Saturday, September 7, 2013

How Bone-conducting Headphones Work

"Let the rhythm pound!" the hiphop group Black Eyed Peas implore in a song titled "Play It Loud." And if you like pop music, chances are you'd like to listen to it that way -- loud.

But whenever you crank up the volume on your portable MP3 player ( and shove those little buds into your ears, you're exposing yourself to sound levels as high as 120 decibels -- a level that's comparable in intensity to a jet engine [source: Science Daily (]. And you may well be paying a high price for your pleasure.

A study published in 2010 in Journal of the American Medical Association found that nearly one in five U.S. teenagers already has some degree of hearing loss, probably as a result of listening to loud music while wearing those buds [source: Ostrow (].


But if you think you'll never again be able to listen to your favorite Foo Fighters or U2 songs while jogging, don't despair. What if you had a way to listen to music on a portable player without putting anything in your ears? As it turns out, it can be done. All you need is a set of bone-conducting headphones, a gadget designed to transmit sound directly to the innermost part of the ear that sends nerve impulses to the brain -- even while bypassing portions of the ear ( Folks call these magical devices "bonephones."

How Bone Conduction Works

To understand how bone conduction works, you first have to understand how we hear sounds, which we do in two ways:

Bone-conducting headphones deliver sound through the bone, bypassing the eardrum.
PHOTO: Bone-conducting headphones deliver sound through the bone, bypassing the eardrum.

Sound ( travels in waves through the air. Normally, sound waves travel through several structures in the ear, before being translated and transmitted through our nervous systems to our brains ( First, the waves enter the outer ear, or pinna, which is the big flappy piece of cartilage that helps to focus the sound. From there, the sound goes into the air-filled middle ear, which includes the auditory canal and the eardrum, a flap of skin that vibrates when exposed to the energy from sound waves. On the other side of the eardrum, there are three small bones, the ossicles, which are attached to it. They transmit the vibration to the cochlea, a fluid-filled structure that takes those vibrations and converts them to electrical impulses that are sent along the auditory nerve to the brain [source: Hass (].

Ear buds deliver sound through the outer ear, the eardrum and on to the cochlea.
PHOTO: Ear buds deliver sound through the outer ear, the eardrum and on to the cochlea.

But that's not the only way our body can process sound. Sound waves can also be transmitted through the bones in your head. When the bones vibrate, the sound reaches the cochlea, just as it would by going through the middle ear and eardrum, and results in the same sort of nerve impulses being transmitted to your brain. This method of sound transmission is called bone conduction [source: Walker and Stanley (].

The great 18th- and early-19th-century composer Ludwig Van Beethoven, who suffered hearing loss apparently caused by thickening of the structures in his middle ear, may have been one of the first people to develop a bone-conducting device help him hear music. He attached a rod to his piano and then connected it to his head, so that it transmitted the vibration of his playing directly to his cochlea [source: Mai (]. Bone-conducting headphones are built around this same concept.

Bone-conducting Technology
Ludwig van Beethoven may have been one of the first people to develop a device for hearing through bone conduction.
PHOTO: Ludwig van Beethoven may have been one of the first people to develop a device for hearing through bone conduction.
Hulton Archive/Stringer/Getty Images

Once the 20th century age of electrically amplified ( sound hit, inventors started developing bone conduction hearing devices to aid people with hearing loss, or who had to work in loud environments.

In 1935, for example, inventor Edgar Hand was granted a patent for a special telephone ( equipped with a headband that attached the receiver to a user's head, so that it could transmit the vibration of a caller's voice through the bones [source: Hand (].

In the 1940s and 1950s, numerous inventors patented hearing aids that utilized bone conduction.

In 1957, Clairdon Cunningham, an engineer for defense contractor General Dynamics, even used the principle of bone conduction to develop an ingenious communication helmet that could be worn by pilots who had to converse over the roar of jet engines [source: Cunningham (].

In the early 1980s, an inventor named James P. Liautaud was granted a patent for a device that enabled people to listen to the radio or taped music while skiing, running, bicycling or doing other active sports, without wearing headphones that might interfere with their safety. He patented a music player that a user wore in a belt around his or her waist. Wires from the device were connected to tiny speakers, which were attached to clothing over the user's collarbone. While the music actually could be heard through the ears, bone conduction was involved as well [source: Liautaud (].

In 1994, another inventor, H. Werner Bottesch, took the concept a bit further and received a patent for a set of stereo music headphones designed for bone conduction. His device attached just behind the user's outer ears, so that it transmitted sound through the mastoid bones of the user's skull. Bottesch also came up with the idea of selectively amplifying certain frequencies of sound that don't transmit through bone as well as others [source: Bottesch (].

Since then, bonephones have become even more sophisticated. But are they a safer alternative than ear buds to protect your precious ears?

Are bone-conducting headphones better than regular headphones?
Bonephones transmit sound through the bones in the skull rather than through the outer ear and eardrum.
PHOTO: Bonephones transmit sound through the bones in the skull rather than through the outer ear and eardrum.
Digital Vision/Getty Images

Bone-conducting headphones began to show up on the consumer market in the early 2000s, and since then, they've gained popularity. For potential purchasers, the key question remains: Do they really protect your ears from damage? The answer seems to be "yes." Deborah Price, a doctor of audiology and vice-chair of the Audiology Foundation of America, told Wired in 2004 that bone conduction is "very safe" [source: Weir (].

Whether bonephones sound as good as conventional earphones or ear buds is a trickier question to answer. Some users complain that the sound just isn't as good as with conventional air conduction. Ars Technica reviewer Ben Kuchera, for example, wrote in 2009 that one brand of bonephones gave him "a tinny, low-volume sound" with no discernible bass and a lot of distortion, particularly when he raised the volume so that he could hear individual voices in a podcast [source: Kuchera (].

Since bonephones are relatively new, scientists are still studying the technology to determine how well we can hear with them, whether they present certain sounds better than others, and how well they can reproduce the illusion of three-dimensional sound. But already, some of their findings make the technology look pretty promising. Skeptics have contended that lateralization, also known as stereo separation -- that is, the illusion that different sounds are coming from different locations -- isn't possible with bone conduction. But recent research by scientists at the Georgia Institute of Technology scientists found that subjects equipped with bonephones actually heard as much lateralization as they would through conventional headphones placed over the ear. The same researchers are working on ways to further improve the 3-D quality of bone-conducted sound [source: Walker and Stanley (].

Only time will tell whether bone-conducting headphones eventually deliver a higher quality of sound ( In the meantime, bonephones serve as a perfectly viable alternative for headphone users who are concerned about potential hearing damage caused by ear buds.

Author's Note

I grew up in the 1970s and 1980s, when listening to rock music was almost like a religion. And most of my favorite performers -- from big stadium acts like the Rolling Stones, Peter Gabriel and U2 to punk bands like the Ramones, X and the Stooges -- play really, really loud. Nevertheless, amazingly, even in my mid-50s, I still have pretty good hearing. One reason, I think, is that I always avoided crowding the stage like other fans did, because I found that I could hear the music better if I stayed farther away from the speakers. In addition, whenever I went to nightclubs with overpowering acoustics, I always made sure to wear earplugs -- or, if I forgot to bring them, to stuff a damp wad of toilet paper in each ear. In the digital music age, I've always avoided wearing ear buds and stuck with the big clunky old-school headphones that make me look like a 1920s wireless telegraph operator. Even so, I'm still careful never to turn the sound up all the way.

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