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Phonation Lab 2
Vocal Jitter
To the instructor: Vocal jitter (also known as perturbation) is the cycle-by-cycle variation in
fundamental frequency of the voice. It’s a useful measure for clinical change but one that’s hard
to comprehend at first blush. This lab is designed to walk someone through the process of
calculating vocal jitter, though (of course) we want a computer to really do that.
Vocal Jitter lab
Following is a “waveform” of speech, with the periods for each cycle marked off in milliseconds
(ms). Note that I’ve marked the cycles at the positive peak for clarity, rather than the zero
crossing.
Period 1 2 3 4 5 6 7 8
12 ms 7ms 6ms 8ms 12ms 10ms 6ms 10ms
|——|—-|—-|–—|——|—-|—-|—–|
/\ /\ /\ /\ /\ /\ /\ /\ /\
/ \ / \ / \ / \ / \ / \ / \ / \ / \
/ \/ \/ \/ \/ \/ \/ \/ \/ \
You need to calculate how much the vibration changes on a cycle-by-cycle basis. To do this,
subtract the duration of the first period from the second and put the answer here:
(12 ms − 7 ms) = ______ ms
Now subtract the second period from the third:
(7 ms − 6 ms) = ______ ms
Do the same for the rest and enter all your calculations in the left side of the following table.
Difference Absolute (unsigned) value
Period 1–2
Period 2–3
Period 3–4
Period 4–5
Period 5–6
Period 6–7
Period 7–8
Sum ______ divided by number of differences = ______
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You are going to add these together, but first you need to take the absolute value of each. If there
is a negative sign (−) on any, simply ignore it. Another way to think of this is to always subtract
the smaller number from the larger so that the result is always positive. Place the absolute value
(without − sign) in the column on the right. Now add those up and divide by the number of
differences you added to get average variability.
What is the average variability for the “waveform” above? ______ ms
(You already have calculated it earlier.)
A more meaningful measure is percentage variability: This is a measure of the percentage
difference relative to the period of the fundamental frequency. To find the average fundamental
frequency, simply add each period and divide by the number of periods. For the “waveform”
above do the following:
Period 1 + Period 2 + Period 3 + Period 4 + Period 5 + Period 6 + Period 7 + Period 8 = ______ ms
Average period = ______ / number of cycles (8) = ______ ms
You calculate average percent jitter as:
Average variability divided by average period multiplied by 100
For the waveform above,
(Average variability) ______ / (Average period) ____ × 100 = ____%
What is the percent of variability by our simple formula? ______% (You just calculated this.)
This percent of variability can also be called vocal jitter or perturbation. While there are many
formulas that are more exotic than this one, this simplified formula will serve for our purposes.
In summary, the steps to measuring the degree of variability of vocal fold vibration are as
follows:
A. Figure average difference between cycles of vibration
B. Get average period of vibration
C. Get percent of variation (percent jitter)
1. Divide average difference by average period