|What sort of mod did you do on your ER4P?
I use pierced yellow hearing-protection foam tips which enable flush driver outlets; the filter tubes are filled with fine lambswool fibres instead of the original filter membrane, which makes for tunability of the sonic balance over a wide range (so don't trash used filters!). The thus exposed filters need to be protected by nylon gauze -- which on its part has to be changed every 20 hours or so. Also the foam plugs need replacement from time to time, as they become inflexible and stiff with use (as e.g. those in the picture). And finally the wool-fibre filter filling may call for replacement after a certain period of time (300-500 hours?)
This modification has even made the P-to-X adapter superfluous, as well as any passive or active equalization: the frequency response seems completely flat to my ears -- it lacks the upper-midrange hump of the stock Etys, and the lower-treble resonance is reduced to insignificance. Only in comparison to circumaural headphones do I notice a certain metallic treble coloration, but without this it's just very close to perfection -- at least for outdoor use.
The goal behind the modifications was:
1) to introduce a «time-smearing» of the filter: The original green filter membrane represents a sharp change of acoustic impedance within the sound canal in the time domain -- hence creates a disturbance perceivable as resonance (also implying reflections back to the driver membrane and from there back to the filter, etc.). The wool fibres create a softer and broader impedance change and thus have a less resonant and reflective effect, which leads to a smoother treble. The other advantage is that you can fine-tune the sonic balance by means of the fibre density.
2) to reduce the length of the narrow section within the sound canal caused by the standard tips, especially obvious with the silicon tips: Instead of the full diameter of the ear canal, there's only a fraction of it available, and this over a length of several millimeters. The silicon tips even make it narrower than it is in the filter tube. This design property also creates an unwanted disturbance for the sound propagation with again the consequence of a change of acoustic impedance, hence reflections, transient- and frequency-response corruption. Whereas the ideal state would be full ear-canal diameter from the driver membrane to the ear drum.
A simpler mod which promises half the effect of my mods is to cut the foam tips to ~66% of their original length to get the filters almost flush with the foam surface (there should be a reserve of 1-2 mm, though). Be aware that the filters need more frequent exchange, since they're more exposed that way.
(That's how it started: link to an older post.)
Edited by JaZZ - 5/22/10 at 8:05am