Right. So, I bought the ER3XR after people kept creaming their pants when talking about Etyimotics and their IEMs.
They keep getting referred as 'flat' and 'neutral'.
I'm not massively knowledgeable about how the inner ear works, but my understanding is that a flat frequency response does not sound flat to you inner ear, so actual sound reproduction needs to be done with a totally not flat response in order to sound flat.
Problem is, how do we know it's flat?
My ER3XR sounds very mid forward, but not artificial. However, I don't know if it actually sounds natural or it's just my ear perceiving this.
Can anyone shed some light into what an actual neutral/flat/pancake IEM sounds like?
Do I perceive the ER3 as mid forward just because all other headphones/IEM are have recessed mids?
There is no flat FR reference for IEM(or headphones for that matter) because variations between people are expected and found. It's that simple.
If those variations were tiny and mostly unnoticeable, we would settle on a definition of neutral and move on. But it's not the case in practice. Some variations can be very obvious between 2 listeners using the same IEM or same headphone.
Let's say your outer ear has a shape that boosts the 6kHz area a lot(more than the super average human ear does). When you insert the IEM, you bypass that boost you have learned to consider normal and neutral. So if the IEM only boosts the 6kHz area the way a perfectly average ear(or a dummy head) would, you will subjectively feel like the IEM has slightly recessed 6kHz.
Another of many similar examples, ear canal length: let's say the average dude has an ear canal that's around 2.5cm. If you happened to have a rather big head with a 3cm long canal, assuming everything else equal(which probably isn't the case), that alone could shift the main ear canal resonance down by more than 500Hz. While a tiny ear inserting that IEM as deep as anybody else, would most likely need the "3kHz bump" centered above 3Khz(maybe 3.5 or something) to feel the neutral midrange tuning that person is used to when listening to real sound sources around him/her throughout the day.
When you insert your IEM, you block quite an area of the ear canal. The remaining space might resonate at maybe 6 or 7Khz, while the natural open ear canal was resonating around 2.5 or 3kHz. A so called "neutral" IEM must have taken that into account when tuning the IEM. But if it did it for some ultra average ear, the values will be shifted compared to your own perceived flat. How much and how noticeable noticeable will depends on who's using the IEM.
And of course, depending on how you insert the IEM, your obstructed ear canal will not resonate at the frequency the "flat" IEM predicted it would. If you insert the IEM like anybody else, with your canal being longer in my example, the remaining space will also be bigger, and the resonance will exist at a lower frequency compared to average human ear predictions.
If you keep looking, you will keep finding reasons why different people hear differently with IEMs. So a universal concept of neutral can be either arbitrary for the sake of having a reference on a graph, which is something we do need to make sense of FR graphs. Even if it doesn't sound flat for most people(kind of what diffuse field compensation has been for a few decades). Or be some averaging of neutral for many listeners, meaning it will effectively sound neutral only for a portion of the population.
IMO we'll only be able to settle the matter of flat the day we get better tools for customized responses. Ideally they'd be based on some HRTF capture of each listener, but I'm open to more creative or subjective alternatives. Until then, a neutral IEM is a subjective concept and a nice EQ is your best friend to help find your own neutral.