The Sounding Board in St. Paul’s Cathedral.
The form of the sounding board is, I think, a novelty, but I am led to believe that it is an approach to the true form for the purpose. Flat sounding boards have been most commonly tried, but they are now generally discarded. A parabolic sounding board behind the preacher has been used with a certain effect, but the advantage is limited to those in the direction of the axis of the curve; and in these cases, action and reaction being equal, the preacher can sometimes hear the criticisms of his audience, if any should be uttered tolerably loud in the proper direction. Besides which, a shell of this description would have had little effect in stopping the objectionable echoes. To meet these difficulties, a curve was selected which has the property of distributing uniformly in every direction so much of the preacher’s voice as can be advantageously reflected. The figure is hyperbolic, the axis being perpendicular over the preacher. The diameter is ten feet, and so much of the voice it receives is reflected as if it came from a point about four feet from the preacher. It would be interesting to know, but I am not aware that sufficient experiments have been made to ascertain, what amount of sound is reflected from a given material. The harder the material, obviously the better the result. In this case hard organ pipe metal has been used. I observed a marked increase of audibility of the voice at a place where the assistance of the reflected sound is obtained over a point equidistant from the pulpit, but too high to receive the reflected wave from the sounding board.—F. C. Penrose.