I have a problem in understanding the vibration propagation in an object. Supposing I have an object with an exciter positioned inside it. If I make the exciter runs producing vibrations at a certain frequency making the object vibrates, do the waves have the same frequency or during the propagation it can change?
As the wave propagates through the object, it will reflect in on itself interfering and creating a waveform different from that caused by the exciter. The waveform at any given location on the object depends on the object's geometry and the consistency of the exciter.
In general, the vibration frequency will remain the same. In some circumstances, such as intermittent contact ("rattle") the frequency can change or higher harmonics can be added. But, generally, the frequency stays the same as the wave propagates through the structure.