 Complete the following phrase:  Popular loudspeakers included the Altec Lansing Duplex 600-series coaxial loudspeaker, studio monitors from the 1940s to the 1980s, and the Altec Voice of the Theatre line of loudspeakers widely used in movie theaters, concert halls, and also in rock concerts from the 1960s to the 1990s, such as custom designs used at 