Electrophones are the last category of musical instruments in the Hornbostel-Sachs System since its first publishing in 1919. This category is placed as the fifth category in the Hornbostel Sachs System. It is also the last category to date. The electrophones category was added to the Hornbostel-Sachs musical instrument classification system by Sachs in 1940, to describe instruments involving electricity. Sachs divided his 5th category into 3 subcategories:
- 51 = electrically actuated acoustic instruments; e.g., pipe organ with electronic tracker action.
- 52 = electrically amplified acoustic instruments; e.g., acoustic guitar with pickup.
- 53 = instruments which make sound primarily by way of electrically driven oscillators; this includes keyboards, synthesizers.
- The last category includes instruments such as theremins or synthesizers, which he called radioelectric instruments.
Francis William Galpin provided such a group in his own classification system, which is closer to Mahillon than Sachs-Hornbostel. For example, in Galpin’s 1937 book A Textbook of European Musical Instruments, he lists electrophones with three second-level divisions for sound generation by oscillation, electro-magnetic and electro-static.
As well as third-level and fourth-level categories based on the control method. Sachs himself proposed subcategories 51, 52 and 53 in his 1940 book; The History of Musical Instruments – the original 1914 version of the system did not acknowledge the existence of his 5th category.
Present-day ethnomusicologists, such as Margaret Kartomi and Terry Ellingson suggest that, in keeping with the spirit of the original Hornbostel Sachs classification scheme, if one categorizes instruments by what first produces the initial sound in the instrument, that only subcategory 53 should remain in the electrophones category. Thus, it has been more recently proposed, for example, that the pipe organ [even if it uses electric key action to control solenoid valves] remain in the aerophones category, and that the electric guitar remain in the chordophones category, and so on.