Patent Publication Number: US-6343134-B1

Title: Loudspeaker and horn with an additional transducer

Description:
This application is a continuation-in-part of an application, filed Jan. 28, 1998 under Ser. No. 09/014,700, now U.S. Pat. No. 6,038,326. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention relates to a loudspeaker including a compression chamber, a first electroacoustic transducer disposed inside the compression chamber, a horn and a second electroacoustic transducer disposed outside the compression chamber. 
     U.S. Pat. No. 4,138,594 teaches a small dimension low frequency loudspeaker which includes a folded exponential horn which provides a unitary curved sound path from an electro-acoustic transducer at the throat of the horn to a volume into which sound is radiated at the mouth of the horn. The length of the horn is such that, at an exponential rate of expansion between the throat and the mouth, the mouth, when it is bounded by at least one planar surface, such as a floor, a ceiling, and/or walls of a room, has adequate area to enable reproduction of low audible frequencies. The low frequency loudspeaker has an effective low end cut-off frequency of 55 Hz. U.S. Pat. No. 4,210,223 teaches a low frequency loudspeaker apparatus includes a folded exponential horn which is divided to provide a bifurcated curved sound path from at least one electroacoustic transducer that is positioned at the throat of the horn to a volume into which sound waves are radiated that is located at the bifurcated mouth of the horn. The mean length of the folded exponential horn is such that, at an exponential rate of expansion between the throat and the bifurcated mouth, the area of the mouth is adequate for reproduction of low frequencies in the audible range. The low frequency loudspeaker apparatus has an effective low end cut-off frequency of 38 Hz. and affords 99 dB SPL output at three meters with one watt input which corresponds to about 20% efficiency measured in free space. Presence of a single boundary surface, such as a stage floor adjacent the mouth of the folded exponential horn, improves amplitude response by 3 to 6 dB. A small dimension low frequency folded exponential horn loudspeaker has a unitary sound path for direction of acoustical waves from an electroacoustic transducer to a volume into which the acoustical waves are radiated. 
     High fidelity sound reproduction requires reproduction of low audible frequencies. W. B. Snow, “Audible Frequency Ranges of Music, Speech, and Noise,” Jour. Acous. Soc. Am., Vol. 3, July, 1931, p. 155, for example, indicates that high fidelity sound reproduction of orchestral music requires that the frequency band should extend to as low as 40 Hz. It is well established that loudspeakers, in order to reproduce a given frequency range, must have dimensions based on the wavelength which corresponds to the lowest frequency in the range. In the case of one type of loudspeaker, the exponential horn loudspeaker, for example, the area of the exponential horn mouth is determined on the basis of the wavelength of the lowest frequency to be reproduced. At an early date, to obtain high fidelity sound reproduction with exponential horn loudspeakers, and, in particular, the inclusion of low audible frequencies, large exponential horn loudspeakers were constructed. For example, theater loudspeakers as large or larger than eight feet in length and four feet by four feet in transverse dimensions were built in order to obtain reproduction of low audible frequencies. Later, the outside dimensions of the exponential horns were reduced by folding, but even then the dimensions of the mouths were large for reproduction of low audible frequencies. More recently, folded exponential horn loudspeakers with reduced mouth dimensions have been used in proximity to boundary surfaces, such as a floor, a ceiling, and/or walls of a room, to increase the effective mouth area so that low audible frequencies are reproduced while at the same time the dimensions of the low frequency loudspeakers are minimized. See, for example, Sandeman, U.S. Pat. Nos. 1,984,550, 2,310,243 and 2,373,692, and Klipsch, “La Scala, ” Audio Engineering Society Preprint No. 372, Apr. 1965. The low frequency folded exponential horn loudspeakers, such as those which are disclosed in the above-cited references, have small dimensions and, when their mouths are located proximate planar surfaces, enable reproduction of low audible frequencies. However, each of these low frequency folded exponential horn loudspeakers is structurally complex due to the structure of the folded exponential horn which defines the sound path from the electroacoustic transducer to the volume into which sound is radiated. Perhaps the simplest construction appears in the above-cited Audio Engineering Society publication. In that construction, the folded exponential horn is bifurcated to define a double sound path. Due to the complex structure, the production of high fidelity, small dimension, low frequency folded exponential horn loudspeakers has required considerable craftsmanship. High quality control in manufacture has been necessary to assure that the construction meets specifications. 
