Have you seen the size of an inductor used in a bass crossover?
These are active circuits, not passive. What you "figure" doesn't have any technical logic as far as I can see, but I could be wrong too.
For me that, and transducer characteristic, is all you've got, as I put in my post. The problem here will be consistently making them acceptably close to all the same and whether it has any merit, given how good single driver IEMs are.
Maximum SPL hasn't been a limit on any IEM I have heard of, including all the single driver ones. Is distortion likely to be less? I am not so sure, given the inevitable complexity.
Exactly, that is what I want to see, it would also, as ever be interesting to see how consistent they are from earpiece to earpiece if there are a lot of drivers, ducts, resonators and so forth. Can they make the all the same? is always a question with complex devices IME.
I can see why 3 ways is a gain in loudspeakers for SPL and distortion. Not sure even 3-ways is needed in IEM, never mind more, except for marketing, of course.
I would just like to see measurements. I am an engineer and so far nothing about multi-way IEMs looks convincing to me.
1. No, perhaps you could figure out what inductor properties would be necessary for a tiny driver and we could google for small ones that match?
2. Passive crossovers = a circuit board with pieces soldered onto it. Having a machine do reflow soldering with tiny componants doesn't make it active, unless they are using an amplification circuit in which case it would need a power source. Tiny components on a circuit board that I'm describing are still passive/subtractive. Here's an example: (Other clear custom IEMs have electrical componants in them not on a board like this that look like resistors and capacitors.)
3. I'm glad we agree driver and chamber/port/membrane differences would work to acoustically tune things. Maybe measurements will prove it wrong, but I live with the assumption that different drivers would be best utilized for different frequencies and would optimally operate with different bore sizes, back chambers, ports, side channels, etc. which would be part of my argument for multiple drivers even if there were no such thing as tiny crossover components.
4. Max SPL is something consumers seem to be demanding, and companies are aiming for. JBL, Genelec, Klipsch, and many others make speakers and IEMs that go mind numbingly loud. Doesn't benefit you or I, sure, but we know that distortion increases with volume, so it is a given that if you split the loudness between multiple devices that don't have to handle the SPL alone the distortion goes down. I've only seen measurements ever indicate increasing distortion with increasing SPL, and lower distortion with lower SPL, and adding a subwoofer for example, leading to less distortion in a two way system trying to play deeper bass at higher levels.
5. Tyll at InnerFidelity used to do measurements of multi driver IEMs and single driver IEMs, headphones, etc... Anything decent seems to match the left side to the right side very well, and I haven't noticed a difference with multi vs single driver units. The most famous variance I recall was with the Focal Utopia units not matching well, and perhaps some Campfire Audio stuff that is also measured on SBAF.
6. There are tons of measurements available here:
https://www.stereophile.com/content/innerfidelity-headphone-measurements
I don't think it will prove much besides "some multi driver IEMS and some single driver IEMs are bad, and some multi driver and some single driver IEMs are good." Just like with speakers. Some 3 ways sound better than some 2 ways, some 3 ways sound better than than 4 way systems, some 2 way systems sound better than 4 way systems, etc.
I'll concede that I don't think there is an inherent need for multiple drivers in an IEM, though I don't think that has anything to do with not being able to have them work together, miniaturizing crossover components, being limited to acoustic tuning.
Marketing wise companies will market whatever they have, whether it's and "XWB driver, xtra wideband phase coherent dynamic driver" in a Sennheiser IEM, or "eigh drivers splitting the lows, mids, and highs, time aligned with FreqPhase technology" by JH Audio. Both don't tell us about the measured performance of the unit, so both should be equally scoffed at.
Personally I like multiple drivers with separate signal paths because it opens the door for an active crossover, or a tweakable passive crossover: With JH Audio earphones for example you can boost or lower the bass driver output with an inline "pod" that is permanently attached. You can also get aftermarket stuff to separately adjust the bass, mids, and treble separately. Perhaps there is a way to do that with a single driver too by splitting the frequencies in a small crossover, having potentiometers or switches, and re-combining the signals.