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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A simple loudspeaker with an acoustic maze. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers The variety of options for the acoustic design of loudspeakers allows you to realize the possibilities of the electrical path to varying degrees. The most widespread among radio amateurs are such designs as "closed box" and "phase inverter" - they are quite simple to manufacture. Somewhat less often, acoustic design with passive radiators, acoustic impedance panels (PAS) is used, and very rarely - an acoustic labyrinth. There are some complexities in its design. This article proposes a description of a loudspeaker with an acoustic labyrinth, which successfully combines ease of manufacture and good, according to the author, sound engineering parameters. The labyrinth is a series of internal baffles installed in the loudspeaker housing, forming zigzag channels through which sound vibrations from the rear side of the head cone pass to the outlet of the labyrinth. If the length of this "passage" is close to X/2 at the lowest frequency (at 50 Hz X/2 = 3,4 m), then the radiation from the outlet of the labyrinth will be in phase with the radiation from the front side of the diffuser. In other words, the labyrinth improves the reproduction of the low-frequency portion of the audio range. Another advantage of the labyrinth is that the rear side of the diffuser emits sound vibrations practically into open space, which eliminates compression and the associated increase in the resonant frequency of the woofer. Relatively short labyrinth legs (much smaller than X) prevent the formation of standing waves, and covering them with sound-absorbing material acts like an EAS. The cross-sectional area of the labyrinth channel is usually made close to the area of the diffuser. For many years, designers have sought to build speakers with a circular directivity. Recall that this characteristic depends on the ratio 1/X, where 1 are the linear dimensions of the loudspeaker. In a box with size I, with a ratio of 1/X s 1 (i.e., at frequencies not higher than 1 kHz), the width of the directivity characteristic is approximately ±50 °, and for a ratio (1/X - 10 (i.e., at frequencies of about 20 kHz) - less than ±20°. To expand the radiation pattern of loudspeakers in the region of higher sound frequencies, even in the postwar years, it was proposed to use scattering gratings and acoustic lenses. Despite the difficulties in manufacturing such devices, they are still used by some companies (for example, AIWA). However, it is completely and simple to solve the problem of obtaining a circular directional characteristic of loudspeakers. only recently with the help of scattering cones mounted opposite vertically located RF heads [1-3]. Circular directivity characteristics allow you to get a uniform sound field in the horizontal plane over the entire frequency band. In such a "spatial" sound, the stereo effect area is significantly expanded. In view of the foregoing, a do-it-yourself loudspeaker with an acoustic labyrinth was developed. No thick plywood is needed for its body. nor chipboard, pasted over from the inside with scarce sound-absorbing materials. It is made in the form of a rigid vibration-resistant cylinder, which excludes any vibrations of its walls. The section of the cylinder with the heads installed in it is shown in Fig. 1.
The loudspeaker used woofer head 16 - 35GDN-1-4, midrange 8 - ZOGDS-117 (ZOGDS-1-8 is also possible). HF 3 - 6GDV-4-8. The cylinders of the MF block 10 and the woofer block 15 are glued from wallpaper. The total thickness of their walls is 10 mm. The rigidity of the cylinders is increased by gluing plywood discs inside the body, on which the bass and midrange heads are fixed, and segments 17 (Fig. 2), turned 180° relative to each other and forming an acoustic labyrinth. For acoustic damping of bass and midrange heads, soft felt 5...6 mm thick made of artificial wool is used. It is manufactured by the Research Institute of Non-woven Materials (Serpukhov) under the name "needle-punched heat and noise insulating fabric" (TU-RF17-14-13-127-95) and is sold in hardware stores. The use of this material allows you to significantly smooth out the frequency response of bass and midrange heads.
The inner disks are made of plywood (can be chipboard) 16 mm thick (top panel 1 and bottom base 19 of the loudspeaker, disk 9 for fastening the midrange head 8 and the base of the midrange unit 10, disk 13 for fastening the low-frequency head 16) and 10 mm (segments 17 of the acoustic labyrinth). In the lower base 19 of the loudspeaker housing (Fig. 3), 22 holes 20 with a diameter of 18 mm were drilled for the output of sound vibrations, after which a stretched fabric was glued to it, which performs the functions of the PAS.
