Why were electrophones added to the Hornbostel-Sachs classification system?

The Hornbostel-Sachs system was originally developed by Erich von Hornbostel and Curt Sachs to classify musical instruments based on their acoustical principles—specifically, the material or mechanism that produces the initial sound.12

The class of electrophones was added to this system later to account for instruments where sound is produced or modified by electronic means. In these instruments, vibration is generated through oscillating electric circuits, or the sound is produced by conventional means (such as a vibrating string) and subsequently electronically amplified. This addition was necessary to accommodate the technological evolution of musical instruments that did not fit into the original four categories of idiophones, membranophones, aerophones, and chordophones.13