Page 42 - Spring 2015

Concorde booms and the Mysterious east Coast noises
Figure 7. Relative strength on the ground from a sonic boom return- ing from the thermosphere as a function of the lateral distance from the aircraft ground track. Adapted from Rogers and Gardner (1980).
and frequency that it is unlikely that they are either respon- sible for the east coast events or likely to disturb the public.
With regard to Garwin’s destruction-of-the-thermosphere hypothesis, results for acoustic Mach number versus altitude for Garwin’s model and the Rogers and Gardner model are plotted in Figure 8. The red curve is Garwin’s model, and the black solid line includes only nonlinear attenuation. The black dashed line includes both linear (L) and nonlinear (NL) attenuation. The acoustic Mach number for the Rogers and Gardner model never exceeds 0.2. Ninety percent of the wave's energy is attenuated below 100 kilometers, with 99% attenuated by the time the wave reaches the turning point.
Rogers and Gardner concluded that the secondary booms from the Concorde did not have sufficient amplitude or en- ergy to produce a deleterious effect on the thermosphere.
Rogers and Gardner completed their model in June of 1978. Press’s process for an independent review of their work in- volved the JASONS who were asked to look into the prob- lem, The JASON team, which included Garwin, developed a simple plane wave model that included only nonlinear stretching and attenuation (no spreading, caustics refrac- tion, or linear attenuation). Despite its simplicity, the JASON model (MacDonald et al., 1978) produced results consistent with those of Rogers and Gardner. They concluded that Rog- ers and Gardner’s results and conclusions were correct.
Upper Atmospheric sound speed
Secondary sonic boom events in the form of “thumps” and low-frequency “rumbles” were once again reported in the New England area during the summer of 1978. Preliminary measurements by the Department of Transportation, Trans- portation System Center (DOT/TSC) (Rickley and Pierce, 1979) suggested some correlation with incoming Concorde flights into John F. Kennedy International Airport. In the summer of 1979, a secondary sonic boom detection and as- sessment program was conducted by the US DOT/TSC in New England (Rickley and Pierce, 1980). A large database of measurements was obtained regarding secondary booms.
The results of these tests showed that the upper atmospheric temperature and winds along with the aircraft operating conditions played an important role in whether and where the secondary booms will impact the ground. It is stated that a principal mechanism causing such long distance effects is refraction caused by wind and temperature gradient effects at altitudes between 20 and 60 kilometers (the stratosphere) and mesosphere (see Figure 6). Sound waves that carry up- ward traveling sonic booms to such altitudes can be bent back toward the ground if these gradients cause the sound speed to increase with altitude. Such downward refrac- tion can also take place in the thermosphere, but the high attenuation and lengthening of the shock duration at high altitudes would render such thermospheric refracted arriv- als much less likely to be audible by the time they return to the ground. This is consistent with the conclusions of Rogers and Gardner (1980).
Rickley and Pierce (1980) applied the simplest model of sonic boom propagation based on geometrical acous- tics that predicts that secondary booms will reach the
Figure 8. Acoustic Mach number versus altitude for initially down- ward sonic boom from Concorde. Red line, Garwin's (1978) model; solid line, Rogers and Gardner (1980) model including nonlinear losses only; dashed line, Rogers and Gardner model including both linear and nonlinear losses. Adapted from Rogers and Gardner (1980).
40 | Acoustics Today | Spring 2015