A bat uses ultrasound at 40kHz to locate a stationary insect. If bat is flying directly towards the insect at speed of 4m/s at what frequency does the bat receive the ultrasound reflections from the insect? Speed of sound = 340m/s. I went frequency = frequency of source x (velocity of wave+velocity of bat)/(velocity of wave) So 40 = f x (340+4)/(340). This however gives me the wrong answer of 40.4kHz compared to the textbook answer of frequency = 41kHz. Using any doppler formula rearrangement doesn't lead me to 41kHz, so what have I done wrong?