Two-way loudspeaker with bass reflex. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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The dynamic heads of the Danish company Peerless were previously known to me from the literature. In addition, the pages of the American magazine "SPEAKER BUILDER" published a description of several high-end loudspeakers using the heads of this company. Having seen the samples "live" at the exhibition "Russian Hi-End 99" (in the exposition of the firm "Arkada"), I decided to use the same ones in a new two-way loudspeaker. The most preferable for my purposes with the optimal quality/price ratio turned out to be dynamic bass-mid drivers with a diameter of 176 mm and tweeters with a diameter of 105 mm. The technical documentation indicates the full name of the dynamic heads, containing information about their purpose and design features. I consider it useful in this case to give an exhaustive decoding of the names of the heads (in brackets - the nomenclature number used in commercial documentation).

LF-MF head - 176 WR 33 102 SD AL 8 (850122):

• 176 - overall diameter in mm;
• WR - "bass" head with a rubber diffuser suspension;
• 33 - voice coil diameter in mm;
• 102 - magnet diameter in mm;
• SD - three-layer diffuser made of composite material;
• AL - the presence of a special device that limits distortion at large amplitudes of the diffuser oscillation;
• 8 - nominal resistance in Ohm.

HF head - 105 DT 26 72 SF FF 8 (812774):

• 105 - overall diameter in mm;
• DT - "dome" high-frequency head;
• 26 - voice coil diameter in mm;
• 72 - magnet diameter in mm;
• SF - impregnated fabric canopy;
• FF - the presence of magnetic fluid in the gap of the magnetic system;
• 8 - nominal resistance in Ohm.

The maximum long-term power of these heads is 100 and 70 W, respectively.

Loudspeaker design begins with the calculation of the acoustic design of the woofer-midrange dynamic head. For this, the author used the parameters measured on two head samples (see Table 1).

Table 1

Taking into account the active resistance of the filter coil and the lead wires (0,7 Ohm), the value of the quality factor Qts=0,44 was adopted for the calculation. According to the results of computer simulation, a phase inverter with a working volume of 18 liters and a tuning frequency of 42 Hz was chosen as an acoustic design. The specified tuning frequency is achieved using a tube 11,5 cm long and 5 cm inside diameter, glued from paper to a wall thickness of about 5 mm.

The housing drawing is shown in fig. 1. The body is made of 18mm thick furniture plywood. In order to reduce the vibration of the walls, the body is pasted over from the inside with hydroglass isolator. To increase the rigidity of the structure, there are two jumpers in the case, which divide the internal volume into three compartments. The top two are filled with the most fluffy piece of low-density synthetic winterizer (with an area of ​​1,4 m2 per body). The lower compartment, where the phase inverter pipe is located, is covered from the inside over the hydroglass isol with artificial fur with a pile length of 10 mm.

On fig. 2 shows the frequency response for sound pressure, taken on a sinusoidal signal in the near field and characterizing the operation of the phase inverter. The dashed and gray lines show the frequency response of the radiation of the head and the bass reflex tube, respectively, and the solid line shows the resulting frequency response of the loudspeaker. The dip in the frequency response of the head allows you to evaluate the area of ​​\u30b\u45bdecreasing the amplitude of the cone oscillations (and nonlinear distortions) due to the action of the phase inverter. A noticeable limitation of the diffuser stroke occurs in the frequency range from XNUMX to XNUMX Hz.

The frequency response of the loudspeaker in the low-frequency region is obtained as a result of the interaction of the radiation of the dynamic head and the phase inverter tube, taking into account phase relationships, and at each frequency is calculated as the vector sum of these radiations. It should be noted that the resulting frequency response cannot be obtained by summing the ordinates of the two lower curves. The frequency response of the loudspeaker at a frequency of 40 Hz relative to the frequency of 150 Hz is 6,5 dB, which seems quite acceptable for a loudspeaker with a useful volume of 18 liters.

The development of a crossover filter for the used dynamic heads is greatly facilitated, firstly, due to the even frequency response of the low-midrange head up to 5 kHz with a smooth decay at the edge of the range, and secondly, due to good damping of the high-frequency head resonance. The resonant frequency of the HF head is 1170 Hz, which allows you to select a crossover frequency of about 2500 Hz. On fig. 3 shows the electrical circuit of the first version of the crossover filter. The filter includes an R1C2 circuit for compensating the inductance of the voice coil of the LF-MF head, a divider R2R3 that aligns the heads with recoil, and two first-order links connected in series with the heads.

At first glance, the out-of-phase inclusion of heads, which is characteristic of second-order filters, is unusual. From the point of view of electrical ratios, in a crossover filter, such an inclusion of heads should lead to a dip in the frequency response near the crossover frequency. However, when developing a loudspeaker, it is much more important to take into account the effect on the resulting frequency response in terms of sound pressure of the radiation unevenness of the heads working together with the filter sections. On fig. 4 shows the frequency response of individual heads (thin lines) and the resulting loudspeaker frequency response taken on a sinusoidal signal. The frequency interval in which the resulting frequency response is formed by the radiation of two heads, we call the region of joint radiation.

For the case under consideration, the region of joint radiation is practically limited within 1 ... 3,6 kHz. The crossover frequency lies within the region of joint radiation, and its value can be conditionally taken at the point of intersection of the original frequency response, i.e., at a frequency of about 2,5 kHz. Near the crossover frequency, the slope of the frequency response is close to 12 dB per octave, which is typical for second-order filters. Rice. 4 clearly shows that a filter with first-order electrical links behaves acoustically, together with the heads, as a second-order filter. This explains the anti-phase inclusion of the heads.

On fig. 5 shows the electrical circuit of the second version of the filter, in which a third-order link is connected in series with the RF head.

Characteristics in fig. 6 are similar to those shown in fig. 4, but the frequency response was removed for the second filter option. It is designed to reduce the co-radiation area. From the experience of previous developments, I know that the reduction of this area in most cases improves the localization of sound sources in the stereo panorama. This turned out to be true in this case as well. In addition, the tonal balance has changed - some emphasis has appeared in the upper part of the mid-frequency spectrum. My assessment is, of course, subjective, so readers who dare to repeat this development have the opportunity to choose a filter option according to their own preferences.

It is best to characterize the sound quality of the developed loudspeaker in comparison with another - on the dynamic heads of the VIFA company ("Radio", 1999, No. 2). The most noticeable difference is in the reproduction of low-frequency sounds: with Peerless heads, the bass is much more powerful, but it is somewhat inferior to VIFA heads in terms of working out the details of the sound image. Otherwise, it's hard to give preference to one of the loudspeakers, since the sound of the VIFA set seems to me softer and more comfortable, and the sound with Peerless drivers is more open and dynamic. Of course, these differences allow you to choose one or another speaker in accordance with the musical preferences of the listeners.

Summing up the work done, it remains to give a short list of the main parameters of the developed loudspeaker.

Table 2

On fig. 7 shows the frequency response of a loudspeaker in one-third octave noise bands. The characteristic of the impedance module is also given there.

Author: S.Bat, Moscow

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