How to read these vowel diagrams?
In pronouncing any given vowel, the recorded frequency spectrum will show distinct amplitude peaks (of the frequency spectrum). These peaks are called formants, and it is commonly accepted that the two lowest frequency formants taken together are enough to characterise the vowel.
The first formant F1 is the lowest frequency formant, and the second formant F2 is the second lowest frequency formant, etc.


Spectrogram of American English vowels [i, u, ɑ] showing the formants F1 and F2. The darkness of the band corresponds to the height of the amplitude/frequency plot; darkest areas show local peaks. From https://en.wikipedia.org/wiki/Formant#/media/File:Spectrogram_-iua-.png

The F1 frequency (range) corresponds to the perceived openness/height of the vowel, and the F2 frequency (range) corresponds to the perceived backness of the vowel.


Wikipedia IPA vowel chart

To demonstrate the effect of the F1 and F2 components on vowels, you can use artificial vowel generators which take these parameters to produce audible vowels, e.g. http://auditoryneuroscience.com/vocalizations-speech/two-formants.

As an example, consider the

Chengdu /o/ sound, which the graph says is at F1 ≈ 440, F2 ≈ 600 Hz

Pengzhou /o/ sound, which the graph says is at F1 ≈ 550, F2 ≈ 875 Hz


The corresponding artificial vowels are (unmute the audio in the imgur image)

Chengdu

Pengzhou



Produced from auditoryneuroscience.com with a fundamental frequency of 110 Hz