Bone conduction is the conduction of sound to the inner ear primarily through the bones of the skull, allowing the hearer to perceive audio content even if the ear canal is blocked. Bone conduction transmission occurs constantly as sound waves vibrate bone, specifically the bones in the skull, although it is hard for the average individual to distinguish sound being conveyed through the bone as opposed to the sound being conveyed through the air via the ear canal. Intentional transmission of sound through bone can be used with individuals with normal hearing—as with bone-conduction headphones—or as a treatment option for certain types of hearing impairment. Bones are generally more effective at transmitting lower-frequency sounds compared to higher-frequency sounds.
Bone conduction is also called the second auditory pathway and not to be confused with cartilage conduction, which is considered the third auditory pathway.
Bone conduction is one reason why a person's voice sounds different to them when it is recorded and played back. Because the skull conducts lower frequencies better than air, people perceive their own voices to be lower and fuller than others do, and a recording of one's own voice frequently sounds higher than one expects (see voice confrontation). [1] [2]
Musicians may use bone conduction using a tuning fork while tuning stringed instruments. After the fork starts vibrating, placing it in the mouth with the stem between the back teeth ensures that one continues to hear the note via bone conduction, and both hands are free to do the tuning. [3] Ludwig van Beethoven was famously rumored to be using bone conduction after losing most of his hearing, by placing one end of a rod in his mouth and resting the other end on the rim of his piano. [4]
It has also been observed that some animals can perceive sound and even communicate by sending and receiving vibration through bone. [5]
Comparison of hearing sensitivity through bone conduction and directly through the ear canal can aid audiologists in identifying pathologies of the middle ear—the area between the tympanic membrane (ear drum) and the cochlea (inner ear). If hearing is markedly better through bone conduction than through the ear canal (air-bone gap), [6] problems with the ear canal (e.g. ear wax accumulation), the tympanic membrane or ossicles can be suspected. [7] This method was first discovered by Italian physician Hieronymus Capivacci. [8]
The first bone conduction hearing aids were invented in the 15th century. Italian physician Girolamo Cardano realized that when a rod was placed between someone's teeth and attached the other end to a musical instrument, the person could hear the music despite their hearing loss. [9] This method was used by Beethoven, as his hearing deteriorated towards the end of his life. [10] In the 1820s, French physician Jean Marc Gaspard Itard improved on this device by attaching the other end of the rod not to a musical instrument but to the mouth of another speaker. This invention was known as the Rod of Itard. [8] In 1923, Hugo Gernsback created a new kind of bone conduction hearing aid called the "Osophone", [11] which he later elaborated on with his "Phonosone". [12] Bone conduction hearing aids have also been fitted to glasses, which fit tightly to the side of the head. [13]
In the 1970s, a team of doctors in Gothenburg, most notably Anders Tjellström, had the idea to implant a bone vibrator plate into the mastoid bone with an adjoining screw that allowed an external audio processor to be attached to conduct sound. The first three patients were implanted in 1977. The device gave good results and became known as a bone-anchored hearing aid, or BAHA. [9] In 2012, this idea was taken a step further by the introduction of the BONEBRIDGE device. Whereas a BAHA implant is a percutaneous device that requires the screw abutment to protrude through the skin, the BONEBRIDGE is a transcutaneous device and is fully implanted under the skin. In this case, the audio processor is held in place by magnets. [14]
Bone conduction devices are suitable for patients with conductive or mixed hearing loss, with a functioning cochlea but problems with the outer or inner ear that prevent sound vibrations from reaching the cochlea. [15] This can be caused by conditions such as atresia, microtia, Goldenhar syndrome or Treacher Collins. [16] Bone conduction is also a good option for someone who cannot use traditional air conduction hearing aids. [15]
Bone conduction devices are also used to help people with single-sided deafness, who have a non-functioning inner ear on one side. In this situation, the device picks up sounds on the non-functioning side and sends them as vibrations through the bone to the functioning cochlea on the other side. [17]
There are many different types of bone conduction hearing aids but most of them work on the same principle and comprise necessary components like microphones, signal processing, energy supply and a transducer that generates vibrations. The microphone of the hearing aid picks up sound signals from the environment. The signal is then optimized and transmitted to the transducer, which generates vibrations. Depending on the specific bone conduction hearing aid system, the vibrations are either sent directly through the skull bone, or through the skin towards the inner ear. Finally, the inner ear picks up the vibrations and sends them to the auditory cortex in the brain.
