of connections to the driver. This winding usually served a dual role, acting also as a choke coil, filtering the power supply of the amplifier that the loudspeaker was connected to. AC ripple in the current was attenuated by the action of passing through the choke coil. However, AC line frequencies tended to modulate the audio signal going to the voice coil and added to the audible hum. In 1930 Jensen introduced the first commercial fixed-magnet loudspeaker; however, the large, heavy iron magnets of the day were impractical and field-coil speakers remained predominant until the widespread availability of lightweight alnico magnets after World War II. First loudspeaker systems In the 1930s, loudspeaker manufacturers began to combine two and three drivers or sets of drivers each optimized for a different frequency range in order to improve frequency response and increase sound pressure level. In 1937, the first film industry-standard loudspeaker system, "The Shearer Horn System for Theatres", a two-way system, was introduced by Metro-Goldwyn-Mayer. It used four 15″ low-frequency drivers, a crossover network set for 375 Hz, and a single multi-cellular horn with two compression drivers providing the high frequencies. John Kenneth Hilliard, James Bullough Lansing, and Douglas Shearer all played roles in creating the system. At the 1939 New York World's Fair, a very large two-way public address system was mounted on a tower at Flushing Meadows. The eight 27″ low-frequency drivers were designed by Rudy Bozak in his role as chief engineer for Cinaudagraph. High-frequency drivers were likely made by Western Electric. Altec Lansing introduced the 604, which became their most famous coaxial Duplex driver, in 1943. It incorporated a high-frequency horn that sent sound through a hole in the pole piece of a 15-inch woofer for near-point-source performance. Altec's "Voice of the Theatre" loudspeaker system was first sold in 1945, offering better coherence and clarity at the high output levels necessary in movie theaters. The Academy of Motion Picture Arts and Sciences immediately began testing its sonic characteristics; they made it the film house industry standard in 1955. In 1954, Edgar Villchur developed the acoustic suspension principle of loudspeaker design. This allowed for better bass response than previously obtainable from drivers mounted in larger cabinets. He and his partner Henry Kloss formed the Acoustic Research company to manufacture and market speaker systems using this principle. Subsequently, continuous developments in enclosure design and materials led to significant audible improvements.