US 3457370 A Abstract available in Claims available in Description (OCR text may contain errors) July 22, 1969 c. P. BONER IMPEDANCE CORRECTING NETWORKS Filed Dec. 3,457,370 IMPEDAN CE CORRECTING NETWORKS Charles Paul Boner, Austin, Tex., assiguor to C. P. Boner and Associates, Austin, Tex., a partnership Filed Dec. 30, 1965, Ser. No. 517,655 Int. Cl. H04m 1/58, l/00 U.S. Cl. 179-1 1 Claim ABSTRACT OF THE DISCLOSURE In a sound reenforcing system, impedence correcting networks for connection between a plurality of loud-speakers and a crossover network, whereby the loudspeakers and said correcting networks will properly load the crossover network so that its performance will be the same as if it were loaded at its design impedance. This invention relates generally to impedance matching networks. More particularly it relates to impedance correcting networks for properly coupling a plurality of loudspeakers, or drivers therefor, to a crossover network. It .is common practice to divide the lsound spectrum delivered by loudspeakers into two or more bands, with each band being reproduced by one or more loudspeakers capable of functioning most effectively in that band. Conventionally the spectrum is divided electrically before power is delivered to the loudspeakers -by a `crossover network. The electrical performance of such crossover networks, terminated in their respective load impedances, is a function of such impedances. For example, if a crossover network using loudspeaker loading on its output terminals is designed for a crossover frequency of 500 cycles per second and for a terminating impedance of 16 ohms, its crossover properties may be such that the power delivered to a bass loudspeaker at 500 cycles per second is 3 decibels below that delivered to the 16 ohm load at 500 cycles per second. Similarly, the power delivered by the crossover network to a high frequency loudspeaker at 500 cycles per second may be 3 decibles lbelow that delivered to the 16 ohm load at 500 cycles per second. In this ideal case the sum of the electrical powers delivered to the two 16 ohm loads at the 500 cycle per second crossover frequency is the same as that delivered to the low frequency load well below the crossover frequency, and to the high frequency load well above the crossover frequency. Also, the crossover network produces a 180 phase shift between the high and low frequency loudspeakers. If, however, a crossover network is designed for a certain terminating pair of load impedances and the loudspeakers providing the load do not have such impedances, then the performance of the network will be less than ideal and the power delivered to the high and low frequency loudspeakers will not follow the ideal curve, i.e., additional low frequency power will be delivered to the high frequency loudspeaker and additional high frequency power will be supplied to the low frequency loudspeaker, in a band of frequencies around the crossover frequency.4 nted States Patent O 3,457,370 Patented July 22, 1969 ice Moreover, the phase shift between the output of the crossover network and the high and low frequency loudspeakers will be distorted. If the variation of the loudspeaker impedance with frequency, in the vicinity of the crossover frequency, is sufficiently great, then an otherwise ideal crossover network will be improperly loaded and its output to the loudspeakers will depart substantially from that to be expected. Phase relationships will also vary rapidly with frequency throughout the crossover region. Variations in phase with frequency will result in phase variations ,in the sound field produced by the loudspeakers. Inasmuch as feedback in a sound reinforcing system is known to be caused by the existence of a gain greater than unity and by a phase shift of 21m-:0 in the room containing the sound system, where n ,is the standard notation in technical literature for any integer, the disruption of the normal and relatively smooth performance of a loudspeaker-loaded crossover network by variation in impedance of the loudspeakers with frequency would be accompanied by a number of feedback modes in the total system. The term feedback mode is a standard term in contemporary technical literature in acoustics and electroacoustics, eg., Journal of the Acoustical Society of Ameri-ca and Journal of the Audio Engineering Society, to denote a frequency at which a sound system is selfoscillatory. It has been demonstrated experimentally that a Sound system embodying a crossover network which drives both bass and treble loudspeakers exhibits severe feedback in the region around the crossover frequency. For example, in a number of systems having a 500 cycle per second crossover frequency, a number of feedback modes occur in the vicinity of crossover, i.e., more or less in the range 400 to 600 cycles. Although these feedback modes may be eliminated by