Compared to their regular earbuds, the experience of listening to music using Etymotic's custom-molded earths is something like the difference between sitting in an amazing sports car and actually driving one. I've had the opportunity to demo an ER4 unit over the last month and was impressed with the range, clarity and quality they offer in such a compact package. A music industry standard since their release in the '90s, Etymotics was the first manufacturer to introduce in-ear earphones and their current range includes a variety of eartip styles and sizes. I found their standard flanged eartips to fit snugly and provide reasonable sound isolation—until I tried the Custom Fit solution.
Available as an add-on for many of their models, the Custom Fit program starts with a visit to an audiologist to have impressions of your ears taken—an eerily blissful process that takes about 20 minutes. They send the impressions to ACS Custom, a company that specializes in making ear molds for a range of audio uses. Roughly 2-4 weeks later, you receive the soft silicon ear tips by mail, ready to be placed on your earphones.
Because the Custom Fit eartips sit deep in the ear canal, you almost have to screw them in. It's really just a quarter turn, and once they're in, you know it—the world around you melts away as music consumes your core. I found the difference between the flanged tips and the Custom Fit ones to be remarkable. In addition to significant audio quality improvement and sound isolation, the bespoke silicon tips are so comfortable they disappear.
While the earphones ($300) and Custom Fit tips ($100) aren't cheap, they're more affordable and the modular design makes them more flexible than many of the other custom professional monitoring solutions out there. And conveniently, ACS will keep your ear impressions on file in case you need to reorder.