Electronic music
After World War II, when tape recorders had been invented and were becoming popular, composers started to use them to make music.  The tape recorder was needed for the performance.  Composers used them to combine lots of different sounds.  Sometimes it was music played on ordinary (acoustic) instruments which was then changed in some way by the tape recorder.  Sometimes they took sounds from everyday life such as the sound of water, traffic noise or bird song.  All these noises were put together in the way the composer wanted by using the tape recorder.  Tapes of sounds were often cut into pieces, then the pieces were 'spliced' – put back together in a different order. The results were often very interesting, but there were problems.  Some people asked: “Is it music?”  Others thought it was boring to just look at a tape recorder during a concert instead of being able to watch live musicians play.