Triple speaker system. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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In the three-way speaker design developed by the author, measures were taken to optimize a number of operational characteristics - the directivity of the emitters, the conjugation of their phase characteristics and the suppression of parasitic vibrations of the cases. The advantage of the design is the possibility of connecting bandpass loudspeakers with separate cables to a common or separate power amplifiers.

The difference between the described speakers and typical acoustic design options is that each of the three audio frequency bands has its own case of an individual shape dictated by certain requirements. This speaker (photo in Fig. 1) is distinguished by the ability to adapt the directivity in each band and the total phase response, optimizing the sound quality, taking into account the acoustic characteristics of a particular room and the placement of listeners in it.

Fig. 1

The main emphasis in the development of the housing design for low-frequency heads was placed on minimizing resonance phenomena and reducing the level of vibrations. At the same time, rare and scarce materials and labor-intensive technologies associated with gluing curvilinear shapes were not used in the manufacture of speakers. When repeating the design, it is permissible to use close analogues of the applied dynamic heads, but it is important to ensure the requirements for the shape, vibration and sound insulation of the cases, which are set out in the article.

The external configuration of the enclosures of the speaker parts (shapes, the presence of chamfers to smooth corners) and the methods of connecting their elements contribute to damping and have a positive effect on the acoustic characteristics of the speaker. The sound quality is also significantly affected by the adjustable polar pattern. The advantages of this three-block speaker include the possibility of shifting the acoustic centers of the strip heads in the vertical plane. It is easy (and advantageous) to connect multiband UMZCH with active crossover filters to such speakers.

Main Specifications

• Rated resistance, Ohm.......4
• AC operating frequency band, Hz......30...22000
• Speaker sensitivity, dB/W m...90
• Crossover frequencies, Hz......500 and 3000

To work with the described speaker, the recommended nominal power of the amplifier in the LF band is 150 ... 200 W, and in the MF-HF band - at least 70 W.

AC cabinet design

Drawings of the AC design are shown in fig. 2, 3. Blocks for MF and HF heads are made according to the recommendations [1, 2] in the form of a separate unit, its tilt angle in the vertical plane is related to the distance from the listener's seat to the speaker. It easily regulates the conjugation of the radiation phase in the area of ​​separation of the LF and MF bands by shifting these blocks in depth relative to the front plane of the case for low-frequency heads. The tweeter is mounted in a spherical housing and mounted on a hexagonal housing for the midrange head. The fastening design of the HF head unit provides the possibility of adjusting its directivity in two coordinates, regardless of the orientation of the MF head unit.

Fig. 2

Fig. 3

In the MF and HF bands, filters and dynamic heads are used, which have proven themselves in designs. For the HF band, a VIFA D26TG-35-06 dynamic head was chosen, and for the MF band, an H143 head from SEAS. This choice was due to their successful use in acoustic systems by S. Batya [3, 4]. The low-frequency unit uses inexpensive KENWOOD KFC-WF255 (P_Mr. = 140 W, R_Mr. \u4d 28 Ohm, operating frequency band - 800 ... 91 Hz, sensitivity - XNUMX dB / W m), intended for car subwoofers.

The stand for the AC loudspeakers with a total weight of three blocks of about 80 kg is served by 25 mm thick laminated chipboard plates, on which two layers of linoleum are glued on both sides. In addition to the aesthetic function, these stands serve as a necessary insulator for the transmission of vibrations through the floor.

The scheme and design of the crossover

The filter for the low-pass heads is a second-order low-pass filter with a crossover frequency of 500 Hz, it is calculated according to the method described in [1]. The frequency response of the woofer was calculated according to the passport data using the Win ISD 0,50a7 program. Filter schemes for midrange and high-frequency heads are similar to those described in [3, 4]. Diagrams of crossover nodes are shown in fig. 4, a, b.

Fig. 4

The crossover of the MF and HF bands is mounted on printed circuit boards 130x127 mm in size (Fig. 5 and 6, respectively), made of foil fiberglass. Capacitors in filters - K73-16 for 63 V and MBGO for 160 V, resistors - C5-5V and C5-16MV. Mounting parts in contact (capacitors K73-16) are damped with silicone sealant. Both filter boards of these bands are installed in the midrange unit and fixed in plastic guides, which are glued to the body through two layers of soft linoleum to eliminate resonance phenomena.

Fig. 5

Fig. 6

The design parameters of the coils are similar to the recommendations given in [3, 4]. The filter parameters for the H143 head are taken from [3] when repeating the design. The frame coils are made of 3 mm thick plexiglass and glued with dichloroethane. Their parameters and structural dimensions are given in the table. It adopted the following designations: D - frame diameter; H and T - winding section dimensions; N is the number of turns; d - wire diameter. After winding, the wire is fixed with varnish, and the coil frames are glued to the board with sealant.

