Shaped Sonic Boom Demonstration shock wave by a chase plane, the SSBD demonstrated a reduction in boom by about one-third. Several of the flights included NASA Dryden's F-15B research testbed aircraft following to measure the F-5E's shock wave signature close-up. During the flights, many shock wave patterns were measured by the F-15B at various distances and orientations from the F-5E. An unmodified F-5E flew a few seconds behind the demonstration aircraft to provide a baseline sonic boom measurement to validate the reduced boom produced by the demonstrator. A U.S. Air Force Test Pilot School Blanik L-23 glider carrying a microphone on the left wingtip,
Speed of sound and thus speed of sound, varies with altitude: The medium in which a sound wave is travelling does not always respond adiabatically, and as a result, the speed of sound can vary with frequency. The limitations of the concept of speed of sound due to extreme attenuation are also of concern. The attenuation which exists at sea level for high frequencies applies to successively lower frequencies as atmospheric pressure decreases, or as the mean free path increases. For this reason, the concept of speed of sound (except for frequencies approaching zero) progressively loses its range of applicability at high altitudes.