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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Acoustic system with a double head. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers Acoustic systems (AS) with dual heads [1] were of interest to many radio amateurs at one time. Many of them opted for just such speakers and, judging by the reviews, they are satisfied with their sound. Some foreign firms also showed interest in twin heads. For example, in 1985, the Jamo company advertised a number of new speakers, claiming in the brochure [2] that their high power and high fidelity in relatively small dimensions were achieved through the use of dual heads. However, the lack of in-depth analysis and, most importantly, practical recommendations for the design of speakers with such heads, as well as the appearance on sale of modern low-frequency compression radiators, somewhat reduced the interest of radio amateurs in dual dynamic heads. Recent studies have revealed new advantages of this type of emitters. By the way, it turned out that its optimal design is the one in which the heads face one another with diffusers, so in the future we will only talk about this option. The main advantages of a double head (compared to a single head) are a smoother frequency response, less non-linear distortion and a smaller required volume of the acoustic design box. The frequency response is smoothed out due to the mutual damping of the heads that make up the double head [1]. Each single head, within the tolerances, has its own, due to the production technology, uneven frequency response, so the frequencies of peaks and dips in their frequency response do not match. In a twin head, some of these peaks and dips cancel each other out. Nonlinear distortions are reduced due to the fact that the double head (unlike the single head) is a symmetrical electromechanoacoustic system. For this reason, the resistance of the air medium on both sides is almost the same, due to the design features and properties of the material, there is no difference in the flexibility of the suspension for heads of some types when the diffuser moves forward and backward. Finally, the asymmetry of the distribution of magnetic induction in the gap of the magnetic system, which adversely affects the level of the second harmonic [3], does not manifest itself in a double head. Of course, there are other ways to reduce the non-linear distortion of speakers. To reduce even harmonics, the Swedish company "Audio-Pro", for example, in the low-frequency unit AC B4-2000 installs two (out of four) low-frequency heads with magnetic systems outside [3]. However, the dispersion of the emitters generates the interference of sound waves and narrows the speaker's radiation pattern. Jamo has come up with a better solution. In the low-frequency link, she used one powerful double head, placing it on a horizontal board (see Fig. 1,a), under which there is a horn that directs the sound towards the listener and coordinates the mechanical resistance of the moving head system with the air [4]. As for the volume of the box, it is reduced due to the fact that the resulting flexibility of the suspension of a double head is halved compared to a single head. The mass of the moving system of the double head increases by the same factor, so the frequency of the main mechanical resonance does not change.
To maintain the calculated resonant frequency of a double head in acoustic design, a box with a volume half that of a single head of the same type is required, as can be seen from the following relations [4]: fi/fg=Sqr(Сг/Ся+1); Cya=1,14V/D4eff, where fb and fg are the resonant frequencies of the head, respectively, in the box and open air space, Cr and Cya are the flexibility of the head and air suspension in the box, V is the volume of the box, Deff is the effective diameter of the diffuser. Since the value of Deff of a double head is the same as that of a single head, in order to fulfill the above ratios with a 2-fold decrease in the flexibility Cr, it is necessary to reduce the flexibility Cya, and, consequently, the volume V by the same amount (compared to two heads installed separately, the volume will decrease 4 times). It would seem that by increasing the number of heads operating per one AS opening, it is possible to further reduce its dimensions [5]. However, in practice, the heads cannot be brought so close that their geometric dimensions do not affect the phase shifts of the sound waves emitted by the extreme heads. In this case, the length of the path of propagation of sound waves from the outermost inner head to the outermost one becomes comparable with the wavelengths of the emitted waves, which ultimately leads to the subtraction and distortion of sound signals (this is why mid- and high-frequency heads cannot be doubled). In addition, the reduction in efficiency in this case will be noticeable. The AC presented to the attention of readers is a bass-reflex loudspeaker with a useful internal volume of 50 liters. A double head made up of 6GD-2 was used as a low-frequency emitter, and 15GD-11 and 6GD-13, respectively, as a medium- and high-frequency emitter. The double head is mounted on an inclined board (see Fig. 1, b), which together with the side and bottom walls of the box forms a horn, which, in the author's opinion, is better directed at the listener than in the Jamo speakers (Fig. 1, a ). In addition, with this arrangement of the board with a double head, the volume of the box is more rationally used, which made it possible to reduce the dimensions and weight of the speakers. The main technical characteristics of the AU Rated power, W ... 12 Passport power, W, not less than ..... 30 Rated electrical resistance. Om... 4 Nominal frequency range, Hz. 30 ... 18 000 Thanks to the use of high-performance low-frequency drivers 6GD-2, the sound volume at a relatively low rated power (12 W) is not inferior to industrial speakers of the S-90 type with a power supplied to them of 30 W. As for the sound quality, most listeners prefer the speakers described below.
