velocity of enregy flow

[alitavakol]

I know that the electromagnetic waves can travel at lower velocities of c (becuse of dispersion and that the sensible thing of light is its energy and its energy travels at the speed of group velocity). can you explain this? is it true for sound also?

 

[aboozar.hamidipoor]

as you know:

we can define a phase velocity and also group velocity for traveling electromagnetic waves. these two velocities have two different concepts. one of them (phase velocity) is only theoritical definition but the other (group velocity) has physical meaning.
a single tone wave could have a phase velocity lower or higher than c but this single tone signal desn't have any information and doesn't contain any energy in physical world, infact every information in physical world has a bandwidth which contains a continuous spectrum of frequencies in frequency domain. when you are gonig to transmit information, you will transmit the whole bandwidth and hence the group velocity is defined. these group velocity should be lower than c ...?!