Frequency of a signal doesn't change when it is penetrating an object because frequency of an em wave doesn't depend on medium as a signal ( basically an em wave ) is produced because of accelerating charges and this acceleration causes change in magnetic and electric field in the space which is self sustaining ( from Faraday and Maxwell's rule ).
Change in frequency of a signal means change in acceleration of the oscillating charge which is independent of the medium in which signal is penetrating.
It may depend on the medium in which oscillating charge is present and also on the energy supplied to it . When signal penetrates an object its speed and wavelength changes in such a way that the frequency remains the same.
It is not the same as resistance in meaning but from the formula they can be said to be similar.