This section is from the book "The National Capitol. Its Architecture Art And History", by George C. Hazelton, Jr. Also available from Amazon: The National Capitol Its Architecture Art and History.
The difficulty of speaking and hearing in this hall was much complained of by the Members from the first time they occupied it, in 1807. The present flooring is tessellated in black and white marble. Some of these squares have accidentally fallen into key with the peculiar form of the ceiling, arch and dome, and now definitely mark the marvelous acoustic properties of the hall, in the way of whispering galleries, curious echoes and ventriloquist effects. These strange echoes have constantly baffled the most skilful efforts of various architects. Their history forms an interesting chapter, not only architecturally but popularly. It is especially amusing to observe how learnedly I^atrobe comments upon them, and how readily he points out for the edification of Congress all the difficulties and their remedies; for, when it fell to his lot to rebuild the old south wing after the fire, though untrammelled in the supervision, his theories * did not solve the difficulty. Thornton always maintained that the chamber would have given no trouble had I>atrobe but followed his original design. The curious echoes still cling to the old hall and reverberate strangely in the ears, like admonitions from the spirits of departed statesmen whose voices once rang out within its walls.
* For reports, see Appendix, pp. 261-273.
There is certainly something ghostly about it, with its circular assembly of mute representatives in bronze and marble and its wonderful whispering walls.
The acoustic properties of the room are truly unaccountable, as it was modeled after buildings successfully used for theaters and auditoriums in Greece and Rome, and is quite similar in design to the French Chamber of Deputies in Paris. Some of the difficulty was obviated, however, by a simple suggestion * of Robert Mills, an architect, who, in 1832, showed the fallacy in the arrangement of the seats by which Representatives were compelled to speak toward the flat wall at the south end of the room, where the Speaker had his desk, near the center of the prostyle. The seats were accordingly reversed with slightly beneficial results, the presiding officer occupying the north end of the room and the Members speaking toward the semi-circular wall.