the sound tube 6 the distance is measured from the distal end 34 of the sound tube 6 to place at which the annular ribs 8 8 u2032 8 u2033 are arranged in fig4 b the female member 30 of the sound tube 6 u2032 and the male member 28 of the sound tube 6 connected to the hearing aid case 32 are joined it can be seen that the annular ribs 8 8 u2032 8 u2033 a received by the cavities 20 of the female member 30 fig5 a illustrates a close up view of the elongated sleeve 18 shown in fig2 and in fig4 the elongated sleeve 18 is attached to a sound tube 6 and the elongated sleeve 18 comprises a first annular rib 8 a second annular rib 8 u2032 and a third annular rib 8 u2033 the height h 1 h 2 h 3 of the annular ribs 8 8 u2032 8 u2033 increases as function of the distance from the distal end 34 of the sound tube 6 the height h 1 h 2 h 3 of the annular ribs 8 8 u2032 8 u2033 are measured from the outer surface 14 of the sound tube the first annular rib 8 is arranged closest to the distal end 34 of the sound tube 6 in fact the first annular rib 8 is arranged at the distal end 34 of the sound.