US 3255842 A
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Description (OCR text may contain errors)
June 14, 1966 B. H. VARDEMAN LOUDSPEAKER Filed March 16, 1965 FIG FIG 2 FIG 4 IN VENTOR.
BRUCE H. VARDEMAN FIG 3 ATTORNEY United States Patent 3,255,842 LOUDSPEAKER Bruce H. Vardeman, 1452 th Ave. SE., Cedar Rapids, Iowa Filed Mar. 16, 1965, Ser. No. 440,220 Claims. (Cl. 181-31) This invention relates to loudspeakers, and more specifically to loudspeakers used for high fidelity sound reproduction.
It is a purpose of this invention to provide a loudspeaker capable of more accurate sound reproduction than has previously been possible.
It is a further purpose of this invention to provide a resonator loading a bass producing diaphragm, one of whose surfaces communicates with the atmosphere only through an additional diaphragm or diaphragms which are preferably less capable of bass production, the other surface of the bass diaphragm communicating with the atmosphere through an opening preferably proximate said additional diaphragm.
It is another purpose of this invention to provide a bass resonator comprised of a pairof chambers of unequal volume communicating with the atmosphere through an acoustic duct, the larger chamber preferably acting as a pair of horns disposed in mouth to mouth relation.
The foregoing and other objects, features, and advantages, of this invention will be apparent from the preferred form thereof hereafter described read in conjunction with the accompanying drawing in which:
FIGURE 1 is a front elevation of a loudspeaker cabinet embodying a preferred form of the invention, a portion of the front panel of the cabinet being broken away;
FIGURE 2 is a section taken along the line 22 of FIGURE 1; t I
FIGURE 3 is a schematic diagram of a preferred form of electrical circuit employed in the loudspeaker of FIG- URE l; and
FIGURE 4 is a schematic diagram of an alternate form of electrical circuit which may be employed in the loudspeaker of FIGURE .1;
FIGURES land 2 illustrate a loudspeaker .10 having its cabinet formed of a number of rigid inner and outer walls. The foregoing consist of a top panel 11, a bottom panel 12, a back panel 13, end panels 14 and 15, and a front panel 16. An inner panel 17 is disposed parallel to and spaced rearwardly of front panel 16 and provided with two circular openings 18 and 19, which are closed by the diaphragms 20 and 21 of midrange drivers 22 and 23 mounted on the rear side of and in axial alignment with openings 18 and 19. A pair of horizontal acoustic ducts 24, 24, having inlets 25, 25" and mouths 26, 26, are formed by top and bottom panels 11 and 1 2, front panel 16, inner panel 17, and a divider panel 27 extending between front panel 16 and inner panel 17. Duct mouths 26, 26' are disposed adjacent the front surfaces of diaphragms 20, 21. Another inner panel 28, angled rearwardly to end panel 15 from approximately the center of inner panel 17, is provided with a large circular opening 29 which is closed by the diaphragm 30 of bass driver 31 mounted in axial alignment with opening 29 upon the rear side thereof. A compression chamber 32 is thus formed by the top and bottom panels 11 and 12, end panel 15, inner panels 17 and 28, and diaphragms 20, 21 and 30. A final internal panel 33 angles rearwardly and slightly toward end panel 15 from Patented June 14, 1966 the junction of panel 28, panel 17 dividing the major portion of the cabinet volume into two unequal acoustic chambers 34 and 35. The rear edge of panel 33 is spaced from back panel 13 to provide an opening 36 connecting chambers 34 and 35. The smaller acoustic chamber 34 is adjacent the bass producing diaphragm 30, the rear surface of which forms a wall portion thereof. The larger acoustic chamber 35 expands smoothly in cross sectional area, from opening 36, along back panel 13 and inner panel 33, then diminishes smoothly along end panel 14 and inner panel 17 to the inlets 25, 25' of ducts 24, 24'. The shape of acoustic chamber 35 is thus in effect that of two horn sections connected at their mouths along line A-A of FIGURE 2. Front panel 16 is provided with two circular atmospheric openings 37 and 38 in axial alignment with the openings 18 and.19 in inner panel 17. A moderate pad of acoustic damping material 39 is stretched across a portion of opening 29 in inner panel 28. Pad 39 is generally rectangular in shape and is attached at its shorter sides to the edges of opening 29 so that it extends across the latter centrally thereof.
