Active Participants In The Process Of Hearing

Birds are especially adept at hearing how sound energy is layered into different frequencies, the overall shape of the sound, rather than attending to the relative pitches that are the particular focus of mammalian ears and brains. Fish and shrimp are immersed in sound as the movement of water molecules directly stimulates their surface hairs and as sound waves flow unimpeded into and through their bodies. On land, insects hear airborne sounds with hairs on their body surface and modified stretch receptor organs in their skeletons, the same organs used by both insects and crustaceans to feel motion and vibration in their legs. Specialized hearing organs independently evolved at least twenty times in different groups of insects. Crickets have drumlike hearing organs in their front legs, but grasshoppers hear through membranes on their abdomens. Many flies hear with a sensor in their antennae. Among moths, hearing organs evolved at least nine different times, resulting in ears on wing bases, along the abdomen, or, in the case of the sphinx moths, on the mouthparts. It is a convenient shorthand to say that the shrimp, fish, bacteria, birds, insects, and I hear the same sound. To hear is a verb that reveals the narrowness of our sonic perceptions and imaginations. They lope, strut, crawl, sidle, wing, creep, sashay, slide, trot, flutter, and bounce. Here is a lexicon that recognizes the diversity of animal motion. But we have an impoverished vocabulary for hearing.

Keep Climbing

These words do little to open our imagination to the multiplicities of sonic experience. When the bony plate in a croaker’s ears slides over a membrane covered in hair cells, what should we name the resulting experience? The ciliary hairs in the lateral lines of the fish are immersed in the water around them, surely yielding a different experience from the movement of a triplet of bones in our middle ears. We lack any word to convey the mystery of the sphinx moth’s mouth palpus when sensing an approaching bat. Without a diverse vocabulary for hearing, our minds lapse into inattention and our imagination is limited. Hobbled by weak verbs, language must draw on adjectives, adverbs, and analogies. A shrimp claw listens spikily, perhaps, through narrowly tuned hairs. The birds’ aural attention, fueled by high body temperatures, is fevered and has a narrower range of pitch perception than ours, trimmed off at its top by a stumpy, uncoiled cochlea. Is bacterial hearing like pressing a trembling thumb into jelly, viscous and enveloping? Yet despite the limitations of language and human sense organs, our experiences of the world are encouragements to imagination. Listening opens our minds to other ways of being. At any place on Earth, thousands of parallel sensory worlds coexist, the diverse productions of evolution’s creative hand. We cannot hear with the ears of others, but we can listen and wonder. At the dock, in my headphones, a whir cuts into the fish and shrimp sounds.

Believe Me

It builds in loudness over five seconds then abruptly ends. Two more turns of the starter and the engine comes alive. The engine’s voice clouds the water, a chug pitched at about the frequency of human speech. The shrimp keep on crackling and their sound joins the outboard in my ears, two textures, one growly, one sparkly, each holding steady. The outboard idles for a minute, then, in an instant, roars. The propellers are spinning, shredding the water. As the boat pulls away, the intensity of the sound wavers, perhaps as the propeller turns toward and away from my hydrophone. Over the next minute, through the hydrophone, I hear the noise climb in frequency, up three octaves from the start, as the engine’s scream fades into the distance. The croaker keeps pulsing its thumping song every ten seconds or so. The silver perch and oyster toadfish fall silent. Like a painter applying a delicate brushstroke to a canvas, my audiologist extends her arm and slides a slender foam plug into my right ear. A thin tube runs from the plug to an electronic console and a laptop.

Too Little, Too Late

A gurgle bursts into my ear. Then the room stills. Winter sun through dusty clinic windows. Odor of floor cleaner and latex. A metal cart clinks far down the hallway. It pulses, repeats, and pulses again, quieter. Then more tones, lower pitched. We’re running down a series. Every time a sound hits my ear, two spikes leap from a trembling horizontal line on a graph on the laptop screen. On the screen, I see the graph twitch with every burst of sound. Sometimes the graph kicks up, but I hear nothing. My audiologist loops the tube and earplug to my left ear. She clicks the machine back on. Then come the tones, working their way through the sequence. Now that I’ve figured out how to read the graph, I stare unblinking at the line, waiting. Just to the left of the two big spikes is a third, a miniature, that pokes up whenever sound floods my ear. It is ankle high to its tall companions, but jabs up always in synchrony with them. For some sounds, even ones that I can hear, the junior spike is absent or merely flutters. The small spike on the graph shows me the hair cells of my inner ear in action. When the incoming double tone hits them, they shoot out a pulse of sound in answer. This reply is too quiet for me to hear, but the microphone picks up its signal. My ears, then, are not passive receivers of sound. They are active participants in the process of hearing, making their own vibrations. One of my favorite exercises with students is to scoop up some slimy ditch or lake water and peer into the lively throng through a microscope. The unaided eye sees only slime. Glass lenses directed at microscope slides reveal dozens of species in every drop. Some species, especially the emerald cells of the larger algae, creep like cargo ships maneuvering in port. Others, tethered by slender tails to fragments of vegetation, pump globular heads back and forth, wafting bacteria into cuplike maws.