The low-pass filter coil contains 259 turns of PEV-2 wire with a diameter of 1,32 mm. The drawing of the coil is shown in fig. 7. It is made of 6mm plexiglass and glued with dichloroethane. Each layer of winding coils of all filters is varnished. The conclusions of each low-frequency head are brought to the niche of the low-frequency filter with a copper stranded wire. The coil is glued in a niche to the body of the LF block with Moment glue. All elements of the low-pass filter are connected by surface mounting with a flexible copper wire with a cross section of 4 mm² minimum length and are installed in the recess of the rear panel (see fig. 3).

Fig. 7

Design and assembly of the LF block housing

When developing the design and choosing materials for the housing and elements of vibration isolation and vibration absorption, recommendations from [5] were taken into account. When gluing chipboard parts, deformations of the workpieces caused by transportation and storage are taken into account. PVA glue was used to fasten the body parts. It must be applied to the sides that will later be glued to the concave side of the other, or vice versa. This avoids deformation of the speaker cabinets for many years of operation. A colorless silicone sealant was used as glue when assembling the case, which does not give a rigid mechanical connection between the structural parts. The thickness of the sealant layer during assembly is limited by L-shaped wire limiters with a diameter of 2 mm, set in increments of 200...300 mm. The assembly of the case is carried out from ready-made, pre-painted and varnished assembly parts. The thickness of the front panel of the bass unit case is 64 mm. This is done to reduce the transmission of vibrations from the speaker cabinet to the speaker cabinet.

The sound pressure level in a typical living room, generated by the radiation of the walls of a typical enclosure, is only about 12 dB lower than that of a cone, at least in the low-frequency part of the AF. At certain frequencies, where the housing walls have resonances, the amplitude of the sound emitted by the housing can even exceed the amplitude emitted by the diffuser [6].

In the design of this speaker, to eliminate these negative factors, methods of vibration isolation and vibration absorption are applied. The effect of vibration isolation is provided by the use of elastic shock absorbers (Fig. 8) when attaching the vibration exciter - LF head (from diffuser 1 to diffuser holder 2) to the front wall of the housing 8, and sometimes the front wall to the side walls 10. Both methods are used in this design. The head diffuser holder is fixed through a solid gasket 7 made of high-quality dense finely porous rubber 4 mm thick, and the screw heads 3 are fixed through metal washers 4 with local support vibration isolators 5, 6 made of the same rubber.

To reduce the transmission of vibration from the body of the dynamic head to the front panel through the fixing screws, they are additionally insulated with bushings 11 made of PTFE, the inner diameter of which is equal to the diameter of the fixing screw, and the outer one is equal to the diameter of the head mounting holes. If necessary, the size of these holes should be increased to match bushings with a wall thickness of at least 1,5 mm. Naturally, all these measures should not affect the tightness of the junction of the heads and the AU case. The front panel itself is attached to the side walls with a thick layer (final section 3x25 mm) of silicone sealant 9, which further reduces the transmitted vibration level to the side and rear walls of the case. The corners were not used when attaching this part.

Fig. 8

The effect of vibration absorption is ensured by the use of a linoleum-foam plastic sandwich 10 mm thick-linoleum, which is pasted over all the internal surfaces of the speaker cabinet, excluding the front panel. A layer of synthetic winterizer, stitched in four layers, performs the function of damping (the whole set is about 35 mm thick as a result). Linoleum must be used as thick as possible (3,5 ... 4,5 mm), high-quality, not reinforced and not insulated (homogeneous material). It is necessary to use foam with maximum density.

First, the linoleum was cut, taking into account the provision of a gap of 40 mm along the perimeter of the inner surfaces of each inner side of the body. Gluing was carried out layer by layer with Moment glue on the structure assembled without a front panel with tapping all sections with a rubber hammer after the glue had dried. After that, pieces of foam plastic 10 mm thick, 50x50 mm in size with a gap of 10 ... 15 mm between them were glued onto the first layer of linoleum. After drying, the second layer of linoleum was glued, and only then four layers of padding polyester, stitched through 20 ... 30 mm.

Any negligence at these stages of work can cause unexplained overtones and resonances. To achieve different characteristics of the side panels, one of them in each speaker has both layers of double-thickness linoleum.

Manufacture and assembly of cases of blocks MF and HF

The body parts of the midrange block are made of pre-veneered chipboard, they have the same angle of 22,5 degrees. and provide complete identity of the geometry, hence the ease of assembly. The midrange block is assembled (see Fig. 1) on PVA glue using two conductors, which are made in sizes similar to the dimensions of the jumper dividing the internal volume of the case into two parts; the only difference is that you need to make holders. The assembly is carried out on a horizontal surface, and the entire structure is fixed with a rubber band until completely dry. After drying, the ends are reinforced with furniture clips, and the longitudinal seams from the inside must be glued with strips of cotton fabric on PVA glue.