The term driver as used herein denotes the combination of diaphragm, frame and driving mechanism (sometimes otherwise known as a loudspeaker). The term Loudspeaker as used herein denotes the complete invention as described, including the cabinet, drivers and electrical circuit. 1
FIGURE 3 illustrates a connection of midrange drivers 22 and 23 in series with respect to one another, and a parallel connection of these with respect to bass driver 31. A bypass capacitor 40 is connected in parallel with midrange driver 22.
FIGURE 4 illustrates an alternate circuit arrangement with the three drivers 22, 23, and 31 connected in series. Bypass capacitor 40" is connected in parallel with drivers 22 and 31 and a bypass choke 41 in parallel with drivers ".2 and 23.
The use of a Helmholtz type resonator to load one surface of a bass producing diaphragm is well known to the art. The combined volume of chambers 34 and 35, together with ducts 24, 24' and atmospheric openings 37 and 38, form a modified Helmholtzresonator loading bass producing diaphragm 30. The shape of chamber 35, being in effect two horn sections connected at their mouths, provides tight coupling between opening 36 and duct inlets 25, 25' at upper bass frequencies. Sound output from ducts 24, 24' should extend upward to about two hundred cycles. the area of opening 36 largely determine this upper cut-off frequency. Chamber 34 is made as small as is practical, and the area of opening 36 is sized for proper cut-off. The low volume of chamber 34 and the shape of chamber 35 combine to provide a much stronger and smoother upper bass output from ducts 24, 24' than is achieved from the usual Helmholtz type resonator.
The flare rates of the two horn sections of chamber 35 are such that they function as horn couplers at upper bass frequencies. Much lower flare rates would be required for them to so function at low bass frequencies. Indeed, the combined volumes of chambers 34 and 35 act essentially as a single acoustic capacitance, the mass reactance of opening 36 being insignificant at low bass frequencies. Location of the drivers at one end of the cabinet provides the sound path length required for the prescribed shape of chamber 35 and for an adequate length of ducts 24, 24' to provide a low resonant frequency.
The volume of chamber 34 and The use of a compression chamber, such as chamber 32, enclosing the non-radiating surface, such as 30', of a diaphragm whose radiating surface, such as 30" has a mas-s air load to atmosphere is also well known to the art. Compression chamber 32 loads the front surface of bass diaphragm 30 and provides for'bass output from ducts 24, 24' below the range provided by the typical Helmholtz resonator. Compression chamber 32 differs significantly from the usual rigid walled compression chamber owing to diaphragms 20 and 21. Midrange drivers 22 and 23 are so chosen that their combined free air displacement at low frequencies is less than that of bass driver 31. They are, in short, less capable of bass production than is driver 31. Also, the electrical impedances of midrange drivers 22 and 23 are so chosen that they are more lightly driven at bass frequencies than is bass driver 31. The presence of the former two reduces the stiffness presented to diaphragm 30 by compression chamber 32 to a value below that which would be presented by a rigid walled compression chamber of equal volume. Or, put another way, the presence of diaphragms 20 and 21 reduces the required volume of compression chamber 32 for a given lower cut-off frequency, which occurs at approximately the resonance of bass diaphragm 30 and its air load mass against the stiffness of both compression chamber 32 and the mounting of bass diaphragm 30. It is not even necessary for diaphragms 20 and 21 to be electrically driven to perform this function. A simple resilient member capable of effecting changes in compression chamber volume at audio frequencies will also serve, its resiliency providing a restoring force tending to maintain it in a position providing an average compression chamber volume. Energy is thus alternately stored and released at audio frequencies by flexing of the member, as well as by compression and decompression of the chamber air.
Bass sound leaving resonator ducts 24, 24' has been amplified by the cabinet structure. A feedback of this amplified bass sound to bass diaphragm 30 is used to further extend and smooth the bass response, and to provide damping of bass sound. Such feedback is achieved by placing the duct mouths 26, 26' closely adjacent diaphragms 20, 21, respectively, so that sound output from the former is impressed upon the latter, and is transmittted through compression chamber 32 to tbass diaphragm 30. The degree of feedback is largely a function of the relation between the projected vibratile area of bass diaphragm 30 and the cross-sectional areas of atmospheric openings 37 and 38. The latter act to restrict the flow of sound traveling from tbass diaphragm 30 to the atmosphere, which restriction increases the feedback sound pressure upon diaphragms 20 and 21. The total area of atmospheric openings 37 and 38 must be less than the projected wibratile area of bass diaphragm 30 in order to provide significant feedback, a practical lower limit being prescribed by the necessity to transmit clean mid-range sound through openings 37 and 38.
