RESPONSE FREQUENCY CHARACTERISTICS OF EARS 319 



cavities of the head. In case the air from the lungs cannot be used for 

 blowing the reed, a bellows may be employed. 



B. Vocoder^^. — The term vocoder is applied to a system for remaking 

 speech automatically from a buzzer-like tone and a hiss-like noise corre- 

 sponding to the vocal cord tone and the breath tone of normal speech. 

 Control of pitch and frequency spectrum obtained from the talker's speech 

 are applied to make synthetic speech copy the original speech sufficiently 

 for good intelligibility, although the currents used in such controls contain 

 only low syllabic frequencies of the order of 10 cycles per second as con- 

 trasted with frequencies of 100 to 3000 cycles in the remade speech. 



FREQUENCY IN CYCLES PER SECOND 



Fig. 13.1. Contour lines of equal loudness for normal ears. Numbers on curves indicate 

 loudness level. Odb = 10~^^ watts per square centimeter. db = 0.000204 dyne per square 

 centimeter. (After Fletcher and Munson.) 



13.4. Response Frequency Characteristics of Ears. — The loudness of 

 a pure tone depends upon the frequency and intensity. See Sec. 13.5 for 

 the definition of loudness. This relation is revealed in the Fletcher- 

 Munson^-^ equal loudness level curves shown in Fig. 13.1. The 1000 cycle 

 tone is the reference tone in these determinations. The loudness level of 

 other tones is the intensity level of the equally loud 1000 cycle tone. These 

 characteristics show that the ear is most sensitive in the region between 



1^ Dudley, Jour. Acous. Soc. Amer., Vol. XI, No. 2, p. 169, 1939. 



^•^ Fletcher and Munson, Jour. Acous. Soc. Amer., Vol. 5, No. 2, p. 82, 1933. 



