42 



NA TURE 



[July 8, 1922 



Guillet, responding to the toast of the French engineer- 

 ing society, replied eloquently, recounting the work 

 that had been done concurrently by French engineers 

 and men of science in the many developments that 

 had taken place during the last century. 



Following the very successful meetings in Paris, 

 members of the Institution journeyed to Liege to 

 participate with l'Association des Ingenieurs sortis 

 de l'Ecole de Liege in the celebration of the seventy- 

 fifth anniversary of the foundation of the Liege Society, 

 which coincided also with the seventy-fifth anniversary 

 of the foundation of the Institution of Mechanical 

 Engineers. In connexion with this anniversarv an 

 international scientific congress and exhibition had been 

 arranged by the Liege Society, and this was opened by 

 the King of the Belgians on June 18. The members 



of the Institution of Mechanical Engineers received 

 invitations to the opening ceremony. The King in 

 his opening address referred in particular to the import- 

 ance of the work of men of science and of engineers 

 in developing the resources of the world. On the days 

 following the opening of the exhibition a number of 

 papers were read at various sections of the Congress, 

 and visits were arranged to works in the neighbourhood 

 of Liege. Representatives of the French engineering 

 society journeyed to Liege with the members of the 

 Institution of Mechanical Engineers, and the association 

 of the three societies proved of the greatest interest. 

 It is believed and hoped that the celebrations will do 

 much to bring about that rapprochement between 

 the three peoples which is so essential for the future 

 welfare of Europe and the world. 



Absolute Measurements of Sound 1 



By Dr. Arthur Gordon Webster, Professor of Physics, Clark University, Worcester, Mass., U.S.A. 



IT is now more than thirty years since it occurred to 

 me to devise an instrument that should be capable 

 of measuring the intensity or loudness of any sound 

 at any point in space, should be self-contained and 

 portable, and should give its indications in absolute 

 measure. By this is meant that the units should be 

 such as do not depend on time, place, or the instrument, 

 so that, though the instrument be destroyed and the 

 observer dead, if his writings were preserved another 

 instrument could be constructed from the specifica- 

 tions and the same sound reproduced a hundred or a 

 thousand years later. The difficulty comes from the 

 fact that the forces and amounts of energy involved in 

 connexion even with very loud sounds are extremely 

 small, as may be gathered from the statement that it 

 would take approximately ten million cornets playing 

 fortissimo to emit i horse-power of sound. 



Before we can measure anything we must have a 

 constant standard. In sound we must construct a 

 standard which emits a sound of the simplest possible 

 character, which we call a pure tone ; it will be like 

 that emitted under proper conditions by a tuning-fork, 

 which is described by saying that the graph represent- 

 ing the change of pressure with the time shall be that 

 simple curve known as the sinusoid or curve of sines. 

 From this connexion we say that the pressure is a 

 harmonic function of the time. Unfortunately, the 

 pressure change is so small that at no point in a room, 

 even when a pel son is speaking in a loud tone, does the 

 pressure vary from the atmospheric pressure by more 

 than a few millionths of an atmosphere. Thus we 

 require a manometer millions of times as sensitive as 

 an ordinary barometer, and, in addition, since the 

 rhythmic changes occur, not once in an hour or day, 

 but hundreds of times per second, if we wish the gauge 

 to follow the rapid changes accurately, we have many 

 mechanical difficulties. 



The problem of a standard of emission has been 

 solved In- a number of persons, including Prof. Ernst 

 Mach and Prof. Ludwig Boltzmann, and Dr. A. Zernov, 

 of Petrograd, a pupil of the celebrated Peter Lebedeff. 

 The problem of an absolute instrument for the reception 

 and measurement of a pure tone has been also success- 



From a Friday evening di 



delivered at the Royal Institute 



fully dealt with by a number of investigators, among 

 whom may be mentioned Prof. Max Wien, of wireless 

 fame, the late Lord Rayleigh, and Lebedeff. But there 

 remains a third step in the process, which is as im- 

 portant as the first and the second. Given the 

 invention of the proper standard source of sound, 

 which I have named the " phone," because it is vox et 

 praeterea nihil, and of a proper measuring instrument, 

 which should evidently be called a phonometer, there 

 still remains the question of the distribution of the 

 sound in space between the phone and the phonometer. 

 Any measurements made in an enclosed space will be 

 influenced by reflections from the walls, and, even if 

 we had a room of perfectly simple geometrical form, 

 say cubical, and were able to make the instruments of 

 emission and reception work automatically without the 

 disturbing presence of an observer, it would still be 

 impossible to specify the reflecting power of the walls 

 without a great amount of experimentation and com- 

 plicated theory. Nevertheless, this is exactly what 

 was done by the late Prof. Wallace C. Sabine, of 

 Harvard University, who employed the human ear as 

 the receiving instrument. Those who have made 

 experiments upon the sensitiveness of the human ear 

 for a standard sound will immediately doubt the 

 possibility of making precise measurements by the same 

 ear at different times, and particularly of comparing 

 measurements made by one ear with those made by 

 another. Nevertheless, Sabine attained wonderful 

 success and was able to impart his method to pupils 

 who carried on his work successfully, so that he was 

 able to create the science of architectural acoustics and 

 to introduce a new profession. Still, the skill that 

 required three or four months to attain by Sabine's 

 method may be replaced by a few minutes' work with 

 the phonometer. 



In order to avoid the influence of disturbing objects, 

 the observer should take the phonometer to an infinite 

 distance, which is manifestly impossible. The method 

 employed was to get rid of all objects except a reflecting 

 plane covered with a surface the coefficient of reflection 

 of which could be measured. For this purpose the teeing 

 ground of a suitable golf course was used. With tin- 

 present instrument it can be determined in a few 

 minutes, if there is no wind. 



NO. 2749, VOL. I 10] 



