The ER-4 MicroPro earphones are the next best thing to live music, with noise isolation far surpassing that of active-noise-reduction earphones. The ER-4's require NO batteries, and can be plugged into any audio source. Their 20-25 dB of external noise exclusion allows you to hear the full dynamic range of recordings without having to play them at unnaturally high and unsafe levels.The ER-4S (Stereo) is the musician's choice. Extremely detailed, well-balanced, and tight bass are just a few words to describe their awesome sound. These phones offer a sonic checkout from the real world!