Dear PF Forum, Sorry I ask this. I should have googled it or doing the experiment myself. If a cars runs 50 m/s and at that time fires a missile, the speed of the missile is 100 m/s so the total speed of the missile is 150m/s, is this right? And if we sit at the back seat of an airplane, flying 250m/s, around 560mph and call the flight attendant, our sound is traveling 340m/s from our seat, is this right? But an observer below, if somehow can 'see' our sound wave, he/she'll see that our sound wave is traveling 590m/s, is this right? What about this. A cars runs 50m/s and suddenly produces an ultrasonic wave. What would we see or hear in the sidewalk? Does the ultrasonic wave travels at 390m/s or 340m/s? Considering the sound wave travels in the same medium as the car, while passenger calling to flight attedant the sound travels in the airplane medium, while an observer observing the sound stays on the ground, different medium.