Bass sound traveling through ducts 24, 24' experiences considerable phase shift between bass diaphragm 30 and atmospheric openings 37 and 38. It reaches the latter openings and diaphragms 20 and 21 simultaneously. The proximity of atmospheric openings 37 and 38 to :bass diaphragm 30 minimizes the phase difference between the output sound and the feedback sound acting upon bass diaphragm 30. The feedback of sound upon bass diaphragm 30 decreases the output level of those frequencies which have experienced a phase shift of between 90 and 270 by the time they have emerged from ducts 24, 24'. The ofttimes prominent resonant frequency which undergoes a 180 phase shift is decreased the most, inasmuch as a 180 phase shift places the output sound from the back sunface 30" of bass diaphragm 30 precisely in phase with movement of its front surface 30. Diaphragm loading is increased. Diaphragm excursion and sound output are decreased. Damping provided by such feedfrequency ranges.
back improves transient decay, as well as smoothness of response. The level of those low bass frequencies which undergo less than a phase shift by the time they have emerged from ducts 24, 24' is increased, inasmuch as such feedback increases the excursion of diaphragms 20 an 21 whereby the stiffness presented to bass diaphragm 30 by compression chamber 32 is further decreased.
Compression chamber 32 and mid-range drivers 22 and 23 are not essential in order to provide the foregoing described feedback, but are essential in order to achieve maximum bass range from the loudspeaker. They all contribute to etficient radiation from the back surface 30 of bass diaphragm 30, while restricting radiation from its front surface 30'. The presence of diaphragms 20 and 21 reduce such cancellations as may tend to occur between the surfaces 30' and 30" of base diaphragm 30. Drivers 22 and 23 themselves further provide midrange and high frequency sound from the loudspeaker. Capacitor 40, by reducing the circuit impedance through driver 23, maintains high frequency response from the latter. 1
Moderately priced drivers of standard design are quite suitable for use in loudspeaker 10. Driver 22 will generally have thehighest electrical impedance and driver 23 the lowest, with that of bass driver 31 being near to that of mid-range driver 23. The acoustic and electrical circuit arrangements of loudspeaker 10 allow ample opportunity for design control of output level over various Driver impedances may be chosen to reduce mid-range output to the same levels attainable at frequencies near the ends of the audio range. Very little of the mid-range energy supplied to ibass driver 31 is radiated as sound from the loudspeaker. At mid-range, when connected as in FIGURE 3, the voice coil of driver 31 acts essentially as a resistance load in parallel with mid-range drivers 22 and 23. This resistance load decreases eificiency, and provides damping of mid-range frequencies. Driver impedances and the value of capacitor 40 are chosen to provide sustained high frequency response from driver 23. Capacitor 40 is also chosen to offset the inductive reactance of driver 23 at high frequencies, while the high inductive reactance of driver 31 minimizes the loss of highs therein.
The dimensions of its cabinet and its bass driver 31 place a practical limit on the lower cut-off frequency of loudspeaker 10. With a given set of drivers, and a given total cabinet volume, the following factors determine optimum design: The volume of compression chamber 32 and the dimensions of ducts 24, 24' should be chosen for resonance at the lowest practical cut-off frequency. While chambers 34 and 35 provide the acoustic capacitance for bass driver 31, the lowest frequency resonce of the latter occurs when the mass of its diaphragm 30 and the latters air load resonate against the stitfness presented to diaphragm 30. This stiffness, in turn, depends upon the mounting of diaphragm 30 and upon compression chamber 32, the stiffness of the latter being an inverse function of its volume. Chamber 35 is preferably made as large as reasonable external dimensions will allow. The area of opening 36 should be chosen for proper level of upper bass frequencies. The areas of openings 18 and 19 in panel 17 should be chosen for proper level of midrange frequencies centering approximately around one thousand cycles. Increasing the areas of openings 18 and 19 increases the level of the affected frequencies. Atmospheric or resonator openings 37 and 38 should generally be made as small as possible without adversely affecting frequencies above mid-bass, and preferably less than the projected vibratile area of diaphragm 30. The area of opening 29 should be approximately equal to the projected vibratile area of bass driver 31. Divider 27 between ducts 24, 24' controls distribution of [bass output between atmospheric openings 37 and 38, and the moderate amount of damping material 39 helps to smooth treble response.
