# [Answer] In the Doppler effect does frequency change? Does wave speed change?

In the Doppler effect does frequency change? Does wave speed change?

An acoustic Doppler current profiler (ADCP) is a hydroacoustic current meter similar to a sonar used to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column. The term ADCP is a generic term for all acoustic current profilers although the abbreviation originates from an instrument series introduced by RD Instruments in the 1980s. The working frequencies range of ADCPs range from 38 kHz to several Megahertz. The device used in the air for wind speed profiling using sound is known as SODAR and works with the same underlying principles. Dynamic real-time path planning in robotics to aid the movement of robots in a sophisticated environment with moving obstacles often take help of Doppler effect. Such applications are specially used for competitive robotics where the environment is constantly changing such as robosoccer. A siren on a passing emergency vehicle will start out higher than its stationary pitch slide down as it passes and continue lower than its stationary pitch as it recedes from the observer. Astronomer John Dobson explained the effect thus: The reason the siren slides is because it doesn’t hit you. In other words if the siren approached the observer directly the pitch would remain constant at a higher than stationary pitch until the vehicle hit him and then immediately jump to a new lower pitch. Because the vehicle passes by the observer the radial velocity does not remain constant but instead varies as a function of the angle between his line of sight and the siren’s velocity: ${\displaystyle v_{\text{radial}}=v_{\text{s}}\cdot \cos {\theta }}$ where ${\displaystyle \theta }$is the angle between the object’s forward velocity and the line of sight f…