# About the speakers with dual heads

A few years ago, Amateur radio operator A. Zhurenkov proposed to reduce the lower bound of loudspeaker range of reproduced frequencies to use dual head [1]. Unfortunately, Amateur radio practice this method of range expansion in the direction of low frequencies is not widely received. And this is due, most likely, the lack of available calculation methods loudspeakers with dual heads. The article is an attempt to fill the gap and give the hams some guidance on the calculation of the speakers with dual heads.

It is known that in the calculation of any loudspeaker normally comes from the parameters used in the head [2]. Double heads leads to a change in only one of these options - equivalent volume. So, when you double heads with equivalent volumes Vэ1 and Vэ2 their total equivalent volume Vэ= (Vэ1i +Vэ2)/4. The whole method further calculation speakers with dual heads is no different from the calculation of the speakers with a single head for the sealed box and the bass-reflex [2].

To accurately determine the equivalent volume of the head is recommended to use the measuring box. If a suitable measuring box to get failed, to determine the equivalent volume of the head (in litres) you can use the approximation formula: Vэ=0,875 *SG* Dэ4 where SG - flexibility of the vibrating system of the head, cm/g, measured according to the method. proposed in [3]; Dэ is the diameter of the diffuser without corrugation, see

Found value Vэ can be used in the calculation box loudspeaker, and after manufacturing to spend more accurate measurement.

A few words about the efficiency of the loudspeaker with dual heads. The dependence of its parameters is described by the expression [2]: efficiency=

,

where C is the speed of sound, K is a dimensionless quantity, a constant for this type of head and acoustic design. V - the value of volume of a box loudspeaker.

This formula shows that the cost reduction of the lower boundary frequency of the playback loudspeaker range is reducing its efficiency.

This, however, is more than compensated by the fact. when doubling heads decrease all kinds of distortions of the reproduced their signal. In addition to the reasons that have been pointed out in [1], this contributes to another important fact. The fact that the irregularity of the sound field inside the box loudspeaker leads to a strong non-uniformity of its frequency response. The uneven distribution of sound pressure inside the box may also cause deformation of the diffuser (especially lightweight and slim) head, which, in turn, contributes to the emergence of nonlinear and intermodulation distortion.

In the case of using dual heads all these unpleasant phenomena occur only on the inner head, external, due to the damping action of the prisoner between the heads of the air is dramatically reduced.

To eliminate the source of these distortions frequency range of vibrations applied to the inner head, depending on the size of the loudspeaker, it is recommended to limit to 100...300 Hz. Loosen the harmful effects of internal resonances of the box on the playback quality can and installation between the heads or on the rear side of the inner head panels acoustic resistance (PAS). In both cases, a PASS should be placed in the holes of diffusionally heads. It should also be borne in mind that a PASS reduces the quality factor of the head, and this can be very handy, because in some cases will allow you to use the amplifier WOOFER PIC without current.

It is known that the sound quality of a loudspeaker depends on the uniformity not only the frequency response, and phase response, Smoothing the phase response anywhere in electrical (by choosing the appropriate separation filters) and in the acoustic paths (in accordance with the recommendations given in [4]).

A certain alignment of the phases of the emitted heads sound vibrations can be achieved, for example, the location of the sound coil heads in the same plane, perpendicular to the acoustic axis of the loudspeaker. However, this measure is often insufficient, especially when using heads with significantly different masses of mobile systems and diffusers made of materials of different density. In the first case this is because the phase shifts introduced by the heads at medium and high frequencies, ceteris paribus, the greater. the larger the mass of the moving system and the second the fact that the phase shifts depend on the velocity of propagation of sound waves on the surface of the diffuser.

These circumstances are forced to push forward low-frequency head towards the mid-frequency and mid-frequency to high frequency. Necessary additional shift heads can be found experimentally by submitting to the input of the amplifier, which operates the loudspeaker, the voltage squared frequency of 0.7 fр (here fp is the frequency partition) and watching the transition of the signal taken from the measuring microphone mounted on the acoustic axis heads.

Given the above considerations, the dual head working in the low-frequency element, set, guided drawing. If you decide to use a dual head and mid-frequency link, then you should use diffusers to each other, as recommended in [1].

A practical example of the use of dual heads can serve as the author designed a two-way loudspeaker, made in the form of a phase inverter. In the low frequency level used dual head GD-2, and in srednetsepochecona - head DDG-42 (DDG-32). It works in conjunction with dual band amplifier rated output power low and high frequency channels 20 and 10 W, respectively. The separation filter (frequency 500 Hz) is similar to that given in [4¦. The output impedance of the low-frequency channel of the amplifier negative - 1.5 Ohms. Nominal range reproduced by the loudspeaker frequency range - 30... 18000 Hz frequency response - no more than 6 LW.

The body of the speaker (HH mm) made of chipboard with thickness of 20 mm. Front wall glued two sheets of chipboard, its thickness is 40 mm. same Is the thickness and cylindrical pads with a diameter of 300 mm of the same material attached to the outer side of the front panel. The hole in the plate with a diameter of 230 mm coincides with the opening in the front panel under low-frequency drivers.

One of them is fixed on the inner side of the front panel, the other on the outside of the lining. Head DDG-42 fortified with the outer side of the front panel above the low frequency node major axis vertically. On the inner side it is covered by a cap, the volume of which (about 2 liters) filled with cotton wool. To increase the rigidity of the box between the front and rear, and between the side walls of the metal struts. The inner walls of the box are covered with felt with a thickness of 20 mm.

Pipe a phase-hole (mounted on front panel) has an internal diameter of 80 and a length of 160 mm, including the thickness of the front wall.

The loudspeaker can be performed in the form of a closed box. In this case, smooth frequency response at low frequencies is obtained at zero output resistance band amplifier, and the lower bound is reproduced by the loudspeaker frequency range rises to 40 Hz. If the speaker to set the crossover networks with frequency section 400...500 Hz, then it can be used with almost any amplifier operating at a load of 4 Ohms.

Fidelity music programs loudspeaker in both versions is very high.

**Literature**

Author: Vladimir Zhbanov, Kovrov, Vladimir region; Publication: N. Bolshakov, rf.atnn.ru