Question about second premise

In the second premise in the video it was written as HASIRS->~UW and then the contrapositive was written as UW->~HASIRS. However, since passage says Imperfect receptor of sound I assumed it should be ~HASIRS->~UW and contrapositive should be UW->HASIRS because of the word "imperfect". While I was able to arrive at the right answer because in my case transitive property looks like this ~HASIRS->~UW->~CE is my second premise correct? If not which words should I be looking for to know if I have to negate a sufficient or a necessary condition. Because in this case I assumed imperfect means I should negate that condition. Thanks