I present to you, from the recent chapter of my bloodymary fic 'On Wanting To Live And Other Choices', the concept of Rocky using thrumming (from the PHM novel) to calm down a panicked and frightened Simon like he's an Eridian offspring because Simon produces his own ultrasound waves that Rocky can hear but Grace can't:
The ‘not dead human’s’ body tensed up, coiled tight like a spring as awareness returned enough to tell him there was a tube down his throat pumping air in and out against his will. His eye opened for a split second, confused and hazy with drugs and, no doubt, pain, before screwing shut again. The telemetry screen flashed rapidly, beeps too loud and too fast. Rocky made a general sound of alarm, high pitched whistling.
“Mary!” Grace barked, reaching out but not knowing where to put his hands or what to do. Scared of making it worse, of causing damage he couldn't take back, “He's awake!”
A thrash. Something that might have been an attempt at a word but was smothered by plastic. On the one half of his face that was visible beneath the bandages and wrappings was an expression Grace could only call sheer terror. He kept his hands to himself, still unwilling to touch someone so fragile and broken now that they were awake and scared, panic in his veins and thumping in his chest.
“It's okay,” He tried as the arms of the NannyBot slid past him, prepped with a syringe of something or other, “Hey. Hey, it's okay, you're safe, you're alright-”
The ‘not dead human’ sobbed although it sounded more like he was heaving, fingers pulling at the thin blanket, catching on the lines taped down and anchored to his body.
“No,” Grace said, “No, please don't do that, you need those-”
Rocky suddenly squatted in his ball and started to thrum once more, that strange, warm sound that vibrated against and through the xenonite. Layers and layers of notes, chords and strums, clicks and pushes of air as through a pipe organ. The segmented vents at the top of his carapace raised and lowered in a fluid ripple: I'm not a threat. Again, his singing was less like unified language and more like feelings, vague phrases and thoughts that Grace only caught snippets of.
“Friend Rocky… friend Grace… watch sleep… safe… ♬♩♪♩♬♩♪♩♩♩♩…”
And Grace watched, astounded, as ‘not dead human’ paused, seeming to register the sound and finding reassurance in its composition. In the way Rocky fell into that ¾ time once more, a gentle swaying waltz, a rhythm that mimicked the way one would cradle an infant and rock them. ‘Not dead human’s’ hand reached out, trembling and uncertain, following the song until it came to rest against the xenonite panels. Rocky only hesitated a moment before leaning forward, pressing part of his carapace where the palm curved around a seam.
“Is okay. Rocky Grace fix. Promise.”
And with that, ‘not dead human’ passed out as the medicine finally kicked in, hand slowly sliding from Rocky’s ball and over the side of the cot where the NannyBot rearranged it and the bedsheet carefully back into place. Grace stared, feeling like he had somehow intruded on a very personal and private moment, “What was that?”
“Thrum for comforting offspring. Offspring small and world is big. Scary. Need much reassurance,” He rubbed a claw over the cut of green-blue stone embedded in one of his legs, “Rocky practise Rocky part of thrum for own offspring one day. When return to mate Adrian we sing together.”
Grace blinked, his chest ached. He'd just witnessed an Eridian lullaby.
