a method for the manufacture of loudspeakers having a ferromagnetic fluid contained in the voice coil magnetic air gap and being confined by the loudspeaker magnet field thereby enveloping the portions of the voice coil that are within the magnetic air gap . a critical viscosity range of the ferromagnetic fluid is required , the viscosity determined by the viscosity of the liquid phase and the concentration of the colloidal ferromagnetic particles which provide the viscous damping forces on the voice coil effective at low and high frequencies . the damping forces are due to shearing stress set up in the fluid by motion of the voice coil in response to electrical excitation and result in surprisingly improved acoustical performance , higher electrical output and freedom from hiss in the loudspeaker . the ferromagnetic fluid also provides an improved heat sink for the voice coil thus improving the electrical power dissipation capability of the voice coil . an opening in the cap over the voice coil permits entrapped air under the cap or dust cover to escape thereby eliminating a noticeable hiss .