I almost know infrasound.
No more than a mile from my home huge freighters push through the deep, cold waters of Puget Sound. On flat black nights the thump-thump of their propellers travels through water, through air, churns into my bed, my bones, and the lowest threshold of my hearing. Born in the bellies and boilers of machines, the mechanical throb carries along rotating shafts that turn the metal blades of propellers, which slice through water like a dull knife hacking flesh: whummmp. ..whummmp…..whummmp.
Everything makes a sound when vibrations travel through a conducting medium, although we may not be able to hear it.
As Morula scuffs dirt, waves of air particles wash out in all directions. They reach my ear and vibrate my eardrum, which excites the three small bones of my middle ear: the hammer, anvil and stirrup. When the last little bone, the stirrup, takes up the vibration, it presses against fluid in my inner ear and creates a tiny sea of waves that tickle the hairs inside the spiral of my cochlea. The tickled hairs trigger auditory nerve cells that shoot electric signals to my brain.
Air, water and electricity in such a small space.
Large without, Morula’s ears are also large within. The bones of her inner ear are massive compared to mine. The combined weight of her hammer, anvil and stirrup totals just over a pound, compared to mine at two ounces. Her ear canal is eight inches long and her eardrum is about one and a half square inches. Maybe this doesn’t seem very big, but my eardrum is thinner than this paper and only one third of an inch square. You would need two hundred and fifty of my eardrums to create a stack an inch high.
Hum with your mouth closed. Now place your hands over your ears and hum again. The vibrations bypass your eardrums and are transmitted through your skull. Wavelengths tingle along your jaw line. Your bones are rattling.
Sounds are louder with a bigger collecting surface. Cup your hands behind your ears and listen as if you were an elephant.