Published on January 10, 2009
The Voice Mechanism Muscular contractions within the chest force air from the lungs through components of the vocal mechanism Stream of air serving as the energy carrier is modulated in its velocity and pressure to produce sounds The modulation is achieved in two distinct ways giving voiced and unvoiced sounds
Fig. 1: Elements of Voice Mechanism
Voiced Sounds Include vowels of ordinary speech and tones characteristic of the singing voice Primary modulating agent is the larynx, across which are stretched the vocal cords -- two membrane-like bands with a slit-like opening which modulates the air-stream as it vibrates Fundamental frequency of modulation is determined by the tension to which the vocal cords are stretched Vocal cords produces a sawtooth type of variation in pressure
Fig. 2a: Pressure output at vocal chords Fig. 2b: Pressure output for vowel sound ‘a’ as in father
Modulation Resonating cavities of the nose, mouth and throat form an acoustic network which modulates the pressure wave A variety of voiced sounds are produced by changing the position of the tongue or altering the configuration of the lips Fig. shows how the output at the vocal cords may be modified so as to produce the vowel sound ‘a’ as in father
Unvoiced sounds Sounds produced without using the vocal cords! Also called breath sounds Produced by steady and forcible exhaling of air, causing turbulences in the stream through irregularities along the vocal tract Include fricative consonants as ‘f’ and ‘s’ ; stop consonants ‘p’, ‘t’, and ‘k’ - modulation using lips, teeth and tongue respectively
Note A Fourier of the voiced sounds shows a large number of harmonically related frequency components Analysis of unvoiced sounds reveals a continuous band of frequencies largely confined to the upper portion of the audible frequency range
Acoustic Power Output of Speech Average speech power in conversation is about 10 microwatts, averaged over time intervals of 2-4 seconds When loud it rises to about 200 microwatts and upon shouting to about 1000 microwatts The speech power associated with whispering is about 0.001 microwatt. whisper-normal-loud-shout
Fluctuations When power is averaged over a short time interval compared to the duration of a syllable (about 0.2 sec), large fluctuations in level are observed as different speech sounds are uttered e.g. the power of the vowel o as in low is about 50 microwatts, whereas the weak consonant v has an average power of only 0.03 microwatt.
Fig. 3: Spectrum level curve for average conversational speech re 0 . 0002 microbar at a 1m for males
Speech Spectrum Average speech spectrum: distribution of speech power with frequency averaged over wider time intervals Ordinate represents pressure spectrum level i.e. SPL in bands of a single cycle/sec width, measured at a distance of 1m from the lips of the talker SPL integrated over entire band from 100 to 10 kHz is ~ 65 dB Total acoustic output ~ 20 microwatts: computed assuming intensity to be uniformly distributed over a 1m radius hemisphere centered on the mouth of the talker
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