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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Acoustic unit with increased efficiency at low frequencies. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers One of the reasons for the poor return of the loudspeaker in the region of low sound frequencies is the interaction of radiation from the front and back sides of the diffuser. To combat this phenomenon, it is necessary to design the loudspeaker, which, providing an optimal acoustic load, separates these radiations. From this point of view, a phase inverter is of interest, in which the radiation from the back of the diffuser is used to increase the output at low sound frequencies. However, a conventional phase inverter operating at frequencies of the order of 40 Hz must have a significant volume and therefore is not widely used. The search for a more successful solution to this problem led the Moscow radio amateur A. G. Presnyakov to the creation of an acoustic unit, which he called "horseshoe" (Fig. 1).
The unit was demonstrated at the XVII All-Union Exhibition of Radio Amateurs. Like a horn, it serves as a waveguide for sound vibrations propagating through it and has an increased efficiency at low sound frequencies. Along with a great advantage, such a unit has a significant drawback. The loudspeaker installed in it turns out to be loaded on a pipe tapering towards the middle, so that behind the diffuser a large-volume pre-horn chamber is formed, as it were. As a result, a number of bursts and dips appear on the frequency response of the loudspeaker sensitivity, worsening its uniformity. Obviously, it is more expedient to make an acoustic unit not in the form of a horseshoe tapering towards the middle, but in the form of a horn folded into a horseshoe (Fig. 2).
The generatrix of the horn, as in the unit of A.G. Presnyakov, has only side walls, its upper and lower covers are parallel. The loudspeaker, installed in the narrow part of the horn, in this case is loaded on the expanding tube. As a result, unwanted resonances are not only eliminated, but the matching of the high radiation resistance of the loudspeaker with the low resistance of the medium is improved. The author has made several such units of various sizes. Two of them are shown in Fig. 3; at the top there is a "small horn phase inverter", with a volume of 50 dm3, working with a loudspeaker 5GD-1, and at the bottom there is a "large horn phase inverter", with a volume of 140 dm3working with a loudspeaker 6GD-1.
Both units can also be used with other loudspeakers. As shown by measurements carried out in the laboratory of electroacoustics of the NIKFI, the units have satisfactory frequency characteristics of sensitivity. One of them is the characteristic of a small phase inverter with a loudspeaker 5GD-1 with and without an acoustic impedance panel (PAS) is shown in Fig. 4.
The frequency response of the sensitivity of a large horn phase inverter with a loudspeaker 6GD-1 was given in the Radio magazine N 4, for 1969, p. 28, Fig. 4. The sound of horn phase inverters has a pleasant peculiar timbre, which is explained by the high radiation efficiency at low sound frequencies. Jazz music performed by small ensembles is especially well reproduced. For high-quality reproduction of symphonic music, the units can be damped with PAS panels (Fig. 3). The PAS is mounted in a cover that covers the large socket of the unit. Holes with a diameter of 10-30 mm or blinds 10 mm wide and the length of the entire cover should be evenly distributed over its entire area. PAS, like any other damping of a moving loudspeaker system, reduces its efficiency, so their use depends on the taste of the radio amateur and cannot be recommended as mandatory. For comparison, the table shows the values of the efficiency of the loudspeaker 4A-28, measured by the method of recording polar radiation patterns for various types of design. As can be seen from the table, the PAS panel reduces the efficiency at low frequencies, however, when working with a horn phase inverter, it remains quite high. A practically horn-type phase inverter makes it possible to use a single loudspeaker to sound a hall that can accommodate 50-70 people, for example, a cafe, restaurant, club or school assembly hall. In a small room (foyer, hall), a horn phase inverter can operate from a standard single-ended bass amplifier with a 6P14P lamp at the output. Own loudspeakers of the device used (tape recorder, radiograms) must, of course, be turned off. In a living room, you can get a significant sound volume when you connect even a transistor radio like "Speedola" to a horn phase inverter without an additional amplifier. Despite the rather complex configuration, the manufacture of the unit does not require special skills and is available to every radio amateur. To do this, you need to have two standard sheets of thick (12-15 mm} and two or three sheets of ordinary thin three-layer plywood. For a cover for a large socket, you will need an additional piece of thick plywood, a cover for a small socket can be made from scrap left after cutting out the upper or lower bass reflex base.You will also need casein glue and 5-6 rolls of elastic bandage (rubber band sold in pharmacies). Work begins with marking the upper and lower bases. Marking the bases is the most responsible operation. You can practice this on a piece of paper first. Then, putting a sheet of thick plywood on the table, from the right near corner, the overall dimensions are applied - the diameter and depth (height) of the loudspeaker, which is supposed to be used in the unit. Leaving a margin of 15 mm on each side, they begin marking (Fig. 2). After a slight narrowing immediately following the loudspeaker, there should be a gradual expansion of the base, ending with a characteristic bell in the left near corner of the plywood sheet. It is desirable that the shape of the bells be symmetrical. Having marked one base, the resulting form is transferred to another sheet of plywood. After that, both bases are cut out and knocked together with nails. Nails should preferably be placed as shown in Fig. 5, then the holes can be reused.
