Minimum Impedance Dip

What the minimum impedance dip is

A loudspeaker's impedance is not one number. The nominal rating on a spec sheet is a summary of a curve that rises and falls with frequency because of the voice coil's inductance, the driver's mechanical resonance, the cabinet and the crossover. Audio Judgement describes a typical 4 ohm nominal driver whose impedance mostly sits around 4 ohms but can reach 50 ohms at a resonance peak or fall to 2.5 ohms in a dip. The minimum impedance dip is the lowest point on that curve inside the rated frequency range. It matters most because, as Dynaudio puts it, the load on the amplifier depends on the minimum impedance and not the average.

How the dip is defined and rated

Dynaudio quotes IEC 60268-5: the lowest value of the modulus of the impedance in the rated frequency range shall be no less than 80 percent of the rated impedance. For an 8 ohm speaker that sets a floor of 6.4 ohms (8 times 0.8), and for a 6 ohm rating a floor of 4.8 ohms. Dynaudio's example is a speaker whose measured curve bottoms at 5.01 ohms, which it therefore rates as a 6 ohm model. IEC's webstore lists IEC 60268-5:2003, the third edition published on May 15, 2003 and covering sound system loudspeakers, as withdrawn with a withdrawal date of April 17, 2026. I did not locate the replacement text, so the 80 percent rule is cited as Dynaudio states it. Dynaudio says many speaker makers have become wary of showing real impedance values, and Archimago's measurements below show labels well above the measured minimum.

Measured dips in real speakers

Archimago's impedance measurements show how far some speakers stray. A Tannoy Mercury mX2 labeled 8 ohms drops to about 4.2 ohms below 40 Hz and again near 1 kHz, which he calls more like a 5 ohm speaker. The Energy C-100B, published as 8 ohms nominal and 4 ohms minimum, measured a minimum of 4.8 ohms, about a 6 ohm speaker by the 80 percent rule. A Paradigm Reference Studio 80 v.3 dips to about 4.5 ohms, a 5.5 ohm speaker. The Paradigm Reference Signature S8 v.3, described by Paradigm as compatible with 8 ohms, falls to about 3.6 ohms from 80 to 120 Hz and again near 20 kHz. His advice: think 4 ohms when choosing an amplifier for it. He adds that almost every speaker he has measured dips below 6 ohms somewhere in the audible band, and that two 8 ohm speakers wired in parallel can easily present 2 ohms for a substantial part of the spectrum.

What the dip does to the amplifier

Current is voltage divided by impedance, so the amplifier's job gets harder as the curve drops. At a fixed 2.83 V, an 8 ohm load draws 0.35 A and 1 W; a 3.6 ohm dip draws 0.79 A and 2.2 W, more than double the current. At the full-power voltage of a 100 W, 8 ohm amplifier, 28.3 V, a 3.6 ohm dip asks for 7.9 A, the equivalent of 222 W if the amplifier can deliver it. A weak amplifier clips, hits current limiting or shuts down, and Dynaudio notes that the lower the impedance is, the more risk there is that the amplifier will shut down. Dynaudio also notes that amplifier makers often write conservative impedance limits on the back panel because of the worst-case thermal tests they run, which is why it advises setting any impedance switch to the high position: the low position only limits the amplifier's output. Because FTC power ratings are specified at 8 ohms, as covered under the FTC Amplifier Rule, they do not show how an amplifier behaves at the dip, which is why 4 ohm vs 8 ohm rated power is a separate question.

Where dips come from

Bass reflex enclosures show two impedance peaks with a valley between them, and the lowest point between the peaks marks the box tuning frequency, which Audio Judgement's examples place at 23 and 28 Hz for two test boxes. That valley is distinct from the dips caused by the crossover. Passive crossovers interact with driver impedance, and Rod Elliott of Elliott Sound Products says that with 12 dB per octave or 18 dB per octave crossovers, impedance compensation is mandatory if the finished system is to perform well. A Zobel network is the usual compensation for a woofer's rising voice-coil inductance. In his worked woofer example the DC resistance is 5.6 ohms, the impedance peak at 57 Hz resonance is 25 ohms and the voice-coil inductance is 901 microhenries. The measured examples above show where dips land: 80 to 120 Hz in one three-way, and below 40 Hz and around 1 kHz in a small two-way.

What to do about it

Choose the amplifier for the minimum, not the label. If a speaker has a published minimum impedance, use that; if it only says "8 ohm compatible," look for a measurement. See impedance for the basics and damping factor for how output impedance interacts with the curve. Check the receiver's manual for its minimum rated load before pairing it with a speaker that dips to 3.6 ohms, and use the receiver match tool to size the amplifier.