Calculating Speaker Impedance

by Tom Thackwray

Speaker impedance, measured in ohms (Ω), is the load or resistance that a speaker puts on an amplifier output. This impedance resists (pushes back on) the current outputted by the amplifier. A high impedance presents a greater resistance than a low one.

Amplifiers are normally rated at 4-16 ohms, and this is generally the recommended speaker impedance range per channel. Modern speakers are typically 4-8 ohms. The issue with impedance is when you start connecting more than one speaker to each amplifier channel. It is perfectly acceptable to connect more than one speaker per channel, but in this case it is very important to understand how this affects the total impedance, and ensure it doesn't drop below the minimum amplifier rating (usually 4 ohms).

So why is total impedance so important?... Running an amplifier with a very low total speaker impedance will allow more current to flow through the speakers from positive to negative. This high current flow forces the amplifier to generate more power, which in turn can result in overheating, thermal cut-out and ultimately damage to the amplifier electronics.

The easiest way to connect multiple speakers to an amplifier is to use a speaker switch, and this is the recommend option when more than two speakers are connected to each channel. However, if wiring multiple speakers manually, it's very important to understand how impedances are affected when adding more speakers.

Series or Parallel - Speakers can be wired together in two ways... in series or in parallel (or a combination of the two). The total impedance is very different in each case, with parallel wiring being the most dangerous as it actually reduces the total impedance as more speakers are added... something which is perhaps counterintuitive. Below are some examples.

Series Speaker Connections

Example - 3x 4 ohm speakers in series:

Rt = 4 + 4 + 4
Rt = 12 ohms

Parallel Speaker Connections

Example - 3x 8 ohm speakers in parallel:

1/Rt = 1/8 + 1/8 + 1/8
1/Rt = 0.125 + 0.125 + 0.125
1/Rt = 0.375
Rt = 1/0.375
Rt = 2.66 ohms

So in conclusion, the series example totalled 12 ohms per channel so this would be fine on a typical amplifier. However, the parallel example totalled 2.66 ohms so this is not okay and could easily overdrive and damage the amplifier.

While it is possible to hard wire speakers in a combination of series and parallel, this does get quite complicated. If in doubt always wire in series as high impedances are less likely to cause amplifier damage than low impedances. However our professional advice is to always use a speaker switch when connecting more than 2 speakers per channel, or when using multiple low impedance speakers (eg. 4 ohms). This will make the connections straightforward and prevent an incompatible impedance damaging your amplifier.