Speech 101 Notes

Speech 101 Notes

Measuring Sound

• Waveform (time vs. amplitude)
• Spectra (amplitude vs. frequency)
• Spectrogram (amplitude vs. frequency vs. time)

Warning: refer to "the energy at different frequencies" and NOT "the frequencies present in a signal"

Sound Characteristics

• Generation of sound requires energy
• Sound sources can be
  • Impulsive (explosion sound) - short burst of energy at all frequencies
  • Noisy (wind) - randomly varying energy at all frequencies
  • Repetitive (buzzing insect, alarm) - energy at the ‘fundamental’ frequency of vibration and its harmonics

Shaping Sound

If a sound wave encounters an obstacle, a proportion of its energy will be reflected.

• Constructive interference results in resonance (i.e the emphasis of energy)
• Destructive inference results in anti-resonance

Resonant frequency $f$ of a cylinder closed at one end and open at the other is given by $f=\frac{(2n-1)v}{4L}$ where,

• $v$ is the speed of sound
• $L$ is the length of the tube
• $n$ is a positive integer (1,2,3,…)

$n=1$ gives the frequency of the fundamental resonance $n=2,3,...$ gives the frequency of the harmonics

The Human Vocal Tract

In the neutral position, the human vocal tract is equivalent to a tube (open one end, closed other end)

The resonances for a 17.5cm (adult male) vocal tract are

• fundamental = 490Hz
• 1st harmonic = 1470Hz
• 2nd harmonic = 2450Hz
• 3rd harmonic = 3430Hz
• etc.

Vocal Source in Humans

Air pressure from the lungs builds up behind closed “vocal folds”

The vocal folds are repeatedly forced apart and pulled together again, producing a series of small pulses of air.

This modulation of the airstream is known as phonation.

The tension in the muscle attached to the vocal folds determines their rate of vibration and hence the fundamental frequency (and perceived pitch) of the voice.

Warning: refer to the fundamental frequency as "Fx" and not "F0"

Voice ‘Filter’

The vocal tract forms a resonator with a complex shape.

Resonances are known as formants.

Speech is produced by using the ‘articulators’ to change the shape of the vocal tract, hence modifying its resonant characteristics.

The principal articulator is the tongue, but the jaw, lips, soft palate and teeth are also involved

Sound Sources in the Vocal Tract

• A noisy sound can be generated anywhere there is partial constriction causing turbulent air flow.
• An impulsive sound can be generated by releasing a blockage (e.g at the lips)
• A repetitive sound can be generated by vibrating an articulator (e.g the tongue)
• A pure tone (e.g a whistle) can be generated by exciting a resonance.