Working daily selling subwoofers and amplifiers at an electronics store, I find myself using these equations several times throuought the day. They are used to calculate the Ohm load, or resistance, when using different wiring patterns for speakers, whether they be series, or parallel
Some amplifiers are 4 ohm stable, meaning they can handle a resistance load of 4 ohms. Other amplifiers are 2 ohm stable. Class D amplifiers are 1 ohm stable.
This equation is used when wiring 2 subwoofers in series.
When R is Resistance, then:
R1 + R2 = Total Load
For example: 4 ohms + 4 ohms = 8 ohm total load.
When wiring speakers in parallel, use this equation.
(R1 x R2) / (R1 + R2) = Total Load
For example: (4 ohms x 4 ohms) / (4 ohms + 4 ohms) = 2 ohm total load.
Using this equation will help you to select the right subwoofers to connect to your amplifier, so that you can get the optimal sound. Most subs are made in both 4 and 8 ohms, so if you can't find the ones you need, ask your retailer. | 