     Consequently, the cost of low frequency folded exponential horn loudspeakers has been high. 
     U.S. Pat. No. 5,212,732 teaches a loudspeaker system of the dipole type, particularly for use in surround sound, reverberation and similar applications. A speaker system includes a pair of woofers having dual voice coil drivers mounted on oppositely facing baffles (e.g., front and rear facing). Preferably, each baffle also includes a high frequency speaker mounted thereon. On a first baffle (e.g., front), both voice coils of the dual voice coil driver and the voice coil of the high frequency speaker are driven in-phase, and on the other baffle (e.g., rear), the second voice coil of the dual voice coil driver and the voice coil of the high frequency speaker are driven out-of-phase from those from the first baffle but in-phase with one another. The coils of the speakers are driven from suitable filter circuits. 
     U.S. Pat. No. 5,212,732 teaches a loudspeaker system of the dipole type, particularly for use in surround sound, reverberation and similar applications. The speaker system includes a pair of woofers having dual voice coil drivers mounted on oppositely facing baffles (e.g., front and rear facing). Each baffle also includes a high frequency speaker mounted thereon. On a first baffle (e.g., front), both voice coils of the dual voice coil driver and the voice coil of the high frequency speaker are driven in-phase, and on the other baffle (e.g., rear), the second voice coil of the dual voice coil driver and the voice coil of the high frequency speaker are driven out-of-phase from those from the first baffle but in-phase with one another. The coils of the speakers are driven from suitable filter circuits. Various forms of loudspeaker systems have been developed, and the types of speakers as well as the technologies involved pertaining to woofers, tweeters, mid-range and other forms of speaker systems are well known. Stereo sound systems using front speakers with or without some form of woofer or subwoofer, along with rear and/or side speakers, have become prevalent particularly for sound systems used to reproduce sound in “home theater” video systems for playing back video motion pictures and similar program material. The typical installation comprises a pair of front speakers positioned to either side of the TV screen, preferably with a center speaker and/or a subwoofer, and along with a pair of right and left side speaker and/or a pair of left and right rear speakers. 
     An Audio Engineering Society (AES) paper entitled “New Factors in Sound for Cinema and Television” by Tomlinson Holman, presented at the 89th Convention of the Audio Engineering Society, Los Angeles, Calif., Sep. 21-25, 1990, and reprinted in the Journal of the AES, Volume 39, No. 7/8, (preprint #2945) notes that the best directivity pattern for the “surround” loudspeakers is not the conventional forward radiating direct radiator, but rather dipolar radiation with the principal lobes of the dipole pointed, not at the listening area, but at the room surfaces with the null in the radiation pattern pointed at listeners, and that the best surround loudspeaker is physically invisible. 
     U.S. Pat. No. 4,733,749 teaches a loudspeaker system for low frequencies has a manifold chamber into which oppositely mounted and aligned woofer units radiate sound. The chamber radiates the sound perpendicularly to the woofer axes, either directly into space or into a horn. An additional back woofer may radiate directly in the perpendicular direction. An arrangement of speakers for a low-frequency sound reproduction is system particularly adapted for high power output and has manifold for coupling multiple low frequency loudspeakers, in a single sound-radiating enclosure. Multiple loudspeakers are often used in sound applications requiring high acoustic power output (sound volume), such as in theaters or arenas, or for studio and stage monitoring, discotheques and the like. In many sound systems, several components, such as driver/horn assemblies or cone/enclosure loudspeakers, are used for sound reproduction across the entire range of audible sound, with different devices covering the bass (low-frequency), midrange and high-frequency portions of the sound spectrum. 
