For a long time, linguists defined vowels by two qualities: height and backness.

The vowel [i] is very high while [a] is very low.

The vowel [i] is also very front while [u] is very back.

As it turns out, vowel production is far more complicated than that. Without getting into the technical details of it, height and backness are good approximations of the actual speech sounds F1 and F2. That's how we're able to plot different people's vowels objectively in a table as I have done below. So, yeah, feel free to explore. You can either look at all the vowels of a single speaker or look at a single vowel across all speakers.

Vowel (Monophthong) Analysis

There's a lot of interesting data in this table. I will point out a few things that I find neat.

Sam: Sam has an incredibly symmetical vowel space. It's actually very uncanny. The vowels [e] and [o] have nearly the exact same height. Same with [ɛ] and [ɔ]. Very nearly a perfect triangle from him.

Fret: You may not have noticed it but Fret's vowel space is unusually large. Consider the vowels [e] and [ɛ]. They are much further apart for him than for the other speakers. I think this is a direct consequence of German's massive vowel inventory. Because German distinguishes so many different vowels, they need to be more spread out to actually be distinct from one another. A funny consequence of this is that his /e/ sounds nearly identical to my /i/.

Caesar: As for my speech, it's rather standard in a lot of ways. Like all English speakers, my /e/ and /o/ are actually diphthongs [ej] and [ow]. And the vowel we analyze as /a/ is actually [ɑ]. And like most Americans, I have the caught-cot merger. /ɔ/ has merged with /a/ in my speech. In all other respects, howevr, my English is rather distinctly Californian. You may have noticed this actually. If you look at the whole of my vowel space, it's strangely centralized. This is because of the "California vowel shift".

My KIT vowel /ɪ/ has lowered all the way to [e]. It is now indistinguishable from the opening of FACE [ej]. I can't think of any other dialect with this feature. Most English speakers only have [ej] and the Scottish dialect has [e] insstead but almost no dialects have both. Beyond that, my FOOT vowel /ʊ/ has centralized and is now pronounced like the British STRUT vowel /ʌ/. Both /o/ and /u/ are fronted for me. /o/ is said [əw] and /u/ is now somewhere in the neighborhood of [ʉ].

Diphthong Analysis

This one isn't done, so feel free to just ignore it. I still need to analyze Sam's diphthongs. Portuguese has 10 and I simply don't feel like adding every single one of those right now.

For now I'll just point out a few things. Because English and German are closely related, our pronunciations are pretty similar. The CHOICE vowel /oj/ is best described as [ɔj]. For both me and Fret, this vowel ends in the neiborhood of /ɪ/. But for him that actually means [ɪ] and for me that means [e]. This is why my diphthong barely changes height at all while his is distinctly pointing upward. Our vowels /aw/ are different as well. For Fret this is [aw] or maybe [ɑw]. This is heard in some English speakers but in General American (and in my speech) it's more typical for this diphthong to open with the TRAP vowel [æ]

It's not pictured here because it would clutter the image but my speech features something called "American raising". When next to a voiceless consonant, /aj/ changes from [ɑj] to [əj]. So my speech has bide [bɑjd] but bite [bəjt].