Sentence:  The resonances of the vocal tract are called <WSD>formants</WSD>. Trying to hear the separate <WSD>formants</WSD> in a vowel is difficult. We are so used to a vowel being a single meaningful entity that it is difficult to consider it as a sound with separable bits. But it is possible to say vowels so that some of their component parts are more obvious. 
Meaning of the word marked with <WSD> tags:  A band of frequencies, in a sound spectrum, that have a greater intensity; they determine the quality of a sound; especially the characteristic sounds of the consonants.