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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Acoustic system VERNA 100А-10. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers When building the model 100A-10, the task was to manufacture speakers with tonal balance and realistic, surround sound. The loudspeakers use SEAS and Peerless dynamic drivers. The design of the housings provides a low level of vibration at all frequencies of the operating range. The author allows the use of some heads of domestic production. Model 100A-10 is structurally made in a classic style - a rectangular case with heads located on the front panel. Low-frequency design - phase inverter, which allows you to increase the return at the lowest frequencies. The loudspeakers of the system have platforms on which they rest with steel cones built into the housing. The platforms are a laminated plywood and chipboard construction that minimizes the vibrations transmitted by the enclosures to the floor surface. In the upper plane of the platform, four steel disks are installed at the corners - for setting the speaker cabinet with their cones, and in the lower plane - four disks (also at the corners) made of dense felt. The usefulness of such a body suspension is confirmed by numerous experiments and analysis of the work of similar products from different manufacturers. The set of dynamic heads for 100A-10 is selected taking into account the tasks. Practical assistance was provided by Georgy Krylov, a specialist from the Arkada firm [1]. To reproduce the range of 30 ... 400 Hz, well-proven dynamic bass heads Peerless 850136 (Denmark) are installed. They have four-layer polypropylene cones, good transient response, 89 dB sensitivity and a low self-resonant frequency (about 28 Hz). Their operation in a given frequency band is ensured by placing these two heads in a housing with an internal volume of 60 dm3. In the selected design, the maximum displacement of the diffuser turned out to be ±4 mm, which made it possible to obtain fast and very deep bass - at a frequency of 30 Hz at a level of -4 dB. A pair of woofers easily copes with low frequencies when power is supplied up to 150 watts. H522 SEAS heads (Norway) are installed to reproduce the midrange band. They also have polypropylene cones (the mass of the moving system is only 6,5 g) and powerful magnets, a sensitivity of 91,5 dB/W/m and a maximum cone displacement of ±3 mm. "Bullet", installed in the center of the diffuser, equalizes the phase-frequency characteristic of the head in the band 1,9...4 kHz. To reduce the effect of the head's own resonance on the transient response in the midrange region, the windows of the diffuser holder are pasted over with one layer of synthetic felt, as was done in [2]. Such an acoustic impedance panel definitely outperforms an electrical fundamental resonant frequency rejection (such as a notch filter) as the head damping is not impaired. The H522's head has good detail even when playing high-level sounds. It is important that the H522 and 850136 heads form a matched pair. The midrange heads are paired with the low-frequency heads with a first-order filter (6 dB per octave), which significantly "stabilizes" the technical and subjective assessments. Good coordination of dynamic and frequency characteristics in the MF-HF region was achieved with the H1149 SEAS HF head. The tweeter has an impregnated fabric dome, a sensitivity of 92 dB/W/m and a muted camera on the back of the dome. It is distinguished by a smooth, without peaks and dips, a frequency response up to 21 kHz and a light, detailed sound. Turning on the head through a third-order crossover filter (attenuation of 18 dB per octave) at a sufficiently low (about 700 Hz) fundamental resonance frequency made it possible to select the operating frequency limit at a frequency of 2730 Hz. This frequency turned out to be the most optimal for pairing this head with mid-frequency emitters with a sufficient power reserve. All drivers are installed through rings made of natural medium-density felt 5 mm thick. It is clear that the felt here plays the role of a damper, reducing the vibration level of the front panel of the case. The double-layer cabinet (its own weight is about 54 kg) with damping perforated panels and stiffening ribs contributes to the reproduction of musical programs with amazing naturalness. Main Specifications
It is known that in order to reproduce deep bass, a case of high rigidity and resistance to vibrations, which mainly propagate from bass heads, is needed. The body of the model 100A-10 (the design drawing is shown in Fig. 1) is assembled from 10, 12 mm thick plywood and 16 mm chipboard. The front panel is made of three layers: plywood-chipboard-plywood (12+16+12 mm), the rest - two-layer: plywood-chipboard (12+16 mm). Blanks for panels are glued with PVA and held under pressure for four days.
