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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Coaxial head in the center channel loudspeaker. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment The author proposes to use the advantages of a coaxial dynamic head in the center channel loudspeaker of a home theater speaker. Connoisseurs of good sound, who already have a high-quality stereo floorstanding speaker at their disposal, will certainly not be satisfied with the "one-box cinema" speakers, and the purchase of a full-fledged 5.1 acoustic set can create not only a financial problem, but also difficulties with its placement. It can be assumed that among the readers of the magazine there are few people who can allocate one room for listening to music, and another for a home theater (DC). Placing both sets in the same room will inevitably turn even a spacious living room into a kind of overstocked warehouse of audio and video equipment. Partially remove the severity of these problems can be a combination of systems, ie, the addition of an existing stereo speakers to a set of 5.1. The most difficult task in this case is the selection of the center channel loudspeaker. Unlike the rear speakers, which can be an inexpensive bookshelf speaker, and a subwoofer, the choice of individual center channel speakers is limited. The way out in such a situation may be to independently manufacture it. Unfortunately, there are very few publications on the independent manufacture of a loudspeaker for the center channel of a recreation center; one of them is the article [1]. In the design described there, dynamic heads from SEAS are used. The Norwegian company SEAS specializes in the production of medium and high class dynamic heads. Its extensive range is formed by combining varieties of cases, diffusers, suspensions and magnetic systems. Therefore, the heads of such manufacturers with formally different parameters often have a similar ("proprietary") sound. After reading this article, there was a desire to repeat the construction described in it. Before starting the manufacture of the center channel loudspeaker, the specifications of the heads were carefully studied, calculations were made, and as a result of the analysis, it was decided to make a completely different design, although using the products of the same SEAS. The following is a description of such an "alternative" center channel speaker. Perhaps the article will also be useful to those readers of the journal who decided to repeat AS A. Demyanov. In the analysis and development, the author was guided by the following set of requirements, which could be called the conditions of reasonable sufficiency for solving the problem: - sound corresponding to the level of Hi-Fi;
Refusal to manufacture a loudspeaker from article [1] in favor of an alternative option is due to the following reasons. The lower cut-off frequency given in the speaker specification (65 Hz, -3 dB) seems overly optimistic. The calculation gives a significantly higher cutoff frequency, and at the frequency of 65 Hz indicated by the author, the frequency response decay exceeds 6 dB. The following column parameters were used in the calculation: useful volume - 10 l, phase inverter setting - 63 Hz (internal diameter of the phase inverter port - 5,4 cm, length - 12 cm). The parameters of the H149 head were taken from the manufacturer's website [2]. In terms of quality / price, the described design does not seem to be optimal. The use of four identical LF-MF heads increases not only the maximum level of return at the lowest frequencies, but also the price - when buying from Russian SEAS dealers, a set of heads will cost more than 7000 rubles. The absence of magnetic shielding in this loudspeaker precludes its use in conjunction with a CRT TV. The loudspeaker has a high manufacturing complexity. The disadvantages also include a narrow radiation pattern in the horizontal plane.
On the last, perhaps, it is necessary to dwell in more detail. The design, in which the tweeter is located between the MF (or LF-MF) heads, has become the de facto standard for the center channel loudspeaker and is used in most of these products. As a result, acoustic parameters were sacrificed for design; with this design, the speaker radiation pattern can almost always be described as unsatisfactory. On fig. As an example, Figure 1 shows the frequency response of a typical Karat CM7DC center channel loudspeaker from the German company CANTON [3] at various angles of deviation of the measuring microphone from the loudspeaker axis. At angles of +/-30° (blue curve), instead of an almost flat initial characteristic taken on the radiation axis (red line), a jagged frequency response with periodic peaks and dips is obtained, starting from a frequency of 500 Hz. This is the result of the interference of sound waves emitted by two LF-MF heads. The column described in [1] is no exception. On its front panel, the heads are located in one line, which, perhaps, is justified from the standpoint of technical aesthetics, but leads to an excessive sharpening of the radiation pattern in the horizontal plane. With a deviation from the axis of only 22 °, the sound pressure from the two extreme heads already at a frequency of 1 kHz turns out to be antiphase. At a frequency of 2 kHz, the same thing happens at half the angle. Thus, the center channel loudspeaker with four midrange drivers provides the correct tonal balance only for the audience (listeners) sitting opposite.