Different bone conduction devices contain different features. Here are the key ones.
Surgical bone conduction devices consist of an internal implant and an external audio processor used to transmit sound. They require surgery in order to implant the device, which is usually done as an outpatient procedure under general anesthetic, [18] however this depends on the device being implanted and the health condition of the patient.
Non-surgical devices only consist of the external audio processor. The processor simply vibrates, making both the skin and the bone vibrate, conducting the vibrations through to the cochlea. Non-surgical devices are ideal for children, who may not be old enough for implantation surgery or who have temporary conductive hearing loss caused by glue ear or ear infections. [19]
There are various ways to attach non-surgical bone conduction devices to the skin, including headbands, adhesives [20] and bone conduction glasses. [13] Devices include the ADHEAR from MED-EL, [21] the BAHA Start from Cochlear, [22] BHM's contact mini or contact forte [23] and the Ponto Softband from Oticon Medical. [24] Unlike headbands or glasses-based devices, adhesive devices do not need to apply pressure against the head in order to transmit the vibration. Because of this, users of adhesive devices report wearing their device for longer each day. [25]
A transcutaneous bone conduction device transmits sound signals, either electronic or mechanical, through the skin. In other words, there is closed, intact skin between the external audio processor and the internal implant. The processor is held in place over the implant using magnetic attraction. Transcutaneous devices currently on the market include the BAHA Attract, [26] and Osia [27] from Cochlear and the BONEBRIDGE from MED-EL. [28] With a percutaneous device, part of the implant (known as the abutment) protrudes through the skin. The audio processor then snaps onto the abutment, providing a direct connection to the implant. [29] Percutaneous devices include the BAHA Connect from Cochlear [30] and the Ponto from Oticon Medical. [31] Percutaneous devices have been associated with skin complications, ranging from slight redness to the formation of granulation tissue and recurring infection. The most serious complications might require further surgery or abutment removal and subsequent reimplantation. [32] One study into skin problems with percutaneous implants revealed a complication rate of up to 84%. [33] In another study a meta-analysis of complications with osseointegrated hearing aids showed that revision surgery is required in up to 34.5% of cases. [34] Transcutaneous devices were later designed to avoid or reduce recurring skin complications. [17]
An active bone conduction device is one where the implant generates the vibrations that directly stimulate the bone. With a passive bone conduction device, the vibrations are generated by the audio processor before being passed through the skin or an abutment to reach the implant and the bone. [35] The main active bone conduction devices available are the BONEBRIDGE from MED-EL [28] and the Osia from Cochlear. [36] Both are active transcutaneous devices. The external audio processor picks up sound vibrations and transmits them electronically through the skin to the internal implant, which directly and actively vibrates the bone. These vibrations are conducted through the skull bone to the cochlea and are processed as normal. [35]
The main passive bone conduction devices are the BAHA Attract [37] and BAHA Connect [38] from Cochlear, the Ponto from Oticon [39] and the Alpha 2 MPO from Medtronic. [40] The BAHA Connect and Ponto are passive percutaneous devices, whereby the audio processor is fixed onto an abutment placed through the skin. The audio processor vibrates, sending the vibrations via the abutment to the implant and then through the bone to the cochlea. [35] The BAHA Attract and Alpha 2 are transcutaneous devices but they work in a similar way. The audio processor vibrates, sending mechanical vibrations to the implant through the bone. However, unlike with the percutaneous devices, the vibrations from the audio processor pass through the skin before they reach the internal implant. These vibrations are then conducted through the skull bones to the cochlea and are processed as normal, just like with an active device. [35]
Active transcutaneous and passive percutaneous bone conduction devices tend to deliver better sound quality than passive transcutaneous ones. Passive transcutaneous devices send sound vibrations through the skin, and as they pass through the skin, they lose some of their strength, causing signal attenuation of up to 20dB. [17] To counteract this, passive transcutaneous devices may require the use of strong magnets that squeeze the skin to achieve optimal conduction. This can lead to pain and irritation of the skin and soft tissue between the two magnets, and in worst cases cause necrosis. [17] A study found that major complications—defined as complications requiring active management, such as post-operative seroma, hematoma, wound infections, skin ulcerations, and dehiscence—were found in 5.2% of cases. [41]
