Basically, impedance is the amount of current that will flow through a speaker at a certain voltage. It is measured in Ohms (Ω). The actual impedance of a speaker is not constant across all frequencies. So, for convenience we use the term “nominal impedance”, which refers to the impedance that a speaker presents to an amplifier at a reference frequency.
A speaker typically has an impedance rating of 4Ω, 8Ω, or 16Ω. Generally, the lower the impedance of a speaker, the more power will be developed by the connected amplifier. For example, a 4Ω speaker will extract more power from your amplifier than an 8Ω speaker. If you connect a speaker with an impedance lower than the amplifier’s output rating, the amplifier can overheat and damage the power output section. It is important to learn how to connect multiple speaker cabinets safely without damaging the speakers or the amplifier in this way.
Here is a simple rule of impedance: When two speakers with the same impedance are wired in parallel, the total system impedance is cut in half, and when two speakers with the same impedance are wired in series, the total impedance is the sum of the speakers individual impedance.
Hartke HyDrive HD speaker cabinet input jacks are parallel connections. The formula to calculate the total impedance of a parallel speaker system is:
1/Rt = 1/R1 + 1/R2 + 1/R3 + … 1/Rn
(R is the rated impedance of a speaker cabinet)
If all speakers have the same impedance, the total impedance will be equal to the impedance of a single speaker divided by the total number of speakers. For example, if you have two 4Ω speakers connected in parallel, the total impedance is 4 divided by 2, or 2Ω. You should be careful when connecting speakers in parallel to an amplifier. The impedance can quickly fall below safe levels. This is especially true when connecting speakers in parallel to a bridged amplifier.
Typical Parallel Speaker Impedance Calculations:
16Ω + 16Ω = 8Ω
8Ω + 8Ω = 4Ω
4Ω + 4Ω = 2Ω
4Ω + 8Ω = 2.7Ω
8Ω + 16Ω + 16Ω = 4Ω