A hearing aid is an electroacoustic body worn apparatus which typically fits in or behind the wearer's ear, and is designed to amplify and modulate sound for the wearer. Earlier devices, known as an "ear trumpet" or "ear horn", were passive funnel-like amplification cones designed to gather sound energy and direct it into the ear canal. Similar devices include the bone anchored hearing aid, and cochlear implant.
This was the first type of hearing aid invented by Harvey Fletcher while working at Bell Laboratories. Thanks to developments in technology they are now rarely used. These aids consist of a case, an ear mold, and a cord. The case contains the amplifier components. The case is about the size of a pack of playing cards and is worn in the pocket or on a belt. The ear mold is connected to the case via a cord. Because of their large size, body worn aids can provide loud amplification. This made them appropriate for profound hearing losses. Today, body aids have largely been replaced by Behind-The-Ear (BTE) instruments.
BTE aids consist of a case, a tube and an earmold. The case is small and made of plastic. It fits behind the pinna (ear). The case contains the amplification system. The tube is used to route the sound from the hearing aid case to the earmold. Typically the sound is routed acoustically. Sometimes the sound is routed electrically. In that case, the speaker is located in the earmold instead of in the case. The earmold is usually custom made or made of other pliable fixture that contours to the individuals ear. The color of the BTE components (case, tube and earmold) range from inconspicuous skin tones to bright colors and optional decorations.
BTEs can be used for mild to profound hearing losses. BTEs have several advantages over other types of hearing aids. One advantage is that they tend to be more durable. This is because the electrical components are located outside the ear. This reduces the amount of earwax and moisture that the electrical components are subjected to. Another advantage is that BTEs can be connected to assistive listening devices, such as classroom FM systems. Lastly, if the earmold no longer fits the user, the earmold can be replaced for a fraction of the price of a new hearing aid. BTE hearing aids are typically prescribed for children. This is because children need a durable hearing aid, often need assistive listening devices, and often outgrow the size of the earmold.
Recent innovations in BTEs include miniature BTEs with thin hair-like sound tubes (see open-fit devices below). These are often less visible than In-The-Ear aids (ITEs). They use a larger vent than other hearing aid types. This keeps the ear canal more open, which allows sound to enter the ear without being amplified. This is helpful for listeners with normal hearing in the lower frequencies. Miniature BTEs are generally used for mild to moderate high frequency losses.
These devices fit in the outer ear bowl (called the concha); they are sometimes visible when standing face to face with someone. ITE hearing aids are custom made to fit each individual's ear. They can be used in mild to some severe hearing losses. Feedback, a squealing/whistling caused by sound (particularly high frequency sound) leaking and being amplified again, may be a problem for severe hearing losses. Some modern circuits are able to provide feedback regulation or cancellation to assist with this. Traditionally, ITEs have not been recommended for young children because their fit could not be as easily modified as the earmold for a BTE, and thus the aid had to be replaced frequently as the child grew. However, there are new ITEs made from a silicone type material that mitigates the need for costly replacements.
At a first glance, these devices are similar to the BTE aid. There is however one crucial difference: The speaker ('receiver') of the hearing aid is placed inside the ear canal of the user and thin electrical wires replace the acoustic tube of the BTE aid. There are some advantages with this approach: Firstly, the sound of the hearing aid is arguably smoother than that of a traditional BTE hearing aid. With a traditional BTE hearing aid, the amplified signal is emitted by the speaker (receiver) which is located within the body of the hearing aid (behind the ear). The amplified signal is then directed to the ear canal through an acoustic tube, which creates a peaky frequency response. With a RITE hearing aid, the speaker (receiver) is right in the ear canal and the amplified output of the hearing aid does not need to be pushed through an acoustic tube to get there, and is therefore free of this distortion. Secondly, RITE hearing aids can typically be made with a very small part behind-the-ear and the wire connecting the hearing aid and the speaker (receiver) is extremely inconspicuous. For the majority of people this is one of the most cosmetically acceptable hearing device types. Thirdly, RITE devices are suited to "open fit" technology (see below) so they can be fitted without plugging up the ear, offering relief from occlusion.
