CONTENTS xiii 



Chapter Page 



3. Line Microphone: Useful Directivity on the Line Axis. Two 



Lines and a Pressure Gradient Element 218 



4, Ultradirectional Microphone 220 



C. Directional Efficiency of a Directional Sound Collecting System. . 221 



9.7 Wind Excitation and Screening of Microphones 222 



9.8 Nonlinear Distortion in Microphones 223 



9.9 Transient Response of Microphones 224 



9.10 High Sensitivity Microphones (Motional Impedance) 224 



9.1 1 Thermal Noise in Microphones 225 



X. MISCELLANEOUS TRANSDUCERS 



10.1 Introduction . 226 



10.2 Telephone Receivers 226 



A. Bipolar Telephone Receiver 226 



B. Crystal Telephone Receiver 230 



C. Dynamic Telephone Receiver 230 



D. Inductor Telephone Receiver 232 



10.3 Phonographs 233 



A. Mechanical Phonograph 233 



B. Phonograph Pickups 234 



1. Crystal Pickup 235 



2. Magnetic Pickup 236 



3. Dynamic Pickup 238 



C. Distortion in Record Reproduction 238 



10.4 Electrical Musical Instruments 240 



10.5 Hearing Aids 241 



10.6 Sirens (Compressed Air Loud Speakers) , 242 



10.7 Supersonics 243 



10.8 Seismic Detectors 243 



XI. MEASUREMENTS 



11.1 Introduction 245 



11.2 Calibration of Microphones 245 



A. Response Frequency Characteristic 245 



1. Pressure Response 245 



(a) Pistonphone 246 



(b) Thermophone 247 



(c) Electrostatic Actuator 248 



2. Field Response 249 



B. Directional Characteristic 250 



C. Nonlinear Distortion Characteristic 251 



D. Phase Distortion Characteristic 252 



E. Electrical Impedance Characteristic 253 



F. Transient Response Characteristic 253 



