The optional Yapalong 4000 open mic conducts sound to the inner ear through the bone of the skull, bypassing the outer and middle ear (where the eardrum is located) and directly stimulating the inner ear (hearing organ). It does not obstruct the outer ear like conventional headset to avoid any loss of the surrounding sound. Meanwhile it is easy to wear and more hygienic.  Dual-pin audio plug - 3.5 and 2.5 mm stereo (two prongs).. SHIPPING: 4-6 business days, via UPS Ground