500 



ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 1924 



system mentioned above. By means of distortionless controls op- 

 erating on the amplifier the loudness of the reproduced speech is 

 varied through a very wide range. By inserting electric-wave filters 

 in the circuit the speech waves can be distorted in known ways. For 

 example, a low pass filter suppressing frequencies above 1,000 cycles 

 is connected in circuit and articulation tests made to find the intel- 

 ligibility carried by frequencies between and 1,000 cycles. For 

 experimental purposes it is practicable to construct such filters in 

 which the suppressed frequencies are diminished to one-millionth 

 or less of the values which Avould otherwise obtain, while the passed 



Loudness 



Fig. 6. — Articulation — Loudness characteristic for speech 



frequencies are scarcely affected. Similarly high-pass filters can be 

 made which suppress all frequencies up to a certain marginal region 

 and pass those above it. Such filter structures are made by the 

 proper combination of suitable inductance coils and condensers. 



Studies of interpretation further require an experimental method 

 for measuring the ability of the ear to understand transmitted 

 speech sounds with ditt'erent conditions of loudness and distortion 

 The method developed consists in pronouncing detached speech 

 sounds into the transmitting end of the experimental system and in 

 having observers at the receiving end write the sounds which they 

 hear. Comparison of the observed sounds with those called shows 



