A CO U STIC A L INSTR UMENTS 



405 



strip of aluminum with circuit terminals at its two ends. This ribbon 

 is placed in a magnetic field so that the lines of force lie in the plane of 

 the ribbon and perpendicular to its long dimension, as shown in Fig. 7. 

 Motion of the ribbon set up by sound waves will then generate a 

 potential between its terminals. This type of microphone construc- 

 tion was first suggested by Reinganum.*^ It was developed into a 

 practical form by Gerlach ^^ and Schottky.^"* They apparently pre- 

 ferred to shield one side of the ribbon so that the instrument operated 

 as a pressure microphone. H. F. Olson, ^^ recognized the greater 

 simplicity of the instrument in construction and in operation if both 

 sides of the ribbon were freely exposed to the air. Constructed in this 



Fig. 7 — Ribbon microphone. 



way the instrument is virtually a velocity microphone at least at low 

 frequencies. At the higher frequencies the ribbon with its surround- 

 ing structure almost completely shields the rear from sound reaching 

 the front of the ribbon at perpendicular incidence. Under these con- 

 ditions the instrument operates substantially as a pressure microphone, 

 but even at these frequencies sound reaching the instrument from a 

 direction parallel to the plane of the ribbon is without effect. Curves 

 published by Olson on the directional characteristics of this micro- 

 phone show that in a plane perpendicular to the axis of the ribbon the 

 variation of response with direction follows approximately a cosine 

 law, where the angle is measured from a line drawn normal to the plane 

 of the ribbon. The relationship is more complicated over a plane 

 passing through the axis of the ribbon. 



*'-Phys. Zs. 11, 460 (1910). 



^^Phys. Zs. 25, 675 (1924); Wiss. Ver. Siemens-Konz 3, 139 (1923). 



^^Phys. Zs. 25, 672 (1924). 



^^ Jotir. Acous. Soc. Amer. 3, 56 (1931). 