When hammering together the bases, the nails should not be driven in all the way so that they can be easily pulled out. It is better to finish the highlanders with a bastard file, but in such a way that there are no chips of the upper layers of plywood. After processing, the bases are separated.
The side walls are made of three layers of thin plywood glued sequentially on top of each other. For this purpose, a sheet of thin plywood should be cut into strips across the fibers of the outer layers. The length of the plywood strip should be 40-60 mm longer than the length of the generatrix of the cover (machining allowance). The width of the strip determines the height of the unit. It is found based on the diameter of the loudspeaker, twice the thickness of the base, a margin of 20-30 mm and, finally, a processing allowance. After making six strips of plywood, eight racks must be cut from wood. The length of the racks should be equal to the height of the unit from the inside, their cross section is 60X60 mm. Racks are installed on a flat surface and one of the bases is placed on them (see Fig. 5). After this, the bases but with the existing holes are nailed to the racks. To prevent the plywood from bending when gluing the side walls, the location of the posts at the edges of the sockets must coincide with the side generatrices of the unit. Similarly, the second base is nailed to the racks, having previously aligned it with respect to the one nailed with the help of a joiner's corner. Before applying the glue, it is useful to lightly moisten the plywood with water. It is more convenient to glue the first layer of side walls together. A strip of plywood is glued to the ends of the bases prepared in the same way, starting from the middle, tightly wrapping the unit with an elastic bandage coil to coil. Due to the tension of the rubber, thin plywood fits snugly to the bases around the entire perimeter. Glue drying time 6-8 hours. The second and subsequent layers of plywood of the side walls are glued in the same way, but now the entire surface of the strips to be glued should be lubricated with glue. Having glued the body of the unit, the nails are pulled out, the fastening racks are removed, and the holes from the nails are tightly clogged with wooden sticks, the protruding ends of which are cut flush with a knife. After that, proceed to the final finishing of the unit. The protruding edges of the side walls are filed with a jigsaw and processed with a bastard file. The openings of the sockets are processed in such a way that covers cut in place from thick plywood can fit snugly on them. Having adjusted the covers, you need to install them in place. To do this, at the corners of the sockets from the inside, steel corners should be fixed on screws or screws and threads for M4 screws should be cut into them. Screws threaded through the flare covers will hold them firmly in place. The unit with the bell caps installed should be sanded until a smooth surface is obtained. In conclusion, the outer surface of the unit can be pasted over with fine wood veneer and polished. However, this work requires certain skills. In the absence of veneer, you can pre-select the wood pattern on the outer layers of plywood, varnish the unit and polish it. In order for the covers to fit snugly to the edges of the sockets, strips of felt or fine cloth must be glued around their perimeter. If the unit is supposed to be used without PAS, then a frame should be cut out of the cover for a large socket. A hole for the loudspeaker is cut out in the small cover. Both covers can be covered with a not very dense fabric, and so that holes are not visible through it, it is useful to paint the outer surface of the socket covers with ink diluted with water. Author: Ing. V. Shorov; Publication: N. Bolshakov, rf.atnn.ru
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