Page 24 - Summer 2018

The New Age of Sound
The other method of producing auditory perspective used loudspeakers. To reproduce the experience of listening to music in an auditorium at some distant room, the problem is one of producing an exact duplication of the pattern of vi- brations in the air at the recording site at every correspond- ing point in the receiving site (Fletcher, 1934). This idea be- comes more complicated when the recording and receiving room shapes do not align well, but it provided a theoretical framework in which to pose the problem.
This line of research involving loudspeakers occupied the researchers during their residency in Philadelphia. In one perceptual study, the engineers Steinberg and Snow asked how many microphones and loudspeakers were necessary for a percept of auditory perspective. Using one of the rooms at the Academy of Music, microphones were set up at the left, center, and right sides of the stage and hidden behind a curtain. Words were uttered from various positions on the stage, and naive participants indicated where they thought the talker was located. The best performance came from a three-speaker setup, where each of the left, center, and right microphone inputs were played through a separate loud- speaker (Steinberg and Snow, 1934). This gave reasonable lateral and depth localization, although the accuracy of the participant’s guesses was far from perfect. Interestingly, whenever the quality of the left and right transmissions was mismatched, listeners showed a pronounced bias toward the most natural side, apparently weighting the cues provided by level differences in accordance with the perceived reliability of the source. Steinberg and Snow’s (1934, p. 17) summary of this experiment shows a practical attitude toward their findings, “The application of acoustic perspective to orches- tral reproduction in large auditoriums gives more satisfac- tory performance than probably would be suggested by the foregoing discussions.” In other words, this will do the trick.
The Grand Unveiling of
Stereophonic Sound
On the evening of April 27, 1933, Harvey Fletcher welcomed a distinguished crowd at Constitution Hall in Washington, DC. Under the auspices of the National Academy of Sci- ences, an audience of presidential advisors, senators, and congressional representatives gathered for a musical per- formance. On the stage were three loudspeakers. Stokowski was stationed at a control panel in the hall (Figure 4). His musicians remained in their rehearsal room in Philadelphia awaiting their cue.
Figure 4. Leopold Stokowski (left) and Harvey Fletcher (right) at Constitution Hall in Washington, DC, on the eve- ning of their wire transmission demonstration. From the Bell Telephone Laboratories Archive.
In the first act of the show, the audience listened to a scene wired from Pennsylvania to Washington. On the left-hand side of the stage in Philadelphia, a handyman constructed a box with a hammer and saw. From the far right, another worker proffered suggestions to his friend. “So realistic was the effect that to the audience the act seemed to be taking place on the stage before them. Not only were the sounds of sawing, hammering, and talking faithfully reproduced, but the auditory perspective enabled the listeners to place each sound in its proper positions, and to follow the move- ments of the actors by their footsteps and voices,” wrote an observer (Fletcher, 1992, p. 184). Next, a soprano sang Com- ing Through the Rye as she weaved across the stage in Phila- delphia. At Constitution Hall, the phantom of her voice “ap- peared to be strolling on the stage” (Fletcher, 1992, p. 184).
The show ended with an unforgettable duel in the dark be- tween two trumpet players separated by more than a hun- dred miles. The two traded licks from their opposite posts in Constitution Hall and the Academy of Music in Philadel- phia. But the audience was none the wiser. “To those in the audience there seemed to be a trumpet player at each side of the stage before them. It was not until after the stage was lighted that they realized only one of the trumpet players was there in person” (Fletcher, 1992, p.184). The crowd was left in awe.
This was not simply a show of tricks. It was the grand pub- lic unveiling of Fletcher’s ambitious and laborious project,
22 | Acoustics Today | Summer 2018