The Elephant in the Room
"My roommate got a pet elephant. Then it got lost. It's in the apartment somewhere."
It started with insects. Caitlin O'Connell-Rodwell, PhD, an ecologist at Standford University, was working on her master's degree in entomology. Part of her research included recording love songs of Hawaiian planthoppers. They were communicating seismically (with really, really low sounds) through their limbs. Later, O'Connell-Rodwell was observing elephants in Namibia when she noticed the herd freezing, flattening their ears, and raising up on their toes. After field tests, she concluded that elephants can listen through their limbs and recognize warnings from particular elephants or herds miles away. Sensing low vibrations in the ground, elephants have been known to travel hundreds of miles toward storms for water, and flee threatening helicopters a hundred miles away.
Just like the deep blue sea, the deep seismic audio world holds many secrets. Scientists can detect infrasounds from as low as 2 Hz to 20 Hz, or in layman's terms, 2 to 20 vibrations a second, which is below what most subwoofers can generate. What lurks below? Earthquakes, tremors, thunder, explosions, and that kid in the neighborhood with the stereo system worth more than the car it's in. Scientists are hoping to predict earthquakes, volcanos, tsunamis, and other catastrophic natural events by detecting and measuring these infrasounds. One interesting method is to record vibrations near a volcano, then speed up the recordings 60 times to pitch them into the range of human hearing. Some of these sped up recordings sound like popping that increases in intensity, revealing the earth shifting from lava and gas buildups. Some mimic a pipe organ, which comes from air forced against the crater walls during rumbles.
All these rumblings got me thinking about challenges we recording engineers face - low vibrations from outside the studio. When we built our current studios, 90% of our effort was to prevent low frequencies from entering the rooms. It's easy to contain high frequencies within the studio with absorption and diffusion. But low frequencies can permeate almost any barrier. When teaching, I've often referred to this acoustical observation as the "flea and the elephant." Like a flea, high, treble sounds have very small wavelengths and can easily bounce around a room but be absorbed by closed cell foam or insulation. Low frequencies, like an elephant, are lumbering, massive, and can just go where they please. A glass door can keep out a flea, but not an elephant. That requires lots of dense mass.
Normally, we think of sound only traveling through the air. But it also travels through solid objects – much, much faster. For instance, sound lumbers through air at about 770 mph. But through concrete, it scoots along at 3,500 mph; wood at 4,000 mph; and steel at 5,800 mph. These are very dense mediums compared to air, thus the efficiency. And as our big floppy eared friends have shown us, sound can travel great distances through solids.
A real-world example of this is a distant door slam in a house. You may not directly hear the door, but you feel the slam in the floor. The structure of the house allows sound to travel through the wood frame, floor, and drywall, all of which are connected together. Another example is from old western flicks where the train robber puts his ear on the railroad track to hear the train coming.
So the elephant in the room is: How do we keep all those low frequencies from busting into our studio uninvited? Well, we know that sound is really just molecules bouncing into each other like dominos. If we remove a few of those dominos, the falling stops. In studios, we decouple all of our solid surfaces by floating floors and wall frames on rubber. We can isolate drywall by floating it off the wall frame with special hangers. And, we can dampen most sound waves before they enter with limp, dense material such as insulation, lead rubber sheets, or sand. Many believe a solid brick or concrete wall will completely block low sound waves. They can't, and might even amplify low frequencies, especially if they are near by.
As in real life, it's much easier to keep a flea out of your house than an elephant. Though I think I would get along with an elephant better. Unless he brought fleas with him.
To learn more, check out this article about professor O'Connell-Rodwell.
There are different types of seismic waves, as ScienceNews for Students explains.
Learn about volcano music here.