I am a high school teacher and I just came across this issue when I began teaching at a new school. I am positive that if I am stationary and a 500 Hz siren comes towards me at 30 m/s, then the observed frequency will increase. I am 99% positive that if the 500 Hz siren is stationary and I move towards it at 30 m/s, the observed frequency will be the exact same. At my new school, we use the formula: Frequency observed = frequency [343 / (343 +/- v)] Obviously the 'v' was the relative velocity of the two objects and adding/subtracting depended on their motion. This made 1st paragraph I described above work out correctly. Whether I move towards a siren or the siren moved towards me, then the observed frequency was the same. At my old school, we used the formula seen on Wikipedia: Frequency observed = frequency [(343+/-Vr) / (343 +/- Vs)] This is what confused me. Using the example above, if a 500 Hz siren comes towards (a stationary) me you would get: Frequency observed = 500 [(343 + 0) / (343 - 30)] = 547.92 Hz If I move at the same speed towards a stationary 500 Hz siren, you get: Frequency observed = 500 [(343 + 30) / (343 + 0)] = 543.73 Hz How can those be different? The relative velocity between the two is the same. Anyone know what I am doing wrong? Which way is the correct way?