To understand accessibility options for people who are D/deaf or hard of hearing, you have to have some understanding of the physical process of hearing. It helps to know the types and degrees of hearing loss, assistive technology choices, and developing research as well. Since this topic is so complex, we are dedicating a series of blog posts to the subject.
One in eight people aged 12 years or older in the United States has hearing loss in both ears. At age 65, one out of three people has hearing loss. My mom has moderate hearing loss and my dad is profoundly deaf in one ear. You or someone you know likely has some degree of hearing loss, even if you aren’t receiving or providing communication access services.
Understanding those we serve helps us to be better providers. The severity and type of hearing loss an individual experiences influences how they interact with others, and how they use communication services.
We’ll begin this series by talking about the process of hearing. The auditory system includes the outer, middle and inner ear, as well as auditory pathways to and from the brain.
The Process of Hearing
We’ll start with these three sections of the ear:
- The outer ear
- The middle ear
- The inner ear
The outer ear
The outer ear consists of the pinna (the ear we see) and the ear canal. Hearing begins when sound waves in the air enter the ear canal through the pinnae. The ridges and folds of our ears collect sound vibrations and allow us to pinpoint what direction a sound is coming from. These sound waves travel through the ear canal to the eardrum. This process of sound being transmitted to the eardrum is called air conduction, because the sound waves are in the air. We’ll review bone conduction, a secondary mode of hearing, in the next section.
The middle ear
The middle ear transmits sound waves from the outer to the inner ear. The middle ear is an air-filled cavity between the eardrum (tympanic membrane) at the end of the ear canal and the inner ear’s oval window (a membrane-covered opening). Sound waves from the ear canal hit the eardrum. The eardrum moves back and forth, which causes three tiny bones in the middle ear to move. These bones, the smallest in the body, mechanically transfer sound waves to the inner ear.
The inner ear
The inner ear is filled with fluid. It contains the cochlea (the hearing part of the inner ear) and the semicircular canals of the labyrinth (responsible for balance). Sound is transmitted to the cochlea when the bones of the middle ear move and push the oval window. This causes hair cells in the cochlea to bend back and forth. This leads to changes within the hair cells, sending electrical signals to the brain through cochlear nerve fibers that rest below the hair cells. These are called ascending (afferent) signals because they are ascending to the brain. Signals descending from the brain are called efferent signals.
This animated video by medical illustration student Brandon Pletsch shows how sound waves travel from the outer ears to the cochlea:
Bone conduction is a secondary mode of hearing where vibrations of the skull stimulate cochlear fluids. Our own voice is partially transmitted through bone conduction. Vibration of the vocal chords cause the skull to vibrate, which stimulates the cochlea. Whales hear through bone conduction.
Bone conduction hearing has been known about since antiquity. It’s been said that Beethoven, who became deaf later in his life, used bone conduction by holding a rod between his teeth and touching it to the piano, though this is debated. Modern technology offers bone conduction headphones and hearing aids that synthesize sound waves into vibrations that can be received directly by the cochlea.
Central Auditory System
So far we’ve only talked about the ascending signals that are sent from the ear to the brain. We don't just hear with our ears. We "hear" with the auditory center of our brain, too. The cochleae convert sound waves to electrical signals that the brain can recognize and understand. Some basic cues the ears send to the brain include timing cues, frequency/pitch cues, and intensity or loudness cues. The brain has a hard task; the ears break these cues down into simple units, and then the brain has to put all of those units back together so it can interpret where the sound is coming from, what the sound is, and what the sound means. My dad says that the speed, cadence, and pronunciation of a person’s speech affects his comprehension.
In addition to interpreting sound, our brain sends out descending (efferent) signals that control parts of our ears. The brain balances hearing, helps us hear in noisy environments, and contracts muscles in the middle ear in response to loud noises (similar to the eyes' blinking reflex). Researchers discovered that these efferent signals from the brain help protect loss of auditory nerve fibers in the cochlea. We know much less about these descending (efferent) signals from the brain to the ears, but research is ongoing.
For the next blog post in this series, we'll look at different types of hearing loss. I invite you to talk to people in your life who are D/deaf or hard of hearing. Ask them about their particular type of hearing loss and how you can help make conversations easier to understand. My parents usually don’t talk much with our family about their hearing loss. To prepare for writing the next post, I asked them some questions. I very much appreciate their openness about their challenges and look forward to sharing with you what I’ve learned.
Dana Pinney has been a TypeWell transcriber with Strada since 2011, joining the staff in 2015. Dana has a B.F.A. and a background in editing and word processing.