Significantly better spatial uniformity of radiation is provided by loudspeakers, where only one head with a coaxial arrangement of midrange and high-frequency cones is used; they can also be expected to have good tonal balance in a real room. Center channel loudspeakers with a coaxial head are widely used in their cinema sets of the middle and highest price categories by English companies KEF and TANNOY - firms that are highly respected among audiophiles. It has long been known that the use of a coaxial head provides the best possible directivity characteristics at problematic frequencies - in the area of \u2b\u9bthe joint operation of the midrange and high-frequency links. On fig. Figure 3 shows the frequency response of the Q7C loudspeaker manufactured by KEF [9], taken under the same conditions as for the Karat CMXNUMXDC mentioned above. The advantage of speakers with QXNUMXC in terms of uniformity of the directivity is more than obvious. This was the main argument for the development and manufacture of a center channel loudspeaker based on a coaxial head. Unfortunately, it is not possible to purchase KEF and TANNOY heads in Russia. Of the dynamic heads available to the Russian radio amateur, as far as the author knows, only SEAS produces such products. Therefore, the P17RE / XTVF (H653-04) coaxial magnetically shielded head was chosen based on the assumption that it would be better combined in sound with the author's front speaker, where P17REX / P (H602) heads with an identical cone and suspension were used as mid-range . It should be noted that two full-fledged heads are structurally combined in the selected head, but this design has nothing to do with cheap "coaxial" head blocks widely used in car audio. Main technical parameters Rated / minimum resistance, Ohm 4 / 3,5
On fig. 3 shows the appearance of the loudspeaker. The body has a simple rectangular shape. The edges of the front panel are slightly rounded, but this, as, of course, everything else related to the exterior finish of the case, is a matter of taste.
The frequency response graph of the loudspeaker is shown in fig. 4. Since the measurements were taken at home, the influence of the room could not be avoided. Therefore, the characteristics obtained should not be considered absolutely accurate, but they are quite enough to evaluate the resulting result. The measurements were carried out using the frequency-sliding tone method using the free software RMAA 5.5 [4] and the ECM8000 microphone from BEHRINGER, installed at a distance of 60 cm from the front panel surface on the head axis, and then at an angle of 30° to it. The black line corresponds to 0° and the red line to 30°.
The results obtained clearly demonstrate the advantages of a coaxial head - the frequency response, taken at an angle of 30°, looks even more preferable, having less unevenness than the axial one. Of course, this does not apply to the highest (above 15 kHz) frequencies. On fig. 5 shows a plot of the loudspeaker impedance modulus. It can be seen from it that the resulting tuning frequency of the phase inverter is approximately 40 Hz.
On fig. 6 shows a crossover filter diagram. The band separation frequency is chosen equal to 4,5 kHz, which is due to the relatively high frequency of the natural resonance of the RF head (1,8 kHz). For the same reasons, a third-order high-pass filter was applied. To equalize the return in the working band of the head, a resistor R1 is introduced into the HPF circuit. Capacitor C1 compensates for the drop in head recoil at the highest frequencies. The low-pass filter, together with the natural decay of the return of the low-frequency head at frequencies above 5 kHz, makes it possible to match the frequency response of the heads in the region of the crossover frequency without significant unevenness.
