The present invention relates to an electro-acoustic communications unit (10) and a portable communication device (20, 30) for producing a desired frequency characteristics in alert mode and phone mode, comprising a housing with a wall (12) defining an interior, having a certain volume and an exterior of the housing, an acoustic driver (14) for generating acoustic signals, said acoustic driver (14) being mounted to the wall (12), an acoustic port (16, 26), having a length and a cross-sectional area, said port (16, 26) penetrating the wall (12) and connecting the interior of the housing with the exterior of said housing, where the volume, length and cross-sectional area are dimensioned in relation to the acoustic driver (14) in a way such that said electro-acoustic communications unit achieves a desired frequency characteristics when engaging the exterior end (22, 32) of said acoustic port of the communications unit with a user's ear (18).