Loudspeaker impedance is one of the most misrepresented specifications in audio. A speaker labelled as “8 ohm” does not present a flat, constant 8-ohm load to the amplifier across its entire operating range. What it presents is an impedance curve — a continuously varying load that peaks and dips across frequency, sometimes dramatically.
A typical dynamic loudspeaker will show an impedance peak near its bass resonance frequency, which can reach 30, 40, or even 50 ohms. It will then drop through the midrange — often to the nominal value — before rising again in the upper frequencies. Many speakers dip below their nominal rating at certain frequencies. A speaker nominally rated at 6 ohms may present a 3-ohm load in the bass region, which demands significantly more current from the amplifier.
This matters because amplifiers are not indifferent to load variation. A tube amplifier with an output transformer is particularly sensitive to impedance swings — the transformer is typically optimised for a specific impedance, and large deviations alter the frequency response and damping characteristics of the amplifier. Solid-state amplifiers are generally more tolerant of varying loads, but those with high output impedance will still produce audible frequency response changes when the speaker impedance varies significantly.
The minimum impedance point and the phase angle at that frequency together determine the true difficulty of the load. A speaker with a 3-ohm minimum combined with a highly capacitive or inductive phase angle at that same point is a genuinely demanding load, regardless of its nominal rating.
Before pairing a speaker with an amplifier, look beyond the single-number impedance specification. Where does the impedance dip? How low does it go? What is the phase behaviour at that point? These questions matter far more than whether the nominal rating says 4, 6, or 8 ohms.