298 BELL SYSTEM TECLINICAL JOURNAL 



type of receiver is removed, the throat insert of the receiver modified 

 and a simple structure mounted in place of the horn coupler having an 

 opening effectively that of the human mouth. In this structure is 

 mounted an acoustic-resistance element. This has a mechanical 

 resistance^ of approximately 41.0 mechanical ohms per cm.^ and 

 below 5000 c.p.s. a reactance of less than 10 per cent. There is 

 mounted on the structure replacing the horn coupler, a guard ring 

 which serves as a reference plane for measurements of distance between 

 the artificial mouth and instruments under test. The location of this 

 reference plane has been empirically determined so as to correspond to 

 the plane of the lips of a human mouth. 



At low frequencies the radius of the opening of the mouth is small 

 compared to the wave-length. Hence in effect a point source of 

 sound is approached. Under these conditions the radiation resistance 

 is small. As the frequency increases the radiation resistance increases 

 until the wave-length has decreased to a value approximately three 

 times the radius of the opening. At this frequency and above, the 

 radiation resistance is approximately constant at about 41.0 ohms 

 per cm.^ The output impedance of the artificial mouth is high with 

 respect to the radiation impedance at low frequencies. Inasmuch as 

 the impedances are not matched except at the higher frequencies the 

 output power should be approximately proportional to the radiation 

 resistance. However, this relationship is modified by the resonances 

 of the instrument. The acoustic resistance reduces these resonances 

 and also serves to reduce the reaction on the artificial mouth which 

 might arise by placing an instrument close to and directly in front of it. 



The response-frequency characteristic of the mouth measured at the 

 guard ring is shown in Fig. 3. In order that this response may be 

 uniform over the important speech frequency range of 100 to 7500 

 c.p.s. the characteristic shown in Fig. 3 is equalized with the network 

 indicated in Fig. 1. The resulting response of the artificial voice is 

 shown in Fig. 4. 



Another important requirement is that the artificial mouth shall 



be capable of delivering without non-linear distortion sound outputs 



corresponding to what may be termed loud talking for human beings. 



Because it is desirable to operate at frequencies as low as 100 c.p.s. 



it is necessary to supply a comparatively large amount of electrical 



energy to the artificial mouth. To accomplish this an amplifier has 



been employed as indicated in Fig. 1 which makes it possible to obtain 



* "Methods of High Quality Recording and Reproducing of Music and Speech 

 Based on Telephone Research," J. P. Maxfield and H. C. Harrison, Bell System 

 Technical Journal, July, 1926, p. 506. 



