Nature of Sound – Background

If a tree falls in the forest, a bird sings, or a beaver slaps its tail, does it make a sound if no one is there to hear it? To think about this riddle, we need to understand the nature of sound – what it is and how we perceive it. Sound is important in our lives and the lives of other animals. Our sense of hearing helps us learn about and monitor our surroundings. And for animals that chirp, croak, bark, or talk, sound provides a highly effective means of communication.

Sound is what we hear when something is vibrating. Pluck a rubber band or a guitar string, and we hear a sound. How does the sound reach our ears? As the string vibrates, oscillating rapidly back and forth, it pushes or compresses the air particles next to it, and they push neighboring particles which push their neighbors in turn, so that a wave of compression moves outward from the guitar string. As the string swings back in the opposite direction, the air particles move back into their vacated places, only to be compressed again by the next oscillation of the string. Sound waves move outward in all directions, and when they reach our ears, we hear a sound. Sound can travel through liquids and solids as well as gases like air. In fact, because the molecules in liquids and solids are closer together than in air, sound waves travel faster  through them – four times faster through water and fifteen times faster through iron or steel. Putting an ear to the tracks, a train could be detected from far away. One place a sound wave cannot travel is in a vacuum where there are no molecules of air to compress.

Airborne sound can make a thin membrane, like our eardrum, vibrate. Our outer ears or pinnae funnel sounds into our ear canals to the eardrum, causing it to vibrate. The eardrum separates the outer ear from the middle ear, an air-filled chamber containing three tiny bones, the hammer (malleus), anvil (incus), and stirrup (stapes), named for their shapes. The vibrating eardrum causes the air in the middle ear to vibrate which causes the hammer, anvil, and stirrup to vibrate in turn. These bones amplify the sound. Next to the stirrup is another small membrane, the oval window, which leads to the inner ear. When the stirrup vibrates, it causes the oval window to vibrate which transmits the vibrations to the inner ear. The inner ear contains the cochlea, a tiny fluid-filled organ shaped like a snail, and the semicircular canals, which are important for balance. The cochlea is lined with tiny hairs that activate nerves. Each of the 20,000 hairs responds only to a particular frequency of vibration. When a hair is triggered by a sound wave, it sends an impulse to the brain via the auditory nerve. Our brain processes this information, enabling us to identify many different sounds.

Sounds can be higher or lower depending on the speed of the vibrations – the faster the oscillations, the higher the pitch. The size and weight, or mass, of a vibrating object is important too. For example, violins sound much higher than bass fiddles. On a violin, pressing down on the neck effectively shortens the part of the string that is free to vibrate. As the string is made shorter, the sound gets higher, and the vibrations get faster. Thicker strings sound lower and vibrate more slowly than thinner strings. Humans have a fairly wide range of hearing; we can detect sounds from twenty to 20,000 vibrations per second. Dogs and cats can hear higher than we can, which may help them hunt for small rodents by listening for their high-pitched squeaks. Bats, animals that hunt in the dark, can hear even higher sounds, some as high as 200,000 vibrations per second. They use echolocation for finding their way or locating flying insect prey, emitting high-pitched pulses of sound that bounce off objects in their surroundings. Some moths can hear the bat’s pulses and take evasive action to avoid being caught.

We hear echoes too, of sounds reflected off large, hard surfaces like cliff faces or walls. Sound travels through normal air at about a thousand feet per second. When we clap our hands we first hear the sound wave that travels the short distance from our hands to our ears, and then we hear the sound wave that travels the longer distance to the wall and back. Our ears can detect two sounds if there is at least a fifteenth of a second between them, so we must be at least thirty feet from the wall to hear an echo. Shorter sounds make better echoes than long, drawn-out sounds, which is why bats emit short high-pitched pulses. Sound can also be absorbed or dampened by things like leaves in a forest, snow falling, or porous materials like cloth or acoustic tile.

Sound is an effective way of communicating, especially in places and at times when visual cues are absent. Sounds can travel in the dark, underwater or underground, in a dense forest, through openings, around obstacles, and over long distances. The drumming of woodpeckers, hooting of owls, and songs of songbirds and frogs are all examples of sounds used to attract mates and defend territories. Raucous mobbing calls of jays and crows help to drive off predators. The faint, high-pitched notes of chickadees are contact calls, helping them stay in touch with their flocks, as are the honking cries of geese as they fly in formation. The growl of a dog keeps us at bay, and the sounds of a baby animal tell its parent when it is hungry, in distress, or contented.

Animal ears come in different shapes and sizes. Many mammals like hares and coyotes have large ears that help funnel sounds to the ear canals. Birds have no pinnae but have ear openings hidden behind their feathers. The feathers can swivel around to pick up different sounds. Frogs have an eardrum-like membrane, the tympana, located behind the eyes. Crickets have eardrums on their knees, and moths have them on their abdomens. Honeybees and fruit flies have hearing organs on their antennae. Snakes have no ear openings but can sense vibrations through their jaws when resting their chins on the ground and airborne sound through their skin and bones.

Animals use many different parts of their anatomy to produce sound. In every case, some part of their body vibrates, creating sound waves that travel through air, water, or the ground. Grasshoppers and crickets stridulate, rubbing one part of the body against another. Crickets rub the edge of one wing against a row of pegs on the other wing, while grasshoppers rub a wing against a back leg. Some soldier termites knock their heads against the tunnel walls, sending out alarm signals like Morse code to recruit helpers. Rattlesnakes shake their rattles, porcupines rattle their quills, and beavers slap their tails on the water. All these sounds communicate specific messages to other animals.

Many animals have a larynx, an organ in the windpipe. Within the larynx are vocal chords, membranes that vibrate when air flows across them. In frogs, sounds from the larynx are amplified by inflating air sacs that act as resonating chambers. Tiny spring peepers can produce earsplitting calls. Elephants have a larynx that is eight times the size of ours and produces very low rumbles, below the level we can hear, to communicate over great distances. We humans can modify the sounds we make in a seemingly infinite variety of ways by adjusting the flow of air, and the shape of the larynx, mouth, and tongue. But usually we can only produce one sound at a time. Birds have a syrinx, a bony, two-sided organ that allows some to produce two different sounds at the same time.

For many animals the sense of hearing and the ability to produce sounds are crucial for survival and a means of complex communication. Understanding the nature of sound helps us make sense of the many noises in our environment. As to the age-old riddle, we know a tree falling in the forest will produce a sound wave, and there will be many ears, large and small, to hear it, even if no person is around.


Suggested Reading

Elliot, Lang, Guide to Wildlife Sounds. Mechanicsburg, PA: Stackpole Books, 2005.

Friend, Tim, Animal Talk: Breaking the Codes of Animal Language. Free Press, 2004.

Kroodsma, Donald, The Singing Life of Birds. Boston: Houghton Mifflin, 2005.

Rossing, Thomas D., F. Richard Moore and Paul A. Wheeler. The Science of Sound, Addison Wesley, 2001.

Tocci, Salvatore, Experiments with Sound. Scholastic Library, 2001.

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