ABSTRjiCTS OF TECHNICAL ARTICLES 389 



pulses and time division to multiplex the channels. The eight message 

 circuits which each AN/TRC-6 system provides are high-grade telephone 

 circuits and can be used for signaling, dialing, facsimile, picture transmission, 

 or multichannel voice frequency telegraph. Two-way voice transmission 

 over radio links totaling 1,600 miles, and one-way over 3,200 miles have 

 been accomplished successfully in demonstrations. 



Further Observations of the Angle of Arrival of Microivaves? A. B. 

 Crawford and William M. Sharpless. Microwave propagation measure- 

 ments made in the summer of 1945 are described. This work, a continua- 

 tion of the 1944 work reported elsewhere in this issue of the Proceedings of 

 the I.R.E. and Waves and Electrons, was characterized by the use of an 

 antenna with a beam width of 0.12 degree for angle-of-arrival measurements 

 and by observations of multiple -path transmission. 



The Ejffect of Non-Uniform Wall Distributions of Absorbing Material on the 

 Acoustics of Rooms} Herman Feshbach and Cyril M. Harris. The 

 acoustics of rectangular rooms, whose walls have been covered by the non- 

 uniform application of absorbing materials, is treated theoretically. Using 

 appropriate Green's functions a general integral equation for the pressure 

 distribution on the walls is derived. These equations show immediately 

 that it is necessary to know only the pressure distribution on the treated 

 surfaces to predict completely the acoustical properties of the room, such 

 as the resonant frequencies, the decay constants, and the spatial pressure 

 distribution. The integral equation is solved approximately using (1) 

 perturbation method, and (2) approximate reduction of the integral equation 

 to an equivalent transmission line. Criteria giving the range of validity of 

 these approximations are derived. It was found useful to introduce a new 

 concept, that of ^^efective admittance,''^ to express the results for the resonant 

 frequency and absorption for then the amount of computation is reduced 

 and the accuracy of the results is increased. The absorption of a patch of 

 material was found as a function of the position of the absorbing material 

 and was checked experimentally for a convenient case, an absorbing strip 

 mounted on the otherwise hard walls of a rectangular room. Particular 

 attention is given to the case where the acoustic material is applied in the 

 form of strips. The results may then be expressed in series which converge 

 very rapidly and are, therefore, amenable to numerical calculation. Ap- 

 proximate formulas are obtained which permit estimates of the diffusion 

 of sound in a non-uniformly covered room. In agreement with experience, 

 these equations show that diffusion increases with frequency and with the 



^ Proc. I.R.E. and Waves and Electrons, November 1946. 

 ^Joiir. Aeons. Soc. America, October 1946. 



