If the acute or midrange frequencies require small car speakers, whose diaphragm moves little, the low and low frequencies pose very different constraints. To reproduce the first two octaves of the audio spectrum, it is necessary to use a loudspeaker of large diameter, “charged” in a large volume of air and equipped with a membrane capable of important excursions because to produce serious, you have to move a large volume of air. 

The lower the frequencies, the more the sound wave requires distance to develop (several meters in practice). Thus, few speakers are cut to seriously tackle the low end. Hence the usefulness of a box, or even when the place is missing or the time is late, a vibrator home theater.

The 20 cm speaker of the subwoofer is powered by an amplifier delivering 1200 watts of RMS power.

To move a large volume of air, there are two complementary solutions:

• a large diameter loudspeaker,
• A loudspeaker with a diaphragm with high clearance.

How to interpret the characteristics of a subwoofer?

Diameter of the speaker

In general, the more important it is and the better the ability of the subwoofer to go down in the audio spectrum. But this is not an absolute rule, some modest diameter speakers, but well designed and implemented, give better results than other larger ones. The usual diameters for subwoofers range from 16 to 30 cm for home use, and often from 38 to 46 cm for professional systems. In any case, it is advisable to choose a subwoofer with a loudspeaker larger than that of the main speakers, or at least equivalent.

Generally, a large diameter speaker allows the subwoofer to go lower in the bass.

Bandwidth

This is the essential element since it is to extend this bandwidth that we add a subwoofer. This is the range of frequencies reproduced by the subwoofer. It is expressed in Hertz, for example from 25 Hz to 120 Hz. But beware, it is essential that the sound pressure is linear in this range. In order to choose correctly, you must ensure that the bandwidth reproduced is in a +3 dB to -3 dB range. Beware of boxes whose characteristic is not clearly indicated. Because to restore bass at 25 Hz with 16 dB of attenuation is absolutely useless it would be imperceptible.

Sound pressure

A subwoofer produces slow, loose waves that are more felt by the listener’s body than heard. So we speak of sound pressure (SPL in English, for Sound Pressure Level), expressed in decibel (dB). For a Hi-Fi system, a box capable of producing 110 dB (or more) over its useful bandwidth is usually sufficient. On the other hand, used with 5 or 7 speakers, a subwoofer should be able to produce ideally 120 dB of sound pressure.