Bose CustomTune technology is a proprietary Bose technology that optimises the audio playback you hear based on the unique physical properties of your ear and ear canal. It works by playing a specially designed calibration tone as the welcome chime each time you power on the headphones while wearing them. It then measures how the shape of your ear uniquely modifies that sound. The headphones use that information to adjust their sound to your ears, creating a more balanced, authentic audio experience.