October 28, 192 2 j 



NA TURE 



575 



American Research on Acoustics. 



By Alan E. Munby. 



THE Wallace Sabine laboratory of acoustics, a 

 photograph of which is here reproduced (Fig. 1) 

 is situated at Geneva, Illinois. It is a three-story 

 building of brick and concrete specially erected for 

 its purpose and forms a unique design, consisting 

 of two structures under one roof, an inner room or 

 sound chamber completely insulated from an outer 

 shell. Figs. 2 and 3 show a plan and section of the 

 building, the main feature of which is the sound 

 chamber 27 ft. by 19 ft. and 19 ft. 10 ins. high. Here 

 the original intensity of the sound is measured. The 

 walls of this chamber are of 18-inch brick coated with 

 cement outside and with wood fibre plaster inside, 

 and the room as shown in the section has a separate 

 concrete foundation. From this room half-way up 



Fig. 1. — Riverbank labo: 



its walls three small testing chambers are provided 

 furnished with heavy steel doors to exclude sound 

 completely. Materials to be tested are placed across 

 these chambers, when the doors are opened to admit 

 sound from an organ in the sound chamber. The 

 organ is a complete 73 pipe instrument giving all the 

 tones of the musical scale from C 64 to C 4096. It is 

 operated electrically by the observer, -who notes the 

 time before a sound becomes inaudible in the test 

 chamber. To ensure equality of sound distribution 

 in the sound chamber a large steel reflector mounted 

 on a central shaft is made to revolve in the room on 

 a vertical axis. The main work, up to the late Prof. 

 Sabine's death, has been connected with the calibration 

 of the sound chamber and its instruments. This 

 laborious undertaking completed, the activities of the 

 laboratory should rapidly command a wider interest. 



The present director of the laboratory, Prof. Paul E. 

 Sabine, has recently published the results of an investiga- 

 tion on the nature and reduction of noises as occurring 

 in business offices. Scarcely anything has been done 



NO. 2765, VOL. I io] 



in the way of investigation on the subject of noise, 

 though the topic is obviously of wide interest. Prof. 

 Sabine begins by pointing out that the sound-absorbing 

 qualities of any material vary widely with pitch, and 

 instead of attempting to apply data obtained for 

 musical sounds, he wisely deals with the matter de 

 novo, taking the actual sources of sound, such as the 

 click of a typewriter, as the source for experimental 

 purposes. A distinction is drawn between sounds in 

 the open air and those in which reflection takes place, 

 as in a room, from the point of view of the effect 

 of the noise of one operator upon another. All but 

 two or three per cent, of sound waves falling on a 

 hard plaster wall are reflected, and in an experiment 

 cited there were found to be 500 reflections before a 

 given sound reached final decay. It would seem, 

 therefore, that as much absorption as possible by 

 walls and ceilings should be aimed at to prevent 

 these reflections. 



An important point brought out by these investiga- 

 tions is that the absorption efficiency of a given material 

 for both musical sound and noise is greater when the 

 material is employed in small units. In discussing 

 practical measures Prof. Sabine alludes to linings of 

 felt for walls, covered with some fabric, to light porous 

 tiles and plaster, citing a plaster recently developed 

 which is a much better absorber than ordinary plaster. 

 He even makes a distinction between painted and 

 unpainted walls, the general tendency of paints being 

 to fill up a porous surface and thus decrease sound 

 absorption, and numerical data are given showing the 

 relative value of various surfaces in absorbing the 

 sound of a typewriter. In these experiments the 

 difference of power of absorption of a given material 

 for various sounds, though existing, was found to be 

 small. 



Prof. Sabine has made a separate and special 

 investigation of the absorption of sound by rigid 

 walls and finds that the refraction effect on the passage 

 of the sound into the new medium is of only trifling 

 importance. His experiments have recently been 

 further extended to tests upon artificial aids to hearing. 

 He classifies the types of instruments commonly used 

 and describes investigations to measure the difference 

 of times during which residual sound may be heard 

 with and without a particular instrument as a measure 

 of the increase in loudness produced by that instrument. 

 His results are illustrated graphically. It was observed 

 that the highest tones in every case were less loud with 

 instruments than without, suggesting that the short 

 wave lengths enter the small cavity of the external 

 ear better than do the air columns of instruments. 

 With certain instruments also the lowest tone (frequency 

 128) was less well heard than without their aid. Prof. 

 Sabine does not consider the prospects of improvements 

 in alleviating extreme deafness to be good, but points 

 the way by reference to the amplification of telephone 

 currents by the thermionic tube, and he suggests a 

 joint attack on the problem by physicists and physi- 

 ologists. 



Another series of experiments on sound-proof parti- 



