From the calming sound of birds chirruping to a warning honk of a car, your sense of hearing is integral to your understanding of the world around us. Certain sounds can help you sleep, calm your mind, cause feelings of panic or stress, or even evoke nostalgic memories. You might wonder why this happens and how hearing occurs.
How we detect sound
It all starts with the outer ear, which is also referred to as the pinna or auricle. Sound travels through the air as sound waves. The pinna acts as a funnel to channel that sound into the ear where it can be converted to a neural signal and recognized by the brain. The positioning of your ears is also important in how we perceive sound. Having your ears symmetrically positioned on the sides of your head allows for binaural sound localization. This means when a sound wave travels through the air the ear closest to the source will detect the sound first, then later by the other ear. While this time difference is incredibly short, several areas in your brainstem recognize the signal from each ear. This allows you to know where the sound is coming from and reflexively turn toward a startling or threatening noise. Other animals, such as cats, have mobile ears positioned more toward the tops of their heads, which lets them have more precise sound localization.
How we interpret sound
Once a sound wave enters the ear, it travels down the ear canal to the ear drum, or tympanic membrane, which marks the beginning of the middle ear. Inside the middle ear reside the three smallest bones in your body—the malleus (hammer), incus (anvil) and stapes (stirrup)—collectively called the auditory ossicles. The ossicles work together to transmit vibrations from the tympanic membrane to the inner ear, where those vibrations can be converted to electrical signals.
The cochlea is a fluid-filled snail-shell–shaped bone and the main hearing component of the inner ear. Within the cochlea is an organ called the corti. The corti is covered in specialized cells called hair cells. Despite their name, hair cells are not made of keratin like the hair on your head; they are sensory cells that act as gates to nerve cells.
When the hair cells bend, they allow chemicals to flow into the nerve cells, triggering action potentials, or firing of the neurons. The auditory information is decoded as it travels through the brain to the primary auditory cortex located in the temporal lobe. This is where the brain interprets what you have heard. On the corti, hair cells are organized so cells at the base of the cochlea bend and respond to higher frequencies while cells at the top (apex) respond to lower frequencies. This is called a tonotopic map and is how the brain can interpret the pitch of a sound.
When someone begins to lose their hearing, it can significantly alter how they interact with the world. Hearing aids have helped people with hearing impairment since the first electric hearing aid was used in 1898. Hearing aid design has since evolved and have greatly improved over the years. The U.S. Food and Drug Administration has recently allowed hearing aids to be sold over the counter, greatly increasing their accessibility to the millions of people in the U.S. who experience hearing impairment.
Gillian Kelly is a PhD student in the Department of Medicine, Division of Nephrology at the University of Alabama at Birmingham. Her research primarily focuses on early-life stress and the cardiovascular system.