1. The problem statement, all variables and given/known data Firstly, whilst reading all this more complicated (and far more interesting) physics in other threads I feel like a dunce posting something which is probably of 'lower' level here as it were, but here goes: If we have a vehicle like a police car (with siren going) moving at a constant velocity, and then another vehicle (carrying an observer) travelling in the same direction and moving at the same speed as the police car, would the observer witness any Doppler effect? 2. Relevant equations Not really relevant. Logic only. 3. The attempt at a solution If neither the observer or the police car were moving there wouldn't be Doppler effect obviously, but when the police car is moving he's effectively moving through his own waves and therefore creating a higher pitch of sound and consequently a red shift. Does this only occur when the observer is at a different velocity compared to the emitter, or does it also occur when they are both moving at the same velocity? -------- Sorry for clogging up your board with easy questions, couldn't find a site which was going to be as efficient with an answer as this one :P! Thanks in advance.