A bat is chasing an insect. if the bat, moving at 6m/s emits a 40kHz sound and recieves back an echo at 40.4 kHz. What speed and direction is the insect moving? take the speed of sound to be 343m/s Okay sooo. ive mucked around with this a fair bit so i know before f'>f that, so the insect is moving towards the bat f' = f( (v + Vd) / (v- Vs) ) so the bat is both the source and detector i tried to write it in terms of the insect so Vd = Vb - Vi and Vs = Vb - Vi so i get Vi = f( V + Vb) + f' ( -v + Vb) all over f + f ' which = 4.29 m/s is that correct?