Tennis Service Speed Meter is an app that measures the tennis serve speed in real time. It is intended as a training tool, to be used to test/improve service technique, racquet type, strings type/tension, etc. and to monitor improvements in time. Note that in order to work properly it should be used in the right conditions and following the correct instructions.
The app measures the time between the moment the racket hits the ball and the moment the ball hits the ground: such time interval is then combined with the length of the ball trajectory (you have to specify where the ball hits the court in order to accurately calculate such distance) in order to report the ball speed.
No other relevant noises (except for the racket hitting the ball and the ball hitting the ground) should be present while measuring the service speed (they could interfere with the measurement process).