The response of an object as a function of frequency versus aspect angle is shown below. All munitions have some version of cylindrical symmetry, so the plots display a repetition of behavior due to the symmetry.
If you hit a steel pipe with a hammer, it will ring with a particular frequency; the response shown below is simply a more complex version of that ringing: the munition, while in general, has some cylindrical symmetry, is more complex than a simple pipe.
The potential of the acoustic response to provide not just detection, but also identification is high. When possible, the site manager will not only know locations of ordnance of concern but also exactly what they are, which for remediation, is a game changer.