From The Miracle of Feedback by Mario van Horrik in hz-journal:

People may use feedback occasionally, as a small part in a bigger whole, like guitar feedback in pop music. That’s not what this is about. We are using feedback as a conceptual basis for our work. We are searching for the ‘bone’ of sound. We are not scientists, so our characterization of types of feedback maybe not be accurate or complete; however it is based on our practical experience. The types of feedback that I am acquainted with are:

• Direct acoustic feedback: direct contact between speaker and sound source (for instance when you place your electric guitar against the speaker box).
• Indirect acoustic feedback: the process that takes place through the air (microphone in front of speaker)
• Industrial feedback: the system of measuring and controlling, mostly performed by sensors, and electric circuits.
• Improvisation: play acting (controlling) and reacting (measuring) between 2 or more people simultaneously.
• Combinations of the above.

For more on Mario van Horrik’s and Petra Dubach’s work with feedback, read Hz.