An in-ear earphone includes a body shaped to fit in the wearer's ear, a nozzle extending from the body towards the ear canal of the wearer's ear, the nozzle including an acoustic passage to conduct sound waves to the ear canal of the wearer, an ear canal sealing structure extending from the nozzle, and a sensor coupled to the nozzle. The sealing structure includes a thin sheet of material forming a hollow shape surrounding the nozzle and sensor, and the body, nozzle, sealing structure, and sensor are arranged such that when the earphone is located in the wearer's ear, the sensor faces the tragus of the ear, and the sealing structure simultaneously forms an acoustic seal to the entrance to the ear canal and provides optical coupling between the sensor and the tragus with minimal air gaps between the sensor, the sealing structure, and the tragus.
