Rules for phonemes: /m/ and /n/

When phoneme [m],[n] are found before another consonants, the voice or voiceless vowel consonant has an effect on the length of both the vowel and the nasal consonant.

This is very similar to the lengthening and shortening of the vowel in examples like: seed / siːd/; seat / siːt /; so we have the rules as follows:
– Before weak voiced consonant ( z d . ..) [ m and n ] are lengthened and before strong voiceless [ p t k s ] , [ m and n ] are shortened./