In Bandoneonbook (2003) by Hans W. Koch the acoustic feedback between the small internal microphone and loudspeaker of a computer is played by opening and closing the laptop, similar to how one would play a bandoneon. The music software Max (at those times still called MaxMSP) is controlled by the keyboard of the laptop and by pressing some keys the feedback will be filtered in different ways. A piece which can only be performed with a titanium-powerbook series of 2001!
Working with a similar principle as in Windy Gong Ute Wassermann developed a kind of loudspeaker dress , through which her voice was sounding (see both pictures above from 1989).
In 2015 she composed a new piece called InPutOut using these loudspeaker dresses for Les Femmes Savantes (Andrea Neumann, Sabine Ercklentz, Ana Marie Rodriguez and Ute Wassermann are performing). The performers wear a loudspeaker connected to a metal plate.
Each performer also holds two microphones: one to send the sound of the voice to the metal plate loudspeaker, the other one to amplify either the voice directly or the metal plate, which is excited by the voice. The signal of the second microphone is amplified through a small PA system. Holding both microphones in front of the metal plate, a feedback between microphone and metal plate loudspeaker occurs, amplified by the second microphone, which is connected to the PA system. A clear example of this feedback through the metal plate can be heard at 5’00” in the video documenting the piece. The microphone movements in this piece reveal fantastic possibilities of voice colouring, sometimes reminding me of ventriloquism.
Audio Ballerina’s by Benoît Maubrey:
Different Version of LapStrap (since 2010) by Steffi Weismann, all using portable loudspeakers: