Created on 19.11.2006 15:58.
Author: M. Sirotyuk, Moscow.
The loudspeaker proposed by the author can be used in the loudspeaker both with an additional low-frequency radiator (subwoofer), and without it. The loudspeaker case is glued from paper, the head is coaxial from Pioneer. It is not a problem to buy it in the Russian markets.
Modern broadband small-sized heads, powerful enough (up to 100 watts), with a coaxially mounted high-frequency “buzzer” and a simple filter, make it possible to obtain common-mode radiation in the area of joint operation, which brings the loudspeaker closer to a point, non-directional radiator. These mini-heads have a number of advantages.
For example, a loudspeaker (LH) with one crossover frequency in the higher frequency part of the band (about 7 kHz) has an advantage over multiband, in particular three-way, with low crossover frequencies. High-order crossover filters introduce phase distortion, cause rise and fall in crossover frequencies, which distort the timbres of the sound.
The unevenness of more than 1 dB at mid frequencies is especially noticeable by ear, so the crossover frequencies of the first-order filter must be shifted to the upper part of the range, which is easy to do with a wide-band head. In addition, the head with a small diffuser diameter (d) retains the piston operation area in a wide band in the absence of oscillatory modes, and also expands the directivity characteristic of the head radiation under the condition d / λ≤1 (where λ is the sound wavelength).
Of the many models of small-sized broadband drivers available on the market, the author preferred to use the universal two-way coaxial head TS-E1070 from Pioneer, recommended for installation in a car. Its broadband metallised cone is made from injection-molded polypropylene and has a soft butyl rubber surround.
The flexible upper suspension reduces the resonant frequency of the head, and the elongated voice coil and the large diameter of the centering washer, which also acts as a diffuser and improve linearity. The dome diaphragm of the coaxially located high-frequency head is also metallized. The technical characteristics of this head are given below, and its frequency response is shown in Fig. 1 (company data).
|Maximum musical power, W||80|
|Rated power, W||25|
|Diffuser diameter, mm||74|
|Diffuser suspension diameter, mm.||100|
|HF-head diameter, mm.||24|
|Sensitivity dB / W (1m)||90|
|Operating frequency range with unevenness 14 dB, Hz||45-30,000|
In fig. 2. Shows a sectional view of the loudspeaker housing.
It consists of a cylinder 4 made of glued layers of paper wallpaper with a total thickness of 8 mm on a frame of plywood segments 6 (Fig. 3), bottom 9 (Fig. 4) and cover 3 (Fig. 5). The plywood segments located in the housing form an acoustic labyrinth.
This design makes it technologically simple to obtain at the same time a rigid body and a labyrinth. This housing is vibration-free, has high losses, and the labyrinth lengthens the path of the sound wave from the back of the cone.
Of course, the length of this labyrinth is not enough to completely cover the λ / 2 path at a low resonant frequency of the head 2 (1 is a protective and decorative mesh) in order to come out of the hole in the rear wall 9 of the loudspeaker in phase with oscillations from the front of the diffuser. It is only a fraction of λ / 2. It is better to consider this labyrinth as a very narrow, in comparison with the length of the propagating wave, open pipe (Ø << λ) with large viscous losses on the walls of the labyrinth and the body, pasted over with sound-absorbing material 5.
The resistance to acoustic losses in it depends on the absorption coefficient of the absorbent, the effect of which begins to increase after about 500 Hz, and reaches a maximum when its volume is 70% of the volume of the case.
In this case, significant acoustic damping is achieved already at medium frequencies, where a flat frequency response is especially important.
Soft felt 5–6 mm thick, made of artificial fiber, was used as a sound-absorbing material. In this felt, the resistance is close to the wave resistance of air and, at the same time, the damping of oscillations is great. Felt is sold in hardware stores as “sound absorbing material and insulation.”
Another important advantage of the labyrinth is the connection of the rear side of the diffuser of the head with the open space, due to which there is no compression in the system and the accompanying increase in the resonant frequency of the head.
Although the GG device is clear from Fig. 2, a few words must be said about the assembly sequence. Previously, all the segments must be pasted over with felt. They are separated by uprights 8 (fig. 6) made of any hard material (metal, plastic, wood or glued rolled paper). A system of segments, racks and a bottom made of plywood 10 mm thick with a tie 10 (metal bar 4 mm in diameter) is assembled into a frame.
Previously, rings with a diameter of 20 mm made of felt are put on the racks 7. At the bottom 9, the outlet holes are sealed with a protective metal mesh 7. In addition, holes are drilled in the bottom for terminals or wires. On the side (end) surface I make grooves into which the wires going to the electrodynamic head are laid and glued.
Plywood parts (see Fig. 3-5) are cut with a cutter. On the end surface of the frame made of segments, bottom and cover, a layer of thick paper (for example, whatman paper) is glued with PVA glue, and pre-cut strips of felt are glued at the same time, tightly spaced between the segments, bottom and cover. This complex operation requires special care.
First of all, the gluing of the end surface to the paper should be done well: glue is applied abundantly on the ends and paper. Layers of paper wallpaper are glued onto the dried structure, each after the previous layer has dried. The roll is pre-cut into three parts (with a wallpaper width of 550 … 600 mm). Two loudspeakers will require one roll of paper wallpaper 11 m long and 1.5 kg of PVA.
The housings are pasted over with a layer of decorative material, such as artificial leather. A rubber ring with an outer diameter of 130 mm and an inner hole with a diameter of 98 mm (for example, cut from an old car camera) is placed between the head 2 and the cover 3.
Since the axis of the ГГ should be located horizontally, a hinge is installed on the back of the end wall, with which the ГГ is attached to the wall or through a small bracket to the furniture.
The assessment of the quality of sound reproduction in the GG was carried out by ear. The sound of such a mini-loudspeaker turned out to be surprisingly good: clear, distinct, “transparent” sound, the absence of any resonances or extraneous overtones. The sound was even slightly better than that of the GG, described earlier (“A simple loudspeaker with an acoustic labyrinth,” Radio, 2000, No. 2, pp. 13-14).
This is due to the fact that in the design of a large size, heads are used that are noticeably lower in quality) – their sound is not so bright. True, its advantage is …