First, the frame was assembled - side, top and bottom panels with a selected quarter at the attachment points. The structure is reinforced from the outside with self-tapping screws 70 mm long into the ends of the upper and lower panels. Screws are screwed into the side panels every 50 mm and fixed with epoxy glue filled with fine (screened) chipboard sawdust. Hats recessed 5...7 mm from the surface are covered with nitro putty flush with the panel plane. Then, based on specific dimensions, the front and rear walls of the case were made, as well as a box for midrange and tweeter heads. In addition, perforated panels and stiffeners were manufactured and installed inside on the front and rear walls. Perforated panels 5, 6 and 7 each have 20 evenly spaced holes with a diameter of 20 mm. Panel 8 has 24 holes with a diameter of 30 mm. All of them are made of plywood 20 mm thick. Stiffening ribs 4 - from the same plywood. The last step is to assemble the case. Here is such a fastening method: the adhesive joints of the panels are immediately fixed with self-tapping screws every 50 mm. Perforated panels 5, 6, 7 and 8 are also fastened with screws. The glued panel 3 serves to increase the rigidity of the top panel. In three corners of the body, beech wooden blocks are installed, which somewhat increase the panel connection area and distribute vibration loads at the panel connection points. From the ends and along the length, these bars are also fixed with screws. Plywood panels (20 mm thick) installed between the woofers and the MF-HF box serve to reduce vibrations of the woofers and reduce the interference of acoustic vibrations inside the cabinet. Panels 1 and 2 serve as an additional damper in the lower part of the case and as an element of external design. In these panels, pipes of a phase inverter are installed, which is tuned to a frequency of 33 Hz. On the rear panel at the bottom in hole 9 there is a port with gold-plated input clamps - "Input 1" and "Input 2". The front panel is upholstered in matte black genuine leather. In addition to aesthetic qualities, the skin surrounding the tweeter has a beneficial effect on reducing interference in sound radiation at frequencies above 2,5 kHz. The body is covered with 3 mm thick beech veneer with a carefully matched pattern texture. The veneer is tinted, covered with seven layers of semi-matt varnish and polished to a high gloss (photo in Fig. 2). Four steel cones 12 mm high and 10 mm in diameter are installed in the bottom panel of the case at the corners, which are part of a flange 70 mm in diameter and 6 mm thick.
In the upper plane of the stand-platform there are round steel inserts (Fig. 3), which serve as a support for the body cones. In turn, dampers made of natural hard felt with a diameter of 70 and a thickness of 8 mm are glued on the lower plane of the platform. The stand-platform is designed to minimize vibrations going to the floor, and thereby acoustically "suspend" the speaker cabinet (after numerous experiments, the author did not find the real advantages of massive stone slabs-stands). The stand is made of alternating layers of plywood and chipboard with a total thickness of 56mm. The design after applying PVA was kept under pressure for four days. The outer surfaces are coated and finished in the same way as the speaker cabinets. The internal surfaces of the body and perforated panels are filled with PVA glue, the total thickness of the dried layer is about 5 mm. Such a coating, firstly, eliminates the inevitable interaction of plywood with moisture present in the air and, as a result, the possible drying out of the panels. Secondly, and no less important, a 5 mm plastic layer serves as another vibration damping layer. The free surfaces of the woofer box are covered inside with natural felt of medium density 17 mm thick. Panel 8 is also pasted over with it, with the exception of holes and the space between stiffeners 4. Pasting is done in such a way that it forms a radius of about 3 cm in the corners. The inner surfaces of the MF-HF box are also pasted over with the same felt. It should be remembered that natural felt is the most effective sound absorber in the band 100...20000 Hz. Adhered to the panel under pressure, it has good vibration damping properties, which is especially valuable when choosing the thickness of the panels is limited. One of the reasons for the extremely rare use of felt in speakers is its high cost. Rolls of cotton batting 80 mm in diameter and 160 mm long are glued into the vertical corners between the rear and side panels, over the felt. Like perforated panels, bolsters also introduce acoustic losses at low frequencies. Together with the felt, the rollers significantly reduce the effect of reflected waves on the rear surfaces of the low-frequency cones. This acoustic damping significantly reduces the frequency response unevenness in the widest frequency range and extends the playback stability in the lowest bass region. Low frequencies become more articulate and deep. The most important point can be called the acoustic damping of the midrange emitter, the sound of which is mainly determined by the subjective (and objective measurements in a muffled chamber) assessment of the sound of the speaker as a whole. This important purpose is served by perforated spacer panels, dividing the total volume into three parts. The shape of the MF-HF box largely evens out the residual reflected resonances on the back surface of the MF head. The volumes behind the midrange and treble heads are filled with combed cotton wool weighing 70 g each. Along with the acoustic radiation panel, such damping made it possible to practically eliminate internal acoustic resonances, bringing together the frequency response of the heads in the box and in the air. In the loudspeaker crossover (the electrical circuit is shown in Fig. 4), first-order filters (attenuation of 6 dB per octave) are used for the midrange and low-frequency bands, which minimally affect the phase-frequency characteristics of the speakers and keep the tonal balance "seamless" on crossover frequency of 400 Hz. RC circuits of elements R1-R4, C1-C5 for bass drivers and R9-R12, C11-C13 for midrange drivers stabilize their input impedance.
