1. Introduction 2. Sonic Boom Generation 3. Atmospheric Effects on Sonic Boom Propagation 4. Design Modifications and Maneuvers 5. Comments # References |
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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:
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
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:
Height (feet) |
Speed of Sound (miles per hour)
|
0 | 760 |
10,000 | 735 |
20,000 | 707 |
30,000 | 679 |
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.
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.
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
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).
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
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.