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 Rotunda of the Capitol exhibits a striking example of the truth of this position. Any attempt to speak in this room, results in the utter confusion of the voice, simply because the point which describes the dome is elevated so high, (being on the top of the great cornice, that the rays of sound striking the dome are reflected, and (as soon as they pass the cornice level) cross each other, and then are subject again to reflection from the walls, so that by the time they reach the ear, the original sound is broken and scattered in various directions, striking the ear at sensible moments of time.
Could we elevate ourselves so as to stand on a level with the cornice or spring of the dome, and there speak, the voice would be found distinct, strong, and clear. At this level, were a light enclosure constructed, this dome would be one of the most perfect whispering galleries in the world, equal to that of St. Paul's, London, famed in the annals of travellers.
It is to be regretted that we should be deprived of witnessing so great a curiosity as this splendid expanse of dome presents, and which is so well calculated to develop the theory of sound, when it is in our power to enjoy it by the construction of a simple balustrading, or enclosed walk, around the circle on the top of the great cornice, and opening a communication with it through one of the stair-ways above :—permit me to call the attention of the committee to this interesting subject.
I come now to the third fundamental error in the plan of the Hall, namely, the location of the Speaker's chair, and consequently those of the members.
From the facts and reasonings previously stated and referred to, it will readily be seen, by examining the plan, that the Speaker's chair is exactly in the reversed position to where it ought to stand. If it is true that a circular surface of wall is better adapted for the transmission of sound than the straight surface, which cannot be doubted, except we will not receive the testimony of ancient and modern practice in the construction of rooms, expressly designed for public speaking, for these invariably are found to assume the circular form ; therefore, if this circular line is broken in any way, a proportionate defect arises in the capacity of the room to support the voice and convey it distinctly to the ear; and it also follows, that, in speaking, the direction of the voice should be towards the circular surface, and not the straight. If we refer to the position of the speakers in theatres, we will find that they all speak to the circle ; and if we examine the Legislative Hall of France, (which we have said was of a similar form to our Hall.) we will find that the orator speaks to the circle, the tribune from whence he speaks being located expressly to meet this necessity. The evidence of the fact that the speakers should speak to the circle, is to be found in our own Hall, for it is only when they do this that the voice is comparatively distinctly heard ; and it is well known that little or no difficulty occurs in hearing what is said from the chair, or from the clerk's desk. These facts are sufficient to satisfy us of the propriety and advantage of reversing the present arrangement of the Speaker's chair and member's seat, so that the latter should front the circle. Independent of the benefit which would result to hearer and speaker by doing this, there would be other advantages gained, namely : getting rid of the disagreeable effect of the light shining into the eyes, and almost blinding the vision. Every one is sensible of this on entering the Hall, and must be satisfied that it is an evil. Again : The members will front the audience, which certainly is most agreeable to those who address the Chair: this House being the popular branch of the Legislature, the people would wish to hear what is said by their representatives.
The different experiments which have been made at different times to rectify the evils complained of in the Hall, go to prove the correctness of the principles herein advocated on the conveyance of sound, 1st. The introduction of draperies between the columns tended to shut out, in a great degree, the return of the voice from the walls behind, which was favorable, as the echoes from the surfaces are mostly what are termed dissonant, or reaching the ear at different periods of time. These curtains being of an unelastic substance, destroyed or deadened the sound.
Though this plan effected a partial remedy of the evil complained of, it was at a sacrifice of so much surface of wall, which, under a different form, would have tended to increase the strength and distinct utterance of the voice. 2d. The spreading of the canvass cloth over the whole Hall, so as to shut off the reflections of the voice from the dome, went to prove the importance of this form of ceiling to hearer and speaker ; for as long as this cloth canopy existed, it so completely (as in the case of the draperies) absorbed the sound of the voice, that it could scarcely be heard ; and, further, it went to prove that were a fat ceiling to take its place, the evil complained of, instead of being remedied, would be increased. To say nothing of the serious injury in point of beauty, which the Hall would sustain were aflat ceiling to supercede the present domical one, there would be a positive reduction in the powers of this ceiling to sustain the voice, for this simple reason, that, in the place of a ring of consonant echoes which the present ceiling can be made capable of giving, there would be but one reflected from the flat ceiling, and, consequently, the voice would lose its support in the ratio of the difference in the number of consonant echoes. It has been well, therefore, that the Hall has escaped being disfigured by such an useless canopy. The members, when in their seats, have, no doubt, sometimes been startled by the sudden sound of a voice as from one close by, and been astonished when they looked for the speaker to find him at the opposite end of the room. The secret of this phenomenon lies in the domical ceiling, and the mathematician would be able to trace the person speaking (among several speakers) by calling to mind that principle in acoustics which determines the angle of incidence to be equal to the angle of reflection. Now, this fact goes to confirm the truth of the doctrine we have endeavored to establish, namely, that sound is transmitted like light in straight lines, and not in undulatory lines. In further proof of this, certain points might be selected for both speaker and hearer in the Hall, where the whole force of the speaker's voice would fall on the ear of the hearer; and these points could be calculated with mathematical precision. Let any member, whilst another is addressing the House, walk along the inner side of the prostyle just behind the columns, and he will reach a point in that line where his ear will be arrested by a powerful impulse of the speaker's voice. Now, let him draw a line so as to strike the circular surface of the dome or wall at any point, and observe the angle, and then draw another line from thence to the speaker, and he will find that the two angles (the angle of incidence and reflection) will be equal.