Impedance 101

Impedance is resistance across a path that includes capacitors and inductors.
A capacitor blocks slow/DC signals but lets fast/high-frequency ones through
An inductor does the opposite — it passes DC but chokes high-frequency signals
A circuit or amp output impedance and speaker's load impedance is an important relationship. When mismatched it can waste power or degrade the signal.
In an amp/speaker relationship, the ideal is to have the amp impedance much lower than the speaker impedance. This is called the damping factor.
A damping factor of 8 is good.
An amp doesn't just set the speaker in motion, it's a dynamically controlled relationship. If the amp impedance is too high, it loses that control, and the sound degrades as the speaker cone wobbles around restrained only by physical design.
Running a 4Ω speaker on an amp rated for 8Ω minimum draws double the current. The amp runs hot, distorts, and can outright fail — either tripping protection circuits or burning output transistors. Do this long enough and you're buying a new amp.
Going the other direction — a 16Ω speaker on an amp expecting 4Ω — won't blow anything up, but you're leaving most of your power on the table and potentially driving the amp into a region where its own distortion goes up significantly.
Multiple speakers compound this fast. Two 8Ω speakers wired in parallel present a 4Ω load. Get this wrong with a cheap amp and you'll see it thermally shut down mid-show, or worse.
Mic impedance mismatching is subtler but can be more insidious because it degrades signal before you've even amplified anything — you're corrupting the source.
The classic rule was impedance bridging: the mic preamp's input should be at least 5–10× higher impedance than the mic's output. This ensures maximum voltage transfer and doesn't load down the mic.
Dynamic mics (like an SM58) are low-impedance (~150–300Ω output) and fairly forgiving. Plug one into a high-impedance input (like a cheap guitar amp's instrument input) and you get high-frequency rolloff — the top end of the sound gets physically filtered out by the impedance mismatch acting like a passive low-pass filter.
Ribbon mics are where this becomes serious. Ribbons are extremely low impedance (sometimes under 30Ω) and the ribbon element itself is a literal strip of thin metal foil. Certain preamps — especially older ones or cheap ones — have poorly controlled input impedance that can interact badly with a ribbon. Some transformerless preamps can actually cause DC offset or reverse current conditions that physically stretch or tear the ribbon.
Phantom power (+48V) is the other major danger zone. Phantom is safe for condenser mics and most modern dynamic mics — it's balanced and equal on both signal pins. But:
Unbalanced dynamic mics wired with certain adapters can have phantom voltage appear asymmetrically across the voice coil, potentially burning the coil out
Vintage or cheap ribbon mics (without proper transformers) can have phantom voltage surge through the ribbon and vaporize it instantly
Improperly wired cables with phantom enabled can send voltage somewhere it shouldn't go — into a preamp output, into a DI box's unprotected input, etc.