to the outlet thereof, with the mean path of the additional acoustic channel and the inlet and outlet thereof being disposed in substantially horizontal alignment;

said outlets of the acoustic and additional acoustic channels have areas which are in the ratio of at least approximately 3:2 and are positioned in the front face of said enclosure for proper phasing of acoustic waves emanating therefrom; and

an additional electroacoustic transducer communicating with said inlet of the additional acoustic channel, the additional transducer producing acoustic waves in response to electrical signals applied thereto over a range of signal frequencies higher than said selected range.