For the soundproofing of the midrange block, two layers of linoleum pasting of internal surfaces (excluding the front panel) and filling of the volume with fluffy cotton wool or padding polyester were used. The block is painted and varnished after assembly. The finished front panel is glued to the body with a sealant. The supports of the midrange block are carved from ebonite, and circles of cork wood are glued to their bases.

The body of the HF block (Fig. 9) is made on a lathe from four layers of plywood to avoid the effect of drying out. In this design, linden wood is used because of the ease of processing and weight. The fastening unit of the RF unit is shown in fig. 10, and in fig. 11 is part of this assembly, a cylindrical support, where 1 is a screw with a hole in which the spring is engaged, as shown in fig. 2. Fixing parts are made of stainless steel.

Fig. 9

Fig. 10

Fig. 11

The idea of ​​such fastening is simple and well proved in operation within six years. The selected direction of radiation of the RF head remains securely fixed due to the fact that a ring with a diameter of 60 mm made of thin (1 mm) rubber is glued to the lower surface of the RF unit. As a wire from the filter board to the head, a twisted pair of copper wire with a cross section of 0,75 mm was used.² in PTFE insulation.

The assembly of the MF and HF blocks is carried out in the following sequence. The assembled structure is attached to the body of the MF block with six screws, then the wire is passed from the filter board of the HF channel, after which, having installed the body of the HF block, a spring is tensioned from spring steel grades OVS, KhVG (wire diameter - 0,8 mm, spring diameter - 5 mm , winding length - 25 mm), fixing it to the hook. The length of the twisted pair should be enough to freely connect the RF head to it.

After that, screws fix the head to the body of the RF unit. Holes for wires in the MF and LF blocks are carefully sealed, followed by a check of the sealing quality. The MF block has two plexiglass covers, which simultaneously serve as holders for the output terminals of the MF and HF blocks.

About the speaker vibration test

The hull resonances of the LF block were checked by measuring the level of vibrations with the AGAT-M instrument [7], therefore, the parameter graphs reflect the dependence of frequency-linear displacements, which adequately show the resonance characteristics of the hull at the control points. The sensor was mounted using a simple non-destructive method. An adhesive tape (thin and high-quality) is glued to the necessary place of the speaker cabinet. After careful lapping and degreasing, the sensor body is glued to it with "superglue" and measurements are taken at the required frequencies. G3-112 was used as a low-frequency generator. First, the level of vibration transmission was measured with a standard woofer mounting design, then with the design described here. With a standard mount (with a rubber seal between the loudspeaker and the cabinet), the level of transmitted vibrations was on average an order of magnitude higher. These measurements also helped to identify and eliminate case resonance by installing a jumper, as recommended in [5].

Connecting and setting up speakers

At the moment, this speaker is operated with two UMZCH ROTEL RB-1070, included in the bi-amping scheme. The left channel of the first amplifier works with the LF block, the right one - with the MF and HF blocks, which are connected in parallel; hence the separate connection cables. The second UMZCH is connected in the same way.

Adjusting the sound according to subjective parameters in a particular room is carried out by choosing the location of the speakers (when speakers are placed in the corners of the room, a noticeable rise in low frequencies occurs), as well as by changing the geometry of the location and direction of the emitters and pairing their phase characteristics [2, 3]. After setting up (when listening to music of different styles in a particular room), it is recommended to fix the position of the midrange and treble units to the bass unit with double-sided adhesive tape.

After adjusting the tonal balance, the speakers showed good localization of the location of the instruments when playing jazz and orchestral music. Heavy and electronic music also sounded good, which was ensured by the introduction of structural features of the woofer cabinet and, as a result, the absence of resonant phenomena. The low-frequency components of the spectrum of musical signals sound at the "right" moment and do not leave resonant overtones, which indicates the effectiveness of the applied measures for vibration isolation and vibration absorption in the design of the bass unit. The stereo effect area is much wider than typical speakers, and, more importantly, brighter (reminiscent of the use of stereo expanders) both horizontally and vertically in the room. The foreground and background of the scene began to distinguish clearly, despite the small dimensions of the room. If, after tuning "by ear", we analyze the location of the midrange and high-frequency units relative to the low-frequency unit, then it is often possible to detect asymmetry in the acoustic characteristics of a particular room.

Literature

1. Aldoshina I. A., Voishvillo A. G. High-quality acoustic systems and emitters. - M.: Radio and communication, 1985.
2. Alekseev I. On the distortion of the frequency characteristics of small-sized acoustic systems and "deep bass". - Radiohobby, 2000, No. 5, p. 5-8.
3. Bat S. Three-way loudspeaker with W21EX001 head. - Radio, 2003, No. 8, p. 15-18.
4. Bat S. Amateur Loudspeakers (Loudspeaker V3). - M.: RadioSoft, magazine "Radio", 2002.
5. James K. Iverson. Theory of resonances of loudspeaker enclosures. - .
6. James Moir. Mechanical resonances in loudspeaker enclosures. MIEE.
7. AGAT-M. Dual channel vibration meter. - Manual.

Author: E. Shalagin

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