The term noise-isolating is bandied about a lot when talking about earphones. Certainly, 'phones from the likes of Shure and Philips do a great job of blocking out external sound thanks to the great foam tips supplied therewith. To a lesser extent the silicone tips that are a feature of just about every canalphone bundle also offer some exclusion from invasive noises. And, of course, this isolation works both ways - enabling fellow passengers in a car, train or plane to remain undisturbed by your undoubtedly impeccable taste in music.
However, as great as these a-few-sizes-should-fit-all solutions are, they never offer quite the level of exclusion you might hope for. There's a simple reason for that: no two persons' ear canals are quite the same. Certainly, foam tips do a decent job of trying to fill these differing shaped ear canals by expanding to fill the available space, but it's still an imperfect solution.
Getting closer to perfection requires a simple extension of common sense: outfit your earphones with tips custom mounted to the exact shape of your own ears ensuring a flawless fit. That's exactly what the Etymotic Research hf2 Custom earphones offer.
The idea is that by using exactly moulded tips, significantly better isolation is provided. Because there is less external interference whatever player is driving the custom-tipped 'phones can be run at lower volume levels which means less distortion and a reduced chance of long term hearing damage.
As purchased - exclusively from the Apple store either online or at a retail location - the hf2 Customs are just a pair of normal hf2s - still £100 a pair - with a voucher in it which is redeemed with a company called Advanced Communication Solutions (ACS) which deals with the business of creating the custom moulds. At £90 adding these tips in effect doubles the price of the phones. Both Etymotic and ACS reckon that's worthwhile.