Hi-fi loudspeaker impedance
Impedance… A mystery for many…
The official norm
The 8 or 4 Ohms commonly indicated in the technical characteristics of the acoustic loudspeakers are based on the N60065 norm, set in the 90s. It only considers the hi-fi loudspeaker impedance at full midrange at a 1000Hz frequency, it will then be said to be 8 Ohms. But contrary to resistance, impedance varies according to frequency.
If impedance measured at 1000Hz is 8 Ohms, the hi-fi loudspeaker will then be said to be 8 Ohms nominal impedance. A hi-fi loudspeaker bandwidth extends from 20 to 20 000Hz, its average impedance or even minimal seen by the amplifier can be very far from the said 8 Ohms, for example 3 or 4 Ohms. Thus the official 8 Ohms (like the 220V of our power supply that may vary from 218 to 240V) remains an official norm but not very indicative. That’s why Focal is one of the rare driver and loudspeaker manufacturer that clearly gives the minimal impedance for each of its products. For example, 8 Ohms nominal impedance for a 5 Ohms minimum.
What does impedance mean?
Even if it’s always a question of Ohm, impedance mustn’t be mixed up with pure resistance, which value remains fixed. As we said, impedance varies according to frequency, going from 3.5 Ohms to 25 Ohms for the same loudspeaker, even more sometimes depending on the technology (sealed box, bass-reflex, etc.).
We can consider that most of modern hi-fi loudspeakers (Focal included) have an average impedance of 6 Ohms. Most of Home Theatre multichannel amplifiers have RMS power often indicated for a 6 Ohm load.
Indicating the minimal impedance under which never a loudspeaker can go (the lowest point in the impedance curve), Focal enables specialized passionate and professionals to have a realistic idea of the impedance.
The modern transistor amplifiers support loads from 2 Ohms to about 10 Ohms. As a result, whether the impedance is 4/8 or even 16 Ohms and beyond, it will be supported. The only impact can be on the amplifier line out power that will more or less vary according to the real impedance. Power tends to increase for the lowest bass frequencies.
In conclusion, we recommend that you take the RMS power or efficient amplifier power as a reference into 8 Ohms, but not the one indicated into 4 or 6 Ohms. In difficult load conditions or at very high sound level, there’s still a margin in terms of available current.