Abstract: A loudspeaker comprises a motor structure which incorporates a magnetic flux control system including a field winding, a controller connected between a voltage source and the field winding and, a polarity reversal switch preferably located across the filed winding. The magnetic flux control system is operative to produce a magnetic flux, which, depending on the level and polarity of electrical current supplied to the field winding, either reinforces or opposes the static magnetic flux produced by the magnet of the motor structure of the loudspeaker, thus altering the motor strength of the loudspeaker system.

Abstract: Automatic gain control in a hearing aid is effected by detecting an input sound level and/or an output sound level and adapting the output sound level supplied by the hearing aid in response to the detected sound level by controlling the gain of the hearing aid towards an actual desired value of the output sound level. The gain control is effected at increases and decreases, respectively, of the input sound level by adjusting the gain towards the actual desired value with an attack time and a release time, respectively, which are adjusted in response to the detected sound level to a relatively short duration providing fast gain adjustment at high input and/or output sound levels and to a relatively long duration providing slow gain adjustment at low input and/or output sound levels.

Abstract: There is provided a microphone apparatus with an adjusting mechanism that prevents wind noise generated at a sound absorbing hole from perceptively inputted in the structure having a microphone built-in at the back of a panel with a sound absorbing hole. A sound box is structured between a sound absorbing hole of a panel and a sound perceptible portion of a microphone, and a movable piece that slidably moves in the sound box is formed. A screw rod is annexed to the movable piece, and a disk screwed with the screw rod is restricted and interposed between a tubular portion, serving as an outer frame of the sound box, and a support portion formed at the lower portion. A part of the disk is exposed to the front surface side from a slit of the panel, and the disk is rotated to move the movable piece, whereby changing a resonant frequency of a ventilation space of the sound box and a ventilation cross.