Anyone who’s ever owned a dog or even played with one knows how sensitive pooches’ ears are. Blow into a dog whistle and pups will go crazy, while humans within earshot won’t hear a thing. Other animals have similarly keen auditory senses, too. Have humans just gotten the short end of the evolutionary stick where hearing is concerned, or does our shorter range of sound-wave frequencies benefit us in some way?
Listen to This
Ears are extraordinary organs, picking up all the sounds around us and translating them into a language our brains can understand. According to Discovery Health Online, hearing is a mechanical process. Whereas our other four senses—taste, smell, touch, and sight—involve chemical reactions in the brain, hearing is based entirely on physical movement.
Sound occurs when an object moves and causes similar vibrations in surrounding matter. That matter can be a solid (earth), a liquid (water), or a gas (air), but most of the time, the sounds we hear travel through the air around us. The vibrations move air particles in the atmosphere, setting off a chain reaction in which air particles disturb other air particles.
There are two kinds of vibrations: compression and rarefaction. Compression occurs when the object flexes out and increases the surrounding air pressure; when it flexes in to decrease the surrounding air pressure, rarefaction happens.
This fluctuating pressure creates waves that move through the atmosphere, and the variations in the frequency of these waves generate different sound pitches. A higher wave frequency, in which the air pressure fluctuates more quickly, produces a higher pitch, whereas slower fluctuations and a lower frequency produce a lower pitch. The level of air pressure in each fluctuation, also called the wave’s amplitude, determines the sound’s loudness.
Our ears direct these sound waves to where we can hear them, sense the fluctuations in air pressure, and translate these fluctuations into electrical signals that our brains can understand. The pinna, the outer part of the ear that’s pointed and has many folds, catches the sound waves and signals to our brains where the sound is coming from by recognizing the direction of the sound wave and altering its pattern by bouncing the wave off the pinna’s folds.
Once the sound waves enter the ear canal, they vibrate the tympanic membrane, or eardrum, a thin, cone-shaped piece of skin about 0.4 inches wide between the ear canal and the middle ear. The middle ear is connected to the throat by the eustachian tube, which helps to maintain the pressure balance of the eardrum as air flows in from the mouth while simultaneously entering through the outer ear. The eardrum itself is rigid and very sensitive; even the slightest change in air pressure will cause it to move back and forth. The tensor tympani muscle also constantly pulls it inward, keeping the membrane taut so that it will vibrate when it’s hit by a sound wave. Higher-pitched sound waves move the eardrum more rapidly, and louder sound moves it even further.
The eardrum is the ear’s complete sensory element; the rest of the ear serves to pass along information the eardrum gathers. The eardrum also protects the inner ear from long, loud, and low-pitched noises. The brain signals to the tensor tympani muscle and the stapedius muscle at the first indication of such a sound, and both contract so that the drum becomes more rigid and does not pick up as much noise from the lower, louder end of the audible spectrum. This reflex also helps to concentrate our hearing on higher-pitched sounds, instead of lower-pitched background noise, and explains why we can (sort of) carry on a conversation in the midst of, say, a rock concert.
On a Different Wavelength
Human pinnae face forward so that we can hear sounds in front of us better than those behind us. Most mammals, on the other hand, have large, moveable pinnae that help them focus on sounds from all directions, in order for them to detect predators and prey more easily. Our pinnae are not as adept at focusing on sound, because they lie flat against our heads and don’t move independently. Cupping our hands behind our ears can help us catch more sound waves, but nowhere near as many as, say, a dog’s ears can.
Animals also hear a broader range of frequencies than we can, which is why humans can’t hear dog whistles. Louisiana State University researchers have charted these frequencies to show that while humans can detect frequencies from 64 hertz (Hz) to 23,000 Hz, dogs can hear from 67 Hz to 45,000 Hz; cats from 45 Hz to 64,000 Hz; and horses from 55 Hz to 33,500 Hz. The most acute hearing in the animal kingdom, however, belongs to mice, bats, beluga whales, and porpoises.
Higher Isn’t Always Better
Humans may not be able to hear sounds from as many directions or at frequencies as high as our mammalian fellows do, but some Israeli researchers have shown that we may be able to detect more precise frequencies than other animals.
Professors Israel Nelken, of the Department of Neurobiology at the Alexander Silberman Institute of Life Sciences at the Hebrew University of Jerusalem; Itzhak Fried, from UCLA and Tel Aviv Medical Center; and Rafi Malach, of the Weizmann Institute of Science, along with their students Roy Mukamel and Yael Bitterman, published a study in the journal Nature in 2008 revealing that neurons in the human auditory cortex respond to frequency differences as small as a quarter of a tone. (In Western music, the smallest interval is half a tone.) This kind of resolution exceeds that typically found in the auditory cortexes of other mammals (besides bats), the researchers say. In short, humans’ range of sound may be limited, but we are better able to distinguish among the sounds we can hear, and therefore to appreciate nuances in musical and linguistic tones.
Do You Hear What I Hear?
It’s probably a good thing that humans don’t hear everything that animals do—our world is full of traffic noises, radio frequencies, cell phone transmissions, and other sounds that would drive us crazy if we heard them all day. In fact, it’s a wonder that dogs and other mammals don’t go bonkers in this loud modern soundscape. Maybe what we don’t know really can’t hurt us.