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

Encyclopedia of radio electronics and electrical engineering / Speakers

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The article proposes a loudspeaker design with imported Tonsil and SEAS heads. Acoustic design - a closed box with a labyrinth filled with padding polyester.

For many decades now, the main question in speaker design has been how to get good bass reproduction without over-sized speakers and without using very expensive drivers? By good reproduction they mean a low cutoff frequency, a high return and the absence of "mumbling", i.e., a low quality factor of the main resonance. The article outlines a variant of solving this issue, although nothing fundamentally new has been invented, the accents are simply placed in a new way. The technical solutions used are described in the book by Aldoshina I. A. and Voishvillo A. G. "High-quality acoustic systems and emitters" (M.: Radio and communication, 1985).

The required low quality factor of the main resonance of the loudspeaker mentioned here immediately excludes such types of acoustic design as a phase inverter and other options for acoustic resonators. We will not consider such types of design as a horn and a transmission line due to their poor predictability, calculation complexity, and considerable size. There remains a simple solution - a closed box. It is generally accepted that the optimal quality factor of the main low-frequency resonance of a closed box is 0,707, although in some places opinions have been expressed that for high-quality sound reproduction, the quality factor should not exceed 0,5. Indeed, from the course of physics it is known that the transfer characteristics of the RLC link with a quality factor of less than 0,5 become aperiodic.

On fig. 1 shows theoretical acoustic pressure vs. frequency for a piston-driven driver in a closed box design.

Curve 1 of the frequency response corresponds to a quality factor equal to 0,5; curve 2 - 0,707; curve 3 - 1. The frequency response of the loudspeaker (curve 1) can be corrected in the amplifier using a simple first-order RC circuit, for example, as shown in fig. 2: curve 1 - loudspeaker characteristic at a quality factor of 0,5, curve 2 - frequency response of the corrective link in the amplifier, curve 3 - resulting frequency response by sound pressure.

It can be seen from the figure that in this way it is possible not only to straighten the frequency response, but also to lower the cutoff frequency. For example, compared to curve 2 in Fig. 1 cutoff frequency is reduced by almost 1/2 octave. The phase-frequency characteristics also shift to the left. On the contrary, characteristics 2 and 3 in Figs. 1 cannot be improved without the use of resonant circuits.

It is clear that the proposed path is not always acceptable, since these loudspeakers can only be used with a specific amplifier, but practice has already shown the undeniable advantages of this method in terms of reproducing bass register musical signals.

Typically, low-frequency heads designed for closed acoustic design have a high quality factor (Qts> 0,5). To obtain a quality factor of the main resonance of the loudspeaker not more than 0,5, such heads are hardly applicable; in this case, it is appropriate to use heads with Qts = 0,3 ... 0,37, which are usually designed for phase inverters. Acoustic design should also be not quite ordinary - it should well dampen both the main low-frequency resonance and higher-frequency ones.

The practical implementation of a "low-Q" closed box is shown in fig. 3. Partitions are installed inside the rectangular case so that the internal volume is a folded labyrinth channel with an exponentially decreasing cross section. In addition, baffles reduce the vibration of the walls. The entire internal volume is filled with a synthetic winterizer with a density of 10 ... 11 g / dm3. The correction depth in the amplifier should be no more than 6 ... 10 dB.

Now a little about crossover filters (crossovers) and the choice of the number of bands in the loudspeaker. Modern dynamic heads quite allow you to get by with two bands. LF-MF heads with composite diffusers based on fibers with a high modulus of elasticity have smooth frequency response, up to frequencies of 2.5...3 kHz. And good HF heads can work starting from a frequency of 1 ... 1,5 kHz. Therefore, a crossover frequency of 2 kHz is quite applicable. Sometimes there are loudspeakers with a crossover frequency between MF and HF even 1 kHz (for example, in AC "Chario Constellation Pegasus"). After all, the lower the crossover frequency, the better the radiation pattern in the vertical plane.

The design of crossover filters is described in great detail in modern literature, for example, in the book mentioned earlier. Only few people pay attention to the remark that the filter works correctly only when it is loaded with an active and frequency-independent load. Therefore, it is absolutely necessary to connect compensation circuits in parallel with the heads, the calculation of which can be found in the same literature. In the case of a two-way speaker with a crossover frequency of about 2 kHz, the tweeter needs a full compensation circuit, and the woofer needs a circuit that compensates only for the inductance of the coil at high frequencies.

As an example, in fig. Figure 4 shows a diagram of crossover filters with compensation circuits for a loudspeaker, the drawing of which is shown in Figure 3. It also shows the types and characteristics of the applied heads.

LF-MF head - Tonsil GTS-20/60 with a four-layer diffuser based on Nomex:

• DC resistance - 6,6 Ohm;
• sensitivity level at a voltage of 2,83 V - 91 dB / m;
• resonant frequency - 40 Hz;
• total quality factor - 0,33;
• electrical quality factor - 0,38;
• mechanical quality factor - 2,5;
• equivalent volume - 26 dm3;
• diffuser holder diameter - 207 mm;
• weight - 1,8 kg.

RF head - SEAS H831:

• recommended frequency range - 2000...25000 Hz;
• nominal impedance - 6 Ohm;
• sensitivity level at a voltage of 2,45 V - 92 dB / m;
• DC resistance - 4,80m;
• resonant frequency - 1050 Hz;
• diffuser diameter - 27 mm.

Despite the fact that third-order filters are applied, the heads are switched on in phase; taking into account the real frequency response of the heads, the frequency characteristics of the radiation in the bands correspond to the fourth-fifth order.

As a result, the following characteristics of the loudspeaker are obtained:

• electrical quality factor of the main low-frequency resonance, taking into account the output resistance of the amplifier 2,4 Ohm - 0,7 ... 0,74;
• mechanical quality factor - 1,3 ... 1,5;
• total quality factor - 0,45 ... 0,5;
• resonant frequency - 50...52 Hz;
• sensitivity level at a voltage of 2,83 V - 90 dB / m.

Taking into account the corrective circuit installed in the amplifier, the lower reproducible frequency is 32 ... 35 Hz (frequency response drop - 4,5 dB, phase shift - 90 °). In a room of 15 m2, where a speaker system of two such loudspeakers is installed, tone signals with frequencies starting from 25 Hz are well heard, and the sound in the bass register is not “overextended”. The frequency response of the impedance module of these loudspeakers is relatively stable, with the exception of the main low-frequency resonance, the resistance deviation from 8 ohms does not exceed ±10%.

Author: A.Ivanov, Ivanovo

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