ITC aids are smaller, filling only the bottom half of the external ear. You usually cannot see very much of this hearing aid when you are face to face with someone. MIC and CIC aids are often not visible unless you look directly into the wearer's ear.  These aids are intended for mild to moderately-severe losses. CICs are usually not recommended for people with good low frequency hearing, as the occlusion effect is much more perceivable.
Extended wear hearing aids are hearing devices that are non-surgically placed in the ear canal by a hearing professional. The extended wear hearing aid represents the first "invisible" hearing device. The concept was invented by Adnan Shennib. These devices are worn up to several months at a time without removal. They are made of soft material designed to contour to each user and can be used by people with mild to moderately severe hearing loss. They work by directing sound into the ear canal from the outer ear (the pinna). Their close proximity to the ear drum results in improved sound directionality and localization, reduced feedback, and improved high frequency gain. While traditional BTE or ITC hearing aids require daily insertion and removal, extended wear hearing aids are worn continuously and then replaced with a new device. Users can change volume and settings without the aid of a hearing professional. The devices are very useful for active individuals because their design protects against moisture and earwax and can be worn while exercising, showering, etc. Because the device’s placement within the ear canal makes them invisible to observers, extended wear hearing aids are popular with those who are self-conscious about the aesthetics of BTE or ITC hearing aid models. As with other hearing devices, compatibility is based on an individual’s hearing loss, ear size and shape, medical conditions, and lifestyle.
"Open-fit" or "Over-the-Ear" OTE hearing aids are small behind-the-ear type devices. This type is characterized by a small plastic case behind the ear, and a very fine clear tube running into the ear canal. Inside the ear canal, a small soft silicone dome or a molded, highly vented acrylic tip holds the tube in place. This design is intended to reduce the occlusion effect. Conversely, because of the increased possibility of feedback, and because an open fit allows low frequency sounds to leak out of the ear canal, they are limited to moderately severe high frequency losses.
The personal programmable, consumer programmable, consumer adjustable, or self programmable hearing aid allows the consumer to adjust their own hearing aid settings to their own preference using any PC. Personal programmable hearing aid manufacturers also have the capability to remotely adjust these types of hearing aids for the customer utilizing remote access technology. Available in most hearing aid styles, these hearing aids differ from traditional hearing aids only in that they are adjustable by the consumer themselves.
Disposable hearing aids are hearing aids that have a battery or power cell that is integrated into the design of the hearing aid and is not meant to be removed. These aids are specifically engineered to use power sparingly, so that this integrated power cell can last far longer than batteries used in traditional hearing aids. By virtue of this long-lasting integrated battery, disposable hearing aids are meant to remove the task of battery replacement and other maintenance chores (adjustment or cleanings). To date, two companies have brought disposable hearing aids to market: Songbird Hearing and Lyric. Both are digital, but are used in very different ways by the consumer. Songbird is a BTE hearing aid that is bought online and worn like any other BTE device. When it runs out, the user replaces it with a new one. Lyric is implanted deep in the ear canal by a professional. When it runs out, it must be removed and replaced with a new one by a professional.
The BAHA is an auditory prosthetic which can be surgically implanted. The BAHA uses the skull as a pathway for sound to travel to the inner ear. For people with conductive hearing loss, the BAHA bypasses the external auditory canal and middle ear, stimulating the functioning cochlea. For people with unilateral hearing loss, the BAHA uses the skull to conduct the sound from the deaf side to the side with the functioning cochlea.
Individuals under the age of 5 typically wear the BAHA device on a headband. Over age 5, a titanium "post" can be surgically embedded into the skull with a small abutment exposed outside the skin. The BAHA sound processor sits on this abutment and transmits sound vibrations to the external abutment of the titanium implant. The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, allowing hearing.
During the late 1950s through 1970s, before in-the-ear aids became common (and in an era when thick-rimmed eyeglasses were popular), people who wore both glasses and hearing aids frequently chose a type of hearing aid that was built into the temple pieces of the spectacles. However, the combination of glasses and hearing aids was inflexible: the range of frame styles was limited, and the user had to wear both hearing aids and glasses at once or wear neither. Today, people who use both glasses and hearing aids can use in-the-ear types, or rest a BTE neatly alongside the arm of the glasses. There still are some specialized situations where hearing aids built into the frame of eyeglasses can be useful, such as when a person has hearing loss mainly in one ear: sound from a microphone on the "bad" side can be sent through the frame to the side with better hearing.
