Beethoven and bone conduction: modern hearing solutions based on old tricks

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Smiling woman with hearing aid
The famous composer Ludwig Von Beethoven lost his hearing at a young age. To keep composing, he actively looked for a way to compensate for his hearing loss. He managed to find a solution by making clever use of the bone conduction of sound. To this day, bone-anchored hearing aids help people stay in tune with the auditory world around them. Researchers at pioneering company Cochlear Limited in Belgium are actively working on the future of this technology.

Beethoven was only 28 years old when he noticed his hearing was degrading. To partly compensate for the hearing loss he experienced, he came up with a creative solution. He would place one end of a wooden stick onto his piano whilst he clamped the other end between his teeth. The notes he played onto the piano suddenly became audible again, as the vibrations were conducted by the stick to Beethoven’s ears. Two hundred years later, we are still using these principles to create hearing aids for people around the world.

The science behind Beethoven’s stick

At school, we learn that sound is a kind of wave that propagates through the air. Our auricle catches these waves and conducts them through the external ear canal until they collide with the eardrum. That collision in turn creates a vibration which is transferred to the small bones connected to the eardrum called the middle ear ossicles – the hammer, anvil, and stirrup. The vibrations from the last ossicle, the stirrup, are converted to a pressure wave once again and travel through the liquid in our snail-shaped inner ear – the cochlea. Here, the wave triggers a response in the auditory nerve, which sends a signal to the brain. This standard process is referred to as ‘air conduction’ hearing.

Cochlear diagram of hearing pathway

The specific type of hearing loss that affected Beethoven meant that hearing through air conduction had stopped working properly. Beethoven’s inner ear was still working fine, however, allowing him to tap into ‘bone conduction’ hearing to partly compensate for his hearing loss.

Although it’s true that we capture sound waves with our auricles, many of the waves that reach us simply collide with our head and other parts of our body. Since the cochlea is embedded into the skull bone, vibrations of the bone will directly result in vibrations of the cochlea. These in turn generate a pressure wave in the fluid inside the cochlea, triggering the hearing nerve.

The wooden stick that Beethoven used simply created a bridge between his piano and himself, allowing the vibrations to be directly transferred to his cochlea and spark up his brain with sound.

Why our voices sound weird when recorded

Although Beethoven and his piano may seem like a unique case, we are all continuously exposed to bone-conducted sound in everyday life. If you’ve ever heard a voice recording of yourself, you might have wondered why your voice sounds so odd, especially when friends and family state that the recording sounds quite normal to them?

The answer lies in bone conduction. When we speak, we hear our voice through both air-conducted sound waves and bone-conducted vibrations, as both reach our cochlea simultaneously. When we hear a recording of ourselves, the bone conduction pathway is less pronounced, with most of the sound arriving from an external source via the air rather than reverberating from our vocal cords through our bodies. Since we’re so used to the combination, recordings leave us with a strange impression.

Read this article to learn about a brain-controlled hearing aid!

Typically, our voice also sounds a bit higher when we hear a recording of ourselves. The sounds best conducted through bone are the lower frequency sounds – the bass sounds. Our voice will therefore sound slightly lower when we hear ourselves speak as compared to how someone else perceives it.

Bone conduction as a hearing solution

Even though Beethoven was using his creative solution over 200 years ago, it took a long time for bone conduction to become a real treatment for specific hearing losses. The benefit of bone conduction is that it provides an alternative for the air conduction pathway, meaning people who have a medical condition in the external ear canal or issues with the ossicles can be helped by bypassing these malfunctioning parts of the ear. The main reason for the delay was a lack of a practical way to implement bone conduction in all aspects of daily life.

The turning point finally came in the twentieth century, when the Swedish professor Per-Ingvar Brånemark identified a way to measure the mechanical stability of an implant through vibrations. In the 1970s, his compatriot professor Anders Tjellström then linked the research to hearing applications, as he was investigating how to treat hearing loss in patients that still have a functioning cochlea. Prof. Tjellström was able to lay the foundation for bone-anchored hearing aids – implants that provide amplified sound to the patient by tapping into the bone conduction pathway.

Bone-anchored hearing aids work by using microphones in a small device to capture sounds from the outside world. This device sends the sound wirelessly through the skin to an implant, which in turn converts the sound into mechanical vibrations. These vibrations are then transferred to a screw surgically implanted in the patient’s skull, triggering the bone conduction pathway. Finally, the cochlea picks up on these vibrations, and the auditory nerve carries the message to the brain.

Cochlear diagram of bone-anchored hearing aids

Since the first patient received their implant 45 years ago, lots of research has been done to understand the bone conduction pathway and describe in detail how exactly it works. We’re not entirely there yet, as it has proven to be a very complex mechanism. Nevertheless, numerous hearing systems have been developed that make good use of the pathway. Thousands of people all over the world are now able to hear better and reconnect with friends and family using this technology.

The future of bone-anchored hearing aids

There are still a lot of bone conduction studies ongoing at multiple research centers all over the world. The technology itself is however only produced by a limited number of companies – currently four in total.

The largest of these is Cochlear Limited, which is historically tied to the pioneering work of Prof. Tjellström. The company is active in Belgium through its research center in Mechelen, where almost 100 engineers and scientists are defining the technologies of tomorrow.

Personally, I’m working as an engineer in a team that is actively investigating bone conduction of sound, trying to translate newly obtained insights into better implantable hearing solutions. In doing so, we are expanding on the early work of Prof. Tjellström to ensure that future Beethovens will be able to keep enjoying music.

Interested to learn more about the bone conduction of sound? Have a look at the website of Cochlear or read an article about bone conduction written by Britannica.