Schematic diagram of the AU (based on the separation filter described in [6]) is shown in fig. 2, the design is shown in fig. 3. Drawer AC 3 is made of chipboard 20 mm thick, pasted over with paper imitating precious woods. The double head 17 is fixed on the board 10, the mid-frequency (12) and high-frequency (16) heads are on the front wall 4. The back wall 15 is removable.
The mid-frequency head is isolated from the rest of the box by box 13, made of plywood 10 mm thick and fixed to wall 4 with corners 11 and screws. The phase inverter tunnel 14 with an internal diameter of 50 and a length of 100 mm is glued from four layers of electric cardboard 0,5 mm thick. In the opening of the front wall 4, it is fixed with glue. The outlet of the horn of the double head 17 is closed with a grate (det. 1, 2), the holes opposite the mid- and high-frequency heads are respectively convex metal grids 6 and 8 with annular decorative frames 5 and 7. Frame 1 is bent from a strip with a section of 5X 20 mm from aluminum alloy, rods 2 with a diameter of 4 mm are made of stainless steel and are glued into holes drilled in increments of 20 mm in the upper and lower sides of the frame. The annular frames of the holes for the rest of the heads, as well as the holes for the phase inverter tunnel, are bent from a strip with a cross section of 5x10 mm from the same material. To fasten the frame of the mid-frequency head 5, four studs with an M3 thread are provided, inserted with glue into holes with a diameter of 3,2 and a depth of 7 mm, drilled in the end of the ring from the side facing the panel 4. Before cutting a hole for the head 12 in the front wall along the outer frame diameter 5 using a circle cutter with a cutter [7] and a chisel, it is necessary to select a groove 20 wide and 2...3 mm deep. When assembling, head 12 is first fixed, then mesh 6 is fixed with wire brackets or nails, and, finally, frame 5 is put in place, which additionally presses the mesh to panel 4. Frame 7 of high-frequency head 16 is fixed in the groove of the front panel with glue. To give the speakers the appropriate look, the outer ends of frame 1 and frames 5, 7 and 9 must be polished to a mirror finish, and their side surfaces (both internal and external) must be painted with black paint. The metal grids 6 and 8, the inner surfaces of the phase inverter tunnel, the horn of the double head and the entire area of the circle under the grid 6, the diffuser holder of the lower head 6GD-2, the part of the diffuser holder of the head 12 facing the listener and the head of the screws fixing it should be painted in the same color. Coils L1 and L2 of the separating filter are wound with PEV-2 1,3 wire on frames with a diameter of 35 and a length of 100 mm. Each of them contains approximately 460 turns (six layers of 75-76 turns). Capacitors C1-C3 - MBGP, MBGO, etc. When installing speakers, pay special attention to the polarity of connecting heads 6GD-2, since in case of an error an acoustic short circuit will occur. Outer head - BA1. To improve the damping of the double head, the inner surface of the speaker box can be glued or upholstered with sound-absorbing material. It is possible to replace heads 6GD-2 with 8GD-1, 15GD-11 with 4GD-8 or 5GDSH-5-4, and 6GD-13 with 3GD-2. The dimensions of the box with such a replacement are preserved. Literature 1. Zhurenoe A. Dual dynamic heads.- Radio, 1979, No. 5, p. 48. Author: A. Zhurenkov, Zaporozhye; Publication: N. Bolshakov, rf.atnn.ru
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