What Is Speaker Impedance?

Speaker impedance is the opposition a loudspeaker presents to an alternating audio signal. It is measured in ohms (Ω).

Common nominal speaker impedances include:

• 2 Ω
• 4 Ω
• 6 Ω
• 8 Ω
• 16 Ω

Unlike an ordinary resistor, a loudspeaker does not have a constant impedance. Its impedance changes with frequency because of the voice coil inductance, mechanical resonance and other characteristics of the driver.

Speaker Impedance vs Resistance

Resistance is normally associated with DC circuits, while impedance describes opposition to an AC signal and can include both resistance and reactance.

Z = R + jX

A speaker's voice coil has resistance as well as inductive reactance. The mechanical system of the speaker also influences its impedance.

Single Speaker Impedance

For a single speaker, the nominal impedance is normally taken from the speaker manufacturer's specification.

Speakers in Series

When speakers are connected in series, their nominal impedances are added:

Ztotal = Z1 + Z2 + Z3 + ...

For example, two 8 Ω speakers in series produce:

Ztotal = 8 + 8

Ztotal = 16 Ω

Amplifier
   │
   │
 ┌─┴─┐
 │ 8Ω│
 └─┬─┘
   │
 ┌─┴─┐
 │ 8Ω│
 └─┬─┘
   │
  GND

Series Speaker Impedance Calculator

Speakers in Parallel

For speakers connected in parallel, the total impedance is calculated from:

1/Ztotal = 1/Z1 + 1/Z2 + 1/Z3 + ...

For two speakers:

Ztotal = (Z1 × Z2)
         ─────────
          Z1 + Z2

Two identical 8 Ω speakers in parallel produce:

Ztotal = (8 × 8) / (8 + 8)

Ztotal = 4 Ω

              ┌── 8 Ω ──┐
Amplifier ────┤         ├────
              └── 8 Ω ──┘

Parallel Speaker Impedance Calculator

Two-Speaker Parallel Calculator

Four Identical Speakers

For four identical speakers, the total impedance depends on how they are wired.

The series-parallel arrangement is often useful when four identical drivers need to present approximately the same nominal impedance as one driver.

Series-Parallel Speaker Wiring

Two pairs of speakers can first be connected in series and then the two pairs can be connected in parallel.

             ┌── 8Ω ── 8Ω ──┐
Amplifier ───┤                ├──
             └── 8Ω ── 8Ω ──┘

Each series branch is:

8 + 8 = 16 Ω

The two 16 Ω branches are then connected in parallel:

Ztotal = 16 || 16

Ztotal = 8 Ω

Series-Parallel Calculator

Amplifier and Speaker Impedance

The amplifier must be capable of operating safely with the impedance presented by the speaker system.

Connecting speakers in parallel decreases the total impedance and can increase the current demanded from the amplifier.

For example, connecting two 8 Ω speakers in parallel produces a nominal 4 Ω load.

If an amplifier is designed for a minimum 4 Ω load, connecting another 8 Ω speaker in parallel would reduce the nominal impedance to:

4 Ω || 8 Ω

= 2.67 Ω

This may be below the amplifier's rated minimum load impedance.

Speaker Power and Impedance

For a simplified resistive load, electrical power can be estimated from:

P = V² / R

or:

P = I²R

Therefore, for the same amplifier voltage, a lower impedance requires more current.

Amplifier Current

The approximate RMS current for a resistive load can be calculated using:

I = V / R

Speaker Impedance and Amplifier Load

The actual amplifier current capability and power rating must always be checked before connecting a low-impedance speaker system.

Speaker Impedance and Crossover Design

Speaker impedance is particularly important when designing passive crossovers.

For a simple first-order high-pass capacitor:

C = 1 / (2πfR)

For a simple first-order low-pass inductor:

L = R / (2πf)

Therefore, changing an 8 Ω driver to a 4 Ω driver changes the required crossover component values.

For accurate crossover design, the actual impedance curve of the driver should be used rather than only its nominal impedance.

Nominal vs Minimum Impedance

A speaker labelled "8 Ω" does not necessarily remain at 8 Ω across the entire audio spectrum.

The actual impedance may fall considerably below its nominal value at some frequencies.

This is why amplifier manufacturers often specify both nominal or rated speaker impedance and a minimum supported load.

Impedance of Multiple Speakers

When multiple speakers are connected together, the resulting nominal load can be calculated mathematically if the individual nominal impedances are known.

However, real speaker systems are more complicated because each driver's impedance varies with frequency.

Therefore, the calculated result should be regarded as a nominal approximation rather than an exact impedance at every frequency.

Common Speaker Impedance Values

Common Mistakes

• Assuming speaker impedance is constant at every frequency.
• Connecting too many speakers in parallel.
• Ignoring the amplifier's minimum load rating.
• Confusing impedance with DC resistance.
• Using nominal impedance as an exact value for crossover design.
• Assuming two speakers in parallel always produce a safe amplifier load.
• Ignoring the impedance of crossover components.
• Ignoring the effect of speaker wiring configuration.

Key Points

• Speaker impedance is measured in ohms.
• Real speaker impedance varies with frequency.
• Series impedances are added.
• Parallel impedances are calculated using reciprocal relationships.
• Parallel speakers reduce the total amplifier load impedance.
• Lower impedance generally requires more amplifier current.
• Speaker impedance affects passive crossover component values.
• Nominal impedance should not be treated as the exact impedance at every frequency.