Published loudspeaker impedance ratings rarely represent actual frequency response

Evidence

Impedance: 8 ohms. This is the kind of specification one sees for loudspeakers. The number may be different, but whatever it is, it is an approximation. For a few—very, very few—loudspeakers it is a good approximation, but for the vast majority it is a poor description of reality. Figure 10.15 shows a typical loudspeaker impedance. It varies substantially with frequency and it matches the rated impedance at four frequencies. The variations are normally of no concern unless they drop to very low values, and tradition requires that a number be specified. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1143

Categorization

• CANDY: A great fun fact: a speaker’s ‘8 ohm’ rating matches reality at only four frequencies — approachable and easy to retell.