Dr.-Ing. Peter Strassacker
Email: peter@lautsprechershop.de

Subwoofer Drive Systems

A good subwoofer driver consists of:
- a sturdy, rigid resonance free basket,
- a hard and rigid diaphragm with little resonance,
- a matching drive system (voice coil within the magnetic field).

But, what is a matching drive system?

A well matched drive system shouldn't be too strong or too weak. This is made clear by the following picture showing the Alcone AC 12 SW4 in a closed cabinet. The drive has been weakened by resistors. in geschlossener Box

You will notice that the flattest frequency response (same volume between 50 and 200 HZ) occurs when a dropping resistor of 6 to 8 Ohm is used.

The same flat frequency response can be achieved by using a weaker magnet.

Rules derived from this fact:

The following statements are general, they don't apply just to the AC12; this driver is, however, quite suitable to illustrate this aspect, since the AC12's drive system is quite strong and may, therefore, be weakened by using resistors or a weaker magnet.

1) If the drive system is strong (upper curve), then the frequency response at higher frequencies slopes gently but early. This example shows at 50 Hz a sound pressure level 8 dB below maximum.
2) Medium strong drive systems (brown curve with 8 Ohm) show a flat frequency response, sloping relatively late.
3) Weak drive systems (light green curve with 14 Ohm) lead to an exaggeration in the vicinity of the cabinet's resonance frequency. Therefore, these drivers show a boomy bass.

The verdict

A matching drive system is needed; in a closed cabinet it should be relatively weak (as pictured above), in a bass reflex cabinet it should be somewhat stronger.

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