PHONOGRAPHS 233 



sure in the ear cavity under these conditions is as follows: the velocity of 

 the diaphragm below 300 cycles must be inversely proportional to the fre- 

 quency; between 300 cycles and 500 cycles the velocity should be inde- 

 pendent of the frequency and above 500 cycles the velocity should be 

 proportional to the frequency. 



The equivalent circuit of a telephone receiver which delivers practically 

 uniform sound pressure to the ear cavity in the presence of a normal leak 

 is shown in Fig. 10.5. The equivalent circuit of the ear is shown dotted. 

 The " V " shaped diaphragm is driven by a straight conductor located in 

 the bottom of the " V." The electrical circuit is the same as in the case 

 of the dynamic telephone receiver. The pressure p may be considered to be 

 independent of the frequency. 



The response frequency characteristic taken on an artificial ear is in- 

 dicated by A, Fig. 10.5. The constants were chosen to give the smoothest 

 response between 60 and 7000 cycles. The response frequency characteris- 

 tic with the receivers feeding a plain cavity is indicated by 5, Fig. 10.5. 

 The small difference between the response with and without a leak indicates 

 the effectiveness of this type of vibrating system in minimizing the effect 

 of the leak between the ear and the ear cap. 



10.3. Phonographs. — A phonograph is a system for the reproduction of 

 sound from a record. To-day, a phonograph usually refers to a system in 

 which a stylus (needle) follows the undulations in the groove of a record 

 and transforms these undulations into the corresponding acoustic or elec- 

 trical variations. The record may take the form of a cylinder or a flat disk. 

 To-day, the flat disk record is almost universally used for entertainment 

 while the cylindrical record is used for dictographs. In the hill and dale 

 or vertical type record the undulations are cut in a direction normal to the 

 surface. In the lateral record the undulations are cut in a direction parallel 

 to the surface of the record. The lateral records are used for home repro- 

 duction. Both vertical and lateral records are used for high quality re- 

 productions as, for example, in transcriptions for broadcasting. The 

 systems used in the recording and processing of phonograph records will 

 be considered in Sec. YIAF. It is the purpose of the sections which follow 

 to consider a mechanical phonograph, phonograph pickups and distortion 

 in phonograph reproduction. 



A. Mechanical Phonograph'^ . — A mechanical phonograph is a mechano- 

 acoustic transducer actuated by a phonograph record and by means of 

 an acoustical system radiates acoustic energy into a room or open air. A 



6 Maxfield and Harrison, Bell Syst. Tech. Jour., Vol. 5, No. 3, p. 493, 1926. 



