ACADEMY OF SC.BKCE8] BIOGRAPHY 11 



The difficulty was remedied in this case by a change in material without change of form, by diminishing the 

 reflecting power of the two side walls. This was done by placing a suitable felt on the plaster walls between the 

 engaged columns, and covering it with a decorated tapestry. 



It is interesting to note that this treatment applied to the lower half of the walls would not have been acous- 

 tically effective. 



The lecture-room of the Metropoltian Museum of Art illustrates the next step in complexity. . . . In this 

 room the reverberation was not merely excessive, but it resolved itself by focusing into a multiple echo, the 

 components of which followed each other with great rapidity but were distinctly separable. The number dis- 

 tinguishable varied in different parts of the hall. Seven were distinguishable at certain parts. 



Certain remedial measures were taken on Sabine's advice, and the result was the one pre- 

 dicted — 



the reduction of the disturbance to a single and highly localized echo. This echo is audible only in the central seats — 

 two or three seats at a time — and moves about as the speaker moves, but in symmetrically opposite direction. 

 Despite this residual effect, and it should be noted that this residual effect was predicted, the result is highly 

 satisfactory to Dr. Edward Robinson, the director of the museum, and the room is now used with comfort, whereas 

 it had been for a year abandoned. 



This is perhaps the most fitting place for reference to Sabine's exceedingly interesting 

 chapter on "Whispering galleries," which is to be found at present only in the volume of his 

 Collected Papers on Acoustics, issued in 1922 by the Harvard University Press. I should 

 like to reproduce here the whole of it, if such a proceeding were consistent with the usual scope 

 of memoirs like the one now in hand, but I can give only two or three short passages : 



It is probable that all existing whispering galleries, it is certain that the six more famous ones, are accidents; 

 it is equally certain that all could have been predetermined without difficulty, and like most accidents, could 

 have been improved upon. That these six, the dome of St. Paul's Cathedral in London, Statuary Hall in the 

 Capitol at Washington, the vases in the Salle des Cariatides in the Louvre in Paris, St. John Lateran in Rome, 

 the Ear of Dionysius at Syracuse, and the Cathedral of Girgenti, are famous above others is in a measure due 

 to some incident of place or association. 



The ceiling of the Hall of Statues [in Washington], with the exception of a small circular skylight, is a por- 

 tion of an exact sphere with its center very nearly at head level. . . . 



In citing this gallery in an article on "Whispering galleries" in Sturgis's Dictionary of Architecture, the 

 writer made the statement that "The ceiling, painted so that it appears deeply panelled, is smooth. Had the 

 ceiling been panelled the reflection would have been irregular and the effect very much reduced." A year or 

 so after this was written the fire in the Capitol occurred, and in order to preserve the whispering gallery, which 

 had become an object of unfailing interest to visitors to the Capitol, the new ceiling was made "to conform 

 within a fraction of an inch" to the dimensions of the ceiling which it replaced. Notwithstanding this care, 

 the quality of the room which had long made it the best and the best known of whispering galleries was in large 

 measure lost. Since then this occurrence has been frequently cited as another of the mysteries of architectural 

 acoustics and a disproof of the possibilities of predicting such phenomena. As a matter of fact, it was exactly 

 the reverse. Only the part between the panels was reproduced in the original dimensions of the dome. The 

 ceiling was no longer smooth, the staff was panelled in real recess and relief, and the result but confirmed the 

 statement recorded nearly two years before in the Dictionary of Architecture. 



Almost any wall-surface is a much more perfect reflector of sound than the most perfect silver mirror is 

 to light. In the former case, the reflection is over 96 per cent, in the latter case rarely over 90. 



On the surfaces of the two mirrors scratches to produce equally injurious effects must be comparable in 

 their dimensions to the lengths of the waves reflected. Audible sounds have wave lengths of from half an 

 inch to sixty feet; visible light of from one forty-thousandth to one eighty-thousandth of an inch. Therefore 

 while an optical mirror can be scratched to the complete diffusion of the reflected light by irregularities of micro- 

 scopical dimensions, an acoustical mirror to be correspondingly scratched must be broken by irregularities 

 of the dimensions of deep coffers, of panels, of engaged columns, or of pilasters. 



From this last passage, and from others declaring that a rough plastered surface acts 

 practically just like a smooth one, so far as sound waves are concerned, one might infer that 

 merely painting a solid wall could have no appreciable effect on its acoustic properties; but 

 this would be an error. Sabine had found the absorptive power of a painted brick wall to be 

 only about half that of the same kind of wall unpainted. This is because of the porosity of 

 natural brick, which enables the sound waves to penetrate the material slightly and so lose 

 a little of their energy. Gradually, through Sabine's suggestions and "the skill and great 

 knowledge of ceramic processes" possessed by Mr. Raphael Guastavino, a kind of tile was 

 developed which "has over sixfold the absorbing power of any existing masonry construction 

 and one-third the absorbing power of the best-known felt." 