Device | Surgical | Non-surgical | Active surgical | Passive surgical | Transcutaneous surgical | Percutaneous surgical |
---|---|---|---|---|---|---|
ADHEAR | x | ✓ | N/A | N/A | N/A | N/A |
Alpha 2 | ✓ | x | x | ✓ | ✓ * (see above) | |
BAHA Attract | ✓ | x | x | ✓ | ✓ * (see above) | x |
BAHA Connect | ✓ | x | x | ✓ | x | ✓ |
BAHA Start | x | ✓ | N/A | N/A | N/A | N/A |
BONEBRIDGE | ✓ | x | ✓ | x | ✓ | x |
Osia | ✓ | x | ✓ | x | ✓ | x |
Ponto | ✓ | x | x | ✓ | x | ✓ |
Bone conduction products are usually categorized into three groups:
One example of a specialized communication product is a bone conduction speaker that is used by scuba divers. The device is a rubber over-moulded, piezoelectric flexing disc that is approximately 40 millimetres (1.6 in) across and 6 millimetres (0.24 in) thick. A connecting cable is molded into the disc, resulting in a tough, waterproof assembly. In use, the speaker is strapped against one of the dome-shaped bone protrusions behind the ear and the sound, which can be surprisingly clear and crisp, seems to come from inside the user's head. [42]
The Google Glass device employs bone conduction technology for the relay of information to the user through a transducer that sits beside the user's ear. The use of bone conduction means that any vocal content that is received by the Glass user is nearly inaudible to outsiders. [43]
German broadcaster Sky Deutschland and advertising agency BBDO Germany collaborated on an advertising campaign that uses bone conduction that was premiered in Cannes, France at the International Festival of Creativity in June 2013. The "Talking Window" advertising concept uses bone conduction to transmit advertising to public transport passengers who lean their heads against train glass windows. Academics from Australia's Macquarie University suggested that, apart from not touching the window, passengers would need to use a dampening device that is made of material that would not transmit the vibration from the window in order to not hear the sound. [44] [45]
Land Rover BAR employed 'military' bone conduction technology, designed by BAE Systems, within their helmets for use within the 2017 America's Cup. [46] The helmets allowed the crews to communicate effectively with each other under race conditions and within the harsh, noisy environment; whilst maintaining situational awareness due to their ears being uncovered. [47]
In March 2019 at The National Maritime Museum, London, British composer Hollie Harding premiered the use of Bone Conduction Headphones as part of a musical performance. [48] The use of the technology allowed the audience to listen to a pre-recorded musical track on the headsets, whilst a live orchestra performed a separate but related musical track. This multilayered effect meant that electronic and digitally-edited sounds could be heard in conjunction with live music without the use of loud-speakers for the first time and that the source of sounds could appear to be close to, far from, or all around the listener.
Research has found that the use of bone conduction headphones can help people distinguish between their own voice and the voice of others. [49] The findings have potential clinical relevance for conditions such as schizophrenia. [49]
Because bone conduction headphones transmit sound to the inner ear through the bones of the skull, leaving the ears free to pick up sound from the environment, users can listen to audio while maintaining greater situational awareness than with acoustic in- or over-ear headphones. However, users may still be less aware of their environment than if not using headphones. [50]
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the organ of Corti, the sensory organ of hearing, which is distributed along the partition separating the fluid chambers in the coiled tapered tube of the cochlea.
Anotia describes a rare congenital deformity that involves the complete absence of the auricle, the outer projected portion of the ear, and narrowing or absence of the ear canal. This contrasts with microtia, in which a small part of the auricle is present. Anotia and microtia may occur unilaterally or bilaterally. This deformity results in conductive hearing loss, deafness.
A cochlear implant (CI) is a surgically implanted neuroprosthesis that provides a person who has moderate-to-profound sensorineural hearing loss with sound perception. With the help of therapy, cochlear implants may allow for improved speech understanding in both quiet and noisy environments. A CI bypasses acoustic hearing by direct electrical stimulation of the auditory nerve. Through everyday listening and auditory training, cochlear implants allow both children and adults to learn to interpret those signals as speech and sound.
A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers such as personal sound amplification products (PSAPs) or other plain sound reinforcing systems cannot be sold as "hearing aids".
