The invention describes an apparatus for software or hardware emulation of acoustic feedback effects. The invention comprises an analog to digital interface (200) for the input, whose output is summed (202) with a feedback of this digital signal passing through an amplifier model (204), a room acoustics model (206) and a string model (210). The summed signal (202) is converted from digital to analog (205) and can then be connected to a standard amplifier. The room acoustic model comprises a volume control (208) where the degree of feedback is controlled, while the string model contains a model (212) of which harmonics to feed back, and finally an algorithm (214) that decides which fundamental frequencies that the incoming digital signal contains.