Loudspeaker rated impedance figures are largely invented marketing numbers

Evidence

Years ago, loudspeaker sensitivity was rated as the sound level at a distance of 1 m for an input of 1 watt. Power input is voltage2 / resistance. Because loudspeakers do not have the same impedance at all frequencies, a sensitivity rating would apply only at a single, or at most a few, frequencies. Figure 16.3 shows the impedance curve for a loudspeaker, specified by the manufacturer as an 8-ohm unit. It is 8 ohms at four frequencies, but looking at the curve it is generally hovering at a level slightly above 4 ohms, dropping to a minimum of 3 ohms. A more realistic rating would have been 5 ohms. But that is an “odd” number in the industry, and such numbers, however true, tend to be avoided. — , Toole, Floyd E - Sound reproduction-…, 1786

Categorization

• CANDY: A fun, tweetable fact: an ‘8-ohm’ speaker is actually only 8 ohms at four frequencies and mostly hovers near 4-5 ohms.
• XREF: connects to: Mass dominates impedance at high frequencies, stiffness at low frequencies — Eric J. Heller, Why You Hear What You Hear- An…, 87

Broader Connections

• Mass dominates impedance at high frequencies, stiffness at low frequencies #expands (0.75) — Eric J. Heller, Why You Hear What You Hear- An…, 87