the use of the process described in U.S. patent application Ser. No. 273,333, filed Apr. 16, 1963, now Patent No. 3,256,391, they are so numerous that `considerable time and expense are required to remove them. An important object of the present invention, therefore, is to provide means for causing a loudspeaker to present to a crossover network a constant impedance independent of frequency throughout the crossover region, whereby said loudspeaker and said means will properly load said crossover network so that its performance will be the same as if it were loaded at its design impedance. Another object of the invention is to provide impedance correcting networks which may be readily interposed between a crossover network and a plurality of loud-speakers, and which will not require adjustment after proper installation has been effected. Other objects of the invention will become apparent as the description thereof proceeds. In the drawings: FIG. 1 is a circuit schematic showing a pair of irnpedance correcting networks connected between a crossover network and a pair of loudspeaker devices; FIG. 2 is a chart showing ideal crossover network performance; FIG. 3 is a circuit schematic showing a modified impedance correcting network for connection between a crossover network and a high frequency loudspeaker; and FIG. 4 is a circuit schematic of a modified impedance correcting network for connection between a crossover network and a low frequency loudspeaker. Referring more particularly to the drawings, and tirst to FIG. 1 thereof, a conventional crossover network is shown at 10. The network 10, which is enclosed by broken line a, includes input terminals 11 and 12 for connection to an amplifier (not shown), low frequency output terminals 13 and 14, and high frequency output terminals 15 and 16. A low frequency, or bass, loudspeaker is shown schematically at 17 and a high frequency, or treble, loudspeaker at 18. To cause the low and high frequency loudspeakers 17 and 18 to present a constant impedance to the crossover network 10, impedance correcting networks 20 and 22, respectively, are employed. For clarity the networks 20 and 22 are enclosed by broken lines 20a and 22a, respectively. The low frequency network 20 includes input terminals 23 and 24, for connection to the terminals 13 and 14 of the crossover network, and output terminals 25 and 26 for connection to the terminals of the loading loudspeaker 17. Parallel conductors 27 and 28 connect the terminals 23 and 25 and 24 and 26, respectively. To match the impedance of the loudspeaker 17 to the low frequency crossover output, impedance elements 30, 31, 32, and 33 are connected in parallel and to the conductors 27 and 28. The element 30 consists of a resistor R which, in parallel with the impedances to the right of element 30 and the impedance of the loudspeaker, loads the low frequency network 10 to its design impedance, essentially independent of frequency throughout the crossover region. The element 31 comprises a capacitor C1 and a resistor in series. The element 32 is an inductance L1 and a resistor R5 in series, and the element 33 is comprised of an inductance L3, a capacitor C3 and a resistor R7 in series. The high frequency impedance correcting network 22 consists of input and output terminals 35, 36 and 37, 38, respectively, connected by parallel conductors 39 and 40. The terminals and 36 are connected to the crossover high frequency output terminals 15 and 16, whereas the terminals 37 and 38 are connected to the high frequency loudspeaker 18. Adjustment of the impedance presented by the high frequency loudspeaker to the high frequency crossover output, so as to make the load impedance on the crossover high frequency output essentially independent of frequency throughout the crossover region, is effected by impedance elements 41, 42, 43, and 44 which are connected in parallel and to the conductors 39 and 40. As in the correcting network 20, the elements 41, 42, 43, and 44 consist, respectively, of a resistor (R2), a resistor and capacitor in series (R4C2), an inductance and a resistor in series (LZRG), and an inductance, a capacitor and a resistor in series (L4, C4, R8). The values of the components of the elements of the networks 20 and 22 would, of course, depend upon the impedances to be matched. Moreover, their wattage ratings would vary, depending upon the power output of the amplifier connected to the crossover network 10. More specifically, the values of the elements of the correcting networks 20 and 22 are so chosen that the loudspeakers 17 and 18 will present constant impedances to the terminals of the crossover network independent of frequency throughout the crossover region. Thus, the combination of the correcting networks and the loudspeakers will properly load the crossover network 10 so that its performance will be the same as if it were