Speaker Connection In Series and Parallel
The first diagram shows two speakers connected in series. Assume that the positive lead from the power source connects to the left speaker's positive terminal, and that the lead connecting over to the speaker on the right also goes to that speaker's positive terminal. The right speaker's remaining terminal is negative and is connected to the negative terminal of the power amplifier. Because there is one path for current that passes through two impedances, the impedances add. Were these 8Ω speakers, the resulting total impedance would be 16Ω. As the higher impedance will reduce current flow for a given voltage, this represents a lighter load to the amplifier than the single speaker.
In the second example, the speakers are wired together in parallel; there are two paths for current to flow. If the speakers are the same impedance, then each will flow the same amount of current, for twice the total amount that the single speaker would otherwise flow (for a given voltage). Therefore, if each speaker is 8Ω, the overall circuit impedance is 4Ω. It should stand to reason that a single 4Ω speaker would flow the same amount of current.
Audio amplifiers are designed to drive a specific speaker load, usually 6Ω to 8Ω for most modern home audio receivers. The percent Total Harmonic Distortion is generally lower in an amplifier driving an 8Ω load, even if designed to drive a 4Ω speaker load. Most home audio speakers produced today are rated at 8Ω impedance.
As you can see in the wiring examples above, how multiple speakers are connected to an amplifier channel has a great effect on the performance of the amplifier. If not rated for a 4Ω load, an amplifier connected to such a load can over heat and shut down, or possibly even be damaged. Also, an amplifier will not provide its best sonic quality driving an impedance load heavier than it is rated for.
Note also that in a Dual Voice Coil speaker (DVC) you have the same possibility of wiring the two voice coils in series or parallel as well. The difference is that the driver has two voice coils to wire in the single driver, rather than two separate speakers. This provides flexibility in its use for your specific setup. But the results are the same with two 8Ω voice coils that add up to 16Ω in series, and in parallel they will result in a 4Ω load for the amp.