What is the difference between an electromagnetic wave and other waves that amounts to the EM wave following the Special Relativity. I have been reading about the Special Relativity for some time, and the arguments provided for light having a constant speed w.r.t. to all observers in all the inertial frames moving relative to each other seem to work on other waves too in my understanding. What is the point I am missing out on ? What separates the other waves from an EM wave regarding the 2nd postulate of Special Relativity ? If it is the EM nature, please elaborate.