Two-way loudspeaker with bass reflex

[ad_1]

Created on 06/10/2008 07:24 PM.

Updated on 16.02.2020 15:59.

Author: S. Bat, Moscow.

The designs created by Sergei Davydovich Bat are invariably popular with the readers of the Radio magazine. The reason for this is their thorough development, which ensures high quality and, which is no less important, good repeatability. We offer his new work to the readers’ judgment.

Danish company dynamic heads Peerless were previously known to me from literature. In addition, the American magazine “Speaker Builder” published a description of several high-end loudspeakers using heads of this company. Having seen the samples “live” at the exhibition “Russian Hi-End 99” (in the exposition of the “Arcada” company), I decided to use the same in a new two-way loudspeaker.

The most preferable for my purposes with an optimal quality / price ratio were the dynamic woofer-midrange heads with a diameter of 176 mm and the HF heads 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. In this case, I consider it useful to provide an exhaustive decoding of the head names (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 composite diffuser;
  • AL – the presence of a special device that limits distortion at large amplitudes of the oscillation of the diffuser;
  • 8 – nominal resistance in ohms.

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 dome;
  • FF – presence of magnetic fluid in the gap of the magnetic system;
  • 8 – nominal resistance in ohms.

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

The design of a loudspeaker begins with the calculation of the acoustic design of the LF-MF dynamic head. For this, the author used the parameters measured on two sample heads (see table).

Head samples 1 2
Resonant frequency, Hz 50.4 49.2
Volume Veq equivalent acoustic flexibility, l 16.2 17.1
Total quality factor Qmf 0.414 0.402

Taking into account the active resistance of the filter coil and the supply wires (0.7 Ohm), the figure of merit Q „= 0.44 was taken for the calculation. Based on the results of computer modeling, a phase inverter with a working volume of 18 liters and a tuning frequency of 42 Hz was chosen as the acoustic design. The specified tuning frequency is achieved using a pipe 11.5 cm long and 5 cm internal diameter, glued from paper to a wall thickness of about 5 mm.

Rice. 1. Drawing of the case of a two-way loudspeaker with a bass reflex

A drawing of the body is shown in Fig. 1. The body is made of furniture plywood 18 mm thick. In order to reduce the vibration of the walls, the body is pasted over from the inside with hydroglass insulation. To increase the rigidity of the structure, the body has two jumpers that divide the internal volume into three compartments. The two upper ones are filled with the most fluffed piece of low-density padding polyester (1.4 m2 per body). The lower compartment, where the bass reflex tube is located, is covered with artificial fur with a pile length of 10 mm from the inside on top of the hydroglass insulation.

In fig. 2 shows the frequency response in terms of 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 from the head and pipe of the phase inverter, 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 estimate the area of ​​decrease in the amplitude of the oscillations of the diffuser (and nonlinear distortions) due to the action of the phase inverter. A noticeable limitation of the diffuser travel occurs in the frequency range from 30 to 45 Hz.

Rice. 2. AFC for sound pressure, recorded on a sinusoidal signal in the near field and characterizing the operation of the phase inverter

The frequency response of the loudspeaker in the low-frequency region is obtained as a result of the interaction of the radiation from the dynamic head and the phase-inverter tube, taking into account the phase relations, and at each frequency is calculated as the vector sum of these emissions.

It should be noted that the resulting frequency response cannot be obtained by summing the ordinates of the two lower curves.

The blockage of the frequency response of the loudspeaker at a frequency of 40 Hz relative to a frequency of 150 Hz is 6.5 dB, which seems to be 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 flat frequency response of the LF-MF head up to 5 kHz with a smooth roll-off at the edge of the range and, secondly, due to good damping of the HF head resonance.

The resonant frequency of the HF head is 1170 Hz, which allows you to select a crossover frequency of about 2500 Hz. In fig. 3 shows the electrical diagram of the first variant of the crossover filter. The filter includes a circuit R1C2 for compensating the inductance of the voice coil of the LF-MF head, a divider R2R3 that equalizes the heads in response, and two first-order links connected in series with the heads.

Rice. 3. Two-way speaker filter

At first glance, the antiphase switching of the heads, which is characteristic of second-order filters, is unusual. From the point of view of electrical ratios, in the crossover filter, such an inclusion of the 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 irregularity of the radiation of the heads working in conjunction with the filter links.

In fig. 4 shows the frequency response of individual heads (thin lines) and the resulting frequency response of the loudspeaker, taken on a sinusoidal signal. The frequency interval in which the resulting AFC is formed by the radiation of the two heads will be called the region of joint radiation.

Rice. 4. Frequency response of separate heads

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 conventionally taken at the point of intersection of the initial 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 acoustically, together with the heads, behaves as a second-order filter. This explains the antiphase switching on of the heads.

In 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 high-frequency head.

Rice. 5. Electrical diagram of the second version of the filter

The characteristics in Fig. 6 are similar to those shown in Fig. 4, but the frequency response is taken for the second filter option. It is designed with the aim of reducing the area …

[ad_2]