Acoustic units for stereo system. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Acoustic units for stereo system. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Speakers

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A stereo system must reproduce a wide range of audio frequencies in each channel with minimal frequency and non-linear distortion. An important condition for this is the choice of loudspeakers and their acoustic design. With good acoustic units, high-quality sound reproduction can be obtained using even relatively simple bass amplifiers.

The sound quality is largely determined by the shape of the frequency response of acoustic units at lower sound frequencies.

With a "smooth" frequency response of the acoustic unit, when the irregularities are "blurred" and do not have a high pitch, the sound has a soft, "not intrusive" character, while all the instruments of the bass group are heard well. Thus, obtaining a "smooth" frequency response in the lower frequencies is crucial for highly artistic sound reproduction.

For a stereo system, it is desirable to select pairs of identical loudspeakers. If the radio amateur has at his disposal low-frequency loudspeakers having a mechanical resonance frequency of the moving system below 45 Hz, then a high-frequency loudspeaker ("tweeter") must be included in a pair with each of them. In the case of using broadband loudspeakers, for example 4GD-28 or 4GD-4, the use of a high-frequency link is not necessary, but it is always preferable to have separate reproduction of lower and higher audio frequencies to reduce intermodulation distortion. In this case, it is necessary to apply a crossover filter, which excludes the supply of low-frequency signal components to the high-frequency loudspeaker.

For the design of acoustic units with a closed back wall, the following recommendations can be given. The walls and the front board should be made of thick (at least 15-20 mm) plywood or chipboard. A low-frequency or full-range loudspeaker should be installed asymmetrically with respect to the axes of symmetry of the box. An irregular shape of the box (non-parallelism of the walls) should be recognized as preferable any other symmetrical form.When the above measures are carried out, the unevenness of the frequency response of the unit in the low-frequency region is reduced (in the region of higher and medium frequencies, this is achieved by selecting loudspeakers).

It is advisable to place the acoustic units of the stereo system, made in the form of boxes with closed rear walls, in the upper corners of the room. (In the case of an elongated room - on a smaller wall). Since the loudspeakers in this case are located much higher than the seated listener, the front boards of the units should be tilted down and turned at the same angle inward.

The author manufactured and tested two (left and right) angular acoustic units. Their volume is 135 dm3. The appearance of one of them is shown in Fig.1 and Fig.2. The ledge on the rear wall is due to the presence of a beam under the ceiling at the installation site of the units. In this design, loudspeakers 4GD-28 and 6GD-1 were tested. Since the 4GD-28 loudspeaker is more widespread, measurements were carried out with it in a larger volume.

Acoustic units for stereo system
Rice. 1

Figure 3 shows the frequency characteristics of the 4GD-28 loudspeaker when installed in the proposed corner unit (solid line) and in a rectangular box of the same volume (dashed line). Both boxes are empty. All frequency responses were measured under conditions equivalent to free space.

Acoustic units for stereo system
Ris.3

Comparison of characteristics shows that the peak height for the corner unit is lower by 3 dB. When listening, however, the nature of the sound of the lower frequencies was found to be unsatisfactory. After that, an acoustic resistance panel - PAS was installed in the lower side wall of the unit (see "Radio" No. 4, 1969, pp. 27-28). The diameter of the panel holes is 30 mm, their number for the 4GD-28 loudspeaker was 9 pcs. The damping fabric was linen. To improve the transient response in the midrange, the reverse side of the loudspeaker was covered with a layer of glass wool. Then, in the same unit, a 6GD-1 loudspeaker was installed with a 21GD-6 high-frequency loudspeaker, covered from the inside with a tight cover. Figure 1 shows the frequency response of these two loudspeakers with FAS and wool.

Acoustic units for stereo system
Ris.4

When listening, the sound of corner units with PAS and cotton wool was recognized as the most pleasant. Units with a 6GD-1 loudspeaker and a tweeter sounded especially good.

Making a corner acoustic unit is practically no more difficult than a regular box with a closed back wall. From the point of view of finishing (gluing veneer, polishing), the corner unit has an advantage over a conventional box, since, in addition to the back, its two side walls do not need to be finished either (Fig. 1 walls "a" and "b"). Structural drawing of the left unit and its dimensions are shown in fig. 5. The right unit should have a slope of the front board to the other side, being a mirror image of the left one. The parts of the boxes cut out of plywood or chipboard are fastened together tightly, without gaps, with the help of rails, glue and screws. The front board is removable and is screwed to the drawer with screws and threaded steel corners mounted on the side walls. For a tighter fit, a strip of felt is placed between the front board and the drawer. Of course, when assembling the box, instead of the front board, you can make its back cover removable.

Acoustic units for stereo system
Ris.5

The reverse side of the loudspeaker can be wrapped with cotton as follows. First you need to take a piece of some fabric and cover it with the reverse side of the loudspeaker already installed on the front board. The fabric keeps the wool on the loudspeaker and protects the cone from it. Then, a strip 30-40 m wide is cut out of cardboard in such a shape that, being laid on the front board next to the loudspeaker, it fits snugly against the diffuser holder. In the case of a round loudspeaker, the strip should be in the shape of a circle, in the case of an elliptical - an ellipse. The cut out strip of cardboard is then laid over the fabric close to the loudspeaker and nailed to the board with small nails along with the fabric stretched over the loudspeaker. After that, on the loudspeaker and around it, on a strip of cardboard, cotton wool or tow is laid in an even layer, and the ends of the protruding fabric are tied into a knot above the magnetic system. Of course, the size of a piece of fabric should be sufficient for this.

Each radio amateur can produce the exterior finish of the unit in accordance with his taste and capabilities. The color of the decorative fabric covering the front board from the outside should match the color of the upholstery or wallpaper.

Author: Ing. V. Shorov; Publication: N. Bolshakov, rf.atnn.ru

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