270 



MEASUREMENTS 



uator. The voltage across the receivers is adjusted until the intensity in 

 the observer's ear seems to be the same as the free wave intensity from the 

 loud speaker. This procedure is repeated at several frequencies, keeping 

 the free wave pressure constant. The reciprocal of the voltage across 

 the phones required to match the free wave sound intensity is proportional 

 to the sensitivity of the receivers at each frequency. 



B. Objective Measureynents 3i-32,33,33a_ — ^he impedance frequency char- 

 acteristic looking through the ear cap of a telephone receiver as normally 

 worn has been investigated by Inglis, Gray and Jenkins. These are 

 shown in Fig. 11.15. An artificial ear and the equivalent circuit which 

 yields approximately the same impedance characteristic is shown in 

 Fig. 11.15. A standard condenser microphone is used to measure the 



Zao'IVo+J^ 



-j-VA^^-V^ 



DIA- 

 PHRAGM ^ 10, 



— 1000 luuuu 



FREQUENCY 

 CROSS-SECTIONAL VIEW 



Fig. 11.15. A cross-sectional view of an artificial ear employing a standard condenser 

 microphone and the equivalent electrical circuit of the acoustical system. The graph shows 

 the resistive r^o and reactive Af^o components of the acoustic impedance, looking into the 

 aperture of the ear cap, as a function of the frequency. CaE acoustic capacitance of the 

 cavity. Me and r^B inertance and acoustic resistance of the leak. 



pressure. A series of slits corresponding to the leak between the ear 

 cap and the ear are represented by the inertance Me and acoustic resistance 

 TaE' The walls of the cavity Cae (4 cubic centimeters) are lined with felt 

 to reduce resonances at the high frequencies. The response frequency 

 characteristic obtained upon the artificial ear in general, agrees quite 

 well with the subjective tests. 



The tests outlined for loud speakers may be performed upon telephone 

 receivers by employing the artificial ear. The same apparatus may of 

 course be used and will not be repeated here. 



The pressure delivered by a telephone to a closed cavity as a function of 



31 Inglis, Gray and Jenkins, Bell Syst. Tech. Jour., Vol. 11, No. 2, p. 293, 1932. 



32 Olson and Massa, Jour. Acous. 'Soc. Amer., Vol. 6, No. 2, p. 250, 1935. 



33 Olson, H. F., Jour. S. M. P. E., Vol. 27, No. 5, p. 537, 1936. 



33^ Olson and Massa, "Applied Acoustics," P. Blakiston's Son and Co., Philadel- 

 phia. 