Recently, a new type of eyeglass aid was introduced. These 'hearing glasses' feature directional sensitivity: four microphones on each side of the frame effectively work as two directional microphones, which are able to discern between sound coming from the front and sound coming from the sides or back of the user . This improves the Signal-to-noise ratio by allowing for amplification of the sound coming from the front, the direction in which the user is looking, and active noise control for sounds coming from the sides or back. Only very recently has the technology required become small enough, in size, to be put in the frame of the glasses. As a recent addition to the market, this new hearing aid is currently available only in the Netherlands and Belgium.
Recent hearing aids include wireless hearing aids. One hearing aid can transmit to the other side so that pressing one aid's program button simultaneously changes the other aid and both aids change background settings simultaneously. FM listening systems are now emerging with wireless receivers integrated with the use of hearing aids. A separate wireless microphone can be given to a partner to wear in a restaurant, in the car, during leisure time, in the shopping mall, at lectures, or during religious services. The voice is transmitted wirelessly to the hearing aids eliminating the effects of distance and background noise. FM systems have shown to give the best speech understanding in noise of all available technologies. FM systems can also be hooked up to a TV or a stereo.
In developed countries FM systems are considered a cornerstone in the treatment of hearing loss in children. More and more adults discover the benefits of wireless FM systems as well, especially since transmitters with different microphone settings and Bluetooth for wireless cell phone communication have become available.
Many theatres and lecture halls are now equipped with assistive listening systems that transmit the sound directly from the stage; audience members can borrow suitable receivers and hear the program without background noise. In some theatres and churches FM transmitters are available that work with the personal FM receivers of hearing instruments.
Most older hearing aids have only an omnidirectional microphone. An omnidirectional microphone amplifies sounds equally from all directions. In contrast, a directional microphone amplifies sounds from in front more than sounds from other directions. This means that sounds originating from the direction the listener is facing are amplified more than sounds from behind or in other directions. If the speech is in front of the listener and the noise is from a different direction, then compared to an omnidirectional microphone, a directional microphone provides a better signal to noise ratio. Improving the signal to noise ratio improves speech understanding in noise. Directional microphones are the second best method to improve the signal to noise ratio (the best method is an FM system). 
Many hearing aids now have both an omnidirectional and a directional microphone. This is because speech often comes from directions other than in front of the listener. Usually, the omnidirectional microphone mode is used in quiet listening situations (e.g. living room) whereas the directional microphone is used in noisy listening situations (e.g. restaurant). The microphone mode is typically selected by using a switch. Some hearing aids automatically switch the microphone mode.
Adaptive directional microphones vary the direction of maximum amplification. The direction of amplification is varied by the hearing aid processor. The processor attempts to provide maximum amplification in the direction of the speech signal. One disadvantage is that the background noise is often speech (e.g. other talkers in a restaurant). This makes it difficult for the processor to select the desired speech signal. Another disadvantage is that noise often mimics speech characteristics, making it difficult to separate the speech from the noise. Despite the disadvantages, adaptive directional microphones can provide improved speech recognition in noise 
Directional microphones work best when the distance to the talker is small. In contrast, an FM system continues to provide a better signal to noise ratio even at larger speaker to talker distances.
Telecoils (T-coils), sometimes referred to as "Telephone Coils", allow audio sources to be directly connected to a hearing aid, which is intended to help the wearer filter out background noise. They can be used with telephones, FM systems (with neck loops), and induction loop systems (also called "hearing loops") that transmit sound to hearing aids from public address systems and TVs. In the UK and the Nordic countries, hearing loops are widely used in churches, shops, railway stations, and other public places. In the U.S.A., telecoils and hearing loops are gradually becoming more common. According to a Hearing Review 2008 survey of hearing professionals: “Respondents said that 62% of their fittings included a telecoil, [an] increase . . . from 37% in 2001.”
