Acoustic characteristics of vowels

This video just has a plain transcript, not time-aligned to the videoIn this video we will consider the waveforms, spectra and spectrograms of vowels.
We have seen that the component frequencies and amplitudes of complex waves are revealed by the spectrum, with harmonics shown as individual peaks, and formants as ranges of amplified and dampened frequencies. These spectral differences result in different qualities that we can hear.
In the case of vowels, there is still more that we can learn from the spectrum and by extension from the spectrogram.
The first and second formants of the vocal tract correlate with perceptions of vowel height and advancement, with particular vowels having characteristic spectral shapes.
Here we see spectra of the vowels [a] [i] and [u] with the first and second formants highlighted.
In [a] F1 and F2 are quite close together, while in [i] they are rather far apart. In [u] F1 and F2 are again close together, but lower in the frequency range than we saw for [a]
The following slides will illustrate the vowel formant patterns in American English vowels, pointing out some generalizations along the way.
These patterns are not just happenstance. The formant positions in the spectrum are the result of the positions of the lips and tongue during vowel articulation.
The relative position of the first and second formants result in the different vowel qualities that we hear.
First, we will look at the relationship between formants and vowel dimension of height. Here we see spectrograms of 8 american English vowels. [i, ɪ, ɛ, æ, ɑ, ɔ, ʊ, u]
Remember that we can describe these vowels using their IPA symbols according to height, advancement and rounding. For example, the [u] vowel is high back and rounded. We will soon see that the first two formants are related to the Height and Advancement of the vowels and we will describe this relationship.
The formants of these vowels are visible as dark horizontal bands at various frequency ranges in the spectrogram.
The first three formants are indicated by arrows on the left hand side of each spectrogram.
If we consider first the high vowels and look at the first formant, we can see that the first first formant is at quite a low frequency for each of these four vowels. (There does not seem to be much similarity across the F2 frequencies of high vowels.) In fact it turns out that the lower the vowel, the higher the first formant will be in the spectrogram.
This relationship between vowel height and the location of the first formant becomes more clear when we compare the lower vowels.
[ɛ, æ, ɑ, ɔ] are all either open or open-mid. Across these four open vowels, the F1 is again rather similar, but if we compare the high vowels, we see that F1 is higher in the low vowels and lower in the high vowels.
The first formant is therefore inversely related to vowel height. Close/high vowels have lower F1, while open/low vowels have higher F1.
Now let’s look at advancement, or how far front or back a vowel is.
The top row of vowels all share the phonetic property FRONT, while the bottom row all share the property BACK.
We have already seen that the first formant is related to vowel height. Now let’s consider the second formant.
If we compare the second formant across the front vowels we see that they are all fairly similar to each other in frequency. By comparison, the back vowels have lower F2 values than the front vowels as a whole.
Therefore, the second formant is directly related to vowel frontness. The more front a vowel is, the higher F2 will be. The more back a vowel is, the lower F2.
We can also consider the relationship between the first and second formants in relation to vowel height. If we look at F1 and F2 of the front vowels, we can see that there is typically a bit of the frequency range that is low in amplitude between the first and second formants. By comparison, the back vowels have first and second formants that are very close together.
This figure summarizes the formant values from the preceding spectrograms in a schematic way to allow for ease of comparison. In this form, we can see that the close vowels have lower F1 than the open vowels. We can also see that the front vowels on the left have higher F2 than the back vowels on the right.
It is also interesting to note that the back vowels all have similar F2 values, suggesting they are roughly equally far back, while the front vowels show decreasing F2 for more open vowels.
This is due to the position of the jaw during the production of these more open vowels, causing them to be produced slightly further back than the close vowels.
Phoneticians use the vowel formant patterns described above to visualize the acoustic relationships between vowels by plotting the first and second formants against each other.
This sort of plot is known as the vowel space. Here we see a plot of the 8 vowels that we have been looking at so far.
This plot puts F1 on the y-axis and F2 on the x-axis, in what is mathematically a rather unusual arrangement, but which results in a figure that looks rather strikingly like the IPA vowel chart.
This view highlights the relationship between the first formant and vowel height, and the second formant and vowel advancement.