     Low-frequency speakers are customarily referred to as “woofers”. A particular sound application may require an especially high power output across the whole audio spectrum. With respect to the low-frequency range, this has been accomplished in the past, in general, by increasing the number of loudspeakers, because of the need to set large volumes of air in motion to create high acoustic power. In order to move large air volumes, the excursion of a moving diaphragm having a given cone area could be increased, but since acoustic distortion increases with increasing excursion once the linear limitation of the loudspeaker suspension is reached, the solution of using multiple loudspeakers is generally preferred. Multiple loudspeakers are conventionally mounted on a front baffle board of a speaker housing or enclosure. The housing may be closed, or may be provided with one or more phase-inverting ports or ducts (as in a bass-reflex type enclosure). Acoustic coupling and addition occurs in such structures at frequencies where the wavelengths are sufficiently greater than the distances between the individual speakers or phase-inverting ports. 
     U.S. Pat. Nos. 4,391,346 and 4,437,540 teach individual speaker units which are set in the walls of a cavity behind a front baffle board. The speaker units are arranged so that the sound-radiating axis of each speaker unit angularly converges on and is concentrated on a point of the central axis of the cavity, just behind the front baffle, toward which the speakers are generally aimed. 
     While such an arrangement may improve mid-range sound reproduction, low-end frequency reproduction is adversely affected, as the cavity behaves like a short acoustic horn having a rapid flare rate, such a horn being incapable of sustaining very low-frequency sounds. A maximum output speaker system for high-volume sound. A more specific object is to provide an efficient arrangement for summing the outputs of a number of individual low-frequency speakers for radiation from a single sound-radiating aperture. The maximum output speaker system minimizes destructive sound interference and maximizes coupling between loudspeakers at low frequencies. The sound-radiating axes of the individual speaker units are not aimed towards the chamber exit. Instead, pairs are aimed directly at or away from each other. This optimizes low frequency erformance without peaking medium-pitch sound. The manifold chamber exit is smaller than the sum of the diaphragm areas of the individual speakers inside the chamber. 
     U.S. Pat. No. 3,903,989 teaches a loudspeaker system which has a cabinet with two compartments, a first of which contains a low-frequency loudspeaker for producing an omnidirectional radiation pattern, and the second compartment, above the first, containing a rotationally adjustable vertically oriented baffle on which are supported additional loudspeaker motors designed to cover the mid-and high-frequency bands of the audio frequency spectrum. The baffle is so shaped and the additional loudspeaker motors located in positions thereon that they operate as high-efficiency gradient or dipole loudspeakers over a significant portion of their respective frequency ranges, whereby the directivity of the loudspeaker system can be controlled by adjustment of the position of the baffle relative to the cabinet. It is conventional in loudspeaker systems to divide the audio frequency range of interest between a plurality of individual loudspeaker drivers mounted in a common enclosure, the higher quality systems utilizing a low frequency driver, or “woofer” for the very low frequencies, a smaller driver for the lower mid-range of frequencies, a still smaller driver for upper mid-range frequencies, and one or more “tweeters” for the high-frequency range. Because the wavelengths of the mid-and high-frequency signals are shorter than those of the low frequency signals, the directivity of the mid- and high-frequency signals of any particular drive is sharper than that of the low frequency signals. Accordingly, the sound field produced by an output signal from a given loudspeaker driver is increasingly narrower with increase in the signal frequency, with the consequence that the mid- and high-frequency signals are severely attenuated in directions offset greater than about 30. degree. to 60. degree. from the central axis of the loudspeaker array, depending on the dimensions of the driver and the frequency of the signal. The nature of this problem is described in detail in a paper by applicant entitled “Broadening the Area of Stereophonic Perception” which appeared in the Journal of the Audio Engineering Society, Vol. 8, No. 2, pp. 91-94 (1960), and a loudspeaker arrangement representing a solution to the problem is described and claimed in U.S. Pat. No. 3,080,012. The problem as it applies to quadraphonic reproduction is described in a paper entitled “Quadrophony Needs Directional Loudspeakers” which appeared in the March 1973 issue of Audio Magazine, pages 22, 24, 26 and 30. 
