Does the received frequency at a location differ from the transmitted frequency when the medium velocity differs at the two locations? To give a concrete example, a sub-sea pinger is transmitting at 37.5 Khz 4,000 metres below sea level and at a particular pressure, salinity, and temperature. A receiver nearer the surface detects the signal at a different pressure, temperature, and salinity. Finally a surface vessel detects the signal under yet another set of pressure, temperature and salinity. For reference the velocity of sound in the oceans can vary by up to 10% due to these factors. My intuitive guess is that sound transmitted in a high velocity domain will increase in frequency when it hits a low velocity domain?