A High Efficiency Receiver for a Horn-Type Loud 

 Speaker of Large Power Capacity 



By E. C. WENTE and A. L. THURAS 



Synopsis: This paper describes a telephone receiver of the moving coil 

 type which is particularly adaptable to the horn type of loud speaker and 

 which represents a notable advance over similar devices at present avail- 

 able. Its design is such as to permit of a continuous electrical input of 30 

 watts as contrasted with the largest capacity heretofore available of about 



5 watts. In addition, measurements show that the receiver has a con- 

 version efficiency from electrical to sound energy varying between 10 and 

 50 per cent in the frequency range of 60 to 7,500 cycles. Throughout most 

 of this range, its efficiency is 50 per cent or better. This contrasts with an 

 average efficiency of about 1 per cent for other loud speakers either of the 

 horn or cone type. Combining the 50 fold increase in efficiency with a 5 or 



6 fold increase in power capacity, a single loud speaker unit of the type here 

 described is capable of 250 to 300 times the sound output of anything 

 heretofore available. 



This device is in commercial use in connection with the V'itaphone and 

 Movietone types of talking motion pictures. As commercially produced in 

 quantities numbering several thousand, an average efficiency of the order 

 of 30 per cent has been realized. 



BEFORE the advent of radio-broadcasting, practically the only 

 loud-speakers in commercial use were of the horn type. In recent 

 years this type has been largely supplanted by others of more compact 

 des'gn. However, where appearance and size are not of prime im- 

 portance, a loud speaker with a horn still has a large field of service, 

 as, for instance, in public address equipment or in systems for reproduc- 

 ing speech and music in large auditoriums from wax or film records. 

 For such uses, the greater directivity obtained by the use of a horn 

 has in some respects definite advantages. In the design of the re- 

 ceiver about to be described we have had in view particularly the 

 requirements for such services, where the following qualifications were 

 deemed of the greatest importance: a good response-frequency char- 

 acteristic up to at least 5,000 p.p.s., large power output without 

 amplitude distortion, high efficiency, and constancy of performance. 



As this paper is concerned with the design of a driving unit, or the 

 receiver proper, and not with the horn, we shall confine our discussion 

 to the operation of the receiver when connected to a tube of infinite 

 length and of the same cross-sectional area as the throat of the horn. 

 An ideal horn should have at its throat the same acoustic impedance ^ 



' The term acoustic impedance as here used may be defined as the ratio of pressure 

 to rate of volume displacement. 



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