Exploratory tests under a variety of conditions were made during Bell Telephone Laboratories' studies on stereophonic reproduction to determine the effect of arrival time as a factor in localization. The recent rapid rise of interest in stereophony appears to warrant a summary of these old results (1934), for which the Laboratories has granted permission. It is shown that arrival time differences have essentially the same effect whether they are generated in the pickup room, listening room, or in the electrical system. They can therefore be made to add or compensate. Small differences (0–3 milliseconds) cause large shifts of localization angle, but the effect becomes relatively constant for larger differences. Time differences can be compensated by level differences. The results of two other experimenters are given for comparison. It is concluded that reasonable agreement is attained, considering the small total amount of data, but that more experiment is required to establish accurate quantitative relationships for practical stereophonic reproduction.
The changes in pitch which result from changes in the loudness of low frequency tones were investigated for several frequencies between 75 and 1000 c.p.s. with a crew of 9 observers. The loudness levels covered the range 20 to 120, and both telephone receivers and a loudspeaker were used as sound sources. Fletcher's pitch standard, a pure tone of loudness level 40, was employed.
All consistent judgments gave pitch shifts downward with increasing loudness, but large differences between individuals were found. Two observers perceived no shifts at any frequency or loudness while three others experienced changes greater than 35 percent at the highest intensity. Appreciable changes with time in individual judgments were found. There appeared insignificant difference between data obtained with telephone receivers or with the loudspeaker. A set of contours of equal loudness level, plotted on coordinates frequency and pitch change in percent, is given. These curves show a pitch change small at 1000 cycles, increasing to maximum at a low frequency and decreasing for still lower frequencies. The frequency of greatest shift increased from about 100 cycles at small loudness to about 200 at loudness level 120.
The Appendix describes automatic test equipment that greatly simplified the testing routine and improved its accuracy.
Stereophonic sound has become of vital importance to industry. The subject has been studied for many years, but the published material is scattered. This paper summarizes the funda-Engineers.
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