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Small speaker. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers A closed-type speaker with a useful volume of each loudspeaker of 12 liters is presented to the readers' judgment. The choice of a closed acoustic design for the woofer is due to the fact that the speakers are designed to reproduce classical and traditional jazz music. These genres use acoustic musical instruments, and many music lovers believe that closed speakers are better than bass reflexes in conveying the character of such instruments in the low-frequency region of the audio range. In addition, small speakers with low frequency response unevenness and modest, but well-articulated bass, as the system develops, can be supplemented with high-quality bass links operating at frequencies below 100...150 Hz. Docking such links with closed speakers seems to be a much simpler task than with phase inverters. In addition, closed speakers are less sensitive to the spread of head parameters, so repeating them at home is much easier. The author would not like to give readers the impression that a closed case is always good, and a phase inverter is always bad. The above arguments in favor of a closed case are just an explanation of the choice he made within the framework of the task. Bass reflexes dominate the market for finished speakers, and many manufacturers prefer to use this acoustic design for woofers. The experience of measuring the parameters of dynamic heads and the study of manufacturers' catalogs allowed the author to conclude that VIFA products have the best price-quality ratio. For small-sized speakers, the woofer of this company M18WO-09-04 with a paper cone and the tweeter D27TG-45-06 with a silk dome were chosen. The main parameters of these heads are given in Table 1. Table 1
After calculating, using the Bass Box 5.1 program, a large number of variants of the closed acoustic design of the M18WO-09-04 woofer, which differ in the values of the total quality factor and usable volume, we chose a usable volume of the case of 12 liters. This choice was made in such a way that with an increase in the electrical quality factor (de-damping of the head) due to the inclusion in series with the voice coil of the ohmic resistance of the separation filter inductor, the connecting cable and the output resistance of the amplifier, the total quality factor of the low-frequency head in acoustic design did not exceed 0,8.
On fig. 1 shows drawings of a loudspeaker cabinet made of 10 mm thick plywood glued in two layers. You can also use furniture plywood 18 mm thick. The body has a jumper that increases the rigidity of the structure. To dampen the vibrations of the body walls, hydrostek-loizol was used, which is a glass cloth, on which bitumen is applied on both sides. With the help of kerosene, which dissolves bitumen well, hydrostekloizol is glued to the inner surfaces of the body. This should be done with great care to ensure good adhesion over the entire surface of the case. The volume of the case is filled with a synthetic winterizer of low density. The heads and the removable rear panel are attached to the walls with screws through polyethylene foam gaskets. The development of a crossover filter for two-way speakers requires finding an acceptable compromise between the unevenness of the frequency response, the directivity pattern and the possibility of overloading the high-frequency head with low-frequency components of the audio signal. The diagram of the separation filter used in the described AC is shown in Fig. 2.
In the circuit of the LF head BA1, a first-order link is used, the HF head BA2 is connected through a third-order filter, the crossover frequency is 1,8 kHz. In the area of joint radiation of the heads, with increasing frequency, a decrease in the frequency response of the low-frequency head is observed. The frequency response of the HF head remains flat. As a result, the total frequency response of the AC loudspeakers has almost symmetrical declines relative to the crossover frequency, despite the use of crossover filters of different orders. The choice of a relatively low crossover frequency contributes to the expansion of the speaker's radiation pattern, since the dome head emits a relatively wide spectrum of frequencies. The speaker parameters were measured with an LMS analyzer from Linear XN155392 with a measuring microphone N152953. The frequency response for sound pressure at frequencies above 400 Hz was taken in the tone burst mode, low-frequency measurements were carried out in the near field. Computer processing of the measurement results was carried out on the basis of the analyzer software.
The dependence of the loudspeaker impedance module on frequency is shown in fig. 3. The loudspeakers have some variation in the height of the resonant peaks of the low-frequency heads, which, when the speakers are powered from a source with zero resistance, corresponds to the values of the total quality factor of 0,527 and 0,536. The indicated values of the quality factor of the low-frequency heads in acoustic design allow the use of speakers in conjunction with tube amplifiers without feedback, which have a relatively high output impedance. The minimum value of the speaker impedance module is 4,1 ohms. On fig. 4 shows the frequency response of two loudspeakers in terms of sound pressure, taken with a microphone installed along the axis of the tweeter. In the frequency range of 60...20 Hz, the frequency response unevenness is +000 dB.
On fig. 5 shows the frequency response of one loudspeaker in terms of sound pressure, taken at different angles to its axis. The decrease in the frequency response in terms of sound pressure when the microphone deviates from the axis is smooth, which indicates the absence of side lobes in the speaker pattern.
Listening to the sound of the speakers was carried out when it worked together with tube and transistor amplifiers. The sound quality rating given by audiophiles turned out to be unexpectedly high. When listening to speakers operating in a very high quality sound path, only distortions were noticed when playing polyphonic vocal parts and musical programs, the spectrum of which is saturated with frequencies from 2 to 3 kHz. Increased distortion near the crossover frequency is a typical drawback of two-way speakers, the manifestation of which can be reduced at the cost of increasing the unevenness of the frequency response and worsening the transmission of the stereo panorama. The correct tonal balance, dynamic sound, good stereo imaging in front and depth, as well as the convincing sound of bass acoustic instruments with a fast attack and soft decay were noted as the advantages of the speakers. The author is grateful to V. Daluda and Yu. Makarov for advice on the production of AS and discussion of the results of the work. Dynamic heads and other components for the described speakers can be purchased from CLASS A Audio Components, tel. 126-11-13. Author: S. Bat, Moscow; Publication: N. Bolshakov, rf.atnn.ru
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