When we look at a loudspeaker's impedance spec, we see it's either 4Ω or 8Ω, right?
Steady Eddie. 4Ω or 8Ω.
Only that's not entirely true.
With few exceptions, those impedance settings are not consistent. In fact, all speakers vary their impedance with frequency. So, when you see a 4Ω rating, it's perhaps helpful to understand that is the average impedance.
Have a look at this rather messy example.
This "4Ω speaker" isn't a flat line, as you can see. And there are plenty of examples of both rising impedance (usually at resonance) and falling impedance (where the designer is sucking more power to get more bass).
In this speaker shown here you see the impedance rising a lot. It's because of multiple resonances. What are those?
Box resonance in a loudspeaker happens when the air inside the box and the speaker cone vibrate at a natural frequency, making the cone move more easily. At this point, the speaker generates a voltage that pushes back against the amplifier, reducing the flow of current. This resistance to current makes the speaker’s impedance rise at the resonant frequency.
So, while this is effortlessly handled by most well designed amplifiers, and there's nothing to lose sleep over, I thought it perhaps helpful to know that the speaker's impedance is anything but steady.
