An electromagnetic vibrator for a bone conduction hearing aid, said electromagnetic vibrator comprising: a magnet arrangement comprising a bobbin assembly comprising a central portion, a coil wound around the central portion and being configured to generate a dynamic magnetic field (φ D) in response to an electric current and at least one permanent magnet (6, I, II, III, IV) configured to generate a static magnetic field (φ S); a vibrator plate arranged in a position with respect to said central portion such that a gap (G 1), extending across a longitudinal axis of the electromagnetic vibrator is formed between the vibrator plate and at least one of said central portion or the at least one permanent magnet (6, I, II, III, IV); an encasing surrounding at least a portion of the magnet arrangement, wherein the encasing comprises a gap adjustment mechanism that is configured to move the bobbin assembly relative to the encasing for adjusting the gap (G 1) between the vibrator plate and at least one of said central portion or the at least one permanent magnet (6, I, II, III, IV), and wherein the gap adjustment mechanism comprises: a first adjustment part, provided at an outer periphery of the bobbin assembly, the first adjustment part comprising one of at least one protruding portion (26, 32) or a plurality of receiving sections (28, 28', 34, 38); and a second adjustment part, provided at an inner side of the encasing, the second adjustment part comprising another of: a plurality of corresponding receiving sections (28, 28', 34, 38) that is configured to operationally cooperate with the at least one protruding portion (26, 32) of the first adjustment part or at least one corresponding protruding portion (26, 32) that is configured to operationally co-operate with the plurality of receiving sections (28, 28', 34, 38) of the first adjustment part. wherein the first adjustment part and the second adjustment part form an adjustment mechanism utilizing a bayonet mount principle or an adjustment mechanism utilizing a ratchet principle, or wherein the first adjustment part comprises a serrated portion and the second adjustment part comprises a male member having a pointed distal portion configured to be received by receiving structures of the corresponding serrated portion. a magnet arrangement comprising a bobbin assembly comprising a central portion, a coil wound around the central portion and being configured to generate a dynamic magnetic field (φ D) in response to an electric current and at least one permanent magnet (6, I, II, III, IV) configured to generate a static magnetic field (φ S); a vibrator plate arranged in a position with respect to said central portion such that a gap (G 1), extending across a longitudinal axis of the electromagnetic vibrator is formed between the vibrator plate and at least one of said central portion or the at least one permanent magnet (6, I, II, III, IV); an encasing surrounding at least a portion of the magnet arrangement, wherein the encasing comprises a gap adjustment mechanism that is configured to move the bobbin assembly relative to the encasing for adjusting the gap (G 1) between the vibrator plate and at least one of said central portion or the at least one permanent magnet (6, I, II, III, IV), and wherein the gap adjustment mechanism comprises: a first adjustment part, provided at an outer periphery of the bobbin assembly, the first adjustment part comprising one of at least one protruding portion (26, 32) or a plurality of receiving sections (28, 28', 34, 38); and a second adjustment part, provided at an inner side of the encasing, the second adjustment part comprising another of: a plurality of corresponding receiving sections (28, 28', 34, 38) that is configured to operationally cooperate with the at least one protruding portion (26, 32) of the first adjustment part or at least one corresponding protruding portion (26, 32) that is configured to operationally co-operate with the plurality of receiving sections (28, 28', 34, 38) of the first adjustment part. wherein the first adjustment part and the second adjustment part form an adjustment mechanism utilizing a bayonet mount principle or an adjustment mechanism utilizing a ratchet principle, or wherein the first adjustment part comprises a serrated portion and the second adjustment part comprises a male member having a pointed distal portion configured to be received by receiving structures of the corresponding serrated portion. a plurality of corresponding receiving sections (28, 28', 34, 38) that is configured to operationally cooperate with the at least one protruding portion (26, 32) of the first adjustment part or at least one corresponding protruding portion (26, 32) that is configured to operationally co-operate with the plurality of receiving sections (28, 28', 34, 38) of the first adjustment part. wherein the first adjustment part and the second adjustment part form an adjustment mechanism utilizing a bayonet mount principle or an adjustment mechanism utilizing a ratchet principle, or wherein the first adjustment part comprises a serrated portion and the second adjustment part comprises a male member having a pointed distal portion configured to be received by receiving structures of the corresponding serrated portion.