The Challenges of Defining an Acceptable Sonic Boom Overland

Abstract

Sonic boom is the ground trace of the pressure disturbance created by the passage of an aircraft, or any other object, flying faster than the speed of sound. A typical conventional (non-minimized) sonic boom time waveform measured at the ground looks roughly like the letter N see Fig.(1) as an example and hence is commonly called an N wave. The distinctive characteristics of conventional sonic booms compared to other types of noise are : 1) the presence of two (or more) shock waves, e.g., large and sudden pressure variations that may be perceived like detonation noise (sonic boom is sometimes also called ”ballistic detonation”) ; 2) the slow variations in time of the part of the sonic boom between the shock waves. This portion of the waveform is slow enough to be inaudible by the human ear. However, because low frequency energy is present, it may induce some indirect noise and other non-audible effects that should be considered when assessing human acceptability to sonic boom. So sonic boom is characterized as simultaneously a loud, low frequency and impulsive noise (according to the definition of CHABA). It is impulsive because of its short duration (of the order of 0.1 to 0.3 ms, closely related to the length of the aircraft) with a relatively distinct termination (the last shock). Moreover the pressure increase through the shock waves takes place over a very short time, defined as the rise time, which is of the order of a few milliseconds. It is loud because of the overall peak overpressure is of the order of 50 to 100 Pa (1 to 2 psf). It is low frequency because the main part of its frequency spectrum is in the infrasonic or low audible frequency range (1-30 Hz) see the pressure spectrum in Fig.(1).