*which is usually based on speakers, and also real instruments which work similarly, in the sense that the perception of bass response is enhanced by feeling the bass as a kick in the chest or a crawl on the skin. It's very different when the drivers are outside the ear canals or in them - you don't get this dispersion of bass, so a lot of people can be shown a headphone graph with a plateau of bass and yet from what they hear, "the bass is weak."
**I mean, to those who want more bass than those who accept the realities of headphone listening, isn't it still fair to say that they are simply seeking to hear what a bass drum sounds like in real life, even if it means going around the flattest possible measurement criteria?
I am not angry, since I see some of your point, but I am just agreeing not all of them.
Etymotic Research lacks bass, slightly, true. However, this is not mainly due to BA issue.
Etymotic Research intentionally aimed FLAT based on diffused field compensated graph with their own standard. And they did really, really good job.
http://rinchoi.blogspot.com/2012/03/etymotic-research-er-4b.html
The problem is that "FLAT IEM" based on diffused field lacks bass. Diffused field compensated means it is tuned based inside of a reverberation chamber, which is a room that maximizes reflection of sound.
We are not listening music in a reverberation chamber, and while speaker which sends a lot of echo/reverberation of bass toward us, IEM doesn't. This is reason why Diffused field compensated IEM lacks bass.
Nowadays, there is Harman HiFi compensated graph, which tries to cover this issue,
However, diffused field compensation was an industrial standard for a long time. And Etymotic Research ER4 series is very very old model. It is released 20+ years ago. And at that time, there was no harman Hifi compensation standard, and diffused field compensation was standard. So at that time, Etymotic Research did their best to make them "flat" based on industrial standard.
What is the point? The point is... yes, ER4 lacks bass, especially sub-bass region but not by too much. (At least better than most of earbuds in the market.) However, that's not because they are using single BA. It is not an issue of single BA issue. That's because Etymotic Research specifically aimed to the sound of FLAT IEM based on diffused field, which is lacking bass.
How can I know this? because Etymotic Research is constantly aiming this goal (at least before Harman HiFi compensation take the place. I do not know how they try to adapt their lineup for the new target) and they are arguably doing the best job for aiming that specific goal throughout whole market. In other words, if they intended to boost sub bass region, I believe they are likely able to do so. They are just not doing it intentionally.
And Phonak PFE 122, which has slightly more bass compared to ER4/arguably appropriate amount, has single BA, and having decent Ultra-high frequency response as well.
http://rinchoi.blogspot.com/2012/07/phonak-audeo-pfe-022.html
(022 with gray filter sounds same as 122)
Plus personally I doubt having good ultra-high frequency matters in cheap market. Sure, there is fit issue for Etymotic Research, but that is mainly for having more ultra-high (over 10khz). Even in expensive IEM market, IEM manufacturers tend to roll-off ultra high. For example, SE846 is one of them
http://www.innerfidelity.com/images/ShureSE846WhiteFilterSample2.pdf
http://en.goldenears.net/30016
I have no freaking idea why they are doing this for 6 BA IEM, and I personally hate to see manufactures are doing this. Not because it sounds terrble, but because making narrower frequency with 6BA IEM sounds awkward as hell. SE846 is not the only one. There are plenty of examples, especially Westone and Shure. Many of their IEM straightly drop after 10khz. I am assuming this is for making sound more relaxing for studio monitoring(but still hate to see them doing this)
However, many people still loves them.
Plus, I doubt not having ultra-high and ultra-low is an big issue for bundle IEM. Nowadays, starting by Apple, bundle earphone quality is getter better and better, but I still believe average bundle earphone quality is crappy as hell. And a lot of people around me do not know what is good sound. Yes, They says they care good sound, but many people around me say Dr. Dre. sounds great. What can I say? Does it sound they really care about frequency range? No. I do not think so. They still believe they care good sound, but they do not even understand what is good sound.
I mean 'good sound' is subjective matter. People can argue "heavily bass boosted sound" is very good sound, so maybe I have to put it in different way.
Do they really cares wide frequency range? Do they care ultra high? I highly, highly doubt so. I doubt they cares even ultra low or not. Especially it is difficult to distinguish difference in ultra low with IEM. As long as frequency range is not extremely narrow, I do not think they care.
Overall , your "likely narrower frequency range if cheap BA is made, and it is unlikely work" is very subjective.
1. in terms of how much narrow frequency range cheap BA will have.
(In which I disargree with your evidence by saying ultra low of ER4 series is intended result of manufacturer)
2. how much bundle earphone market is generous about narrow frequency range.
(in which I disagree with you by suggesting that there are a lot of high-price IEM without ultra-high, and many people love basshead IEM)
Not saying your suggestion is totally wrong, you might be right but it is just mixture of assumption and hypothesis. And there is not enough strong evidence in your argument.
I am assuming BA is naturally more difficult to make, so it is more expensive, and I do not know why(and not much people know why, and I want to know why), but this is MY assumption.
And with strong evidence, you can prove I am wrong, but I do not see that strong evidence right now.
I am assuming it is not mainly due to economic scaling, because the price difference is just too big. At least it seems too big for me.
most of dynamic drivers for earphone costs less than 5 bucks, while single BA cost 20+ bucks(ER4's one ED-9689 cost 20+ bucks. with mass order, little less than 20)
item such as semi-conductor may decrease its price by A LOT by increasing economic scaling.
But there are some items that does not work like that.
Can BA manufacturer decrease its price by A TON? Maybe or maybe not.
But for me, it sounds there gotta be some reasons.
Dynamic driver is easy to make. If you do not care quality at all, then you can make it with fountain drink plastic cap & cup, paper roll, strong magnet, and copper wire.
http://gigglehd.com/zbxe/12406673
Like this.
In fact, many headphone manufacturers produce their own dynamic drivers for their products.
On the other hand, most of BA is not manufactured by IEM manufacturer. There gotta be some reason for manufacturing difficulties other than crossover issue.
I know it is likely harder than ez make-dynamic driver but for 400% price even just for single BA driver, there should be better reasons.
I just do not see your assumption is right. I am not saying you are totally wrong, but it sounds just rough guess.