Why do Bombs make a Whistle Sound as they Fall?
When watching a film or documentary, a falling bomb or a missile always has a descending sound or a whistle. Why is this? Does it mean that if the missile fell down a bottomless hole the sound would go subsonic?
Izzie Clarke spoke to Professor Peter Main from King’s College London to sound out George’s question…
Peter - Most of the missiles shown in documentaries and films refer to the Second World War and for the typical height of those bombers, the falling missiles are accelerating, but not sufficiently to break the sound barrier. That means that apart from a relatively gentle whoosh, they would not naturally make any sound. However, it was in the interests of the bombers to terrify those under attack so, often, an artificial whistle was incorporated into the missile.
Izzie - You hear right - they added a fake whistle. But what does that mean for missiles that travel in this hypothetical endless hole?
Peter - If the missile could fall under gravity further, accelerating all time, after falling about 5,000 metres it would reach the speed of sound and would then emit a sonic bomb, just as supersonic aircraft do when then fly at speeds greater than the speed of sound.
Izzie - This happens when objects travel faster than 343 metres per second. The air molecules are pushed aside with such a great force that it forms a shock wave. It sounds a bit like a thunderclap. So how does the missile sound relative to the pilot?
Peter - In principle, if the pilot of the plane could have heard the whistle, he would have heard it in the way described - a high pitched sound, falling in frequency according to the well known Doppler effect. This is the same effect as when say a police siren changes pitch as it approaches and then passes by, and is due to the motion of the object compressing the wavelength of the sound as it approaches the observer - that is increasing its pitch and stretching it as it moves away.
Izzie - Someone on the ground would actually hear the pitch increase. In other words, it sounds higher and higher as it approaches. So that means those beloved filmmakers are using the wrong sound…
Peter - Well, that’s because the sound has nothing to do with bombs or missiles; it’s a special effect created in the studio. The particular sound with the frequency of the whistle falling has become a cinematic convention, which explains its common use in many films.