May 31, 2013
It is commonly accepted that we hear the world with our ears. Early man must have noticed that when the ear canals are blocked, hearing decreases. Everyone can observe the outer ear and a canal that travels into the head, but what is really happening when we hear a sound, say, a person talking, music playing or a warning alert? There’s probably more going on than meets the eye, you’re guessing, and you are correct.
This article will be the first of a two part series dealing with the complexity of hearing and our body’s hearing system. Here we will deal with the outer and middle ears. Part two will examine the inner ear and brain structures.
Our outer ears have their peculiar shape for a good reason. As sound waves propagate through the air, our funnel-shaped ears can pull in more sound than two small holes on the sides of our heads. They also face forward, as it is most likely we seek to emphasize what we can see in front of us.
When the sound waves travel inward, they cause the tympanic membrane (eardrum) to vibrate. This three-layer membrane of modified skin tissue “transduces” sound energy into mechanical vibrations. Every sound we can detect has the property of “frequency”, or the number of vibrations per second. A high-frequency sound is high-pitched; think of a violin compared to a bass drum. Our eardrum is such a sensitive instrument, it can and does vibrate at the same frequency as every sound we receive ALL the time. The drum also signals the intensity of the sound, by vibrating with more force in response to louder sounds.
Attached to the back of the eardrum is a chain of three tiny bones, the ossicles. These bones vibrate with the eardrum and transmit the vibrations inward with even greater force. The third bone, the stapes (“stirrup”) is connected to the inner ear (the cochlea) by a ligament at the “oval window”. Vibrations into the oval window become fluid waves in the inner ear, as we will see in our next article. The middle ear space occupied by the ossicles is typically empty and air-filled. It connects to the throat via the Eustachian tube, which allows us to “pop” our ears when pressure changes, and allow air into the middle ear and fluid OUT of the ear.
Problems with the outer or middle ears can produce a “conductive hearing loss”, or a disruption in the conduction of sound energy toward the inner ear. Often such losses can be treated medically or surgically, restoring some or all of the original hearing.
In our next blog entry we will explore the inner ear and the auditory pathways leading to the brain.