Composers in Paris were experimenting with electronic music in the 1940s. They called it “Musique concrète” because they used natural, concrete sounds. (“Concrete” in this sense meant the opposite of “abstract” music which was written down for performance). The sounds were played back at different speeds, combined in lots of ways, played backwards or played continuously (repeated in a 'loop'), or played into a mixer and re-recorded onto another tape recorder. The sounds could be filtered. Effects such as vibrato or echo could be added. Sometimes composers used synthesizers which were machines that could make electronic music in real time. They sounded more like normal instruments than the sound effects on a tape recorder.