Low-frequency speakers

For a loudspeaker to be a sufficient source of sound energy, correctly distributed in space, it is essential to separate the waves radiated by the front of the loudspeaker from those emitted by the rear. This is the role of the loudspeaker. While this allows satisfactory results to be obtained at mid-range frequencies, it poses major problems at very low frequencies.

If the enclosure is completely closed, the mass of air trapped inside reacts by compression on the movement of the driver, causing an increase in the apparent stiffness of the diaphragm. As a result, the speaker's low cutoff frequency rises, rendering it incapable of reproducing bass frequencies, let alone extreme low frequencies. The phenomenon is all the more critical as, at very low frequencies, the radiated power, proportional to the radiation resistance, becomes very low.

To compensate for this effect, we try to extend the speaker's bandwidth by coupling the driver's moving element to a mass which, combined with the speaker's stiffness, forms a resonator tuned to a frequency low enough to reproduce low-frequency sound and high enough not to create a hole in the coupling zone.

Several solutions exist, depending on the nature of the mass. It can be :

• that of an opening called a vent (the most common case);

• that of the second loudspeaker ;

• that of a column of air.

The vented loudspeaker is called "bass-reflex". It gives rise to a considerable number of more or less sophisticated variants, depending on the shape and position of the port. The fact that the vent sometimes radiates in phase, sometimes in anti-phase with the loudspeaker poses delicate problems of adjustment and optimization.

Loudspeaker systems using a second loudspeaker can give rise to various families of solutions, depending on whether the complementary loudspeaker is active or passive, and whether it is placed inside or outside the enclosure. In many cases, the active system is the subject of a symmetrical device designed to reduce the effects of excursion non-linearity.

The air column loudspeaker is derived from the labyrinth loudspeaker. The most common are the transmission line enclosure and the quarter-wave enclosure.

It's worth noting that many systems today are obtained by combining these different principles.