Zhurenkov’s speaker system with dual speakers

Acoustic systems (AC) with twin heads^[1] interested in due time a lot of radio amateurs. Many of them opted for just such speakers and, judging by the reviews, are satisfied with their sound.

Some foreign firms have also shown interest in twin heads. For example, in 1985, Jamo advertised a number of new speakers, claiming in a brochure^[2]that their high power and high fidelity with relatively small dimensions are achieved due to the use of double heads. However, the lack of in-depth analysis and, most importantly, practical recommendations for designing speakers with such heads, as well as the appearance on the market of modern low-frequency compression emitters, somewhat reduced the interest of radio amateurs in dual dynamic heads.

Recent studies have revealed new advantages of this type of emitter. By the way, it turned out that its optimal design is one in which the speakers are facing one another by 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, lower nonlinear distortions and a smaller required volume of the acoustic design box.

The frequency response is smoothed out due to the mutual damping of the heads, of which the double^[1]… Each single head, within the limits of permissible deviations, has its own, due to the production technology, the unevenness of the frequency response, therefore the frequencies of the peaks and dips on their frequency response do not coincide. In a double head, some of these peaks and dips are mutually compensated.

Harmonic distortions are reduced due to the fact that a double head (as opposed to a single head) is a symmetrical electro-mechano-acoustic system. For this reason, the resistance of the air environment on its both sides is practically the same, due to the design features and material properties, there is no difference in the flexibility of the suspension for some types of heads when the diffuser moves forward and backward. Finally, the asymmetry of the distribution of magnetic induction in the gap of the magnetic system, which negatively affects the level of the second harmonic^[3], does not appear in the double head.

Of course, there are other ways to reduce speaker harmonic distortion. To reduce even harmonics, the Swedish company “Audio-Pro”, for example, in the low-frequency unit AC B4-200 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 directional diagram of the speaker.

Jamo has found a better solution. In the low-frequency link, she used one powerful double head, placing it on a horizontal board (see Fig. 2, a), under which a horn is located, directing sound towards the listener and matching the mechanical resistance of the head moving system with the air environment^[4]… As for the volume of the box, it is reduced due to the fact that the resulting flexibility of the suspension of the double head compared to a single head is halved. The mass of the moving system of the double head increases by the same factor, therefore 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, which can be seen from the following relations^[4]:

f_{I am}/ f_r = √ (c_G/ c_{I am} + 1);

with_{I am} = 1.14V / D⁴_eff,

where: f_{I am} and f_r – resonant frequencies of the head, respectively, in the box and open air space, c_G and with_{I am} – flexibility of head and air suspension in the box, V – box volume, D⁴_eff Is the effective diameter of the diffuser. Since the value of D⁴_eff a dual speaker is the same as a single speaker to fulfill the above ratios while decreasing the flexibility c_G it is necessary to reduce the flexibility by 2 times with_{I am}, and, consequently, the volume V by the same amount (in comparison with two heads installed separately, the volume will decrease by 4 times).

It would seem that by increasing the number of heads working per hole of the speaker, one can further reduce its dimensions.^[5]… However, in practice, the heads cannot be brought closer together so 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 propagation path of sound waves from the innermost head to the outermost head becomes commensurate with the wavelengths of the emitted waves, which ultimately counting leads to subtraction and distortion of audio signals (which is why you cannot double the medium and high frequency heads). In addition, the decrease in efficiency in this case will become noticeable.

The speaker presented to the readers’ attention is a bass reflex loudspeaker with a useful internal volume of 50 liters. A double head made of 6GD-2 was used as a low-frequency emitter, and 15GD-11 and 6GD-13, respectively, as a medium and high frequency. The double head is installed on an inclined board (see Fig. 2, b), which together with the side and bottom walls of the box forms a horn, which, in the author’s opinion, is more successfully directed at the listener than in the speakers of the Jamo company (Fig. 2, 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 speaker.

Rice. 2 Example of dual speaker placement

Main technical characteristics of the speaker:

Rated power, W	12
Passport power, W, not less	thirty
Rated electrical resistance, Ohm	4
Nominal frequency range, Hz	30-18000
Dimensions (edit)	360x300x800

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 acoustic systems of the S-90 type with a power input of 30 W. In terms of sound quality, most listeners prefer the speaker described below.

Schematic diagram of the speaker (based on the crossover filter described in ^[6]) is shown in Fig. 3, the structure is shown in Fig. 4. The speaker box is made of 20 mm thick particle board, pasted over with paper imitating valuable wood species. The dual head 17 is fixed on the board 10, the mid-frequency (12) and high-frequency (16) heads are on the front wall 4. The rear 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 the wall 4 using corners 11 and screws. The phase inverter tunnel 14 with an inner diameter of 50 and a length of 100 mm is glued from four layers of electrical cardboard with a thickness of 0.5 mm. In the hole in the front wall 4, it is fixed with glue.

The outlet of the horn of the double head 17 is closed with a grating (parts 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 5X20 mm made of aluminum alloy , rods 2 with a diameter of 4 mm are made of stainless steel and glued into holes drilled with a pitch of 20 mm in the upper and lower sides of the frame.

The ring framing of the holes for the rest of the heads, as well as the holes for the bass reflex tunnel, are bent from a 5X10 mm strip of the same material. For fixing the bezel of the mid-frequency head 5, four studs with M3 thread are provided, glued 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 the hole for the head 12 in the front wall along the outer frame diameter 5 using a circular cutter with a cutter^[7] and with a chisel it is necessary to choose a groove with a width of 20 and a depth of 2-3 mm. When assembling, first, the head 12 is fixed, then with the help of wire brackets or nails – the mesh 6 and, finally, the frame 5 is installed in place, which additionally presses the mesh to the panel 4. The frame 7 of the high-frequency head 16 is fixed in the groove of the front panel with glue.

To give the speaker an 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 inner and outer) must be painted with black paint. The same color should be used to paint metal grids 6 and 8, the inner surfaces of the bass reflex tunnel, the horn of the double head and the entire area of ​​the circle under the grid 6, the diffuser holder of the 6GD-2 lower head, the part of the diffuser holder of the head 12 facing the listener and the heads of the screws fixing it.

The coils L1 and L2 of the crossover 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 the speaker, you should pay special attention to the polarity of connection of the 6GD-2 heads, 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 pasted over or upholstered with sound-absorbing material. It is possible to replace 6GD-2 heads with 8GD-1, 15GD-11 – with 4GD-8 or 5GDSh-5-4, and 6GD-13 – with 3GD-2. The dimensions of the box are retained with such a replacement.

Literature:

1. Zhurenkov A. Dual dynamic heads – Radio, 1979, No. 5, p. 48;
2. Jamo Prospectus, Zurich, 1985;
3. Aldoshina I. A., Voishvillo A. G. High-quality acoustic systems and emitters – Moscow, Radio and communication, 1985;
4. Ephrussi M. M. Loudspeakers and their application. Ed. 2nd, rev. and add. – Moscow, Energy, 1976;
5. Zhbanov V. Ways to reduce the size of acoustic systems – Radio, 1987, No. 2, p. 29-31;
6. Raikin L. First, take out the low-frequency speakers – Inventor and rationalizer, 1985, no. 7, p. 40.
7. Raikin L. And the brace, and the circular cutter – Inventor and rationalizer, 1986, No. 2, p, 29.

Radio magazine, 1989, No. 4, pp. 45–47.