MEASUREMENT OF ACOUSTIC IMPEDANCE 271 



the frequency is sometimes used to depict the response of a telephone 

 receiver. The artificial ear shown in Fig. 11.15 may be used for this 

 purpose by closing the slits. 



The artificial mastoid is a system for objectively measuring the response 

 of a bone conduction telephone receiver. In one form, the artificial mas- 

 |.qJ(^33b consists of a rubber block having approximately the same imped- 

 ance as the human head at the mastoid bone. The velocity which the 

 bone conduction receiver delivers to this impedance is measured by a 

 vertical or hill and dale phonograph pickup. See Sec. 10.353. 



11.5. Artificial Voice ^' ^^. — The artificial voice is a term applied to a 

 loud speaker which exhibits some of the characteristics of the human voice. 

 One form of the artificial voice consists of a small dynamic loud speaker 

 enclosed in a spherical case of the volume of the average human head. 

 Using a cone one and one half inches in diameter and back enclosed, it is 

 possible to obtain very uniform response over the range from 60 to 15,000 

 cycles. See Sec. 7.2. Due to the small diameter of the cone, the com- 

 pliance of the case is negligible and the controlling compliance is the cone 

 suspension. See Sec. 7.10. The directional characteristics of this loud 

 speaker are very close to those of the human mechanism. The artificial 

 voice is useful for obtaining the response frequency characteristic of close 

 talking microphones because the reaction of the loud speaker upon the 

 microphone is practically the same as that of the human voice. It is also 

 useful for general development work on microphones because, with the 

 small cone, the irregularities in response can be made less than ±1 db. 

 The power output at a distance of one to two feet is sufficient for most 

 development work on microphones. 



11.6. Measurement of Acoustic Impedance. — There are a number of 

 methods of measuring acoustic impedance. A purely acoustical means for 

 measuring acoustic impedance has been devised by Stewart. This method 

 measures the change in acoustic transmission through a long uniform tube 

 when the unknown impedance is inserted as a branch. 



The acoustic impedance bridge ^^'^^ is shown schematically in Fig. 11.16. 



ss-BHawIey, M. S., Bell Laboratories Record, Vol. 18, No. 3, p. 73, 1939. 



3* Inglis, Gray and Jenkins, Bell SysL Tech. Jour., Vol. 11, No. 2, p. 293, 1932. 



3^ Olson and Massa, " Applied Acoustics," 2nd Ed., P. Blakiston's Son and Co., 

 Philadelphia, 1939. 



36 Stewart, G. W., Phys. Rev., Vol. 28, No. 5, p. 1038, 1926. 



3^ For other systems for measuring acoustic impedance see Olson and Massa, 

 " Applied Acoustics," 2nd Ed., P. Blakiston's Son & Co., Philadelphia, 1939; 

 Flanders, P. B., Bell Syst. Tech. Jour., Vol. 11, No. 3, p. 402, 1932; Morse, P. M., 

 " Vibration and Sound," p. 209, McGraw Hill Book Co., New York, 1936. 



