1921] on Absolute Measurements of Sound 407 



number of persons, including Prof. Ernst Much, 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 successfully 

 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 important as the first and 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 instru- 

 ment, 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 w T as used. With the present instru- 

 ment it can be determined in a few minutes, if there is no wind. 



In 1890 I proposed to use a diaphragm made of paper, which 

 should be placed, shielded on one side, at the point where the sound 

 was to be measured. In order that the effect of the sound should 

 not be distorted, the membrane, instead of having to do any work, 

 as in the case of the diaphragm of the phonograph in digging up the 

 wax, or in that of the microphone in compressing the carbon, was to 

 be perfectly free, but was to carry a small plane mirror cemented on 

 at its centre. In close juxtaposition and parallel with this was the 



