The present vowel reference shows how all IPA vowels can be described acoustically and gives a sound example for each. An essential feature is the frequencies of the two main low resonances (formants F1 and F2), although sometimes other criteria have to be added. The assumption is that IPA vowels, despite their non-acoustic definition, allow an idealized, prototypical acoustic realization. All vowels are considered to be phones, i.e. the phonological aspect of real languages is completely excluded.

Conversely, the vowel reference can be used to define vowels acoustically. This results in the APhA (['aːpʰaː]) alphabet. While the IPA alphabet defines vowels purely in terms of articulation (size of mouth opening, position of tongue), and ignores the resulting sound in the definition, the APhA alphabet does exactly the opposite, defining vowels using acoustic examples and ignoring the question of articulation. APhA also takes the opportunity to provide a more systematic symbolization of vowels compared to IPA and to compensate for some weaknesses (the German "a" can finally be written as [a] instead of [ä]).

In addition to the sound example, the spectral transfer function of a filter is given for each vowel, which can be used to generate the vowel from a vocal cord vibration (source-filter model).