A hearing aid system may include an ear-mould for a child, an output transducer, and an electrical feedback path. At least one electronics card can be supported in the card chamber. The boundary surface is slanted with a predetermined angle to regulate the range of diffraction of the light according to the wavelength. The coil is formed such that the connecting portion is formed on top of the raised portion. The primary assembling pieces andor the secondary assembling pieces form bumps, and the bumps support a light homogenization mechanism.