I've often seen that running a sub at lower impedance will increase distortion. Is it possible to generalize at all in this question? For a given sub, you have to try it out, or is it possible to say that at a final load of 4 ohm, you are unlikely to percieve distortion? I am asking because, if there is a risk, it might affect both amp and sub choice due to monetary limitations.

Given unclipped output from the amp, I highly doubt you'd notice a difference between 1 and 4 ohms, for example, even in higher frequencies.

Lowering impedance lowers efficiency, but you won't notice any extra distortion.

Efficiency of the speaker or of the amp? I see no reason why it would lower the efficiency of a speaker... for example, the efficiency of a dual coil sub is the same with the coils in series and in parallel. With the amp I could see the efficiency dropping a little because the output impedance of the amp is a larger fraction of the total load impedance but that's pretty minor. Are there other effects I'm not thinking of?

Efficiency of the speaker or of the amp? I see no reason why it would lower the efficiency of a speaker... for example, the efficiency of a dual coil sub is the same with the coils in series and in parallel. With the amp I could see the efficiency dropping a little because the output impedance of the amp is a larger fraction of the total load impedance but that's pretty minor. Are there other effects I'm not thinking of?

LOL Yes lower the efficiency of the amp, it actually has a major impact on the amp.

I was under the impression that the output impedance of most car audio amps was very small. What kind of efficiency difference are we talking here (for say, 1 ohm vs 4 ohm)?

I was under the impression that the output impedance of most car audio amps was very small. What kind of efficiency difference are we talking here (for say, 1 ohm vs 4 ohm)?

Depends on the amp, most class D amps are between 80% and 90% efficient, at 4 ohms, and closer to 60% at 1 ohm. With class A/B efficiency is even worse.

Thanks... nice to see some numbers.

I take it from the majority of the answers, that there is little risk of hearing more distortion at 1 ohm then?

Very interesting that with the efficiency at varying ohm loads. Take for example the 1 ohm load where the class D amp. is only approx. 60% effective. Does that mean that if I want to run a sub rated at 1000RMS, I would need a class D amp rated at something like 1400RMS or whatever, to getting close to the subs 1000RMS rated output? That would have some impact on my amp selection later on.

#1 - The RMS rating of the sub is a thermal rating of the coil - that's it. In no way does it mean that you need to send it that much power, that's installation dependent...

#2 - No, the efficiency going down at lower impedance loads has to do with the amount of current being drawn to make that power - an efficiency of 60% means that 60% of the current drawn is converted into usable power, with 40% lost to heat.