Several years ago I worked for the NHS and they replaced every ambulance in the trust with a two tone that omitted a white noise in between tones. This was off the back of this research. However... it turned out that the research was wrong and the company that sold the systems were forced to come and remove them and replace the sirens with the traditional ones. It was concluded that there is no proof whatsoever that white noise is easier to locate and in fact can't be heard by some people with frequency dependant hearing, so more dangerous than traditional two tone. That's why you don't hear the white noise ones on ambulances any more.