For now, the method he used was still rather "crude." He could only set up several membrane surfaces in advance at locations where sounds were certain to occur, then wait for the spreading sound waves to strike them. The membranes would automatically "encode" the sound waves and emit infrasound in his direction. He would then receive it, filter it, and "decode" it.
When the cruise ship had passed by earlier, he had set up imperceptible membrane surfaces near several of the islands along the route. It was an instinctive precaution—"Returning to the Abyss" had accurately locked onto the cruise ship, which meant there were most likely wizards nearby watching and tracking them. He needed to determine whether they harbored hostile intentions.
At the same time, he was also testing this spell, which was still under development.
Using sound waves as a carrier for transmitting information involved many difficulties. For example, sound waves had high requirements for the density of the transmission medium: the denser it was, the farther they could travel.
Seawater was a fairly ideal transmission medium, and nearly all the membrane surfaces Vaughn set up were pressed close to the water.
Muggles also had technology based on similar principles, called underwater acoustic communication.
Another issue was the attenuation of energy during sound transmission, something that could not be overcome. The intensity of sound waves produced by human speech simply could not travel very far, which was why Vaughn used magical membrane surfaces to convert them into infrasound, making use of infrasound's penetrating power.
Lastly, there was noise. At present, the spell could not specifically capture particular waveforms. Once a membrane was placed in an environment, until the magic sustaining it completely dissipated, it would convert every sound that disturbed its surface into infrasound and transmit it.
That required