Sonic bangs

We are now all too familiar with the noises made by aircraft ‘breaking the sound barrier’, not to mention those unfortunate people who have suffered damage to property as a result. These so-called sonic bangs, or booms, are of course, caused by shock waves, generated by an aircraft, and striking the ears of an observer on the ground, or his glasshouses or whatever it may be; but there has been considerable argument as to the exact circumstances which result in the shock waves being heard, why there are often two or more dis­tinct bangs, whether the second one came first, and so on.

Strangely enough many people don’t seem to realise that we were familiar with sonic bangs, and their effects on us and our property, long before aircraft flew at all. A crack of the whip is probably the oldest man-made example; it may not have been responsible for breaking glasshouses, but in the hands of a circus performer it can be a pretty shattering noise. A roll or a clap of thunder is an example from nature of a series of shock waves; and one must have noticed how the bangs produced by aircraft often resemble a short roll of thunder. Explosions, too, produce shock waves, and, although a bombing raid is hardly an appropriate time to analyse such things, there were many unfortu­nate enough to experience during the war the disastrous effects on their ears and property of the shock wave of an exploding bomb, as distinct from the damage caused by the bomb itself. Some, too, may even have noticed the rather weird way in which the different bangs arrived at different times depending on where the observer was relative to the launching and explosion of the bomb. But the nearest thing to the sonic bangs produced by aircraft are the crack of a rifle bullet, or of a shell going overhead, or of the V2 rocket of wartime memory.

If an aircraft were to fly at supersonic speed at a height of a few feet over one’s head the shock waves from wings, body, tail, etc., would strike one’s ears in rapid succession, so rapid that one couldn’t distinguish between them, and (if one remained conscious at all) the impression, so far as noise is concerned, would probably be of a short roll of thunder. The higher the aircraft flew, the less violent would be the noise produced by the shock until it would hardly be noticed at all from the ground; the noise of the engine, and of the aircraft itself, are of course continuous noises which are quite distinct from that of the shock.

An aircraft diving towards the earth at supersonic speed, and at an angle of say 45°, then suddenly slowing up and changing direction, will ‘shed’ its shock waves, which will travel on towards the earth and strike any observers who may happen to be in their path. It is certainly quite clear from schlieren pho­tographs that a bow wave approaches from the front as the speed of sound is approached, and, conversely, goes ahead of the aircraft when it decelerates below the speed of sound.

So far as effects at ground level are concerned, we know that these become less intense with the height of the aircraft; more intense with Mach Number, though not anything like in proportion; that they are affected by the dimen­sions of the aircraft, increasing with its weight and volume, and being of longer duration according to its length; that they are more intense during accelerated flight (when the shocks tend to coalesce) than in steady flight; and that they decrease very rapidly with lateral displacement from the line of flight of the aircraft, in fact they only extend over a certain lateral distance. All this is rather what one might expect, but the problem is complicated because shocks of different intensities may be generated by the body, wings, tail and other parts of the aircraft, hence sometimes the roll as of thunder rather than one or two sharp bangs. One factor that one might not expect is the extent to which the bangs vary according to the conditions in the atmosphere between the aircraft and the ground, the winds, temperature, turbulence and so on. The actual pressures created at ground level are not so great as is sometimes thought; the overpressure in the Concorde boom, for instance, was only of the order of 96 newtons per square metre.

Can anything be done about it? Not much; if we insist on flying at these speeds. We can legislate against supersonic flight other than over the sea or thinly populated areas, but even so aircraft have to reach these areas. Moreover the tendency must be for the weight and size of such aircraft to increase rather than the reverse. Some alleviation can be obtained by control of climbing speeds; and at certain heights the speed of sound may be exceeded without creating a boom at all, the shock wave being dissipated before it reaches the ground. So really there is nothing for it but to fly as much as poss­ible over the sea, and as high as possible, perhaps even really high – as we shall mention in the last paragraphs of this book.

Finally it should be mentioned that the publicity that has been given to sonic booms has tended to make us forget all the other noises created by air­craft, those from the engines, propellers or jets, and from the motion of the aircraft itself; these noises are probably more objectionable than sonic booms to those who live on landing or take-off paths, they are by no means confined to transonic and supersonic aircraft, and there are better prospects of reducing some of these noises as, for example, by using quieter engines.