comprises a permanent magnet embedded within ferromagnetic material with the air gap being within the ferromagnetic material the air gap which will be referred to herein as the coil space is made as short as possible in order to maximize the magnetic field intensity impinging on the coil for a given size magnet for reasons of design simplicity and efficient use of material it is desirable to place the magnetic circuit on the concave side of the diaphragm i e the compression side and construct the phasing plug and surrounding pole piece from ferromagnetic material actually only the outer portion of the phasing plug need be made of ferromagnetic material since no magnetic field lines which impinge on the voice coil pass through the inner portion this means that the voice coil and coil space must necessarily also be located on the concave side of the diaphragm it is possible however to design otherwise so that the voice coil is mounted on the convex side of the diaphragm see for example u s pat no 2 832 844 issued to matsuoka the present invention is not applicable to those designs where the phasing plug and voice coil are located on opposite sides of the diaphragm the embodiment in fig1 thus shows the voice coil 36 being disposed within an annular coil space 42 in which it is free to vibrate in a direction along the longitudinal axis and cause corresponding vibration of diaphragm 34 an annular permanent magnet 14 is embedded within the outer concentric.