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LING 1001
Karin Nault

LING 1001 B  January 23, 2014  Acoustic Phonetics  • Looking at the physical aspects of the sound wave (acoustic characteristics of the sounds) • What is a sound wave? • Disturbance in the air set off by a movement – Vibration • Violin strings • Vocal folds • Not permanent • How? • Air molecules that want to keep a comfortable distance from each other. – One moves to right, the other move to the right as well. Compression = Molecules are more crowded together than usual Rarefaction = Molecules are spread farther apart than usual Periodic wave = Sound wave that repeats at regular intervals Hertz =Unit of measurement - Cycles/second - Hz - 20 – 20 000 Hz = perceived as sound - Highest frequency of telephone: 3 500 Hz • Very little essential information is lost Complex sound waves  • Simple sound waves at single frequencies that combine – See Figure 3, p. 71 • Sound wave produced by vocal folds = complex wave Vocal fold complex sound wave  Composed of • Fundamental wave – Repeats itself at the frequency of the opening and closing of the vocal folds • Set of harmonics – Repeat at frequencies which are multiples of the fundamental. • Vocal fold complex sound wave • Example: – Fundamental wave at 120 Hz – Second harmonic at 240 Hz, third at 360 Hz, … – First harmonic = fundamental frequency (pitch) • Source wave – Source: vibration of vocal folds in response to airflow from the lungs – Filter: vocal tract (acoustic resonator) • Acts as a filter to selectively transmit frequencies (produced by the larynx or within the vocal tract itself) through it. • Filter (vocal tract) Think of it as 1. a tube, closed on one end (glottis), open at the other end (lips) This type is called quarter-wave resonator. 2. a series of air-filled containers hooked up to each other. – Each container transmits certain frequencies. – Each has its own resonating frequencies (RF). – Overall RF differs from each of the single containers. 3. A variable resonator (frequency response changes depending on its shape) • Average male’s vocal tract: 17 cm in neutral position. • Quarter-wave resonator. Therefore, wavelength of the lowest RF is 17 x 4 = 68 cm • Formula to convert wavelength to frequency is Frequency = speed of sound divided by wavelength – Speed of sound: 34 000 cm/s – 34 000/68 = 500 Hz • Higher RFs of the vocal tract are odd-number multiples of the lowest. – Next resonance is at 1500 Hz, then 2500 Hz, then 3500 Hz… – These RFs are called formants. • Sound emerging from the filtering system has the same fundamental frequency and harmonics of the glottal sound. • Amplitudes of the harmonics have changed. – Some harmonics have been amplified, others have been damped.
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