Created on 15.02.2000 14:59.
Author: M. Sirotyuk, Moscow.
The variety of options for the acoustic design of the loudspeakers makes it possible to realize the capabilities 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. Acoustic design with passive emitters, acoustic impedance panels (PAS), and very rarely – an acoustic labyrinth is used somewhat less often. This is affected by certain difficulties in its design. This article offers a description of a loudspeaker with an acoustic labyrinth, which successfully combines ease of manufacture and good, according to the author’s opinion, sound parameters.
The labyrinth is a series of internal partitions installed in the loudspeaker housing, forming zigzag channels along which sound vibrations from the rear side of the head diffuser pass to the labyrinth outlet.
If the length of this “passage” is close to λ / 2 (where λ is the sound wavelength) at the lowest frequency (at 50 Hz λ / 2 = 3.4 m), then the radiation from the exit of the labyrinth will coincide in phase with the radiation from the front of the diffuser.
In other words, the acoustic labyrinth makes it possible to improve 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 the open space, which excludes compression and the associated increase in the resonant frequency of the bass head. Relatively short legs of the labyrinth (much less than λ) prevent the formation of standing waves, and covering them with sound-absorbing material acts like a PAS. The sectional area of the labyrinth channel is usually made close to the area of the diffuser.
To expand the radiation pattern of loudspeakers in the region of higher sound frequencies, it was proposed to use scattering gratings and acoustic lenses in the post-war years. Despite the difficulties in manufacturing such devices, they are still used by some firms (for example, by AIWA). However, the problem of obtaining the circular directivity characteristic of loudspeakers was completely and simply solved only recently with the help of scattering cones installed opposite vertically located HF heads.
Circular directivity characteristics allow you to obtain a uniform sound field in the horizontal plane over the entire frequency band. In such a “spatial” sound, the stereo effect zone is significantly expanded.
In view of the above, a loudspeaker with an acoustic labyrinth, available for self-manufacture, was developed. For its body, neither thick plywood nor chipboard is needed, 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 uses LF head 16 – 35GDN-1-4, SCh 8 – ZOGDS-117 (ZOGDS-1-8 can also be used), VCh 3 – 6GDV-4-8. The cylinders of the midrange block 10 and the low frequency block 15 are glued from the wallpaper. The total thickness of their walls is 10 mm. The stiffness of the cylinders is increased by gluing inside the body of plywood disks, on which the LF and MF heads are fixed, and segments 17 (Fig. 2), deployed 180 ° relative to each other and forming an acoustic labyrinth.
For acoustic damping of the bass and midrange heads, a soft felt 5-6 mm thick made of artificial wool was used. It is manufactured by the Research Institute of Nonwovens (Serpukhov) under the name of “needle-punched fabric heat-and-noise insulation” and is sold in hardware stores. The use of this material allows you to significantly smooth out the frequency response of the LF and MF heads.
The internal disks of the speaker system are made of plywood (chipboard can be used) 16 mm thick (top panel 1 and bottom base 19 of the loudspeaker, disk 9 for attaching the midrange head 8 and base of the midrange unit 10, disk 13 for attaching the woofer head 16) and 10 mm (segments 17 acoustic labyrinth). In the lower base 19 of the loudspeaker housing (Fig. 3), 22 holes 20 with a diameter of 18 mm are drilled for the output of sound vibrations, after which a stretched fabric is glued to it, which performs the functions of the PAS.
Discs and segments are turned on a lathe or cut with a cutter. To mount the bass and midrange heads, holes are cut out in discs 9 and 13. From below, using screws, metal rectangles with dimensions of 25x10x4 mm are attached to them. In the center of these rectangles there are threaded holes into which the posts 7 and 12 are screwed, securing the head to the discs through rubber gaskets.
Collect the loudspeaker in this sequence. First, a “stack” of the labyrinth is mounted from segments 17, the distance between which is fixed by metal bushings 18 with a diameter of 10 mm. To do this, a threaded rod at the ends (not shown in the figure) is passed through the segments and bushings, tightening the “stack” parts, using nuts, under which it is necessary to put dense rubber washers, which provide tension if the segments may dry out.
Before assembly, the segments of the labyrinth 17 are pasted over with felt, in which holes with diameters of 11-12 mm are cut (punched out) so that the sleeves 18 rest directly on the material of the segments. Then, to fix the distance between the “stack” of the labyrinth and the disc 13 of the LF compartment, a technological strip of the required height, cut from corrugated cardboard and rolled into a cylinder, is temporarily installed.
After drilling holes in segments 17 and discs 9 and 13 to pass the signal wires, you can start assembling the LF compartment 15. For this, the “stack” of the labyrinth together with the disc 13 and the cardboard cylinder fixing it is pasted over with one layer of thick paper. Then collect the midrange compartment 10. On its base 9 and the disc, 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 compartment prepared in this way is placed at a distance of 70 mm below the lower segment of the “stack” (also using a strip of industrial cardboard), and another layer of thick paper is glued to both compartments 10 and 15. After this layer has dried, the entire cylinder is pasted over with wallpaper, gradually bringing the wall thickness to 10 mm.
One speaker will require approximately 2-3 rolls of wallpaper 11 m long and 3-4 liters of PVA glue. 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 cut off compartment 10 with a hacksaw.
Before assembling the loudspeaker, all 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 compartment 10 is evenly filled with loosened cotton wool (100–150 g). Further assembly is clear from fig ….