Page 38 - Spring 2015

Concorde booms and the Mysterious east Coast noises
Figure 3. Comparison of Concorde primary and secondary sonic boom signatures and spectra. One pound per square foot (psf) equals 47.88 pascals. Adapted from Holbeche (1972) and Rickley and Pierce (1980).
On the left side of Figure 4 is a ray diagram showing a variety of ray paths that the booms travel for an aircraft in supersonic flight at an altitude of 18 kilometers, trav- eling toward the viewer. The downward- propagating rays, shown by the solid lines, impact the ground to form the primary boom carpet, as in Figure 4. At a lateral distance of about 40 kilometers (25 nauti- cal miles) in the example shown, the rays refract away from the ground and thus de- fine the lateral extent of the primary boom carpet. Also indicated is a secondary boom carpet at about 120-160 kilometers from the flight track, in which the dashed-line rays impact. These dashed-line rays arrive in two different ways: either they travel di- rectly to the secondary carpet as a result of bending in the upper atmosphere or they may first be a part of the primary carpet, reflect upward from the surface, and then bend downward after traveling through a portion of the upper atmosphere.
the right side of Figure 4 are examples of temperature and wind profiles for a given atmosphere. Note that there is a portion of the higher atmosphere in which the temperature increases as the altitude increases, and the associated wave propagation speed thus increases compared with that in the lower portions of the atmosphere, causing upward propagat- ing rays to be curved (refracted) back toward the ground. The wind speed gradient will also influence refraction and may reinforce, or counteract, the effects due to temperature gradient.
Figure 4. Ray path diagram in plane normal to that of flight illustrat- ing the manner in which the atmosphere above and below is involved in developing the primary and secondary boom carpets. One nautical mile equals 1.852 kilometer. From Maglieri et al. (2014).
Early on in the development of the Concorde, there was se- rious concern that its primary carpet boom levels would be too excessive to allow overland supersonic operations. Early commercial flight operations eventually proved this to be the case. Concorde supersonic flying would henceforth be con- fined to overwater operations, primarily Atlantic routes, due its limited range, thereby limiting utilization of the plane. On the other hand, the booms near the lateral cutoff and the secondary booms, which do not have an N-wave char- acter and are much lower in intensity, are not apt to be the source of serious community response problems. Near the lateral cutoff, primary booms usually resemble low rumbles or rolling thunder. Secondary booms, however, are generally not audible (0.1-1.0 hertz) but can cause building vibrations that are readily felt. It will be shown later in this article that secondary booms also played an influential role in further defining Concorde’s operating procedures.
secondary sonic booms
Secondary booms, also referred to as “over-the-top” booms, were more of an unknown quantity during the design-and- development days of the Concorde. They too, like primary booms, are inherent to supersonic flight. The distinct dif- ferences between secondary and primary boom signatures
36 | Acoustics Today | Spring 2015