Fig. 11. Plan of Auditorium Showing Concentration of Sound by the Rear Wall.
Fig. 12. This Figure Taken with [Fig. 9] Shows How an Echo is Set Up on the Stage.
A more comprehensive idea of the action of this wall is shown in [Fig. 11]. This reflected sound was small in amount and therefore not a serious disadvantage.
The cases cited were fairly easy to determine since the bundles of sound considered were confined closely to either a vertical or a horizontal plane for which the plans of the building gave some idea of the probable path of the sound. For other planes, the paths followed could be anticipated by analogy from the results already found. [Fig. 12] shows in perspective the development of the result expressed in [Fig. 9].
A square bundle of sound starts from the stage and strikes the spherical surface of the dome. After reflection, it is brought to a point focus, as shown, and spreads out until it strikes the vertical cylindrical wall in the rear of the balcony. This wall reflects it to a line focus, after which it proceeds to the stage. Auditors on all parts of the stage complained of hearing echoes.
Referring to [Fig. 7], it is seen that the arch over the stage reflects sound back to the stage. [Fig. 13] shows in perspective the focusing action of this overhead arch. [Fig. 14] shows the effect of the second arch. Some of this sound is reflected to the stage and to the seats in front of the stage; other portions, striking more nearly horizontally, are reflected to the side balconies. The echoes are not strong except for high pitched notes with short wave lengths, since the width of the arch is small.
Fig. 13. Perspective of Stage Showing Focusing Action of Arch on Sound.
