222 BELL SYSTEM TECHNICAL JOURNAL 



such devices consists of a high ratio step-up transformer with its pri- 

 mary connected directly across the receiver to be protected. The 

 secondary is connected to a low voltage discharge gap. Any abnormal 

 voltage across the primary operates the discharge gap and the trans- 

 former becomes a low-impedance shunt. A number of field trials of 

 these reducers applied to operator's receivers have been made. While 

 not affording the full degree of protection desired they have been found 

 to reduce substantially the severity of acoustic shocks and it is believed 

 that they will be of considerable benefit in cases where some form of 

 protection against acoustic shocks to operators is urgently required. 



Another device based on the shunting principle consists of opposingly 

 poled copper oxide rectifiers connected across the receiver. These have 

 the property of greatly diminishing impedance with increasing voltage. 

 The problem is to obtain a sufficiently sharp change in impedance 

 with voltage, while avoiding a normal impedance so low as to cause 

 serious transmission losses. As an aid to this end, biasing batteries are 

 under investigation. 



The committee has also investigated the saturating characteristics of 

 a vacuum tube for acoustic shock reduction. The properties of a 

 vacuum tube are such that the output current cannot be increased sub- 

 stantially beyond a definite value regardless of the input voltage. This 

 feature can be made use of to limit shocks by a design which will pass 

 currents substantially without distortion up to approximately the 

 highest value of signal current used, thus cutting down the shock vol- 

 tages which exceed the normal signals. While quite effective, this 

 method involves apparatus which is more bulky and expensive than 

 the transformer and spark-gap type reducer. Telephone repeaters 

 accomplish this result to some extent and are being investigated by the 

 subcommittee, to determine the quantitative reduction of acoustic 

 shock by this means under practical conditions. 



In cases where toll or trunk lines are exposed, an acoustic shock 

 reducing device which could be placed at the ends of the lines would 

 have the advantage of protecting subscribers as well as operators. 

 Development work to obviate certain difficulties in using such a device 

 is under way. 



An effort is being made to develop a telephone receiver which will 

 saturate between the values of current required for effective speech 

 transmission and values of current which produce acoustic shock. 

 This requires a sharp bend in the saturation curve of the iron employed 

 in the receiver magnetic circuit. Until the development of permalloy, 

 this feature was not approachable, but experimental permalloy re- 

 ceivers have now been developed, and, while it has not yet been possible 



