590 BELL SYSTEM TECHNICAL JOURNAL 



method for predicting the loudness established by loudspeakers in enclosures. 

 A loudness-efficiency rating for loudspeakers is suggested, and its application 

 to sound-system engineering problems is described. 



A Sheet of Air Bubbles as an Acoustic Screen for Underwater Noise} 

 Donald P. Loye* and Wm. Fred Arndt. In Pearl Harbor, where there 

 often were eight hundred ships of all kinds, the underwater noise level was 

 high. No place was found where noise measurements could be made satis- 

 factorily, and therefore it was decided that the best arrangement would be to 

 insulate Auxiliary Repair Docks and measure the noise of submarines while 

 they were in the docks. This was done by the development of a suitable air 

 bubble screen across the open end of the dock. Such an acoustic barrier was 

 comparatively easy to install, did not interfere with submarines entering 

 and leaving, kept ocean surface oil out of the dock, insulated against low- 

 as well as high-frequency noises as was required and, after extensive experi- 

 mentation, the noise of the screen was reduced to a level that did not interfere 

 with the noise measurements. The insulation of the screen upon the noise 

 of a nearby submarine charging batteries is illustrated by a phonograph 

 recording. 



A Method of Determhting and Monitoring Power and Impedance at High 

 Frequencies} J. F. Mokrison and E. L. Younker. A method and newly 

 developed devices for determining and monitoring power and impedance 

 levels in transmission lines at high frequencies are explained. Practical 

 considerations influencing accurate determination of power and impedance 

 levels are analyzed, and the previous and newly developed methods of 

 monitoring these important quantities under changing conditions of load 

 are compared. 



Automatic Volume Control as a Feedback Problem? B. M. Oliver. 

 Feedback ampHfier theory is shown to be applicable to the usual a.v.c. 

 system. Expressions are derived for the loop gain in terms of the design 

 requirements and the gain-control characteristic of the controlled amplifier. 

 Using these expressions, the design of an a.v.c. system is quite straightfor- 

 ward and its characteristics, such as regulation and effect on desired modula- 

 tion, are readily predictable. 



^ Jour. Acous. Soc. Anter., March 1948. 



* Of Western Electric Co. 



« Proc. L K. E., February 1948. 



7 Proc. L R. £., April 1948. 



