1. Introduction

Observers of unidentified flying objects report a variety of sound effects associated with the phenomenon. Some report sharp, explosive sound during rapid acceleration or high-speed flight. Others refer to humming, whining or whirring noise while the UFO is hovering or moving at relatively slow speeds (Hall, 1964). Still others mention whistling or swishing sounds suggestive of rushing air.

More remarkable than any of the foregoing, however, are reports that describe the UFO as moving at velocities far in excess of the maximum speed of sound in the earth's atmosphere without producing any noise or shock wave that would normally be expected under such conditions of atmospheric displacement. No characteristic "boom" is heard in these instances.

The absence of a sonic boom in these cases remains a mystery. Possible explanations are that:

1. actual speed was overestimated;
2. a natural atmospheric effect that could suppress the sonic boom was present; or
3. the object or phenomenon did not displace the atmospheric gases through which it was passing at supersonic speeds.

In this chapter we shall present the basic concepts involved in the production of the sonic boom or shock wave resulting from the passage of an object through the atmosphere at speeds greater than that of sound at the altitude of flight. Natural effects that are theoretically capable of rendering such shock waves inaudible at ground level will also be discussed, as will current research aimed at suppression of sonic booms by aircraft design modification and other means.

In general, it would be unrewarding to analyze each UFO report in conjunction with meteorological data to determine if a sonic boom from a particular object flying at supersonic speed would be heard at ground level. The difficulties are two-fold: first, the existing state of knowledge concerning meteorological effects on sonic booms is sufficient only to provide information in terms of statistical probabilities (Roberts, 1967); and second, local meteorological

features which occur between weather observing stations and/or which occur between the times of scheduled observations would not be observed.

2. Sonic Boom Generation

Sound waves are a manifestation of the compressibility of air. A source capable of compressing air produces pressure fluctuations, called sound or compression waves, which travel through the atmosphere. The peaks and troughs of the waves correspond to maxima and minima of the pressure fluctuations. The leading edge of the wave or wave front is approximately spherical in shape, and the pressure disturbance propagates away from the source in a series of concentric spheres. The speed of propagation of these waves, the sound speed, varies with the temperature and pressure of the air through which the waves travel. The maximum value for speed of sound waves is generally at ground level and reaches about 760 mph. The sound speed may show considerable variation in the atmosphere, alternately decreasing and increasing with altitude. A minimum value of 580 mph is reached at approximately 50 miles above the earth's surface. However, these values are principally a function of altitude, but they also vary with the time of day, season and latitude and longitude. The following are approximate average values:

Pressure disturbances are generated whenever a body, such as an airplane, moves through the atmosphere and displaces the air around it. In subsonic flight the speed of the aircraft is less than the local sound speed and the wave disturbances propagate away from the plane in all directions. These pressure variations are generally weak and too slowly varying to be detected by the ear (Carlson, 1966).

An aircraft traveling at supersonic speeds moves faster than the pressure disturbances it generates. When this occurs the plane is always ahead of the wave front and the spherical waves emitted at successive points alongthe flight path become tangent to lines sloping backward from the bow of the plane. These lines form a cone, the surface of which is the shock wave. Shock waves are formed by each protuberance on the plane's exterior. However, with distance, the various shock fronts tend to coalesce into two large shock fronts, usually attributed to the bow and to the tail of the plane. Fig. 1a shows how the fronts intersect level ground from a hypothetical flight path parallel to the ground surface at constant sound speed and with no wind. The indicated abrupt pressure rise and fall is responsible for the sonic booms heard at the earth's surface. Two booms will be heard as the "bow" and "tail" shocks successively pass over an observer but the ear may not always register the separate shocks when they are of different intensities (Carlson, 1966) or when the observer is taken by surprise.

The ratio of aircraft speed to the sound speed at its altitude is called the Mach number. The limiting value at which no sonic boom is heard, because of atmospheric effects, is called the cutoff Mach number (Wilson, 1962). Studies made by Wilson (1962), >Kane (1966) and Roberts (1967) have established that the cutoff Mach number ranges roughly between about 1.0 and 1.3 depending on atmospheric conditions and the altitude of the plane. This means that sonic booms produced by objects moving faster than 1.3 times the sound speed should be heard at ground level.

The angle between the shock front and the ground becomes smaller as the aircraft speed increases relative to the sound speed. In this situation the sonic boom may not be heard at ground level until the plane has passed from view. Wilson (1962) has estimated that the plane may be as much as 25 miles away from the point on the ground where the sonic boom is heard.

3. Atmospheric Effects on Sonic Boom Propagation

When the actual wind and temperature variations that occur in the atmosphere are taken into account, the simple conical pattern of the shock front may become quite distorted. The sound speed generally decreases with altitude between the ground and the plane. Therefore, as a propagating shock wave descends toward the ground, the portion of the wave front closest to the earth moves faster than the portions above. If the sound speed decreases sufficiently rapidly with altitude, the wave front may become perpendicular to the ground. In this situation the shock never reaches the ground because it begins to travel parallel to the ground before it gets there (Carlson, 1966). Physical requirements for such an effect, however, are unlikely, even under extremely abnormal atmospheric conditions. In any event, an object moving through the atmosphere at any altitude parallel to. the earth's surface, at a speed greater than the speed of sound at ground level would inevitably produce a sonic boom.

The decrease of sound speed with altitude also affects the portion of the wave front that spreads out to the sides of the plane. An investigation of the effect by Kane (1966), under conditions of no wind, shows that the lateral extent of the sonic boom at ground level ranges from about 10 to 35 miles on either side of the ground track of the plane. Furthermore, the intensity of the shock wave will be diminished as it spreads out. Consequently the boom will become less intense on either side of the flight track.

