Hearing is one of our five senses. Hearing and the other four senses — seeing, tasting, smelling and touching — all play an important part in our understanding of the world around us.
So what is hearing? Hearing is the ability to perceive sound. Your ears are in charge of collecting sounds, processing them, and sending signals about them to your brain. The brain interprets the signals and determines what sounds we're hearing.
How Do We Hear?
Hearing happens when the ear changes sound waves into electrical signals that are sent to the brain. We don't really hear with our ears — we hear with our brains!
How the Ear Is Put Together
Let's look at how the ear is put together — scientists call this “Anatomy.” The ear has three main parts: the outer, middle, and inner ear. Here's a detailed, interactive model of the ear's anatomy.
Sound waves enter through the outer ear which is also called the “pinna.” That's the part of the ear that we can see. From here the sound moves through the external auditory canal to the middle ear where they cause the eardrum to vibrate (move back and forth). Another name for the eardrum is “the tympanic membrane.”
The middle ear also has three tiny bones called the ossicles. These three bones are named the malleus, incus, and stapes (and are also known as the hammer, anvil, and stirrup). The eardrum and ossicles make the vibrations larger and carry them to the inner ear.
Fun Fact: Did you know that the frog has an eardrum on the outside of its body behind the eye?
The cochlea is a a snail-shaped, fluid filled structure in the inner ear. Inside the cochlea are tiny hair-like bundles. As fluid in the cochlea moves against the sensory hairs, the sound vibrations are changed into nerve impulses. These impulses are then sent to the brain along the auditory nerve.
Different sounds move the hair bundles in different ways. This helps the brain to tell one sound from another.
In this photo [left], the bony wall of the cochlea has been cut away revealing the fluid-filled spiral chambers within. You can see the stapes in the lower left.
The stapes is the smallest bone in the human body. It is only 0.25 to 0.33 cm long [0.10 to 0.13 inches] and weighs only 1.9 to 4.3 milligrams.
Did you know? There are other important structures in the inner ear. They are called the semi-circular canals and they have nothing to do with our hearing. However they are very important for helping us to keep our balance.
Why Do We Have Two Ears?
Two ears located on either side of the head allows us to locate the source of a sound. Try closing your eyes and have someone clap his hands. You will be able to point at that person because your brain is able to compute how much time went by between when your left ear heard the sound and when your right ear heard the sound.
This can work because sound moves through air pretty slowly. It reaches one ear before the other, and your amazing brain can use that information to help you locate the sound!
Sound waves have two characteristics that are both important for our hearing: Frequency and Amplitude.
Frequency determines how low or high a sound appears to us: low like a tuba or high like a flute. Technically, frequency is the number of sound vibrations in a period of time. It is measured in cycles per second (cps), also called Hertz (Hz). Humans can hear sounds waves with frequencies between 20 and 20,000 Hz. Some animals, like dogs, bats, and porpoises, can hear sounds at much higher frequencies than humans. Check out the hearing ranges for selected animals. Find hearing ranges for more animals.
Amplitude is a measure of how big a sound wave is and is expressed in decibels or dB, named after Alexander Graham Bell. Amplitude is the force of sound waves against the ear drum and determines the loudness or volume of a sound. The louder the sounds, the bigger the amplitude of the sound waves, and the more decibels they have.
Sounds we can hear can be very faint like a whisper (30 dB) to very loud like a siren or jet engine (130 dB). Long exposure to any noise above 90 decibels can cause gradual hearing loss. How does this happen? It has to do with the hair cells. Loud sounds and some infections can cause some of the hair cells to die. Once a hair cell dies it cannot grow back.
We are born with only about 3500 of these hairs, so make sure you protect your hearing from loud noises. You can experiment with sound to see how loud is too loud. Find out if a ringing telephone, a band concert, or an electric drill can impact your hearing with this chart of Environmental Noise Levels.