A simple subwoofer in a car speaker. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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 The article describes a simple design of a powerful subwoofer that can be used in car speakers. A specific feature of this design is the use of two identical heads "in a pair".

The task of obtaining high-quality sound in a car interior with a modest budget is considered difficult, but amateurs attempt to solve it with enviable constancy. The next attempt, from the point of view of the author, seems to be quite successful: good reviews have been received about the work of this AS. An important factor should be recognized as the ease of repetition of the design.

The articles [1,2, XNUMX] quite fully formulated and argued the principles for the location of sound sources in the car interior and gave practical recommendations for their installation. Based on these recommendations, we will present a diagram of our sound-reproducing installation.

Shown in the diagram in Fig. 1 amplifier A1 - a four-channel PA of a conventional car radio or CD receiver, made according to a bridge circuit. It is these amplifiers that make up the majority of the models currently produced. Dynamic heads BA1, BA2 and BA7, BA8 are parts of widespread two-way 10-12 cm car loudspeakers with a nominal impedance of 4 ohms. Dynamic heads BA1, BA7 act as a low-midrange link and are mounted in the front doors, directly into the casing or on the podiums. High-frequency heads BA2, BA8 separated from them are mounted on windshield pillars or on a panel. As a rule, sounding this part of the car interior is achieved easily.

To create the most complete sound picture, the so-called "Heffler circuit" has become widespread, when the difference component is extracted from the stereo signal and reproduced by separate rear speakers. The role of spatial "sub-sound" in the acoustic system is performed by two heads 25GDN-1-4 (10GD-34). Many authors recommend that the rear channel circuit be used with a low-pass filter, but I decided to get by with a high-pass filter, since the used heads are not capable of reproducing signals above 5 kHz. The location of these heads is possible on the rear shelf or in the lining of the sidewalls of the car. Thus, the interior of the car is sounded in the frequency range from about 100 to 20000 Hz.

Capacitors C1, C3, C4 are non-polar oxide. Capacitors C2, C5 - any film or metal-paper. In the absence of a non-polar capacitor with a capacity of 220 μF, it can be made, for example, from two anti-series connected polar ones with a capacity of 470 μF for a voltage of 16-25 V.

To reproduce low-frequency sounds (below 100 Hz), a special loudspeaker is used - a subwoofer, in fig. 1 are heads BA5, BA6. Since the sound signal at frequencies below 300 Hz is not localized in the car interior, the low-frequency sound source can be located in the trunk. Naturally, if the car is used as an everyday means of transportation, the subwoofer must be small-sized, not afraid of contact with the cargo placed in the trunk and creating sufficient sound pressure.

It is known that the equivalent volume of the majority of domestic widely available low-frequency drivers with their own resonant frequency below 60 Hz is in the range of 40 ... 80 liters. Obviously a lot for the trunk. There is a technique that allows you to halve the volume of the bass link box - the use of heads in "tandem" inclusion. So for dynamic heads 35GDN-1-8 (25GD-26-8) the equivalent volume is 40...45 l. By placing them on the same axis side by side ("opposite" diffusers), you can build a loudspeaker with a volume of 20 ... 22 liters. The price for this will be some decrease in the sensitivity of the speakers - by about 3 dB.

To compensate for this loss, we will use the experience of foreign subwoofer manufacturers. The well-known company Bose Corporation has patented a type of acoustic design of the low-frequency link, called bandpass ("bandpass"). It is known that speakers of this type have a high efficiency compared to other types of acoustic design.

Let's estimate various acoustic design schemes for our subwoofer. Using the JBL SpeakerShop program, we enter the known Thiel-Small parameters of the dynamic heads 35GDN-1-8 (25GD-26-8) and test various types of speakers.

The graphs generated by the program (Fig. 2) show the frequency response shapes of various types of loudspeaker with a sinusoidal signal of 2,83 V applied to it (to compare the characteristic sensitivity); Let's analyze these graphs. The frequency response of a loudspeaker with a phase inverter shown by the red line is quite satisfactory, but the volume of 39 liters recommended by the program is clearly too big. The blue line corresponds to the frequency response of a 20-liter phase inverter - not bad either. The blue line shows the frequency response of the loudspeaker in a bandpass design (sixth order). Its disadvantage is that the lower limit of the reproducible range is higher than that of a phase inverter. The purple line is the frequency response of the loudspeaker, designed as a closed box with a volume of 20 liters. Its characteristic sensitivity in the operating frequency range is clearly small, so we discard this option.

So, we have two interesting options - a 20-liter phase inverter or a strip one of the same volume. I also note that the interior of the car itself has changing acoustic characteristics (open or closed windows, the number of passengers in the cabin, the presence of luggage, etc.). All this changes the transfer function of the cabin, so the real frequency response of the sound reproduction system will be somewhat different. As a rule, this is expressed in the rise in the frequency response in the region of 40 ... 130 Hz by 2 ... .3 dB (for different types and sizes of car interiors). However, most listeners are fine with it.

What is the advantage of a bandpass speaker over a bass reflex? As can be seen from the graphs, it creates sound pressure in the frequency range of 50 ... 100 Hz by 3 dB more than a phase inverter. In fact, it acts as a narrow-band sound source with effective suppression of sound vibrations outside the operating frequency band, even without an external filter. Such a loudspeaker does not contain protruding parts of dynamic heads or other structural elements. Its main disadvantages are unimportant phase characteristics and a large group delay. But since most big-name speaker manufacturers put up with it, we choose this option too.

For the manufacture of such a subwoofer, the following parts and materials must be prepared: dynamic heads 35GDN-1-8, plywood 15 ... 20 mm thick, sanitary polyethylene pipe with an internal diameter of 44 mm, self-tapping screws 03,5x35 mm, window putty or silicone sealant , cotton.

We cut out and assemble the subwoofer case from plywood, using self-tapping screws for fastening. On the sketches in Fig. 3, for clarity of construction, the front walls are not shown, and when assembled, the subwoofer is a blank plywood case with two holes in which phase inverter pipes are pressed. Screws should be screwed into the panel from the ends, at a distance of about 5 cm from each other. Before fixing the last wall, do not forget to carefully coat the joints and cracks with putty, solder the wires to the heads and remove the wires from the box and fill the chambers with fluffed cotton wool. Remember also if you forgot to make a hole in the middle wall between the heads (Fig. 3). The wool should be positioned so that it does not block the ports of the phase inverters and does not touch the diffusers of the heads. To do this, you can use wire frames and gauze.

The exterior finish of the assembled loudspeaker is performed according to your taste and capabilities. There are a lot of options here: from painting with oil paint to fitting natural leather with inserts made of precious wood and ivory.

Such a loudspeaker is easy to place in the trunk. In the event that it is mounted in a body type "sedan" ("classic" VAZ), the subwoofer housing can be attached to the rear shelf from the inside of the trunk with pipes up, not forgetting to make holes in the shelf so that the phase inverters "breathe" freely into the passenger compartment. In station wagon or hatchback bodies, it is enough just to put the loudspeaker in the trunk - the flimsy trunk shelf does not create noticeable resistance to sound penetration into the passenger compartment.

In conclusion, it can be noted that the subwoofer is capable of creating sound pressure up to 120 ... 125 dB with an input power of up to 250 W, so it is possible to increase the power of the amplifier later.

Literature

1. Shikhatov A. Autosound: we install ourselves. - Radio, 2000, No. 1-7.
2. .

Author: D.Gorbunov, Ufa

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