Drivers: what adding more buys

Adding drivers is the one change in loudspeaker design that gives you something for nothing in the response, and costs you in every other way.

What stays the same

The shape of the curve. Two identical drivers in twice the box have the same f_s, the same Q_{ts}, the same alignment and the same F3 as one driver in one box. The curve does not move; it only goes up.

That is because every quantity that sets the shape scales together. N drivers move N times the mass on N suspensions and present N times the cone area, which leaves f_s and every Q exactly where they were.

What changes

V_{as} multiplies by N. Two drivers want twice the box for the same alignment, four want four times. This is the cost people forget: a second driver does not fit in the cabinet you already drew.

Output rises by 20·log₁₀(N) in parallel, so 6.02 dB for two and 12.04 dB for four, at the same drive voltage. Half of that is the extra cone area and half is the extra current the lower impedance draws.

Excursion falls for the same output. Two drivers each move half as far as one would to make the same sound pressure, which is the real reason big systems use several small drivers rather than one big one. Less travel means less distortion and more headroom.

Power handling multiplies by N, because there are N voice coils sharing the heat.

The impedance changes, and this is the part that bites. See wiring: the level you gain in parallel arrives as a load your amplifier has to drive.

Choosing a number

Pick the count from the output and the excursion you need, then pick the wiring from the impedance your amplifier wants, then check the box volume you have just committed to.

If the box is the constraint rather than the output, more drivers is the wrong lever: look at isobaric mounting, which halves the box instead of multiplying it, or accept a smaller alignment with one driver.