What do diphthongs actually sound like to you guys?

They sound like two separate sounds but ones that blend into each other. Most people don't think about things like diphthongs so if you asked them they might say it's "the vowel sound of this syllable" because the rule they were taught (if they were at all) is that there is one vowel sound per syllable. But if you made them stretch it out you could make them realize it's indeed two distinct sounds, just mashed together.

They sound aaight

Oh, that's really interesting!

This is not exactly the same thing, but I've been mulling over the fact I can't tell the difference between /ai/ and /aj/ in the slightest, or between /au/ and /aw/, like help me out here. Native English speakers, do you hear something different there? Am I missing something? Was I just poisoned by the bad linguistics we were taught in middle school for some reason?

To answer your question, I hear like, a vowel and a half. Doesn't "feel" like one phoneme to my ear.

To me they sound indistinguishable from a sequence of two vowels in hiatus, so to me /ai/ sounds the same as /a.i/, my native language doesn't have any kind of phonemic diphthongs whatsoever.

Always perceived them as a single phone, same with tʃ, and I always perceived ŋ as two

Several other factors that tend to reinforce the single-phonemeness in my brain are (beware the Australian English notation ahead):

• my diphthongs start and end in places pretty different from the monophthong equivalents - the /æ/ in /æ/, /æɪ/, /æɔ/ are all different, similarly the /ɪ/ in /ɪ/, /æɪ/, /ɑɪ/, /oɪ/ are all different (I've seen /ɑɪ/ written /ɑe/ too, but it's still nowhere near my actual /e/). Technically this could just be explained as allophony, but it helps to reinforce the distinction in my brain if I can't pin either part of the diphthong to a specific monophthongal phoneme

• the fact that English diphthongs are so different in different accents, and in ways inconsistent with monophthongs. My /æɪ/ corresponds to American /eɪ/, but /æ/ by itself does not become /e/ (although some American accents may have [eə]). Similarly, /əʉ/ corresponds to American /oʊ/ but I definitely don't pronounce my /o/'s as a schwa.

• the fact that what may be a monophthong in one accent is very often a diphthong in another - or even allophonically within the same accent. It's difficult to think of American English /oʊ/ as a sequence of two vowels when it can easily be pronounced [o] depending on the context. Similarly, Australian English /ʉː/ can surface as something like [ɘ̟ʉ] in some accents.

None of these necessarily prevent any linguistic analysis breaking down diphthong phonemes into sequences of monophthongs, but in a practical sense, they add up and mean that it's difficult for me to see these phonemes as sequences of individual phonemic monophthongs. Of course I'm still very aware I'm saying two different vowel sounds though (unless it's something like [ɪi] where I don't really notice the movement)

A difference between spanish and catalan is that the diphthongs "iu" and "ui" are [ju] and [wi] in spanish (ciudad=[θju'ðað], cuidar=[kwi'ðaɾ]) but [iw] and [uj] in catalan (ciutat=[siw'tat], cuidar=[kujˈða]). Can't for the life of me tell the difference

With English as my native, I find it very difficult to transcribe the diphthongs I make because they sound like one sliding sound rather than two vowels. Phonemically, I recognise it as one sound/letter. When learning new languages though, I find that it depends on the language's orthography. Italian I find very difficult to see it as anything other than two vowels in one syllable. French oi making a [wa] sound makes it sound like one "sliding" vowel to me but ui sounding like [ɥi] sounds like two vowels. I think when the orthography doesn't match the phonology in my mind, I'm more likely to see it as a single vowel. I can tell you though that a lot of English speakers around me have no idea that they're making diphthongs half the time. Trying to get them to pronounce [oː] is brutal. They keep making [oʊ], not understanding the difference

What language? Yes I perceive them as a single phoneme

When kids are learning how to read English, they're taught to think of diphthongs as a single sound.

I wish I could unhear it.

I do hear them as a single phoneme. If I think about it, I can process that the beginning of eye and the end are different, but honestly I think they're like how people understand tones so easily. I'm not hearing the individual sounds, I'm hearing the glides. So rather than hearing one vowel and then another I'm hearing a glide and that transition is like a phoneme to me. I can barely tell the difference between [ae] and [aɪ] for that reason, in the same way that the French have trouble differentating /ɪ/ and /i/ in enɡlish

Hearing two vowels as a single glide causes me a lot of trouble when speaking languages where hiatus is allowed, because I often miss one of the vowels.

of the core vowel sounds we are taught of English

oo - single sound /uw/ and /ʊ/

o - single sound /ow/ and /ɒ/

a - single sound /ej/, /a/ and /æ/

e - single sound /ij/ /ɛ/

i,y - single sound /aɪ/ /ɪ/

u - two distinct sounds /juw/, single sound /ʌ/

It's unclear exactly why we were taught this when the vowels have so much more variation, but as you can see dipthongs are our base conception of what a vowel is. The only one I hear as seperate sounds is u because it begins with a clear consonant sound and ends with the base form of oo

That’s sou clichei

my native language is Italian and we have some diphthongs: ai, au, ei, eu, ɛi, ɛu, oi, ɔi ja, je, jɛ, jo, jɔ, ju, wa, we, wɛ, wi, wo, wɔ and even some triphthongs wja, wjɛ, wjo, jai, wai, jɛi, wɔi but I think we don’t perceive them as phonemes, just the vowels as “shorter”. Maybe some people don’t even realize it, since in our spelling system diphthongs are transcribed as separate vowels: [jɛi̯] <iei>

To me, a native Teutophone and technically native(-ish) Polonophone, diphthongs always felt like vowel-consonant/consonant-vowel sequences. When I was a youngster making up German spelling reforms while bored in math class, I'd use ⟨aj⟩ for German /aɪ̯/, since that's what it was to me: [a] followed by a consonantal glide.

I wouldn't be surprised if diphthongs were a purely phonemic concept, and didn't exist phonetically as distinct from vowel glide sequences.

As an Italian speaker, I also hear them as two distinct phonemes.

This must certainly be because diphtongs in my language (and I assume in yours too) are always written as two graphemes, unlike in English where ⟨a⟩ may be read as /ei/.

My dialect of English only really has offglide diphthongs so I interpret all those structure as vowel-consonant sequences. When I listen to other dialects and languages that have "normal" diphthongs, they sound like a vowel that slides into another vowel.

As a tween when I made my first analysis of the sounds in my speech to make a secret alphabet, I interpreted my gliding diphthongs as single phonemes composed of two phones, the same as the affricates. I didn't have that language at the time to describe what I was doing, but my notebook from back then uses the component sounds to describe them but gives them single symbols.

They don't sound like seperate letters to us unless we slow down the speech thereof

Say /ai/ is ah ee or ah ih

I hear eye as a quick syllable as ī whereas Were i to seperate it it would be eyee eyih

I would not be able to recognize I as a diagraph because of that

Exactly the same for me. I didn’t know English speakers saw it as one phoneme!

Fun fact: years ago, before learning about linguistics, I thought that /æ/ was a diphthong. I grew up in the American Southeast, and when I was 10 to 12, I has this idea in the hack of my head that /æ/ as in "cat" was actually /eə/ or /ejə/. I've known for a long time now that it, obviously, is not, but that idea is probably what sparked my interest in linguistics.