Acoustics of Speech: The Voice Mechanism

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Information about Acoustics of Speech: The Voice Mechanism

Published on January 10, 2009

Author: farhatsurve



Voice, Speech, Voiced, Unvoiced, Speech Spectrum

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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