     U.S. Pat. No. 4,437,541 teaches a controlled dispersion loudspeaker configuration in which a loudspeaker is mounted through a hole in a front baffle forming a seal between the speaker and the baffle. A rear baffle is parallely spaced a predetermined distance away from the front baffle by means of spacers. 
     Acoustically absorptive material is placed between the two baffles and is acoustically open on at least two opposite sides. The sound waves from the rear of the speaker exit from the acoustic material and serve to cancel the sound waves at the sides and rear of the loudspeaker configuration eminating from the front of the speaker. The size of the baffles, as well as the spacing therebetween, bears a particular relationship to the frequency of the sound to be reproduced by the loudspeaker. The inventor incorporates the teachings of the above-cited patents into this specification. 
     SUMMARY OF INVENTION 
     The present invention is generally directed to a loudspeaker which includes a compression chamber, a first electroacoustic transducer and a horn. The first electroacoustic transducer is disposed inside the compression chamber. The horn is mechanically and acoustically coupled to the first electroacoustic transducer. 
     In a first, separate aspect of the present invention, the loudspeaker includes a second electroacoustic transducer which is disposed outside the compression chamber. The second electroacoustic transducer is mechanically and acoustically coupled to the horn. 
     Other aspects and many of the attendant advantages will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawing in which like reference symbols designate like parts throughout the figures. 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a loudspeaker according to the present invention. 
     FIG. 2 is a side elevational view in cross-section of the loudspeaker of FIG.  1 . 
     FIG. 3 is a side elevation view of a loudspeaker which U.S. Pat. No. 4,138,594 teaches. 
     FIG. 4 is a side elevation view of a loudspeaker which U.S. Pat. No. 3,912,866 teaches. 
     FIG. 5 is a side elevation view of a loudspeaker which U.S. Pat. No. 4,313,032 teaches. 
     FIG. 6 is a cross-sectional view of the loudspeaker of FIG. 5 taken along the line  6 — 6  of FIG.  5 . 
     FIG. 7 is a cross-sectional view of the loudspeaker of FIG. 5 taken along the line  7 — 7  of FIG.  5 . 
     FIG. 8 is a schematic drawing of a loudspeaker which includes a compression chamber, a transducer and a straight horn. 
     FIG. 9 is a schematic drawing of the loudspeaker of FIG. 8 which includes a first transducer and a second tranduscer mechanically and accoustically coupled to the compression chamber according to the present invention. 
     FIG. 10 is a schematic drawing of a loudspeaker which includes a compression chamber, a transducer and a flat folded horn. 
     FIG. 11 is a schematic drawing of the loudspeaker of FIG. 8 which includes a transducer mechanically and accoustically coupled to the compression chamber according to the present invention. 
     FIG. 12 is a schematic drawing of a loudspeaker which includes a compression chamber, a transducer and a split bent horn. 
     FIG. 13 is a schematic drawing of the loudspeaker of FIG. 8 which includes a first transducer and a second tranduscer mechanically and accoustically coupled to the compression chamber according to the present invention. 
     FIG. 14 is a schematic drawing of a loudspeaker which includes a compression chamber, a transducer and a cornerless corner folded horn. 
     FIG. 15 is a schematic drawing of the loudspeaker of FIG. 8 which includes a transducer mechanically and accoustically coupled to the compression chamber according to the present invention. 
     FIG. 16 is a perspective drawing of a loudspeaker according to the first embodiment of the present invention. 
     FIG. 17 is a partial perspective drawing of the loudspeaker of FIG.  16 . 
     FIG. 18 is a perspective drawing of a loudspeaker according to the first embodiment of the present invention. 
