# Engineering Acoustics/Vocal Folds

## Introduction

The mechanism of sound production in speech and singing would be the
results of airflow in the human lung system, and is also connected to
the digestive system. The diaphragm action from the lungs pushes air
through the vocal folds, and produces a periodic train of air pulses.
This pulses train is shaped by the resonances in the vocal tract, and
has various frequencies and loudness. Vocal formants are basic
resonances, and can be changed by the movements of the articulators to
produce different vowel sounds. To produce different vowel sounds, the
vocal mechanism is controlled to produce the resonances of the vocal
tract is produced the vocal formants. The vocal tract can be considered
to be a cavity resonator, and the soft palate position, the area of
opening, the tongue positions an shape, and the position of jaw would
established the shape of this cavity. More specifically, voice
articulation is the movement of the tongue, pharynx palate, jaw or lips
that changes the volume of cavity, area of opening, and port length,
which determine the frequency of the cavity resonance. The voice
mechanism can be modeled as the lung and diaphragm being the power
source, along with the larynx, pharynx, mouth and nose. At the end of
the tubular larynx rest the vocal folds, also known as vocal cords.
During speech and singing, the larynx is connected to pharynx, and is
covered by epiglottis during swallowing. The vocal tract acts as a
resonator.

![](Voice_production_organs.png "Voice_production_organs.png")

## The vocal folds

Vocal folds are twin enfoldings of mucous membrane that act as a
vibrator during phonation. Phonation is the process by which the energy
from the lungs in the form of air pressure is converted vibration that
is perceivable to the human ear. There are two methods to phonation. One
is by the air pressure setting the elastic vocal folds into vibration,
which is called voicing. The other is air passing through the larynx to
vocal tract, where airstream gets modified as produces transient of
aperiodic sound waves. In aperiodic phonation, the transient or
aperiodic sound waves generates plosive sound, /t/, where sound is
produced by blocking the airstream and suddenly releasing the built-up
air pressure, fricative sound, /sh/, where a continuous noise type
sounds is made by forcing air through a constricted space, affricate
sound, /ch/, which is a combination of plosive and fricative sound, and
a voiced consonant, /d/, which is a plosive sound followed by a voiced
sound. While vocal folds are open for breathing, the folds are close by
the pivoting of the arytenoid cartilages for speech of singing. Positive
air pressure from the lungs forces the vocal folds to open but the high
velocity air produces a lowered pressure due to the Bernoulli equation
which brings them back together. In an adult male, the vocal folds are
17-23 mm long, and it is around 12.5-17 mm in an adult female. Due to
the action of muscles in the larynx, the vocal folds can be stretched 3
to 4 mm. The frequency of the adult male speaking voice is typically 125
Hz, while the frequency of the adult female voice is about 210 Hz.
Children's voice is around 300 Hz. In comparison to a piano keyboard,
the men's voice would be 1 octave lower than a women's voice, and a
child's voice would be 1 octave higher than an adult women's voice. The
front end of the vocal folds is attached to the thyroid cartilage. The
back end is attached to the arytenoid cartilages, which separates the
folds for breathing. ![](Vocalfolds1.png "Vocalfolds1.png")

## Electrical circuit representation of vocal folds

The vibration model of vocal folds, and the acoustic impedance model of
the vocal can be represented as electrical circuit representation. A
gyrator can be used to combine the acoustic impedance model and the
vibration model, so that the velocity(as potential) from the mobility
analogy for the vibration model can be transferred to the velocity(as
current) in the acoustic impedance model, and where pressure would be
force divided by surface area of the vocal folds tube.

## References

1.  <http://hyperphysics.phy-astr.gsu.edu/hbase/Music/voice.html>

