ELECTROACOUSTIC CALLING UNIT

摘要

An electro-acoustic transducer (2) mounted in a housing (5, 6) in which a volume control is provided by an aperture (7) in the front of the housing (5) and a rotatable shutter plate (11) which is mounted on the front of the housing (5) and which has a plurality of smaller apertures (14). The smaller apertures (14) are positioned in such a way that linear movement of the shutter plate (11) causes a logarithmic change in the exposure of the aperture (7) in the front of the housing (5) to give a logarithmic change in the intensity of the sound output such that the linear movement causes an apparent linear change in the intensity of sound as heard by a human ear.