APPLICATION OF ARTICULATION TESTING 365 



measuring circuit is used alternately to measure the rectified voltage 

 resulting from the speech energy and that from the noise. The volt- 

 ages to be measured are arranged to be negative. The measurement, 

 in principle, is a determination of the amount of positive voltage which 

 must be added to the negative voltage so that the net voltage applied 

 to the grid of a "trigger tube" reaches the operating point of the tube. 

 The positive voltage is obtained from a rotary potentiometer which is 

 driven by a synchronous motor through a magnetic clutch (which is 

 required to start the apparatus in synchronism with the data recording 

 and analyzing equipment). It makes one complete revolution for a 

 measurement of speech and another for the measurement of noise. 



The fraction of a complete revolution which the potentiometer 

 makes between the start of a cycle and the time at which the "trigger 

 circuit" operates indicates the magnitude of the rectified voltage being 

 measured and is arranged to be directly proportional to the number of 

 db that the magnitude of the speech or noise is above some previously 

 chosen reference value. The sum of these fractional rotations for the 

 66 sentences of a list gives a measure of the average speech magnitude 

 during the calling of a list. This sum is obtained mechanically by a 

 totalizing device which is essentially a revolution counter coupled to 

 the shaft of the rotary potentiometer through a magnetic clutch. As 

 each of the 66 sentences is called the totalizer moves ahead. Its final 

 reading shows the average volume for the whole list. Exactly the 

 same operations take place to give the average noise volume through- 

 out the calling of a list. 



The switching of the apparatus from the condition in which it is 

 set up to measure the speech magnitudes to that for the measurement 

 of noise magnitudes is under the control of timing contacts on the 

 shaft of the motor-driven potentiometer. However, since callers may 

 occasionally fail to finish calling a sentence in the allotted time and 

 as a result deliver some speech energy during the time when noise 

 only is supposed to be measured, an auxiliary voice operated control 

 is required to keep the equipment from starting a noise measurement 

 until the speech has stopped. The auxiliary control is operated by the 

 voltage produced by the speech wave in the caller's control circuit at 

 the point where the automatic volume indicator is connected. 



Observers' Circuit 

 A special circuit, shown in Fig. 5, is needed at the receiving end to 

 enable four observers to work at the same time. An amplifier is 

 necessary to preserve the proper relationships between the magnitudes 

 of speech, sidetone noise, and room noise leaking under the receiver 

 cap. This amplifier, which gives a gain of 6 db (offsetting the loss of 



