Signal modulation can be achieved by varying the frequency of a signal right? According to the formula $\displaystyle f=\frac{v}{\lambda}=\frac{3 * 10^8m/s}{\lambda}$ as the velocity changes so does the frequency. If the wavelength of a wireless signal passes from one medium to another (say air to wall) wouldn't this change the velocity and consequently change the frequency and mess up the modulation?