[SilverEars]

If we speak of bigger drivers and moving greater air is harder, it's generally true. You can't really go against physics.

iems can have impedance that dips as low as 4 ohm, but so can speakers. From this, we ask ourselves what are the differences then? It's the power(sensitivity) and therefore the amount of current.

 

[flyte3333]

Here we have an explanation:

"Weidemann's explanation for why battery-powered devices perform best with low-impedance headphones is based on this: Most portable players use from 3-volt to a maximum of 5-volt batteries, that means the signal voltage at the output is low, but they can still deliver high current with low-impedance headphones, and the sound will be fine. However, if you plug in a high-impedance headphone to a portable device the voltage and current will be low, so the headphone won't play loudly at all, and dynamics will be reduced."

https://www.cnet.com/news/headphone...d-to-know-about-low-vs-high-impedance-models/

Additional info:

"It's not an opposite logic, it's rather that speaker range between 4-8 ohms while headphone are more like 30-300. With speakers you'll get amperage problems while headphone will ask more voltage. "

and

"Both are extremes - Speakers have very low impendance to begin with. 8 ohms being fairly standard and can drop to even 1 ohm. Headphones have very high impedance 30 - 600 ohms. So at the extremes 4 ohms or less and above 300 ohms, you are asking a lot more from your amp (for different reasons).

I know with headphones the challenge with high impedance is getting sufficient volume out of the headphones. Since the higher the impedance, the less power (wattage) the amp produces. So if you want to use your headphones with your phone/portable device you want to be closer to 30 ohms. If you plan to use them with a dedicated headphone amp, then 600 ohms is your best bet. "

This speaks to me a little more - in addition to the obvious important sensitivity/efficiency spec you mentioned, for determining drivability.

 

[SilverEars]

Higher the impedance, more volume maybe needed to give it for loudness since high impedance means more voltage required. But ultimately, it's the sensitivity of the headphone determines how much power it needs and how loud it gets for the volume as well(they are inter-related, but case by case, just different scenarios).

If you look at Amp specs, it will tell you how much power the amp can deliver at various impedance of the transducer. And also at what wattage or current that the specific impedance that amp is limited before clipping. But, those power values(amp can put out at different impedance loads) are beyond what something like 600ohm or 300ohm Sennheiser/Beyer headphones require.

600 or 300 ohms are of use when you have significant output impedances like OTL amps, etc.. We have lots of low impedance source options, so it's not necessary. Maybe some built in sound cards had pretty high impedances and noise, and the high impedance probably helps in those situations for those didn't really look into those aspects and simply plugged the headphones into the sound card.

 

[castleofargh]

with speakers, lower impedance is harder to drive

with headphones, lower impedance is easier to drive

don't lose yourself over this. it's hard to clearly explain those rules because those aren't rules, only very poor rules of thumb.

a low impedance driver will let a lot of current go through, the amp needs to be able to provide that amount consistently or we'll have issues. all things considered equal(which they almost never are with various amps, various drivers with various sensitivities), very low impedance driver can be harder to properly drive because of increased needs in current.

on the other side of the spectrum, a high impedance driver will oppose the flow of current significantly in the circuit, making current needs trivial. all we need to know now is if it will go loud enough, which is a voltage matter. double the voltage and your music is 6dB louder. again, with all things equal and they have no reason to be, reaching the same loudness could require higher voltage into a very high impedance driver. a voltage gain that the amp may or may not have been designed to provide.

instead of making a rule for speakers and a rule for headphones that seem to contradict each others, it would probably be better to just say that extreme impedance can create extreme needs.