IEMs are very-high-end personal monitors that are custom molded to your body to fit perfectly in your ear canal. Your ear canals on your left and right side are

IEMs are very-high-end personal monitors that are custom molded to your body to fit perfectly in your ear canal. Your ear canals on your left and right side are different sizes and shapes, so you'll have quite a bit of difference between them. Most importantly, rather than using speakers like headphones, they use a technology called balanced armature drivers which have incredible properties. An example manufacturer is https://alclair.com https://alclair.com I have two sets of theirs, one for master studio tracks, and another for live music that's more geared for guitar and bass playing. They block out nearly all outside noise, some up to 28db of protection, which is wonderful.