The heads are connected electrically in opposite polarity. The crossover filter uses wire-wound resistors C5-16 with a power of 5 W. Capacitors - film K73-16. Choke L1 is frameless and contains 70 turns of PETV-2 wire with a diameter of 1,12 mm. The inner diameter of the coil is 31 mm. height - 20 mm. Choke L2 is wound on a frame with a diameter of 32 mm and a height of 38 mm (an empty coil for solder was used) and contains 110 turns of PETV-2 wire with a diameter of 1,32 mm. In cases of replacement, it is preferable to use capacitors of the K73, K78 series (for example, K73-17, K78-12 or others). Of course, audiophile components are also suitable for the filter: Solen capacitors and the like; chokes and cables made of oxygen-free copper. However, from the objective parameters of the loudspeaker, this will change only one - the price. On fig. 7 shows a drawing of a speaker cabinet. It is made of 18mm thick plywood, which can be replaced with chipboard or fiberboard (MDF). To increase the rigidity of the walls, as well as for the convenience of fastening the rear wall of the case, wooden blocks with a section of 35x35 mm were used. The effective internal volume of the case is about 12 liters.
The phase inverter pipe (type TR-45) and the terminal block are purchased. The hole for the block is not shown in the drawing of the housing. The bass-reflex port is placed on the rear wall of the case, so when wall-mounted, the acoustic design will turn into a closed one. The parameters of the pipe for self-production are as follows: length - 140 mm, inner diameter - 41 mm. With these parameters, the bass reflex tuning frequency is close to 40 Hz, and the calculated cutoff frequency at the level of -3 dB is 50 Hz. The inner surface of the box, except for the front and rear walls, is covered with a layer of polyethylene foam 5 mm thick (not shown in the drawing). The free internal volume is filled with padding polyester. The space around the phase inverter pipe and between the pipe and the head is left free. The filter is fixed on the rear wall of the housing. The proposed design has a low manufacturing complexity, but it can be further reduced if a closed box is chosen as the acoustic design. In this case, the lower cutoff frequency (at the level of -3 dB) will rise to 90 ... 100 Hz (the exact value depends on the amount and type of sound-absorbing filler in the case), which is still acceptable for a center channel loudspeaker. You may have a reasonable question: if the advantages of coaxial heads are so obvious, then why haven't they replaced conventional heads yet? The fact is that in addition to the obvious advantages, coaxial heads also have disadvantages. With a coaxial design, a larger cone acts as a horn for a smaller one. However, such a "horn" is far from optimal: after all, its shape is designed, first of all, to obtain the least uneven frequency response at medium frequencies. The problem of optimizing the shape of a large diffuser for the simultaneous solution of both problems has not been fully solved to date. If we compare the frequency response of the loudspeaker described in this article and the frequency response of the KEF Q9C loudspeaker shown in fig. 2, it is impossible not to notice their surprising similarity in the frequency range above 5 kHz. The frequencies at which the peaks and dips of the frequency response are observed are practically the same for both loudspeakers. This is the peculiarity of the behavior of coaxial heads of this design at high frequencies. However, in situations where high spatial homogeneity of the sound field is required, there is practically no alternative to coaxial drivers, and the center channel loudspeaker is just such a case. The human ear is very sensitive to tonal imbalance in the mid-range, but easily "forgives" uneven frequency response at high frequencies. The loudspeaker uses a modification of the head with the resistance of the voice coil of the low-frequency emitter equal to 4 ohms. On sale, it is found under the commercial name WP172SCOAX. Below are the main parameters of the bass section of this head. Main technical characteristics Main resonance frequency, Hz ...40
Despite the fact that the loudspeaker turned out to be quite bassy, when using it in conjunction with a home theater receiver, it is better to limit the spectrum of the supplied signal from below to a frequency of 80 Hz, recommended by the THX standard. This will prevent the occurrence of intermodulation distortion due to loudspeaker overload with low-frequency signals and increase the "transparency" of the sound at mid frequencies. And let the subwoofer do the bass reproduction better, into which the receiver will send the "cut off" low frequencies. Literature:
Author: D. Gorshenin, Moscow; Publication: radioradar.net
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