Wiring: series or parallel

Two or more drivers have to be connected to each other somehow, and the choice changes what the amplifier sees and how loud the result is.

Parallel puts the full drive voltage across every driver. Impedance falls by the number of drivers: four 8 ohm woofers in parallel are a 2 ohm load.

Series passes the same current through each in turn. Impedance rises by the number of drivers: those same four are 32 ohms.

What it does to the level

Baffle simulates at a fixed voltage, as measurements are made, so the effect shows up directly on the plot.

For N drivers at the same voltage, parallel gives you 20·log₁₀(N) more than a single driver: two are 6.02 dB louder, four are 12.04 dB. Series gives you nothing at all over one driver, because the voltage is divided among them as the impedance rises.

That is not a reason to always choose parallel. The output is coming out of the amplifier either way; parallel simply asks it for more current to deliver more power. What the two arrangements really let you do is place the load where your amplifier is happy.

Choosing

Start from the impedance you want to present, not from the level. Check the impedance curve afterwards, because the nominal figure is not the minimum: a nominal 4 ohm pair can dip to 3 ohms or below, and that dip is what the amplifier actually has to drive.

• Two drivers, 8 ohm each. Parallel gives a 4 ohm load, which most amplifiers are content with. Series gives 16 ohms, which is easy but quiet.
• Four drivers. Parallel is 2 ohms and many amplifiers will object. Two series pairs wired in parallel brings you back to 8 ohms, which is usually the sensible answer for four.
• Mismatched drivers. Do not. Series wiring in particular makes one driver's behavior depend on the other's impedance, and any difference between them turns into a response error.

One thing to get right in the build

Connect them in phase. A driver wired backwards moves against its neighbors and the bass largely cancels: the symptom is a thin, hollow sound with far less output than expected, and it is a wiring error rather than a design one.

The exception is an isobaric pair mounted cone to cone, which is deliberately wired out of phase so that the two cones move the same way.