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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING About loudspeakers with twin heads. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers A few years ago, a radio amateur A. Zhurenkov suggested using dual heads to reduce the lower limit of the frequency range reproduced by a loudspeaker [1]. Unfortunately, this method of extending the range towards low frequencies has not been widely used in amateur radio practice. And this is most likely due to the lack of an available method for calculating loudspeakers with dual heads. The article attempts to fill the gap and give radio amateurs some recommendations on the calculation of loudspeakers with dual heads. It is known that when calculating any loudspeaker, they usually proceed from the parameters of the head used in it [2]. Doubling heads leads to a change in only one of these parameters - the total equivalent volume. So, when doubling heads with equivalent volumes Ve1 and Ve2, their total equivalent volume Ve= (Ve1i +Ve2)/4. The whole procedure for further calculation of loudspeakers with double heads does not differ from the calculation of loudspeakers with single heads both for a closed box and for a phase inverter [2]. To accurately determine the equivalent volume of the head, it is recommended to use a measuring box. If it was not possible to get a suitable measuring box, to determine the equivalent volume of the head (in liters), you can use the approximate formula: Ve = 0,875 * Сg * De4 where Сg is the flexibility of the oscillating system of the head, cm / g, measured by the method. proposed in [3]; De - diameter of the diffuser without corrugation, cm. The found value of Ve can be used in the calculation of the loudspeaker box, and after its manufacture, more accurate measurements can be taken. A few words about the efficiency of a loudspeaker with dual heads. Its dependence on the parameters is described by the expression [2]: Efficiency =
where c is the speed of sound, K is a dimensionless value, constant for a given type of head and acoustic design. V is the given loudspeaker box volume. The above formula shows that the price for lowering the lower cutoff frequency of the range reproduced by the loudspeaker is a decrease in its efficiency. This, however, more than pays off. that when doubling the heads, all types of distortion of the signal reproduced by them are reduced. In addition to the reasons indicated in [1], this is facilitated by another important circumstance. The fact is that the unevenness of the sound field inside the loudspeaker box leads to a strong unevenness of its frequency response. An uneven distribution of sound pressure inside the box can, in addition, cause deformation of the cone (especially light and thin) of the head, which, in turn, contributes to the occurrence of non-linear and intermodulation distortions. In the case of using double heads, all these unpleasant phenomena occur only on the inner head, while on the outer one, due to the damping effect of the air enclosed between the heads, they are significantly weakened. To eliminate the source of these distortions, it is recommended to limit the frequency spectrum of oscillations supplied to the inner head, depending on the size of the loudspeaker, to 100 ... 300 Hz. It is possible to weaken the harmful effect of the internal resonances of the box on the quality of reproduction by installing acoustic impedance panels (PAS) between the heads or on the back of the inner head. In both cases, the PAS is recommended to be placed in the holes of the diffuser holders of the heads. It should also be borne in mind that the PAS reduces the quality factor of the head, and this can be very useful, since in some cases it will allow the use of a low-frequency amplifier without current POS. It is known that the sound quality of a loudspeaker depends on the uniformity of not only the frequency response, but also the phase response. Smoothing of the phase response is achieved both in the electric (by choosing the appropriate crossover filters) and in the acoustic paths (guided by the recommendations given in [4]). A certain alignment of the phases emitted by the heads of sound vibrations can be achieved, for example, by placing the voice coils of the heads in one plane perpendicular to the acoustic axis of the loudspeaker. However, this measure is often insufficient, especially when using heads with significantly different masses of moving systems and with diffusers made of materials of different densities. In the first case, this is explained by the fact that the phase shifts introduced by the heads at medium and higher frequencies, ceteris paribus, are the greater. the greater the mass of the moving system, and in the second case, the fact that the phase shifts depend on the speed of propagation of sound waves over the surface of the diffuser. These circumstances force the low-frequency head to be pushed forward in relation to the mid-frequency one, and the mid-frequency head in relation to the high-frequency one. The necessary additional displacement of the heads can be found experimentally by applying a rectangular voltage with a frequency of 0,7fp (here fp is the crossover frequency) to the input of the amplifier with which the loudspeaker works and observing the transient process of the signal taken from the measuring microphone mounted on the acoustic axis of the heads. Given the above considerations, dual heads operating in the low-frequency link should be installed, guided by the figure. If it is decided to use dual heads in the mid-frequency link, then they need to be positioned with diffusers to each other, as recommended in [1].
A practical example of the use of dual heads can serve as a two-way loudspeaker developed by the author, made in the form of a phase inverter. In its low-frequency link, twin heads 6GD-2 are used, and in the medium-high-frequency link - the head ZGD-42 (ZGD-32 is also possible). It works in conjunction with a two-way amplifier, the nominal output power of the low- and high-frequency channels of which is 20 and 10 W, respectively. The crossover filter (crossover frequency 500 Hz) is similar to that given in [4¦. The output impedance of the low-frequency channel of the amplifier is negative - 1,5 Ohm. The nominal range of frequencies reproduced by the loudspeaker is 30 ... 18000 Hz, the frequency response unevenness is not more than 6 dB. The loudspeaker case (700x400x360 mm) is made of chipboard 20 mm thick. The front wall is glued from two sheets of chipboard, its thickness is 40 mm. The thickness of a cylindrical lining with a diameter of 300 mm from the same material, fixed on the outer side of the front panel, is the same. The hole in the overlay with a diameter of 230 mm coincides with the hole in the front panel for low-frequency heads. One of them is fixed on the inside of the front panel, the other on the outside of the lining. The ZGD-42 head is fixed on the outer side of the front panel above the low-frequency unit with a large axis vertically. On the inside, it is covered with a cap, the volume of which (about 2 liters) is filled with cotton wool. To increase the rigidity of the box, metal spacers are installed between the front and back, as well as between the side walls. The inner walls of the box are covered with felt 20 mm thick. The phase inverted hole tube (installed on the front panel) has an inner diameter of 80 and a length of 160 mm. considering the thickness of the front wall. The loudspeaker can also be made in the form of a closed box. In this case, a smooth frequency response at low frequencies is obtained with zero output impedance of the band amplifier, and the lower limit of the frequency range reproduced by the loudspeaker rises to 40 Hz. If crossover filters with a crossover frequency of 400 ... 500 Hz are installed in such a loudspeaker, then it can be used with almost any amplifier operating at a 4 ohm load. The fidelity of the reproduction of the musical programs of the loudspeaker in both versions is very high. Literature
Author: V. Zhbanov, Kovrov, Vladimir Region; Publication: N. Bolshakov, rf.atnn.ru
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