     FIG. 19 is a partial perspective drawing of the loudspeaker of FIG.  16 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 in conjunction with FIG. 2 a loudspeaker  10  includes a compression chamber  11 , a transducer  12  and a straight horn  13 . The transducer  12  is disposed in the compression chamber  11 . The output of the loudspeaker  10  is a monopole and therefore is omnidirectional. A first transducer  21  and a second transducer  22  are mechanically and acoustically coupled to the straight horn  12 . When the first and second transducers  21  and  22  are added to the transducer  12  their radiation output in the front of the loudspeaker  10  is a dipole and in phase augmentation sharing the monopath of the output of the transducer  12  and their output in the rear of the loudspeaker  10  is out of phase of with the output of the transducer  12 . The combined monopole and dipole produces a cardiode-shaped wave. A typical sealed chamber horn radiation is omnidirectional because the mouth area is small compared to the wavelength it is projecting, i.e. 36 feet at 30 Hz. An infinite horn would be directional. A practical horn rarely exceeds three feet in diameter. Hence the pattern is almost omnidirection in the thirty to ninety eight (30 to 98) Hz region also the effectiveness fall rapidly despite the taper near the thirty (30) Hz cutoff usually beginning an octive higher. Klipsch in his article, entitled “A Low Frequency Horn of Small Dimensions”, published in The Journal Acoustical Society of America, Volume 13, Number 2, 1941, pages 137-144, derives the analytical expression for the volume of a back air chamber. Theoretically, the back air chamber should be about 10-20% larger to compensate for the flexure of the suspended diaphragm and for the immersed volume of the electromagnet of the electro-acoustic transducer. Since a 20% change in the volume of a back air chamber has been found to produce less than approximately 1 decibel of response error and since error toward a smaller back air chamber results in less modulation distortion due to subsonic inputs, the back air chamber preferably has a volume of 2,730 cubic inches, or 44.74 liters, so that the volume is only 2%, rather than 10-20%, larger than the analytical value. 
     Referring to FIG. 3 the loudspeaker  30  of U.S. Pat. No. 4,138,594 includes a compression chamber  31 , a transducer  32  and a folded horn  33  in an enclosure  34 . The transducer  32  is disposed in the compression chamber  31 . The output of the loudspeaker  30  is a monopole and therefore is omnidirectional. 
     Referring to FIG. 4 the loudspeaker  40  of U.S. Pat. No. 3,912,866 includes a compression chamber  41 , a transducer  42  and a folded horn  43  in an enclosure  44 . The transducer  42  is disposed in the compression chamber  41 . The output of the loudspeaker  40  is a monopole and therefore is omnidirectional. 
     Refering to FIG. 5 in conjunction with FIG.  6  and FIG. 7 the loudspeaker  50  of U.S. Pat. No. 4,313,032 includes a compression chamber  51 , a transducer  52  and a folded horn  53  in an enclosure  54 . The transducer  52  is disposed in the compression chamber  51 . The output of the loudspeaker  40  is a monopole and therefore is omnidirectional. 
     Referring to FIG.  8  and page  186  of a chapter on Enclosures in a book, entitled Hi-Fi Loudspeakers and Enclosure, Revised Second Edition, written by Abraham B. Cohen, published by Hayden Book Company Cohen describes a loudspeaker. A loudspeaker  110  includes a compression chamber  111 , a transducer  112  and a straight horn  113 . The transducer  112  is disposed in the compression chamber  111 . The output of the loudspeaker  110  is a monopole and therefore is omnidirectional. 
     Referring to FIG. 8 in conjunction with FIG. 9 a first transducer  121  and a second transducer  122  are mechanically and acoustically coupled to the straight horn  113 . When the first and second transducers  121  and  122  are added to the transducer  112  their radiation output in the front of the loudspeaker  110  is a dipole and in phase augmentation sharing the monopath of the output of the transducer  112  and their output in the rear of the loudspeaker  110  is out of phase of with the output of the transducer  112 . The combined monopole and dipole produces a cardiode-shaped wave. 
