We Live Embedded Within Sensory Distortion

The ear trumpets, pinnae, on either side of our heads, along with the ear canal, amplify sound by fifteen to twenty decibels. This boost is the equivalent to walking across a large room to stand next to someone who is talking. Sound waves also bounce around the cups and folds of the pinnae. This clash of waves cancels out some high frequencies. Push your ear flaps forward. You’ll hear a change in brightness. As we move our heads, the sound reflections shift, cutting out slightly different frequencies. From these subtleties, our brain extracts information about where sound is located on the vertical plane. We edit sound even as it enters the ear canal. When a wave in air hits water, most of the energy bounces back. This is one reason why we can’t hear poolside chatter when we swim underwater. To solve this problem, the tiny bones of the middle ear gather vibrations from the relatively large eardrum and, using the levering action of the longer hammer bone pivoting onto the shorter anvil and stirrup bones, these bones focus vibrations onto a much smaller window leading to the cochlea’s watery tubes.

Maybe I'm Amazed

This conversion both amplifies, increasing the pressure of sound waves by about twenty times, and puts a slight filter on the sound, trimming extremely high and low frequencies. Then the cochlea imposes a more severe filter. The upper and lower ends of our hearing are set by the sensitivity of cochlea. The stiffness of the membrane, the responsiveness of outer hair cells, and the tuning of nerve sensitivities determine not only upper and lower bounds of our perception of pitch but also our ability to discriminate among sound frequencies. Between the notes B and C, for example, we can potentially, if we concentrate, hear twenty additional microtones. But this is only true for quieter sounds. Our ears hear subtle differences in pitch in whispered or spoken words, but for shouts our discrimination of pitch is coarser. Intense sound bucks the cochlear membrane and overwhelms auditory nerves. We have finer discrimination at lower frequencies than high. But for the lower sounds of human speech, we perceive subtle differences among sound frequencies. Nerve signals and the brain’s processing add their own layers of interpretation. Nerves in the cochlea fire when inner hair cells are stimulated.

Nobody's Fault

The width of these ranges and their overlap set another limit on frequency discrimination. The nerve impulses from the cochlea then flow to the auditory nerve through a series of processing centers in the brain stem and then to the cerebral cortex. There, the brain interprets incoming signals in the context of expectations, memories, and beliefs. What passes into conscious perception is an interpretation, not a transcript. This is most vividly illustrated by auditory illusions. By playing different sounds into each ear or by looping sounds to create repetition, pioneering acoustic psychologist Diana Deutsch found that she could trick the brain into hearing phantom words and melodies. These illusions reveal that what we hear emerges from the brain’s attempts to extract order from incoming signals, even when no such order exists. The words and melodies that we hear are partly a product of our background, each of us hearing words and music relevant to our culture. Our brains do not just receive input from the ears, they send out signals to the ears, adjusting the cochlea to local conditions. In noisy environments, the brain suppresses the sensitivity of the outer hair cells, like a hand reaching out to crank down the volume on a loudspeaker. This reduces the masking effect of noise, allowing meaningful sounds to be more clearly distinguished. The hair cells in our ears are less jumpy in a noisy restaurant, for example, than they are in a quiet forest.

Make It Better

These layers of interpretation bias our perceptions of loudness. When we walk on pavement, for example, we perceive the sound as about twice as loud as footsteps on soft grass. This accords with the increase in sound intensity, the amount of energy hitting our eardrums. But in a carpentry workshop, our ears mislead us. The circular saw sounds about twice or three times as loud as the power drill. But the actual sound intensity, the rate at which energy pounds our ears, is about one hundred times higher. The extent of this biased perception depends, too, on sound frequency. The distorted scaling of subjective experience adapted us to subtle differences in the quiet sounds of the preindustrial world. The meanings in human speech, especially the textures of emotion, are conveyed through tiny changes in sound intensity. The same is true for information gleaned from the sounds of wind, rain, plants, and nonhuman animals. Our ears evolved to pay attention to quiet voices and are out of place in persistently loud environments. We’d better appreciate the sonic variegations of this new world and be empowered to protect our inner ears from permanent damage. We also have biased perceptions of sound frequencies. Our ears’ specialization on the intermediate frequencies works well for hearing the speech of other humans and some of the voices of nonhuman animals. But although we can hear many low and high sounds, we have an erroneous sense of their vigor. It is the biases of our ears and nerves that have cranked down the perceived loudness of these high and low frequencies. We live embedded within sensory distortion. There are also many sounds beyond the ken of our cochleas. Some whales and elephants hear down to 14 hertz. Pigeons can hear as low as half a hertz. Mice and rats chatter and sing to one another up to 90,000 hertz. If my feet represent the lowest sounds heard by animals and the top of my head the highest, we humans hear from just above the skin of my feet to the top of my hiking boots. Compared with most mammals, humans and our primate cousins live within a restricted aural world. These low sounds carry for hundreds of kilometers, revealing the dynamics of seas, skies, and Earth. But we cannot hear them and thus live in a sonic world unaware of what stirs over the horizon.