The following specifications provide a preferred example of a loudspeaker incorporating the foregoing design features:
Nominal size 12 inch. Resonance 70 cycles per second. Nominal impedance 8 ohms. Magnet 24 ounce ceramic. Voice coil diameter 1 /2 inch.
Nominal size Sinch. Resonance 95 cycles per second. Nominal impedance 16 ohms. Magnet 10 ounce ceramic. Voice coil diameter linch.
Nominal size 5 inch. Resonance 130 cycles. Nominal impedance 8 ohms. Magnet 5 ounce ceramic Voice coil diameter inch.
Material /2 plywood. External dimensions 13% H x 23 /2" W x Compression chamber 32 460 cubic inches net. Chamber 34 670cubic inches net. Chamber 35 1230 cubic inches net. Opening 36 36.0 square inches. Opening 37 21.5 square inches. Opening 38 12.5 square inches. Opening 18 28.5 square inches.
' Opening 19 9.6 square inches.
Opening 29 85.0 square inches. Duct 24 12.0 square inch cross section. Damping pad 39 /2" x 4" x 10 /2" fiberglass. Capacitor 40 2.0 microfarads.
(The volumes of the physical structures of drivers 22, 23 and 31 were considered in calculating the foregoing net acoustic chamber volumes.)
Capacitor 40 and choke 41 of the alternate circuit shown in FIGURE 4 are chosen to present a high electrical impedance at mid-range. Capacitor 40' should present .a low impedance at and above about eight thousand cycles and choke 41 a low impedance at and below about one hundred and fifty cycles. Bass and high frequency output are thereby increased and the electrical impedance of the entire loudspeaker made more constant over the audio frequency range.
While only one form of the invention is detailed herein, modified forms will readily occur to those skilled in the art. For example, a single mid-range driver may be used in place of the two drivers 22 and 23. A tapped voice coil on the former, with capacitor 40 connected in parallel with a portion of the coil, may be used to sustain high frequency response. A bass driver of oval shape may be used to decrease the loudspeakers height. Small-size loudspeakers of high quality may be inexpensively mass produced by molding the cabinet of suitable plastic material. Such a cabinet would likely have a more rounded appearance, curved surfaces being employed, and all drivers would remain near one end, the ends being regarded as those portions of the cabinet separated by its maximum dimension.
While the invention has been described with reference to particular embodiments and detailed descriptive language has been used it is not so limited. Instead, the following claims are to !be read as encompassing all modifications and adaptations of the invention falling within the scope and spirit thereof.
1. In a rigid walled loudspeaker cabinet including a resonator having an opening to the atmosphere through an acoustic duct, said resonator acoustically loading one surface of the diaphragm of a bass driver, and a compression chamber acoustically loading the other surface of said diaphragm, said other diaphragm surface forming a wall portion of said compression chamber, the improvement comprising at least one mid-range driver having one of its diaphragm surfaces forming another wall portion of said compression chamber disposed in generally opposed relation to said other bass diaphragm surface, said mid-range driver being less capable of bass reproduction than said bass driver, said duct being disposed adjacent the other surface of said mid-range diaphragm effective to impress the sound waves emanating from said duct upon said other surface of said mid-range diaphragm, said duct and said other surface of said mid-range diaphragm together communicating with the atmosphere through said resonator opening, said resonator opening having a cross-sectional area less than the projected vibratile area of said bass diaphragm.
2. The device of claim 1 wherein said bass driver is mounted upon a rigid wall of said compression chamber and communicates with said one of said mid-range diaphragm surfaces through an opening in said wall axially aligning with and of a cross-sectional area substantially equal to the projected vibratile area of said [bass diaphragm, said bass and mid-range drivers being operatively electrically connected with respect to each other.