Disks and segments are turned on a lathe or cut with a cutter [4]. Holes are cut out in discs 9 and 13 to mount bass and midrange heads. From below, with the help of screws, metal rectangles measuring 25x10x4 mm are attached to them. In the center of these rectangles there are threaded holes, into which racks 7 and 12 are screwed, fastening the head to the disks through rubber gaskets. Assemble the loudspeaker in this order. First, a “shelf” of a labyrinth is mounted from segments 17, the distance between which is fixed with metal bushings 18 with a diameter of 10 mm. under which it is necessary to put dense rubber washers, which provide tension in case of possible drying of the segments.Before assembly, the segments of the labyrinth 17 are pasted over with felt, in which holes with diameters of 11 ... 12 mm are cut (cut out) so that the bushings 18 rest directly on the material of the segments. fixing the distance between the "shelf" of the labyrinth and the disk 13 of the LF compartment, temporarily install a technological strip of the desired height, cut out of corrugated cardboard and rolled into a cylinder. After drilling holes in segments 17 and disks 9 and 13 to pass signal wires, you can start assembling the low-frequency compartment 15. To do this, the "shelf" of the labyrinth, together with disk 13 and the cardboard cylinder fixing it, is pasted over with one layer of thick paper. Then the midrange compartment 10 is assembled. On its base 9 and the disk, one layer of felt is preliminarily glued, then subsequent layers are applied to it, fixing them in several places with the help of small nails. The distance between the base and the disk in this compartment is also fixed with a cardboard cylinder and the compartment is pasted over with one layer of thick paper. The midrange compartment prepared in this way is placed at a distance of 70 mm below the lower segment of the "whatnot" (also using a strip of technological cardboard), and another layer of thick paper is glued onto both compartments 10 and 15. After this layer has dried, the entire cylinder is covered with wallpaper, gradually increasing the wall thickness to 10 mm. For one loudspeaker, approximately 2 ... 3 rolls of wallpaper 11 m long and 3 ... 4 liters of PVA glue are required. Each glued layer should dry well. At the end of gluing, the upper and lower uneven ends of the cylinder are carefully cut off. Then compartment 10 is cut with a hacksaw. Before assembling the loudspeaker, all the inner sides of compartments 10 and 15 must be pasted over with a layer of felt 11. Subsequent layers (their number is shown in Fig. 1) are strengthened with small nails. In addition, the entire internal volume of the compartment 10 is evenly filled with loosened cotton wool (100..150 g). Further assembly is clear from Fig. 1. Racks 12 in the upper part have shanks with M5 thread, on which midrange compartment 10 is put on. Rubber washers must be placed under mounting racks 7 and 12. Compartment 10 is also fixed through washers, for which holes of the corresponding diameter are preliminarily cut out in its felt covering (not shown in Fig. 1). Scattering cones 5 and 6 can be machined from duralumin or made from plywood, as recommended in [1]. Their diameters by 20...30 mm should exceed the diameters of the diaphragms of the corresponding heads. The angle between the generatrix of the cone 6 and the horizontal surface is 45°. Racks 4 have threaded shanks, on which cone 6 is installed (cone 5 is glued). The HF head 3 is mounted on racks with 4 shanks of racks 2. The top panel 1 of the loudspeaker is attached to the same racks with screws. The loudspeaker housing (compartments 10, 15 and 21) is pasted over with some kind of decorative material, for example, self-adhesive film or artificial leather. The sound outlet holes are covered with an elastic plastic mesh. The joints between the mesh and the decorative material are closed with cloth or leather tape 14. Separating filters are placed in the cylindrical compartment 21, also glued from paper. More details should be said about them. Often in crossover filters between the bass and midrange heads, a first-order filter is included, consisting only of a capacitor. It is assumed that this is sufficient, since the midrange head has a natural drop in the frequency response in the low and high frequencies. However, a first-order filter (especially at high signal power) can lead to intermodulation distortion, since a poorly filtered low-frequency signal, when it hits the midrange head, heats up its voice coil. Accordingly, the resistance of the coil changes with the frequency of oscillation of the unfiltered low-frequency signal. As a result, the current flowing through the coil of the midrange head will be modulated by this signal, which will lead to the appearance of intermodulation distortions [5]. Therefore, in the proposed crossover filters, not only low-pass and high-pass filters are used, but also a band-pass filter that limits the range of the midrange head. The filter scheme is shown in fig. 4. The frequencies of their section are 500 and 5000 Hz. The filters are mounted on a fiberglass printed circuit board. Capacitors C1 (K76P-1) and C2 (K73-16) are isolated from the board with soft gaskets to reduce vibration, and the frames of the inductors are rubber washers. The axes of the trimming resistors (TPB) are brought to the lower base of compartment 21.
The frequency response of individual sections of the loudspeaker was not taken, since the capabilities of the applied heads and filters are well known. Only the overall frequency response of the loudspeaker was evaluated, especially in the areas of band separation, in order to equalize it with the help of tuning resistors R2 and R4 (Fig. 4). Estimated measurements were carried out on an open loggia using an MKE-3 microphone and an oscilloscope. The microphone was placed at a point at a distance of 1 m from the LF and HF heads. A sound generator was used as a signal source. The measurements showed that the obtained frequency response does not differ from the typical FFR (frequency response of sensitivity) of the microphone: a smooth decline below 40 Hz and above 15 kHz. The limiting noise power of the loudspeaker is 70 W, the electrical resistance is from 5 to 7 ohms. The best assessment of the sound of any sound reproducing complex is still direct listening to the music programs played by it, and it showed that despite the use of medium quality heads in the loudspeaker, the stereo sound was clear and natural and did not tire the ear during prolonged listening, and this indicates the absence of noticeable non-linear and intermodulation distortion. Shown in fig. 5 the photograph illustrates the appearance and arrangement of the loudspeaker with the decorative mesh removed.
Speaker stands can be, for example, furniture ball bearings or tapered legs with rubber gaskets. Literature
Author: M.Sirotyuk, Moscow
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