When wind is present, the wave front progresses at a rate which is the sum of the sound speed and the wind speed. Therefore the effect on the wave front by the temperature decrease is counteracted if a tail wind increases with altitude. If a tail wind decreases with altitude the distortion of the wave front caused by the temperature variation is reinforced, while a head wind produces the opposite effect. The situation becomes more complicated when the horizontal variations of wind and temperature are considered.

Other atmospheric features could produce unusual sonic boom patterns at the ground. Among these are: turbulent air motions in the lowest few thousand feet of the atmosphere, the type of clouds present and their spatial distribution, and temperature inversions. None of these meteorological phenomena have been studied in sufficient detail to produce conclusive results about their effects on sonic booms. However preliminary investigations have been reported (Roberts, 1967).

4. Design Modifications and Maneuvers

Although various government agencies, industrial organizations and university research projects are currently engaged in seeking methods to reduce sonic boom intensities, all known practical supersonic airplane designs will produce sonic booms (National Academy of Sciences, 1967). Furthermore, according to the Academy report, "The possibility that unconventional configurations may be devised which will yield significant reductions cannot be disallowed but, at present, the future must be viewed in terms of small reductions obtained through better understanding of theory, design refinements of conventional aircraft and improvements in propulsive efficiency and operating procedures." Research efforts are continuing in an effort to find an unconventional design, with practical aerodynamic. characteristics, which would minimize or eliminate the sonic boom.

The various research efforts to suppress sonic boom intensities which are under investigation are reviewed below.

The pressure distribution at ground level, shown in Fig. la and lb is the so-called "farfield" signature. The shock fronts emanating from protuberances on the aircraft have little effect on the pressure pulse at ground level. The sonic boom can be reduced, but not necessarily eliminated, if the aircraft climbs at subsonic speeds before making the transition to supersonic speeds at high-altitude cruising levels. Optimization of the arrangement of the various components, such as the shape and position of the wings, may lessen sonic boom intensity. Long, slender and blended configurations appear to offer

the best compromise between maximum aerodynamic performance and low sonic boom levels. Reduction of the peak pressures at ground level by design modifications is also being attempted. For example, a "stretched" design would alter the point at which the various waves form a bow and tail shock. With this type of design a less rapid rate of pressure rise would be produced at ground level and consequently a less audible boom would result (McLean, 1966; NAS, 1967).

Aircraft accelerations and maneuvers at various altitudes cause sonic booms of varying intensities in localize regions at or above ground level. It is possible, during common flight maneuvers, to produce local pressure buildups which may be more than twice as large as those produced by the same aircraft in level, unaccelerated flight. The subsequent "superbooms" occur at isolated points at ground level in contrast to the ordinary booms that move with the aircraft. Limitations on rapid accelerations and maneuvers would reduce the intensity and frequency of "superbooms" but could not be expected to suppress sonic booms altogether (Maglieri, 1966).

In subsonic flight, pressure disturbances propagate ahead of the aircraft altering the airstream in such a way that abrupt pressure changes do not occur. In supersonic flight however, pressure disturbances cannot propagate ahead. In order to prevent the buildup of a shock wave in supersonic flight, the Northrup Corporation is currently working on a method to modify the airstream through an electromagnetic force field concentrated at the nose of the aircraft. This work is still in preliminary stages and experiments have only been undertaken in wind tunnels (Aviation Week and Space Technology, 1968).

5. Comments

Although sonic boom research has progressed rapidly since the early 1950's, the complete suppression of sonic booms at ground level by means of present technology does not appear imminent. This does

not mean that sonic booms are always heard in conjunction with supersonic flight. Some meteorological factors occasionally could reduce sonic boom intensities or, even more rarely, prevent sonic booms from reaching the ground at all. However, the reported total absence of sonic booms from UFOs in supersonic flight and undergoing rapid accelerations or intricate maneuvers, particularly near the earth's surface, cannot be explained on the basis of current knowledge. On the contrary, intense sonic booms are expected under such conditions.

Discussions with Professor Adolph Busemann, Aerospace Engineering Sciences Department of the University of Colorado, have been extremely helpful in the preparation of this manuscript.

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Aviation Week and Space Technology, 88 (1968), p. 21.

Carlson, H.W. and F.E. McLean. "The sonic boom", Int. Sci. Tech, No. 55 (1966), p. 70.

Hall, R.H., (ed.). The UFO Evidence. National Investigations Committee on Aerial Phenomena, Washington, D.C.: 1964.

Kane, E.J. "Some effects of the nonuniform atmosphere on the propagation of sonic booms", J. Acoust. Soc. Amer., 39 (1966), S26-S30.

Maglieri, D.J. "Some effects of airplane operations and the atmosphere on sonic-boom signatures", J. Acoust. Soc. Amer., 39 (1966), S36-S42.

McLean, F.E. and B.L. Shrout. "Design methods for minimization of sonic boom pressure-field disturbances", J. Acoust. Soc. Amer., 39 (1966), S19-S25.

National Academy of Sciences. Report on Generation and Propagation of Sonic Boom. 1967.

Roberts, C., W. Johnson, G. Herbert, and W.A. Hass. Meteorological Investigations in Sonic Boom Experiments at Edwards AFB. National Sonic Boom Evaluation Office, Arlington, Virginia, Dl-Dll, (1967).

Wilson, H.A., Jr. "Sonic boom", Sci. Amer. 206 (1962), 36-43.