Chapter 4: Vocal Fold Oscillation
Equation 4.1. The frequency of oscillation, F_{0},
is the number of backandforth movements made per second. Since
1/2p_{0} is a constant, and since the square roots
of large numbers are larger than the square roots of smaller
numbers, we can see that increasing the stiffness k will
increase frequency, while increasing the mass m will decrease
frequency.
Equation 4.2. Here we have a mathematical definition
of a periodic (i.e., repeating) waveform. If the waveform lasts
for a predetermined time T, then the waveform must be
the same at the time t + T as it was at the time t.
The quantity T is known as the period; it is the
duration of one cycle of the waveform.
Simple Harmonic Motion: Equations 43 through 412
The next ten equations, which all lead up to 412, cover simple
harmonic motion, also known as sinusoidal motion.
The first four equations (4.3 through 4.6) are simple definitions
of the trigonometric functions sine and cosine:
Equation 4.3.
Equation 4.4.
Equation 4.5.
Equation 4.6.
Equation 4.7. This equation makes clear the various
ways in which circular motion can be expressed. Once around a
circle is equal to all the quantities shown.
Equation 4.8. Frequency is the inverse of period.
Highfrequency oscillations have short periods, and vice versa.
Equation 4.9. Oscillation can also be measure in terms
of angular velocity, measured in radians per second. This
is just a simple conversion; multiply F_{0} by 2p to
get the angular velocity in radians.
Equation 4.10. Here's the same expression, but now we're
solving for the angle covered during a given period of time by
the oscillation, in radians. We simply multiply the radian frequency
by the elapsed time.
Equation 4.11.
Equation 4.12. This is the general formula for simple
harmonic, or sinusoidal, motion. As we can now see, there are
three key quantities involved; the amplitude A, the phase q_{0},
and the frequency F_{0}.
Equation 4.13. Here we are solving
for the mean intraglottal pressure, which is the pressure
inside the glottis over the medial surfaces of the vocal folds.
You may refer to the Tutorial
for Chapter 4 to see the quantities in this equation illustrated
in an animated model.
Equation 4.14. The inertance of an air column
is defined as the air density times the length of the column,
divided by its crosssectional area.
Vocal tract input pressure = (inertance) x (acceleration
of the air column)
Equation 4.15. This is Equation 414 written out in
terms of Newton's second law of motion, which states that force
= mass times acceleration. Force is analagous to the vocal tract
input pressure, mass is analagous to inertance, and acceleration
remains the same. If we restate this again with mathematics,
we have:
Equation 4.16. I is once again the inertance, dU/dt is
the rate of change of the flow of air in the air column, and P_{i},
which we are solving for here, is the input pressure to the vocal
tract. From this, we can see that the input pressure will be
positive if the glottis is opening and the rate of change of
the flow (acceleration) is positive.
Equation 4.17: Bernoulli's energy law. Refer
to Equation 36.
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