Why in a word 'lecture' /k/ is unaspirated? Shouldn't it be weakly aspirated because of the fact that it's in unstressed syllable?
From John Wells' blog:
As I put it in LPD, English [p t k] are aspirated
- when they occur at the beginning of a syllable in which the vowel is strong.
They are unaspirated
- when preceded by s at the beginning of a syllable
- when followed by any FRICATIVE, as in lapse læps, depth depθ
- if immediately followed by another plosive as with the k in doctor ˈdɒktə || ˈdɑːktər. The release stage of the first plosive is then usually inaudible (‘masked’).
Otherwise, they are unaspirated or just slightly aspirated. For example, ripe raɪp, shut ʃʌt, lake leɪk; happy ˈhæpi, writer ˈraɪtə (BrE), lucky ˈlʌki; wasp wɒsp || wɑːsp, restinɡ ˈrestɪŋ, Oscar ˈɒskə || ˈɑːskər, lifted ˈlɪftɪd, today təˈdeɪ.
The last case, "unaspirated or just slightly aspirated", is also called weakly aspirated.
The /k/ in lecture is followed by a plosive, so by these rules, it is unaspirated.
But speaking as an English speaker, I am not sure that English speakers actually distinguish between weakly aspirated consonants and unaspirated consonants. We don't hear three levels; using an unaspirated consonant instead of a weakly aspirated consonant is never going to be wrong.