In-ear earpiece and expansion adaptor

An in-ear earphone comprising:

• a housing for receiving an electroacoustic transducer, and
• an expansion adaptor unit coupled to the housing of the in-ear earphone,
• wherein the expansion adaptor unit has an expansion unit having a substantially shark fin shape that includes an upper portion with a curvature for fitting in a crus inferius anthelicis, in an anti-helix and in a concha of an ear of a user and at least one flexible coupling element for coupling the upper portion to the housing of the in-ear earphone,
• wherein the upper portion has a first end and a second end and the at least one flexible coupling element is coupled to the second end, the upper portion having a substantially convex shape beginning at the second end and extending toward the housing and the at least one flexible coupling element having a substantially concave shape beginning at the second end and extending toward the housing,
• wherein the expansion adaptor unit is made of a flexible material,
• wherein the curvature of the upper portion is selected to fit securely in the crus inferius anthelicis and the anti-helix of the ear of the user and wherein the second end of the upper portion bears against the crus inferius anthelicis, and
• wherein the expansion unit has a spring stiffness in a vertical direction of between 0.05 and 0.6 N/mm and a spring stiffness in a horizontal direction of between 0.02 and 0.4 N/mm, the spring stiffness permitting a secure fit of the expansion unit in the ear of the user by bearing against the crus inferius anthelicis at the second end of the upper portion.