Sparky and Chuck, housemates and fellow intelligence analysts in Last of the Annamese, learn to identify by sound the explosions they hear as the North Vietnamese lay siege to Saigon. They can tell mortars from artillery and distinguish between friendly and enemy fire. Toward the end, what starts as distant thunder becomes explosions inside the compound where they’re holed up.
Once again, the experience of Sparky and Chuck is based on my own during the fall of Saigon. Unlike my characters, I never became adept at classifying the weapons being fired based on the sounds of their impacting shells. Sparky and Chuck had both worked closely with mortars and artillery. My time providing signals intelligence support to combat units was spent exclusively with infantry. I certainly saw close up the results of shelling, but I never worked in direct support of artillery.
What I became very good at toward the end was gauging how close the rocket, mortar, and artillery explosions were to me. I listened as they came nearer and knew almost to the hour when shells would land in our compound. The first contiguous hits came on the night of 28 April 1975. By the time I was evacuated by helicopter on the night of 29 April, our compound was taking direct hits. The building next door to us was destroyed, and the western gate to our compound was struck, killing two Marines on guard there. I never suffered a direct hit, but several were close enough that my hearing was permanently damaged. To this day I wear hearing aids.