3. In a rigid walled loudspeaker cabinet including a resonator having an opening to the atmosphere through an acoustical duct forming a portion of said cabinet, said resonator acoustically loading one surface of the diaphragm of a bass driver, and a compression chamber acoustically loading the opposite surface of said bass diaphragm, said opposite diaphragm surface forming a wall portion of said compression chamber, the improvement comprising a resilient member having one of its surfaces forming another wall portion of said compression chamber and its other surface communicating with pression chamber required in order to provide a resonance of said bass driver at a low frequency and a low cutoff frequency of said loudspeaker, said duct and said other surface of said resilient member being disposed adjacent each other effective to impress sound waves emanating from said duct upon said other surface of said resilient member and together communicating with the atmosphere through said resonator opening, said last named opening having a cross-sectional area less than the projected vibratile area of said bass diaphragm.
4. The device of claim 3 wherein said resilient member comprises the diaphragms of apair of laterally adjacent mid-range drivers mounted in axially parallel relation upon a rigid wall of said compression chamber, their first corresponding pair of diaphragm surfaces forming said another wall portion of said compression chamber and their second corresponding pair of diaphragm surfaces communicating with said resonator opening and the atmosphere through a pair of openings in said last named rigid wall of said compression chamber.
5. The device of claim 4 wherein said bass driver is mounted upon another rigid wall of said compression chamber and communicates with said first pair of midrange diaphragm surfaces through an opening in said last named wall axially aligned with and of a cross-sectional area substantially equal to the projected vibratile area of said bass diaphragm, said mid-range drivers being operatively electrically connected with respect to said bass driver.
6. The device of claim 4 wherein said duct comprises a pair of ducts, said ducts being disposed adjacent said secnd pair of mid-range diaphragm surfaces effective to impress the sound waves emanating from said ducts upon each of said second pair of diaphragm surfaces, said pair of mid-range diaphragm openings in said compression chamber wall and said ducts together communicating with the atmosphere through said resonator opening, the cross-sectional area of said resonator opening being less than the projected vi bratile area of said bass diaphragm.
7. The device of claim 6 wherein said resonator opening comprises a pair of openings in a rigid wall of said caibinet spaced from and in axial alignment with said pair of mid-range diaphragm openings in said compression chamber wall, said ducts opening into the space between said last narn'ed cabinet and compression chamber walls, the total cross-sectional area of said resonator openings being less than the projected'vibratile area of said bass diaphragm.
-8. A loudspeaker including a rigid wall cabinet forming a resonator acoustically loading one surface of the diaphragm of a bass driver and having an opening to the atmosphere, said resonator including a pair of interconnected acoustic chambers of unequal volume and an acoustic duct communicating with the atmosphere through said resonator opening, said diaphragm surface forming a wall portion of the smaller of said chambers, the larger of said chambers expanding smoothly in cross-sectional area from its connection with said smaller chamber and thereafter diminishing smoothly in cross-sectional area into said acoustic duct, and a compression chamber having a wall portion formed by the other surface of said diaphragm and a generally opposed wall portion formed by one surface of a resilient member, the other surface of said member communicating with the atmosphere through said resonator opening, said d-uct being disposed adjacent said other surface of said resilient member effective to impress the sound waves emanating from said duct upon said other surface of said resilient member.
9. In a rigid-Walled loudspeaker cabinet including a pair of interconnected acoustic chambers of unequal volume, one surface of the diaphragm of a bass driver forming a portion of the wall of the smaller of said chambers, the larger of said chambers having an opening to the atmosphere through an acoustic duct forming a portion of said cabinet, said cabinet also including a compression chamber having a wall portion formed by the other surface of said diaphragm and another wall portion formed by one surface of a resilient member, said duct and said other surface of said resilient member being disposed adjacent each other effective to impress the sound waves emanating from said duct upon said other surface of said resilient member, said duct and said other surface of said resilient member together communicating with the atmosphere through said atmosphere opening, said last named opening having a cross-sectional area less than the projected vibratile area 'of said bass diaphragm.
10. The device of claim 9 wherein said resilient member comprises the diaphragm of at least one mid-range driver, said bass driver and said mid-range driver being operatively electrically connected with respect to each other.
References Cited by the Examiner UNITED STATES PATENTS Rich 181-31 LEO SMILOW, Primary Examiner.
STEVEN J. TOMSKY, Examiner.