For the H1149 head, a third-order crossover filter was assembled (attenuation 18 dB per octave), which protects well from overloads and optimally matches with two midrange heads. It should be noted here that the tweeter is located slightly away from the axis of installation of the midrange heads, which expands the zone of the stereo effect of the speakers. With this arrangement of heads, the third-order filter is the most optimal. The inductors are wound on plastic frames without cores with wire PEL-1 1,2 (L1 and L2) and PEL-1 0,8 (L3). Coil frames L1 and L2 have a height of 28 and a diameter of 32 mm and contain, respectively, 260 and 90 turns. Coil L3 is wound on a frame with a diameter of 12 and a height of 18 mm and contains 210 turns. The coils are wound with wire turn to turn with insulation between the layers. The layers are impregnated with polyurethane varnish. All elements of separation filters are matched in pairs and differ from the ratings by no more than 0,5%. The elements are mounted on plywood panels and without connecting wires. On one panel, a low-pass filter is assembled, on the other - midrange and high-pass filters. The solderings were made with silver-tin solder, the resistance of each soldering was controlled and did not exceed 0,01 Ohm. The filter elements are fixed with plastic-filled epoxy adhesive, the coils are fixed with M4 screws and also fixed with adhesive. To exclude the occurrence of oxide processes in the places of soldering, all mounting joints are filled with polypropylene varnish. The output of the L1 coil, left according to the scheme, is soldered directly to the input terminal of input 1. The connection of the heads with separation filters is made by pairs of stranded silver-plated wires with double insulation: inside - fine glass fiber fabric, outside - fluoroplastic. The diameter of each stranded group is 2,1 mm. The groups of resistors R5-R8 and R13-R16 are separated in the same way as the elements of the midrange and high-frequency filters to avoid mutual influence. The connections of the MF and HF filters are made with stranded silver-plated wires. Model 100A-10 also has "Input 2", when connected to which the signal passes through a capacitor with a capacity of 880 microfarads. It serves as a first order filter that cuts frequencies below 30 Hz. Connecting through "Input 2" protects the speakers from getting on the bass heads of signals of infra-low frequency and DC voltage. When listening to the same pieces of music with speakers connected to each of the inputs, there was no noticeable difference in bass reproduction. Full subjective listening was carried out with the inclusion of classical compositions: jazz, opera, rock and pop music, ethnic oriental compositions (rich in many percussion and plucked instruments). The general opinion of the listeners is a light, detailed and deep sound, not tiring even with prolonged listening at high power. Separately, the 100A-10 loudspeakers were tested as front speakers in a home theater system. In most cases, a subwoofer (under $2000) was not required. Acoustic design 100A-10 (in fact, the body) is suitable for installing some Russian dynamic heads: 35GDN-1 -8 - as low-frequency ones, 20GDS-4-8 - as mid-frequency ones. While maintaining the good characteristics of 35GDN-1-8 and 20GDS-4-8, it is still recommended to install H1025SEAS or H883SEAS as an RF radiator, which can operate at 2700 Hz. With such a set of heads, it will be necessary to increase the useful volume of the MF-HF box to 3,5 dm3 and increase the amount of cotton wool by 20 ... 30%. To correct the reproduction of the lowest frequencies, you should re-select the length of the phase inverter pipes. Changes will also be required in the block of separation filters. The main thing that should be preserved when replacing radiators is their location relative to each other (at the same crossover frequencies) and the rigidity of the case with a large attenuation decrement. This allows you to show what relatively cheap heads are capable of and whether the CD being listened to is of high quality. Literature
Author: A. Demyanov, Moscow
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