406 BELL SYSTEM TECHNICAL JOURNAL 



A microphone that responds to the velocity of the air particles has 

 equal sensitivity for sound waves traveling in opposite directions. 

 Weinberger, Olson and Massa ^^ have modified the ribbon microphone 

 so that its response is not the same for the positive as for the negative 

 direction of propagation of the sound wave. So modified the micro- 

 phone has a greater response for sound waves coming towards one 

 side of the ribbon than for those coming towards the other side. In a 

 plane wave the magnitude of the pressure and the velocity are propor- 

 tional. For waves traveling in a positive direction the two are in 

 phase and for waves traveling in the negative direction they are in 

 opposite phase. If, then, a pressure and a velocity microphone of 

 equal sensitivity are connected in series the resultant voltage will be 

 double for sound of normal incidence coming from one direction and 

 equal to zero for sound coming from the opposite direction. Wein- 

 berger, Olson and Massa placed an appropriate acoustic impedance 

 over a part of the ribbon so as to give this part the characteristics of 

 a pressure microphone, while the other part of the ribbon was left 

 free so as to function as a velocity microphone. The voltage at the 

 ends of the ribbon is then proportional to the vector sum of the pres- 

 sure and the velocity in the sound wave. In this way a pressure and 

 velocity microphone combination is obtained in one instrument. It 

 is insensitive to sound falling at perpendicular incidence on one side 

 of the ribbon but not to sound propagated in the plane of the ribbon, 

 as in the case of a velocity ribbon microphone. 



Electrical Instruments of Particular Interest In Acoustical 



Studies 



So far our discussion has been restricted mainly to microphones and 

 instruments used in their calibration. There have, of course, in recent 

 years been developed many other devices especially adapted for the 

 quantitative study of particular acoustic problems, but a rather ex- 

 tensive discussion of the microphone has been given because it is 

 an adjunct in almost all of these other instruments. In great part 

 acoustic measurements are today made by first translating sound into 

 a corresponding amplified electric current. The results of measure- 

 ment or analysis of this current may then be referred back to the 

 sound if the characteristics of the translating device are known. The 

 type of analyzer or measuring instrument applied to the electrical 

 circuit depends then altogether upon the kind of information that is 

 desired. Strictly speaking we should classify these not as acoustical 

 but as electrical instruments. In fact, every kind of electrical instru- 



■"■' Jour. Aeons. Soc. Amer. 5, K19 (1934). 



