Symptoms / consequences of pairing an amp with too low of a speaker impedance?

[2FlyingYorkies]

Professional audio reviews are fond of pointing out how low impedance speakers (or ones that have a low dip combined with a high phase angle) need a "robust" amplifier. I'm curious what the sonic outcome of a poor pairing is? Is it always audible, or only when pushing the volume to higher levels?

As a not terribly theoretical example, my Focal Sopra 3's were measured by Stereophile's JA at 2.75 ohms at 96 hz, despite being nominally called an 8 ohm speaker. Some Wilsons (for example) dip lower, and some electrostatics are known to be all but a short circuit. When you pair speakers like this with a nominal 4-ohm amp, what happens? My own personal amp, the ATI AT542NC, claims to be happy with just about any load, but it's a bridged Class D design, which means the amp sees half the speaker impedance - in my case only ~1.4 ohms. I haven't heard anything I would describe as bad behavior from the speakers, so maybe the amp's power supply is robust enough to handle things after all, or maybe I just don't listen loud enough?

What's the science behind this, and what are the symptoms of a poor pairing?

 

[NekoAudio]

These articles may help:

A New Way to Plot Speaker Impedance: the Smith Chart

If the speaker impedance gets too low, the amplifier might not be able to deliver enough current, and will clip. If the speaker impedance is very high, the amplifier might not be able to provide enough voltage gain to play loud enough.

Understanding Impedance Curves & Phase Angles

The phase angle determines how much the current will lead or lag the voltage waveform in a reactive circuit. In an inductive circuit, the current will lag behind the voltage, and you’ll get a positive phase angle. In a capacitive circuit, the current will lead the voltage, leading to a negative phase angle. The phase angle will resultantly determine how much apparent power the speaker will see.

This is also why if you look at the Stereophile frequency response measurements for amplifiers, there is a "simulated speaker load" that shows what the frequency response might be due to a speaker having varying impedance and phase, instead of being a resistive load.

 

[2FlyingYorkies]

So basically it's a situation that manifests as clipping? Ok, that's easier to understand than I was thinking it would be. The articles you linked help too (I knew those engineering degrees would come in handy one day!).

 

[NekoAudio]

So basically it's a situation that manifests as clipping?

Yeah. If the amplifier runs out of juice, so to speak, it is going to distort (output will be a nonlinear function of input). In the worst case, something could get damaged, which is why an amplifier designer may (should) put in protection circuitry.

 

[atmasphere]

Any amplifier driving a lower impedance tends to have higher distortion. You can see this in the specs, and you you can hear it as the increased distortion tends to be higher ordered harmonics, to which the ear is keenly sensitive (as it uses them to gauge sound pressure). Since the ear also interprets distortion as a tonality, the amp will also sound harsher and brighter.