Subwoofer - low frequency loudspeaker. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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An acceptable compromise for reproducing low frequencies in a home environment is the placement of a modern high-quality head with a cone of a large effective area in a relatively small volume - about 65 liters. On fig. Figure 1 shows the frequency response of a bass loudspeaker (also called a subwoofer) with a bass reflex tuned to 23 Hz. The lower cut-off frequency at the level of -3 dB is close to 40 Hz, which allows you to get a very full-fledged sound reproduction.

A passive bass loudspeaker designed for a stereo system should, of course, be two-channel. The use of two identical dynamic heads will require doubling the volume of the box (while maintaining the specified boundary of effectively reproduced low frequencies). But given the practical identity of the low-frequency signals in both channels, you can go the other way. The SPH-300 TC "Monacor" head used in the subwoofer contains two identical coils, each of which is connected to its own channel through a separate filter.

For acoustic matching with a typical small speaker system, the upper operating frequency of the subwoofer should be around 100 Hz. But a simple calculation of a first- or second-order low-pass filter using well-known formulas in this case turns out to be unacceptable due to the complex nature of the load impedance. On fig. 2 shows the dependence of the LF loudspeaker impedance modulus on frequency (without filter). Both voice coils are connected in parallel here, so for individual coils, the resistance values ​​\u23b\u10bshould be doubled. The minimum is around 52 Hz; two peaks are still distinguished: at XNUMX Hz and at XNUMX Hz - the frequency of the main resonance of the head in the box, the latter making it especially difficult to match the head with the filter.

To smooth out impedance unevenness, RCL circuits are usually used, connected in parallel with the coil and tuned to the resonant frequency of the head. In this case, a more economical solution was chosen (Fig. 3): connecting resistors in parallel to each coil. While this does not completely smooth out the peaks, it reduces them to harmless "humps".

The calculation of the second-order filter in relation to such a load was carried out using the Calsod computer program. The role of the resistor R1, connected in series with a non-polar oxide capacitor, is to reduce the quality factor of the LC circuit. The resulting frequency response is shown in fig. 4.

On fig. 5 shows a drawing of the housing; external dimensions are indicated for chipboard with a thickness of 28 mm (you can use ordinary sheets with a thickness of 16 ... All connections are made with glue and screws, the vertical walls are reinforced with a cross-shaped strut. In the upper part of the box there is a layer of mineral wool - with a surface of approximately 20 m0,5; at the same time, free space must remain between the sound absorber and the opening of the phase inverter pipe. The phase inverter is a piece of PVC sewer pipe with an outer diameter of 2 and a length of 80 mm. The outlet, as well as the diffuser of the head, is turned downwards, for which the corresponding holes are sawn out in the bottom of the box. At the corners of the bottom there are legs 365 mm high, made of any elastic material.

The SPH-300 TC head from Monacor was chosen due to its relatively low cost - about 80 USD. Despite this, it has an excellent sound, but it also has a serious drawback: at increased power, strong overtones are noticeable, reminiscent of the coil touching the pole pieces. In fact, the defect is caused by insufficient rigidity of the dust cap and is completely eliminated by impregnating the latter with a suitable composition (you can use KT-30 or MSN-7 glue diluted in gasoline or toluene, followed by drying for a day).

Electrical connections must be made with the utmost care. High-quality thick wires, expensive high-current connectors pay for themselves completely. Capacitor C1 must be of high quality, with flat terminals designed for significant currents (for example, K73-26), but non-polar oxide capacitors can also be used. Special requirements apply to the L1 coil. In order to reduce active resistance, it must be wound with a thick wire on a frame with a diameter of 56 mm, which has a ferrite magnetic circuit (the number of turns depends on the properties of the magnetic circuit and therefore is not indicated). The filter elements are mounted by surface mounting on a separate board, which is desirable to be placed away from the head.

To obtain tonal balance, the main speakers should have a close sensitivity of about 88 dB/W/m. The speaker impedance is 8 ohms (per channel). The power supplied from the amplifier can reach 2x250 watts.

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