CHAPTER XI 

 MEASUREMENTS 



11.1. Introduction. — The rapid progress made in acoustics during the 

 past two decades has resulted in a corresponding advance in acoustical 

 measurements. In applied acoustics, as in any applied science, theoretical 

 analysis and analytical developments are substantiated by experimental 

 verifications. In view of the importance of acoustical measurements, it 

 seems logical to devote a portion of this book to that phase of acoustics. 

 It is the purpose of this chapter to consider the testing of microphones, 

 loud speakers and telephone receivers together with fundamental acous- 

 tical measurements. 



11.2. Calibration of Microphones ^' 2. — A number of different measure- 

 ments are required to determine the performance of a microphone. The 

 most important characteristics which depict the performance of a micro- 

 phone are as follows: 



1. Response frequency characteristic. 



2. Directional characteristic. 



3. Nonlinear distortion characteristic. 



4. Phase distortion characteristic. 



5. Transient response characteristic. 



6. Electrical impedance characteristic. 



In addition to the above characteristics are such factors as the effect of 

 temperature, humidity and changes in atmospheric pressure upon the per- 

 formance of the microphone. Carbon microphones exhibit characteristics 

 peculiar to granular contacts such as carbon noise, packing and breathing. 



A. Response Frequency Characteristic. 



1. Pressure Response. — The pressure response frequency characteristic 



of a microphone is the ratio ejp as a function of the frequency where e is 



the open circuit voltage generated by the microphone in volts and p is the 



sound pressure in dynes per square centimeter upon the diaphragm of the 



^ American Standards Association Sectional Committee z-24, Report on, Cali- 

 bration of Microphones, Jour. Acous. Soc. Amer.^ Vol. 7, No. 4, p. 33, 1936. 

 2 Standards of the Institute of Radio Engineers, 1933. 



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