So I was comparing the IR stretch frequency of ethyne, ethene and allene. Of course triple bond is stiffer than double bond so ethyne has a higher stretch frequency. The problem then is where allene sits when compared to these two. I guess it should be the lowest because I just think of the cumulated double bonds as serial spring, which should give lower spring stiffness than only one double bond in ethene. Obviously, I'm wrong... It turns out allene IR stretch frequency is in the middle of ethene and ethyne. I wonder how might we explain that. Thanks!