Notes on how phase and delay distortions affect the quality of speech, music and sound effects

Phase and delay distortions in the reproduction of speech and music have always been an important consideration in the design of high-quality systems. Early authoritative literature indicated this type of distortion could be disregarded. In 1935, events occurred in which phase delay in a two-way loudspeaker severely distorted sound effects. From this observation, a research program was set up to evaluate the minimum detectable delay that could be tolerated in two-way theater type loudspeakers; the final recommendation was 2 ms. During the period of 1930 to 1940 an intensive redesign of recording equipment was made for lower phase shift, and the quality increase is now long a matter of record.