An interchangeable furniture system for providing an interchangeable furniture element having faces with different appearances. The waveguide substrate has an optical waveguide and a recessed portion for mounting the optoelectronic component, and electrodes are arranged on the recessed portion. The refining is based at least in part on the playing back of audio media during a time period prior to executing the maximum volume refining step. The first sub-domain is formed between one of the two first regions and the third region, the second sub-domain is formed between the second region and the third region, the third sub-domain is formed between the second region and the fourth region, and the fourth sub-domain is formed between the other one of the two first regions and the fourth region. The coupling part and the rear upper section are respectively formed with a slot unit and a hole unit for extension of the fastening unit therethrough.