loaded at its design impedance. If the crossover network exhibits the ideal characteristic at the crossover frequency, as shown in FIG. 2, with a resistive loading on its low and high frequency outputs of 12 ohms, then the correcting networks which are to be connected between the loudspeakers and said crossover network must be so designed that the impedance seen by the crossover network is 12 ohms resistive throughout the crossover region (for example, from 400 to 600 cycles per second for a 500 cycles per second crossover frequency). Examples of such impedance correcting networks are shown in FIGS. 3 and 4. In the example shown in FIG. 3, a high frequency loudspeaker, such as an Altec-Lansing Model 288C, is shown at 45 and is connected to the output terminals of a correcting network 46 consisting of impedance elements 47, 48, and 49 connected in parallel and to the high frequency output terminals of a crossover network such as the network 10, designed for l2 ohm loading. The values of the components of the elements 47, 48, and 49 are such that an impedance of approximately 12 ohms will be presented to the crossover network throughout the crossover region centering at 500 cycles per second, and the loudspeaker will thus properly load said network. To effect such proper loading in the example of FIG. 3, the element 47 utilizes a 5 mh. inductance, an 18 mf. capacitor and a 25 ohm resistor in series. The element 48 consists of Ia 1.1 mf. capacitor and a 20 ohm resistor in series, and the element 49 comprises a 50 ohm resistor. In FIG. 4 there is Shown a correcting network for causing a low frequency loudspeaker, such as an Altec- Lansing Model 515 bass loudspeaker, to present to the low frequency output terminals of the crossover network 10 an impedance of 12 ohms throughout the crossover frequency region. The correcting network of FIG. 4 is shown at 50 and comprises impedance elements 51, 52, 53 and 54 connected in parallel and across input and output terminals 55 and 56, respectively. The low frequency loudspeaker is shown at 57 and is connected to the output terminals 56. The input terminals 55 are connected to the low frequency output terminals of the crossover network 10. To provide proper 12 ohm matching between the loudspeaker 57 and the crossover network 10, the impedance element 51 comprises a 78 mh. inductance, a 15 mf. capacitor, and a 50 ohm resistor in series; the element 52, a 40 mh. inductance and a ,25 ohm resistor in series; the element 53, a 14 mf. capacitor and a 20 ohm resistor in series, and the element 54, a ohm resistor. It will be understood from the foregoing that when the networks 46 and 50, together with their respective loudspeakers 45 and 57, are connected to their proper crossover network output terminals, the impedance seen at said crossover network will be l2 ohms throughout the crossover frequency lregion. Moreover, the irregularities in phase in the outputs of the two loudspeakers will be markedly reduced, with the result that the number of feedback modes in the room containing the loudspeakers as a part of a sound reinforcing system including amplifiers and microphone( s) will also be substantially reduced. The overall result of the use of the above described impedance correcting networks is a more satisfactory sound reinforcing system which may be equalized to control feedback modes at minimum cost. What is claimed is: 1. In combination with a crossover network having a plurality of pairs of divergent frequency output terminals, and a plurality of loudspeakers having divergent frequency responses, an impedance correcting network connected between each pair of crossover network output terminals and one of said loudspeakers, each said impedance correcting network comprising a plurality of impedance element means for matching the impedances of the loudspeakers with the impedance of the crossover network in the crossover frequency region, whereby said loudspeakers will properly load the crossover network, said impedance element means comprising a resistor paralleled by a capacitor series coupled to a second resistor paralleled by an inductor series coupled to a 5 6 third resistor paralleled by a capacitor-inductor- OTHER REFERENCES reslstor serles Combmation- Crowhurst, Norman H.: Loudspeaker Crossover De- References Cited sign, pp. 43-45 in Radio-Electronic Magazine. Published July 1952. TK 6540 R24. Class 179-l.2. UNITED STATES PATENTS 5 Terman, F. E.: Radio Engineers Handbook. McGraw- 1,674,683 6/ 1928 Hahnemann 179-12 Hill Book Co., Inc., 1943, pp. 249-251, TK6550 T42. 2,270,644 1/ 1942 Blackman 333-32 2,802,054 g/1957 Conley 179 1 2 KATHLEEN H. CLAFFY, Primary Examiner 2,957,944 10/1960 De Monte 333-32 X 10 V. C. WILKS, Assistant Examiner FOREIGN PATENTS U.S. Cl. X.R. 744,300 2/1956 Great Britain. 333-32 Patent Citations
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