496 BELJ. SYSTEM TECHNICAL JOURNAL 



increased reverberation. The data obtained thus far are insufficient 

 to permit assignment of quantitative values to the importance of these 

 two factors. 



At the present state of the art, the most important requirement of a 

 recording or reproducing system is its frequency characteristic. This 

 involves two factors — intensity x'crsus frequency, and phase distortion 

 versus frequency. The effect of the second of these factors is not thor- 

 oughly understood but as it is closely related to the production of tran- 

 sients it has to be considered, as mentoined above. The system to be 

 described is, however, relatively free from violent phase shifts within 

 most of the range covered, but does have some undesirable phase-shift 

 characteristics with small accompanying transients near its limiting 

 cut-off frequencies. 



Frequency Requirements 



The frequency range which it would be desirable to cover if, it were 

 possible, with relatively uniform intensity for the transmission of 

 speech and all types of music including pipe organ is from about 16 

 cycles per second to approximately 10,000. 



It may be interesting to examine the record requirements for a band 

 of frequencies this great. For the purpose of this illustration, a lateral 

 cut record will be assumed although in all the factors except the time 

 which the record will run, the arguments apply in a similar manner to 

 the hill-and-dale cut. Since, for mechanical reproduction, the sound at 

 a given pitch is radiated by means of a fixed radiation resistance, it is 

 necessary that the record must be cut with a device the square of whose 

 velocity is proportional to the sound power. Under these conditions, it 

 is seen that for a given intensity of sound the amplitude is inversely 

 proportional to the frequency of the tone, and that a point will be 

 reached somewhere at the low end of the sound spectrum where this 

 amplitude will be great enough to cut from one groove into the adja- 

 cent groove, or in case of vertical cut, to cut so deeply that with present 

 materials the wax will tear instead of cut away with a clean surface. 

 This means that there is an inherent maximum amplitude beyond 

 which it is not commercially feasil)le to go. Similarly the minimum 

 radius of cur\aturc of sine wa\'es of various frequencies cut at constant 

 velocity is inversely proportional to the frequency, so that as higher 

 and higher frequencies are reached the radius of curvature becomes 

 smaller and smaller until finally it l)ecomes too small for the reproduc- 

 ing needle to follow. There is, therefore, an inherent limit at the upper 

 end. 



In order to extend these limits, it is necessary in the case of the low 



