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Maybe my understanding is somewhat simplistic. But wouldn't there be more variance introduced in the CIEM process as opposed to a universal since ear impressions are now required by a competent audiologist and the tuner now has to tune a custom mold which still matches up to the tuner's reference model (whatever that may be, if at all) and/or reference frequency/phase response chart. Tuning multiple drivers with inductive/capacitive filters must be difficult for a custom IEM simply considering where the crossovers will now have to be placed in order to match the intended/original/universal IEM frequency/phase response. I think I just confused myself there....
For that matter how do they (the tuner(s)) determine if the CIEM is tuned correctly and sounds as it should to their ears if it is based on a unique mold specific to the user? Someone school me.
No doubt, there are other challenges in CIEM that's very different from universal IEMs. However, also keep in mind that CIEM is like a 20-ish year old industry now, these are problems that they've been addressing for years, and they're still continuing to improve and innovate on.
(As a side note, Sensaphonics posted this pic to Twitter, a CIEM they made for NASA... from waaaay back when before they made soft silicone IEMs: https://twitter.com/Sensaphonics/status/349903501343735810/photo/1)
Getting the right fit
I think the hardest variance to overcome is still the fit. Yes, you do need to go to a good audiologist and get a good impression made. I had been lucky enough that all the impressions I've got over the years (several UE and Sensaphonics) have all been done by audiologist that are closely in touch with the IEM industry, either directly trained by UE or Sensaphonics, or had long standing history for working with musicians.
There was one time when out of desperation of not being able to locate the audiologist I went to before (she was out of town working with various bands during touring season), that I went to a standard hearing aid audiologist and tried my best to explain to them what I needed, I basically knew right away that the impression didn't look right, and the fitting wasn't going to work. Instead of sending in the impression, I waited another month for the audiologist I trusted to be back in town.
So with custom IEM, if you don't get a good fit, you won't get a good seal, and you won't get good sound. This is a huge challenge that's hard to overcome. There are actually a few 3D scanner that's used by audiologists for hearing aids. I remember reading about one that uses an insert that gets filled with saline, so it can even expand and measure the flexibility of your ear's inner walls, to determine how tight they should make the hearing aids. However, these equipments are still quite a bit away from being usable for IEMs, because they only capture the inner ear, and seems like none have been developed to capture the outside ear as well.
Also, it's not as simple as just taking the impression and making a mold from it. The CIEM actually should be made a little larger than your impression as to get a snug fit. Usually this process of building up from the impression to a larger size is a manual process, done by people who are trained with years of experience. There's nothing easy about this process, it's more art than science.
Tuning the sound
I've had the opportunity to tour Ultimate Ear's facility where they make all their custom IEMs. So I can't attest that every single CIEM manufacturer put as much care, attention, and detail to their build process. However I've seen it with my own eyes that Ultimate Ears does, and JH Audio has to do something similar as well for their Freqphase tuning process.
Once the shells are created the tuning process is relatively straight forward. The drivers are designed with specific positioning and sound tube routing in mind. There are going to be slight variance due to the shape of the user's ear, however they only deviate by matters of millimeters. If there are enough discrepancy to cause serious driver arrangement issues, they would just not make it. A good example is Veronica Belmont's recent UE-11 Pro purchase, she actually wanted a UE-18 Pro, and UE told her that's just not possible because they can't fit six drivers into the shape of her ear.
They literally sit there, put the IEM up to a microphone pickup, and test the frequency response of each driver individually, and tune them until they're within 1% of the spec. I forgot how many engineers would have to re-test the previous engineer's work, whether it passed through two or three people... totally forgot. Either way, there's a redundant process here to make sure everything passes within spec.
"Reference plane"?
What every CIEM company will tell you is to make sure you have an impression that gets around the "second bend" of the ear canal. The reason is so that CIEM's can be build to insert deeper into your ear canal and reduce the variance caused by the acoustic chamber between the last bit of ear canal and the ear drums. Given that most of us probably don't insert any universal IEMs beyond the first bend in our ear, which do you think is actually a more stable reference plane to tune the sound for?
BTW, triple-flange tips do insert deeply into the ear and up to the second bend. The problem is a lot of people found triple-flange tips uncomfortable.. and kinda nasty when they pull out chunks of ear wax.. It wasn't a very marketable thing for IEM companies to require triple flange tips for optimal sound.
Sound isolation of CIEM vs. IEM (also build materials)
It's true that sponge/comply tips with universal IEM do provide really good isolation. However I think people are missing the fact that there are actually different construction material for CIEMs as well. What people usually forget, is that sound isolation isn't just about the seal, but it's also about the body material of the IEM and its property to absorb or transfer sound. Hence acrylic CIEMs aren't as good as good as sound isolation, because they actually transfer sound better than universal with sponge/comply tips. Here's a basic rundown of different CIEM constructions:
Acrylic - Most CIEM uses this because it's the easiest to build, you can easily tweak drivers because they're built in two half-shells which is easily sealed. By far the most popular method to build CIEM, and much easier if you want to fit a lot of drivers. It's the least isolating out of all the CIEMs.
Acrylic w/ heat-activated soft acrylic tips - Very, very few CIEM uses this construction now. I had one of the early UE that had this construction. The heat-activated soft acrylic tip gets really nasty over time, and it kinda warps in shape too after a while. I wouldn't recommend these type of construction at all. The sound isolation is the same as full acrylic, the slightly softer tip doesn't make any difference.
Soft silicone - Very complex to build, because it has to be all done in one piece, you can't just make two half and bond them together. Has the advantage of being the tightest fit (since it's soft and can flex), and probably the most vocalist friendly if you're a musician, because the material can move more as you open your jaw wide. The soft silicone material absorbs a TON of sound. It's by far the most isolating out of all IEM/CIEM.
Acrylic shell + soft silicone tip - tbh, I have no idea what these are like, I never had one a CIEM with this combination. I know some companies make them... makes me wonder how they bond the hard acrylic portion to the soft silicone.
So overall, isolation wise, I'd go:
Soft silicone CIEM > universal w/ sponge tip > acrylic CIEM > universal w/ silicone tip