After a three-hour concert by her favorite Norwegian metal band, Anja finds it difficult to hear her friend rave about the show. It sounds like he's speaking from across the room, and it’s tough to make out his muted voice over the ringing in her ears. By the next morning, the effect has mostly worn off, but Anja still has questions.
What caused the symptoms? Is her hearing going to fully recover? And can she still go to concerts without damaging her ears?
To answer these questions, we first need to understand what sound is and how we hear it. Like a pebble creating ripples in water, sound is created when displaced molecules vibrate through space. While sound vibrations can travel through solids and liquids, our ears have evolved to process vibrations in the air.
These waves of air pressure enter our ear canals and bounce off the eardrum. A trio of bones called the ossicular chain then carries those vibrations into the cochlea, transforming waves of air pressure into waves of cochlear fluid. Here, our perception of sound begins to take form.
The waves of fluid move the basilar membrane, a tissue lined with tens of thousands of hair cells. The specific vibration of these hair cells and the stereocilia on top of each one determine the auditory signal our brain perceives. Unfortunately, these essential cells are also quite vulnerable.
There are two properties of sound that can damage these cells. The first is volume. The louder a sound is, the greater the pressure of its vibrations.
While the ear’s upper limits vary from person to person, close range exposure to sound exceeding 120 decibels can instantly bend or blow out hair cells, resulting in permanent hearing damage. The pressure of more powerful sounds can even dislocate the ossicular chain or burst an eardrum. The other side of this equation is the sound’s duration.
While dangerously loud sounds can injure ears almost instantly, hair cells can also be damaged by exposure to lower sound pressure for long periods. For example, hearing a hand dryer is safe for the 20 seconds you’re using it. But if you listened for 8 consecutive hours, this relatively low-pressure sound would overwork the stereocilia and swell the hair cell’s supporting tissue.
Swollen hair cells are unable to vibrate with the appropriate speed and accuracy, making hearing muffled. This kind of hearing loss is known as a temporary threshold shift, and many people will experience it at least once in their lifetime. In Anja’s case, the loud sounds of the concert only took three hours to cause this condition.
Fortunately, it's a temporary ailment that usually resolves as swelling decreases over time. In most cases, simply avoiding hazardous sounds gives hair cells all they need to recover. One temporary threshold shift isn’t likely to cause permanent hearing loss.
But frequent exposure to dangerous sound levels can lead to a wide range of hearing disorders, such as the constant buzz of tinnitus or difficulty understanding speech in loud environments. Overworked hair cells can also generate dangerous molecules called reactive oxygen species. These molecules have unpaired electrons, driving them to steal electrons from nearby cells and cause permanent damage to the inner ear.
There are numerous strategies you can adopt for preventing hearing loss. Current research around earbud headphone use suggests keeping your volume at 80% or less if you’ll be listening for more than 90 minutes throughout the day. Noise-isolating headphones can also help you listen at lower volumes.
Getting a baseline understanding of your hearing is essential to protecting your auditory system. Just like our eyes and teeth, our ears also need annual check-ups. Not all communities have access to audiologists, but organizations around the world are developing portable hearing tests and easy-to-use apps to bring these vital resources to remote regions.
Finally, wear earplugs when you’re knowingly exposing yourself to loud sounds for extended periods. An earplug’s effectiveness depends on how well you’ve inserted it, so be careful to read the instructions. But when worn correctly, they can ensure you'll be able to hear your favorite band for many nights to come.
