While it is not officially in any published form of the Hornbostel–Sachs system, and hence, lacking proper numerical subdivisions, it is often considered a fifth main category.
The fourth HS number describes instruments that make sound from matter in its gaseous state (air).
A number of instruments have been invented, designed, and made, that make sound from matter in its liquid state.
Another category of instruments has been invented that use matter in higher energy states, such as plasma, to produce the initial sound.
It has been proposed that music synthesizers that perform computation, and/or that work by recording and playback of sound samples, or the like, be referred to as quintephones.