How Bone-Conducting Headphones Function

"Let the rhythm pound!" the hip-hop group Black Eyed Peas urge in their song "Play It Loud." And if you're into pop music, you probably enjoy turning up the volume. But every time you increase the volume on your portable MP3 player and jam those tiny earphones into your ears, you expose yourself to sound levels reaching as high as 120 decibels – which is as intense as the sound of a jet engine [source: Science Daily]. And that high volume could come at a cost to your hearing. A study published in 2010 in the Journal of the American Medical Association found that nearly 20% of U.S. teenagers have experienced some hearing loss, likely from listening to loud music through those earphones [source: Ostrow].

But don't worry, you don't have to give up your favorite Foo Fighters or U2 tracks while running. Imagine if you could listen to music on a portable device without placing anything in your ears? Well, it's possible. All you need is a pair of bone-conducting headphones, designed to send sound directly to the inner ear and transmit nerve signals to the brain – even bypassing parts of the ear. These revolutionary devices are often called "bonephones."

How Bone Conduction Functions

To grasp how bone conduction works, it’s important to first understand how we perceive sound, which occurs in two primary ways:

Sound moves in waves through the air. Typically, sound waves pass through various structures in the ear before being converted and transmitted by the nervous system to our brains. Initially, the waves enter the outer ear, or pinna, the large flappy cartilage that helps direct the sound. From there, the sound travels into the air-filled middle ear, which contains the auditory canal and the eardrum, a skin flap that vibrates when exposed to sound wave energy. Behind the eardrum, three tiny bones called the ossicles are connected to it, passing the vibration to the cochlea, a fluid-filled structure that converts the vibrations into electrical impulses sent along the auditory nerve to the brain [source: Hass].

However, sound can also be processed differently by the body. Sound waves can travel through the bones in your skull. As the bones vibrate, the sound reaches the cochlea, just like it would if traveling through the middle ear and eardrum, resulting in the same nerve impulses being sent to the brain. This type of sound transmission is known as bone conduction [source: Walker and Stanley].

The renowned 18th- and early-19th-century composer Ludwig Van Beethoven, who experienced hearing loss likely due to thickening of the middle ear structures, might have been one of the first to invent a bone-conducting device to help him hear music. He attached a rod to his piano and connected it to his head so that the vibrations from his playing were transmitted directly to his cochlea [source: Mai]. Bone-conducting headphones are built upon this very principle.

Bone-Conducting Technology

As the 20th century ushered in the age of electrically amplified sound, inventors began developing bone conduction hearing devices to assist those with hearing loss or people working in noisy environments. For example, in 1935, inventor Edgar Hand was granted a patent for a special telephone featuring a headband that connected the receiver to the user's head, enabling the transmission of the caller's voice vibrations through the bones [source: Hand]. In the 1940s and 1950s, several inventors secured patents for hearing aids that utilized bone conduction. In 1957, Clairdon Cunningham, an engineer at General Dynamics, applied the principle of bone conduction to create a communication helmet for pilots who needed to converse over the deafening sound of jet engines [source: Cunningham].

In the early 1980s, James P. Liautaud, an inventor, was granted a patent for a device that allowed people to listen to music or radio while skiing, running, cycling, or engaging in other sports, without the need for headphones that could obstruct their safety. He created a music player worn around the waist with wires connecting to small speakers placed on the user's clothing over the collarbone. While the music could still be heard through the ears, bone conduction was also part of the experience [source: Liautaud]. In 1994, H. Werner Bottesch took the concept even further, receiving a patent for stereo music headphones designed for bone conduction. These headphones rested just behind the ears and transmitted sound through the mastoid bones of the skull. Bottesch also innovated by selectively amplifying frequencies that are less effectively transmitted through bone [source: Bottesch].

Since then, bonephones have become much more advanced. But are they a safer option compared to earphones when it comes to protecting your precious hearing?

Are bone-conducting headphones superior to traditional headphones?

Bone-conducting headphones began appearing on the consumer market in the early 2000s, and their popularity has grown since. The burning question for many potential buyers is: Do they truly safeguard your ears from harm? The answer seems to be "yes." In 2004, Deborah Price, an audiologist and vice-chair of the Audiology Foundation of America, told Wired that bone conduction is "very safe" [source: Weir].

Whether bonephones provide the same sound quality as traditional earphones or earbuds is a more complex question. Some users report that the sound isn't quite as good as with conventional air conduction. For example, Ars Technica reviewer Ben Kuchera wrote in 2009 that one particular brand of bonephones delivered "a tinny, low-volume sound" with no noticeable bass and a lot of distortion, especially when he cranked up the volume to hear voices in a podcast [source: Kuchera].

Since bonephones are still in the early stages, researchers continue to explore the technology to assess how well we can hear with them, whether certain sounds are better represented, and how effectively they can recreate the sensation of three-dimensional sound. However, some initial findings already make the technology seem quite promising. Critics have argued that lateralization, or stereo separation – the effect where sounds appear to come from different directions – is not achievable with bone conduction. Yet, recent studies conducted by scientists at the Georgia Institute of Technology revealed that people using bonephones experienced lateralization similar to what conventional over-ear headphones provide. These same researchers are now working on ways to enhance the 3-D sound experience of bonephones even further [source: Walker and Stanley].

Only time will tell if bone-conducting headphones will ultimately offer superior sound quality. For now, however, bonephones present a perfectly viable option for users who are concerned about the potential hearing risks of traditional ear buds.