hi ivanete, 

here are the descriptions:

http://www.degruyter.com/view/j/rela.2014.12.issue-2/rela-2014-0006/rela-2014-0006.xml

http://ifa.amu.edu.pl/~krynicki/teaching/cg/_06_phonetics_vowels.pdf

hope they help.

Polish doesn't really have diphthongs. It has vowel-glide sequences (e.g. raj 'paradise'). However, the glide in such sequences is clearly consonantal. 

One article in English http://wa.amu.edu.pl/psicl/files/2/05_Kalisz.pdf

and one in Russian (attached)

Schwartz is right about Polish diphthongs being phonologically vowel-glide sequences, though the Russian article Polish Diphthongues includes rising sequences [ło].  Saying that the j is "clearly consonantal" is a bit strong, as it's based on a theory of features.  Jakobson called j [-vocalic, -consonantal], so you be the judge.  More importantly, I assume the original question (about acoustic descriptions) is aimed at the phonetics, in which case feature assignment is not in play.  Frankly, I don't know how you tell the difference between vowel-glide and diphthong phonetically on any a priori basis.

In vowel-glide sequences, you can typically identify a formant steady-state in the spectrogram that correspond to the vowel target. In true diphthongs, such steady-states are typically absent. For example, in Polish /ej/ as in 'rejs', you can usually isolate a stable portion of the /e/, before the transition to the /j/. By contrast, in Englihs race, with a true diphthong, it looks a little bit as if they entire vowel is a 'transition'. 

But how do you isolate the literacy factor, where people are providing a spelling pronunciation?  Native speakers claim that two words spelled differently are pronounced differently because they're spelled differently:  Czy trzej czesi trzymali trzy trzciny?

Dear Ivanete,

Phonetically speaking, diphthongs in Polish are derived from nasal vowels (see Wierzchowska’s studies). Underlying sequences of a vowel+vowel or a vowel+glide are not regarded as diphthongs.

Concerning the phonetic differences between a glide in a vowel+glide sequence and the second part of a diphthong, it is usually the case that the second part of a diphthong is shorter (and there might be differences in amplitude and formant transitions as well).

Best,

Marzena