i agree unreservedly

She rubbed her eyes. "Believe it or not, Dr. Grace, I kind of like you. I don't respect you very much, but I do think you're a fundamentally good man."

"Easy for you to say when you're not the one being murdered! You're murdering me!" Tears rolled down my face. "I don't want to die! Don't send me off to die! Please!" She looked pained. "I don't like this any more than you do, Dr. Grace. If it's any consolation, you'll hailed as a hero."

echolocation jamming - some moths produce ultrasonic clicks in the same frequency range as bats to confuse them

fur based camouflage - moth fuzz absorbs sound, reducing echo

environmental camouflage - bugs resting on rough surfaces produce a much lower contrast acoustic shadow, making them harder to identify. The best frequency of roughness probably depends on creature size.

acoustic deflection - the twisted tails of luna moths scatter sound in a way that displaces their echo center (acoustic center of mass) so bats target their tails instead of body

mimicry/acoustic aposematism - imitating the sounds made by predators or toxic prey

damping structures - Submarines use soft rubber anechoic tiles to absorb or dissipate incoming sonar. It also dampens outgoing sound from activity within the submarine.

Sonar shadows - differences in ocean temperature or salinity result in different densities. At the interface between the two, impedance mismatch causes sonar to bounce off, creating a shadow zone where animals (or submarines) can hide. Predators can also hide here, as the refraction index allows sonar out but not sonar in.

Beamforming - body structures allow bats and dolphins to produce a directional sonar to hunt. Owl faces collect sound and direct it to their ears. Many prey animals communicate by focusing sounds in a specific direction, lessening the signal to unwanted listeners like predators.

acoustic transparency - internal structures allow sound waves to simply pass through them, rendering them invisible to sonar. This one is pure speculation on my part, but it seems obvious given that many fish are transparent to light.

EXTREMELY LOUD FISH - these fish use it for communication, but I imagine incredibly loud sounds could be painful or disorienting to predators with an extremely sensitive sense of hearing. sound also travels farther in higher density mediums

body structures have a resonant frequency. Imagine if you were trying to hunt an deer and it started making a noise that vibrated your eyeballs or gave you a migraine.

And because I eventually got sidetracked, here's a bunch of sound based sciences and metrology that eridians would probably have crazy advanced versions of. I don't know anything about materials science so I certainly missed some obvious fields.

Nanoacoustics - using sound to characterize materials, manipulate structures (like cells and drops of fluid), and to sense things at the nano scale, typically using sound in the gigahertz to terahertz range. Many of the following fall under the category of nanoacoustics.

Acoustic microscopy - exactly what it sounds like. A microscope but using sound. It lets you see the internal structure of tiny things. A 10MHz to 2GHz transducer pings the object and then listens to the reflected sound. An acoustic microscope can have a resolution similar to an optical microscope. (I used to do this for my job.)

Nanoacoustic subsurface atomic force microscopy - basically what the eridians do but at the nano scale. Tap the surface with a really tiny needle, producing a broadband signal, and then see what reflections bounce back. Has single-digit nm scale resolution. This is really cutting edge stuff. Link is a feasibility study from 2025.

(Medical ultrasound) Ultrasound, tomography, transmission tomography

Acoustic metamaterials/phononic crystals - engineered to transmit, trap, or attenuate waves at selected frequencies, while permitting others to pass. Applications include earthquake mitigation and shielding against blasts. The human history of metamaterials began with manipulating electromagnetic waves.

Modal analysis - used to find the natural frequencies and standing wave patterns of an object. Every object has an infinite number of these, determined entirely by its mass and stiffness. (My pet headcanon is that eridians are tuned (pun intended) to do this naturally and use it to identify other eridians, the same way humans are tuned to identity faces. After tapping or clicking to produce a broadband signal, they would analyze not only the echo, but the vibrational response after excitation.)

Acoustic tensor tomography - evaluates structural integrity and internal non-homogeneous stress-strain fields

Ocean acoustic tomography - measuring underwater temperatures, salinity, and ocean currents with sonar. Eridians would probably figure out how to get a lot more properties out of it.

Ballistocardiography - a no-contact method of observing mechanical function of the heart

Elastography - a class of medical imaging diagnostic methods that map the elastic properties and stiffness of soft tissue using ultrasound. Since they are hard shelled, eridian applications would likely be more oriented towards materials and manufacturing. Maybe the research of soft-bodied creatures, to a much lesser extent. They would definitely want to put Grace in one.

We use IR spectroscopy (light) in gas analyzers. The eridian equivalent device would probably be surface acoustic wave sensors (SAWs). The device would contain hundreds or thousands of tiny SAWs, each with a special coating to make it sensitive to a particular molecule or class of molecules. Wave propagation across the surface changes depending on molecules that adsorb to the surface.

Enjoy! I spent so many hours doing research.

COMPOSURE [Impossible: Failure] - "Would it make you feel any better if I was dead?"

grace says "yes" (<- never going to happen): she stoically accepts "that's fair" and then informs him she will not be doing that, but she respects him for being honest with her

spider

puppet shows

makes his materials by weaving