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Exactly. Removing the damper also reduces acoustic impedance and increases treble, but increases it in higher parts, more linearly and does not cut down bass.
The foam filter acts in the 3-7kHz area, while the damper acts in the 7-infty kHz area. Also the foam has a tendency to smoothen out peaks, the damper does not. The foam also increases apparent volume of the air by slowing the speed of sound - making for a virtually larger enclosure.
There's more bass presence with dual dampers because you've cut off the treble so much, you had to increase volume to compensate. Am I right?
Actually, I didn't have to increase the volume. Anyway here is my theory on this:
Ok, neither of these filters act directly on the bass region at all (Sub Bass: 20-60 Hz, Bass: 60-250 Hz) so this leads me to think that more dampening leads to more bass. Either you dampen in the White foam 3-7kHz region or the T4000-filter 7kHz - infinity kHz region. We don't know by how much dB's these filters dampen - this has to be measured, but they do dampen.
So what I wanted from the mod was a more analytical sound like in Etymotic ER4-S but with better bass presence. I thought this might be possible because dynamics are more capable of producing more bass than BA:s in general - surely this has to do with dynamics being able to push more air. So removing the white foam bumps the 3-7kHz region by a little, enough to make it noticeable (human hearing is more sensitive in this region). Sure you have to compensate by putting on an extra layer of T4000 otherwise you loose too much bass (again more dampening should provide more bass). And in reality how much can an extra layer dampen, I don't know because I don't know by how many dBs it is dampening. In general it is more difficult to hear higher frequencies (not only that but measuring instruments have also a harder time with measuring them - look at how frequency curves tend to behave after 10kHz). Higher frequencies provides sparkle and ‘air’ of a sound but could also lead to fatigue if overdone.
Going back to Etymotic ER4-S - this IEM is known as an analytical one. And it actually has its most presence in the 3-7kHz giving it this analytical sound character. So removing the white foam on the RE-400 tilts it more to the analytical side.
A note about linearity:
In an ideal world the perfect headphone would be linear. This is what I have believed anyway...
Sennheiser when designing the HD-650:
"...but engineer Axel Grell found that when the ‘phones measured flat, they sounded harsh. So he very carefully tuned the response to have notches at 5kHz and 16kHz."
Most ideal headphones/IEM's today tend to have linearity up to 2kHz, then after that it is more what you prefer: analytical, warm and so on. Everyone has their taste in what it should sound like. Frequency curves doesn't give the whole picture so don't judge everything by that. There are many more factors that make a difference.
So why did I try the mod - well as I said before it was kind of an accident but when you think about it - Hifiman put in 3 years of hard work and research into this driver and housing shell. They didn't do all that just to make a 99 USD IEM out of it - so why not try a little mods with it. Not only that but it is completely their own driver design also so they can finetune everything the way the want to do it. Also because of it being a completely new driver design it won't sound like the older generation or anything else - imo this is a step in the right direction. As with all Hifiman IEM's the sound will grow on you and you will appreciate it more with time (I actually found it a little boring at the beginning). Don't judge it by its price.
Again everybody has their preference so I'm not saying the mod is awesome or anything but I happen to like it.