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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Dual speaker loudspeaker. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers It is desirable that the improvement in the sound of speakers is not achieved through the use of very expensive or scarce dynamic heads.
The search for a compromise between cost and quality led me to the idea of developing a double cabinet loudspeaker, the manufacturing process of which can be divided into two stages. I suggest that you first make mid/high boxes through which you can listen to music, and then supplement the system with bass links. The joint design of such a mid/high box and the bass part will save you the trouble of matching and reworking the mid/high boxes in the second stage. In addition, many audiophiles who have small speakers will benefit from the results of the development of bass units described in this article to improve their speaker systems.
Closed-type MF/HF boxes use Peerless type 850100 woofer/midrange drivers (about $65 per pair), two in each; and HF emitter 812774 (about 70 ye per pair).
The low criticality of the closed case to the spread of the parameters of the heads makes the reproduction of the loudspeaker good. The drawing of the body of the MF/HF box is shown in fig. 1: On the front panel, the tweeter is located between the woofers. This arrangement expands the loudspeaker's horizontal radiation pattern and improves the localization of sound sources. The case is made of MDF 16 mm thick, its panels are connected with screws and PVA glue, and hydroglass isol is used as an anti-vibration coating. The internal volume is filled with a low density synthetic winterizer. The crossover circuit of the mid/high box can be seen in Fig. 2: This section uses 3,2rd order filters to achieve a crossover frequency of XNUMX kHz. Such filters are then used to reduce the area of \uXNUMXb\uXNUMXbthe joint radiation of the heads and provide reliable protection of the HF head from overloading with medium frequencies.
The frequency response for the sound pressure of the mid/high-frequency box is shown in fig. 3. In the frequency range 85 - 20000 Hz, the unevenness is +2,5 dB. and the dependence of the impedance modulus of the MF/HF part on frequency is shown in fig. 4. Thus, the minimum impedance value falls at a frequency of 5 kHz and is 3.75 ohms, and the characteristic sensitivity of the midrange/high-frequency link is 88 dB/W/m.
You can quite successfully listen to music without bass links through mid/high boxes, but you will have to put up with their two main drawbacks: the lack of convincing bottoms and some increase in distortion when there is a powerful bass in the program. The first problem arises due to the rapid decay of the sound pressure frequency response at frequencies below 100 Hz. The second trouble is connected with the output at low frequencies of the amplitude of oscillations of the voice coil of the LF / MF heads beyond the linear section of the stroke, which causes the appearance of additional distortions not only at low, but also at medium frequencies. The disadvantages considered are inherent to one degree or another to all small loudspeakers that use woofer / midrange speakers with a diameter of 100 - 130 mm with a small linear section of the voice coil free play.
Table 1
To cope with these phenomena, it is necessary to unload the mid/high boxes from low frequencies, for the reproduction of which separate bass links should be used. They use dynamic heads 850148 from Peerless (185,8 ye per pair), working in a phase inverter. Table 1 shows the measured parameters used to calculate the acoustic design for two copies of the 850148 heads, as well as data from the manufacturer's catalog. The following notation is used in the table: Fs - resonant frequency in free space, ots - full goodness, You - volume equivalent to acoustic flexibility. The measured and reference values differ quite significantly, in other words, the phaeo inverters calculated according to the catalog parameters turn out to be unsuitable for the heads whose characteristics are indicated in the table. In this regard, I recommend undertaking the manufacture of phase inverters to those readers who have the opportunity to measure the parameters of the heads and calculate the phase inverters according to the results obtained. The following values were taken to calculate phase inverters: Fs = 30 Hz. Ots = 0.3. Vas = 100 l.
Qts increases compared to the measured values due to the effect on the total quality factor of the ohmic resistance of the crossover inductor, the output resistance of the amplifier and the cable resistance. As a result of the calculation, an acoustic design with a volume of 40 liters and a bass reflex tuning frequency of 39 Hz was chosen. Drawings of the body of the bass unit are shown in fig. 5: the case is made of MDF 16 mm thick with an anti-vibration coating of hydroglass isol; in the phase inverter, a pipe with an inner diameter of 70 mm and a length of 110 mm is used. In the case you will find four jumpers that connect the front, bottom and top panels with the side ones, one of the jumpers divides the case into two parts: the top is the dynamic head, and the bottom is the bass reflex pipe. The hole in this jumper is covered with padding polyester. The top of the case is filled with a low-density padding polyester, in the lower part the internal surfaces are covered with this material. With such a filling of the case, the quality factor of the phase inverter is close to 3.
On fig. 6 shows the frequency response for sound pressure, characterizing the operation of the phase inverter. There is a dip in the frequency response of the dynamic head radiation with a minimum at a frequency of 39 Hz, corresponding to the phase inverter setting. The frequency response of the radiation of a bell-shaped pipe with a smooth maximum is the vector difference of these two radiations and gives the resulting frequency response of the bass link. The considered frequency responses were taken with the crossover switched on, the diagram of which is shown in Fig. 7. The 850148 bass head is powered through a first order filter with a cutoff frequency of 200 Hz. MF / HF box is connected through capacitances C3, C4 and resistor R2. Changing the value of the resistor R2 allows you to correct the tonal balance if necessary. When considering the bass crossover circuit, one feature should be noted: for a mid/high box with a nominal impedance of 4 ohms, a capacitance of 200 uF is required for a crossover frequency of 132 Hz, while the total capacitance of C3 and C4 is 40 uF.
The fact is that starting from 200 Hz, the impedance module of the MF/HF box begins to increase, reaching a value of 100 ohms at a frequency of 17 Hz. In this regard, a capacitance of 40 microfarads is sufficient. Having passed a maximum at a frequency of 100 Hz, the impedance module decreases rapidly as the frequency decreases, providing effective unloading of the MF / HF box from low frequencies with a capacitance of 40 μF. On fig. 8 shows the sound pressure response of the bass and midrange/treble evens working together. The use of additional bass heads made it possible to expand the band of effectively reproducible frequencies down to 45 Hz and reduce distortion at medium frequencies, unloading the midrange / treble box from low ones.
Table 2
For readers who have difficulty in measuring the parameters of the heads and calculating the phase inverters, I suggest a variant of the closed-type bass unit using heads 850140 (139,4 USD per pair), having the same connecting dimensions as 850148. requires changes, only the hole for the phase inverter pipe is excluded, the bass crossover also remains in its original form. Closed-type woofers are significantly less critical to the spread of dynamic head parameters than a phase inverter, therefore, when it is manufactured without the use of measuring equipment, there are much more chances to get a good result. Calculations showed that the volume of 40 l turned out to be close to optimal for 850140 heads operating in a closed case. Table 2 shows the results of measurements of the parameters of two copies of 850140 and data from the manufacturer's catalog. In the case of the bass section, with the phase inverter hole closed, the 850140 heads had resonant frequencies of 48,7 and 49,6 Hz with full quality factors of 0,67 and 0,68, respectively. For a closed acoustic design, the indicated values of the total quality factor are close to the optimal value of 0.707, at which the decrease in the frequency response in terms of sound pressure at the resonance frequency is 3 dB. The bass links described above can be used with various midrange / treble boxes, choosing C3, C4 and R2 values \uXNUMXb\uXNUMXbbased on the results of listening. In conclusion, I wish success to readers who decide to reproduce the described construction. Publication: cxem.net
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