are the opposite of monophtongs like /i/, /ɪ/, /ɛ/, /æ/, /u/, /ʊ/, /ʌ/, /ə/, /ɚ/, /ɔ/, /ɑ/. In the case of a monophthong there is only one vowel in the syllable. In po-lice or pho-to-gra-phy there is only one vowel in one syllable. In the case of diphthongs we have two vowel in one syllable: /eɪ/, /oʊ/, /aɪ/, /aʊ/, /oɪ/. In fla-vor we have an insinuated i after the a. (Attention: The spelling is completely irrelevant in this context. A phonem, a sound, can be presented by any graphem. Also there is only one vowel in cow, it is a diphtong, because in the pronunciation there is kind of a an a followed by something like an u.)
The IPA (International Phonetic Association) represents the different phonems more or less exactly and sometimes they are useful, because even if we have an audio example, what is almost every time the case in online dictionaries, it can be difficult to here the differences.However the IPA signs are not infallible. Between an open e as in cat and a closed e as in exercise there are a lot of phonems in between and not all of them have an IPA sign. Voiced and unvoiced, to give another example, are not a secure criterion either. We have as well a little bit voiced. The s after a vowel for instance is very often a little bit voiced, because any vowel is voiced, but the IPA sign knows only voiced or unvoiced. And obviously sometimes we have a broad range of officially accepted pronunciations and what we find in dictionaries as IPA transcription represents the standard the IPA believes to be the most relevant.
This said the IPA signs are helpful. If someone can't here the difference between the French [ɑ̃] in words like an, envergure etc. and the Portuguese [ɐ̃ ] in words like alemã, samba he can look at the IPA signs to become more sensitized and hear the word again. (On this website he can here it ten thousand times.) The French [ɑ̃] by the way doesn't exist in Portuguese. In the following chapters we discuss about all the nasal sounds in detail. At the end there is a summary.