Unilateral hearing loss (UHL) is a type of hearing impairment where there is normal hearing in one ear and impaired hearing in the other ear.
Presbycusis, or age-related hearing loss, is the cumulative effect of aging on hearing. It is a progressive and irreversible bilateral symmetrical age-related sensorineural hearing loss resulting from degeneration of the cochlea or associated structures of the inner ear or auditory nerves. The hearing loss is most marked at higher frequencies. Hearing loss that accumulates with age but is caused by factors other than normal aging is not presbycusis, although differentiating the individual effects of distinct causes of hearing loss can be difficult.
Demant A/S is a Danish multinational company involved with hearing care, hearing aids, audiometric equipment and personal communication devices.
A bone-anchored hearing aid (BAHA) is a type of hearing aid based on bone conduction. It is primarily suited for people who have conductive hearing losses, unilateral hearing loss, single-sided deafness and people with mixed hearing losses who cannot otherwise wear 'in the ear' or 'behind the ear' hearing aids. They are more expensive than conventional hearing aids, and their placement involves invasive surgery which carries a risk of complications, although when complications do occur, they are usually minor.
Cochlear is a medical device company that designs, manufactures, and supplies the Nucleus cochlear implant, the Hybrid electro-acoustic implant and the Baha bone conduction implant.
A contralateral routing of signals (CROS) hearing aid is a type of hearing aid that is used to treat a condition in which the patient has no usable hearing in one ear and minimal hearing loss or normal hearing in the other ear. This is referred to as single sided deafness.
Electric acoustic stimulation (EAS) is the use of a hearing aid and a cochlear implant technology together in the same ear. EAS is intended for people with high-frequency hearing loss, who can hear low-pitched sounds but not high-pitched ones. The hearing aid acoustically amplifies low-frequency sounds, while the cochlear implant electrically stimulates the middle- and high-frequency sounds. The inner ear then processes the acoustic and electric stimuli simultaneously, to give the patient the perception of sound.
Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is auditory science.
Geoffrey R. Ball (born 1964) is an American physiologist specializing in Biomechanics and the inventor of the VIBRANT SOUNDBRIDGE active middle ear implant – a medical device designed to treat his own hearing loss.
SoundBite Hearing System is a non-surgical bone conduction prosthetic device that transmits sound via the teeth. It is an alternative to surgical bone conduction prosthetic devices, which require surgical implantation into the skull to conduct sound.
A direct acoustic cochlear implant - also DACI - is an acoustic implant which converts sound in mechanical vibrations that stimulate directly the perilymph inside the cochlea. The hearing function of the external and middle ear is being taken over by a little motor of a cochlear implant, directly stimulating the cochlea. With a DACI, people with no or almost no residual hearing but with a still functioning inner ear, can again perceive speech, sounds and music. DACI is an official product category, as indicated by the nomenclature of GMDN.
Cochlear Bone Anchored Solutions is a company based in Gothenburg, Sweden that manufactures and distributes bone conduction hearing solutions under the trademark Baha. The company was founded in 1999 under the name Entific Medical Systems. When Cochlear bought the company in 2005, the name was changed to Cochlear Bone Anchored Solutions. The acronym "BAHA" was trademarked into Baha, as it is not considered a hearing aid by insurance companies.
MED-EL is a global medical technology company specializing in hearing implants and devices. They develop and manufacture products including cochlear implants, middle ear implants and bone conduction systems.
Cartilage conduction is a pathway by which sound signals are transmitted to the inner ear. In 2004, Hiroshi Hosoi discovered this pathway and named “cartilage conduction”. Hearing by cartilage conduction is distinct from conventional sound-conduction pathways, such as air or bone, because it is realized by touching a transducer on the aural cartilage and does not involve the vibration of the skull bone. Therefore, cartilage conduction is referred to as the “third auditory pathway”.
Treatment depends on the specific cause if known as well as the extent, type, and configuration of the hearing loss. Most hearing loss results from age and noise, is progressive, and irreversible. There are currently no approved or recommended treatments to restore hearing; it is commonly managed through using hearing aids. A few specific types of hearing loss are amenable to surgical treatment. In other cases, treatment involves addressing underlying pathologies, but any hearing loss incurred may be permanent.
A middle ear implant is a hearing device that is surgically implanted into the middle ear. They help people with conductive, sensorineural or mixed hearing loss to hear.
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