SPEECH 59 



axon. Speculations are rampant on the mechanism of this transposition. 

 Transduction through changing electrical potentials across the receptor cell 

 wall is currently a very popular generalization; but reliable details of mech- 

 anism, unfortunately, are too few. 



SPEECH 



Three resonators, or vibrating cavities, are responsible for the organized 

 noise which we call speech. They are (1) the vocal chords, which close the 

 exit used by air exhaled from the lungs; (2) the throat and the mouth; and 

 (3) the nasal cavity. The vocal chords, the tongue, and the lips control the 

 changes in vibration which are induced in the exhaling air stream and which 

 are the sounds of speech. The combination of these three moving parts, each 

 of which can take several different shapes, gives remarkable versatility in the 

 production of sound. 



The fundamental sounds of speech are divided into six classes: pure 

 vowels, diphthongs, transitionals, semivowels, fricative consonants, and stop 

 consonants. The subject of phonetics is well known, is heavily illustrated in 

 any good dictionary, and needs no review here. 



Amplitude and intensity are controlled mainly by the rate of expulsion of 

 air, although secondary resonators such as the head and the chest play a 

 small role. 



Speech sounds have been analyzed on many people by the Bell Telephone 

 Laboratories, for obvious reasons. Some of the results are contained in the 

 classic book by Fletcher. 3 For instance "oo" as in "pool" spoken by men 

 (by women) has a mean fundamental frequency of 140 cps (270 cps), a mean 

 low frequency of 411 (581 for women), scattered high frequency of 3700 

 (4412 for women). All speech sounds have been carefully recorded and ana- 

 lyzed, and the sounds of the "average man" used for microphone design. 



The fundamental speech sounds have a power. When one talks as loudly 

 as possible without shouting, the average speech power is about 1000 micro- 

 watts (1 nw = 0.000001 w) at the source. When one talks in as weak a voice 

 as possible, without whispering, it drops to 0.1 fiw. A very soft whisper has 

 a power of about 0.001 ^w. Very loud speech is ~20 db over average speech 

 power; a soft whisper is ~40 db under average. 



NOISE 



High-intensity noise has become one of the most disturbing problems of 

 the modern way of life. Noise is usually defined as any unwanted sound, 

 and hence the classification is highly subjective. High-intensity noise is 

 usually defined as any unwanted sound greater than 85 db (see Table 3-1). 



