268 



MEASUREMENTS 



terns. In this case the phase shift may be several hundred degrees (equiv- 

 alent to a sound path difference of several feet). See Sec. SAB. 



F. Impedance Frequency Characteristic. — The impedance character- 

 istic of a loud speaker is the impedance at the input terminals as a function 

 of the frequency. The plot of the characteristic should also include the 

 resistive and reactive components. 



A one-to-one ratio impedance bridge may be used and should be capable 

 of measuring the impedance at the full power output of the speaker. The 

 power input should be included with every impedance characteristic. If 

 the impedance characteristic varies with power input, it is desirable to 

 show a series of impedance frequency curves for various inputs. Other 



SQUARE 



WAVE 



GENERATOR 



CM * 



LOUD SPEAKER MICROPHONE 



M/l/lM 



CATHODE 



RAY 



OSCILLOGRAPH 



Fig. 11.14. Schematic arrangement of the apparatus employing a square wave generator 

 and a cathode ray tube for indicating the transient response characteristics of acoustical 

 apparatus such as microphones or loud speakers. 



methods may be used as, for example, the three voltmeter and a known 

 resistance method. 



G. Transient Response Characteristic. — The measurements in the pre- 

 ceding sections have been concerned with steady state conditions. In all 

 types of sound reproduction the phenomena is of a transient character. 

 For this reason it is important to measure the response of the system to a 

 suddenly applied force or voltage. The Heaviside Operational Calculus is 

 a very powerful tool for predicting the performance of a system to a sud- 

 denly applied force or voltage. See Sec. 7.16. 



The apparatus for investigating the transient response of an audio 

 system is shown schematically in Fig. 11.14. The output of a square 

 wave generator is fed to the apparatus to be tested. The output of the 



