It's all about the bass. Many body sounds, especially heart sounds, can only be heard at low frequencies. For over-the-ear headphones, or on the ear headphones, use models that have excellent bass. We recommend Beats Pro over-the-ear and AKG Y40 on-the-ear headphones. Noise-canceling headphones usually have their own built in amplifiers, so you can get great results with those. For in-the-ear headphones, you must select earbuds that form a good seal in your ears, addressing two problems simultaneously. First, you need a great seal to get good bass. Second, sealed ear tips block out external noise. We recommend the high-quality earbuds included with the Thinklabs One. If you would like to try other models, we like the Bose SoundTrue earbuds.