     Referring to FIG.  10  and page  186  of the book, entitled Hi-Fi Loudspeakers and Enclosure, a loudspeaker  210  includes a compression chamber  211 , a transducer  212  and a flat folded horn  213 . The transducer  212  is disposed in the compression chamber  211 . The output of the loudspeaker  210  is a monopole and therefore is omnidirectional. 
     Referring to FIG. 10 in conjunction with FIG. 11 a transducer  221  is mechanically and acoustically coupled to the flat folded horn  212 . When the transducer  121  is added to the transducer  212  its radiation output in the front of the loudspeaker  210  is a dipole and in phase augmentation sharing the monopath of the output of the transducer  212  and their output in the rear of the loudspeaker  210  is out of phase of with the output of the transducer  212 . The combined monopole and dipole produces a cardiode-shaped wave. 
     Referring to FIG.  12  and page  186  of the book, entitled Hi-Fi Loudspeakers and Enclosure, a loudspeaker  310  includes a compression chamber  311 , a transducer  312  and a split bent horn  313 . The transducer  312  is disposed in the compression chamber  311 . The output of the loudspeaker  310  is a monopole and therefore is omnidirectional. 
     Referring to FIG. 13 in conjunction with FIG. 12 a first transducer  321  and a second transducer  322  are mechanically and acoustically coupled to the split bent horn  312 . When the first and second transducers  321  and  322  are added to the transducer  312  their radiation output in the front of the loudspeaker  310  is a dipole and in phase augmentation sharing the monopath of the output of the transducer  312  and their output in the rear of the loudspeaker  310  is out of phase of with the output of the transducer  312 . The combined monopole and dipole produces a cardiode-shaped wave. 
     Referring to FIG.  14  and page  186  of the book, entitled Hi-Fi Loudspeakers and Enclosure, a loudspeaker  410  includes a compression chamber  411 , a transducer  412  and a cornerless corner folded horn  413 . The transducer  412  is disposed in the compression chamber  411 . The output of the loudspeaker  410  is a monopole and therefore is omnidirectional. 
     Referring to FIG. 14 in conjunction with FIG. 15 a transducer  421  is mechanically and acoustically coupled to the cornerless corner folded horn  412 . When the transducer  421  is added to the transducer  412  its radiation output in the front of the loudspeaker  410  is a dipole and in phase augmentation sharing the monopath of the output of the transducer  412  and their output in the rear of the loudspeaker  410  is out of phase of with the output of the transducer  412 . 
     The combined monopole and dipole produces a cardiode-shaped wave. Referring to FIG. 16 in conjunction with FIG. 17 a first loudspeaker  510  includes a compression chamber  511 , a first electroacoustic transducer  512  and a flat folded horn  513  and a rectangular structure  514 . The electroacoustic transducer  512  is disposed inside the compression chamber  511 . The first loudspeaker  510  also includes a second electroacoustic transducer  521  which is disposed outside the compression chamber  511  and is mechanically and acoustically coupled to the flat folded horn  513 . 
     Referring to FIG. 18 in conjunction with FIG. 19 a second loudspeaker  610  includes a compression chamber  611 , a first electroacoustic transducer  612  and a flat folded horn  613  and a rectangular structure  614  with a rear extension  615 . The electroacoustic transducer  612  is disposed inside the compression chamber  611 . The second loudspeaker  610  also includes a second electroacoustic transducer  621  which is disposed outside the compression chamber  611  and is mechanically and acoustically coupled to the flat folded horn  613 . 
     From the foregoing it can be seen that a loudspeaker has been described. It should be noted that the sketches are not drawn to scale and that distance of and between the figures are not to be considered significant. 
     Accordingly it is intended that the foregoing disclosure and showing made in the drawing shall be considered only as an illustration of the principle of the present invention.