A T-coil consists of a metal core (or rod) around which ultra-fine wire is coiled. T-coils are also called induction coils because when the coil is placed in a magnetic field, an alternating electrical current is induced in the wire (Ross, 2002b; Ross, 2004). The T-coil detects magnetic energy and tranduces(or converts) it to electrical energy.
Although T-coils are effectively a wide-band receiver, interference is unusual in most hearing loop situations. Interference can manifest as a buzzing sound, which varies in volume depending on the distance the wearer is from the source. Sources are electromagnetic fields, such as old (not flat) computer monitors, older flourescent lighting, some dimmer switchers, and airplanes.
The states of Florida and Arizona have passed legislation that requires hearing professionals to inform patients about the usefulness of telecoils.
In the United States, the Hearing Aid Compatibility Act of 1988 (HAC Act) requires that the Federal Communications Commission (FCC) ensure that all telephones manufactured or imported for use in the United States after August 1989, and all “essential” telephones, are hearing aid-compatible (through the use of a telecoil).
"Essential" phones are defined as "coin-operated telephones, telephones provided for emergency use, and other telephones frequently needed for use by persons using such hearing aids." These might include workplace telephones, telephones in confined settings (like hospitals and nursing homes), and telephones in hotel and motel rooms. Secure telephones, as well as telephones used with public mobile and private radio services, are exempt from the HAC Act. "Secure" phones are defined as "telephones that are approved by the U.S. Government for the transmission of classified or sensitive voice communications."
In 2003, the FCC adopted rules to make digital wireless telephones compatible with hearing aids and cochlear implants. Although analog wireless phones do not usually cause interference with hearing aids or cochlear implants, digital wireless phones often do because of electromagnetic energy emitted by the phone's antenna, backlight, or other components. The FCC has set a timetable for the development and sale of digital wireless telephones that are compatible with hearing aids. This effort promises to increase the number of digital wireless telephones that are hearing aid-compatible.
Direct Audio Input (DAI) allows the hearing aid to be directly connected to an external audio source like a CD player or an assistive listening device (ALD). By its very nature, DAI is susceptible to far less electromagnetic interference, and yields a better quality audio signal as opposed to using a T-coil with standard headphones.
Every electronic hearing aid has at minimum a microphone, a loudspeaker (commonly called a receiver), a battery, and electronic circuitry. The electronic circuitry varies among devices, even if they are the same style. The circuitry falls into three categories based on the type of audio processing (Analog or Digital) and the type of control circuitry (Adjustable or Programmable).
The Cetera algorithm works to preserve the miniature differences between the sound intensities in both ears in order to preserve the 3D nature of the sound. When the algorithm is used with a hearing aid it makes the hearing aid almost unnoticeable to the user.
Hearing aids are incapable of truly correcting a hearing loss; they are an aid to make sounds more accessible. Two primary issues minimize the effectiveness of hearing aids:
Ordinary hearing aids are Class I "restricted devices" under the Federal Food and Drug Administration ("FDA") regulations adopted pursuant to the Medical Device Amendments of 1976 (21 U.S.C. § 360k (a) (2005)). The 1976 statute explicitly prohibits any state requirement that is "different from, or in addition to, any requirement applicable" to regulated medical devices (which includes hearing aids) which relates "to the safety and effectiveness of the device." 21 U.S.C. 5 360k (a) (2005). Inconsistent state regulation is preempted under the federal law. Under this grant of federal statutory authority, the FDA began to establish uniform conditions of sale for hearings aid in the late 1970s. In a series of rulemaking proceedings from 1977 to 1980, the FDA established federal rules governing hearing aid sales, and addressed various requests by state authorities for exemptions from federal preemption, granting some and denying others.
Under the 1977 FDA rule, hearings aids may be sold by mail under defined conditions. As "restricted devices" under FDA regulation, hearing aids sold through advertising that is false or misleading are deemed "misbranded" under the Medical Device Amendments and may subject the seller to penalties. The Federal Trade Commission also regulates the interstate marketing of hearing aids to protect consumers.
In addition, some states specifically regulate and license the sale of hearing aids by mail.
Several industrialized countries supply free or heavily-discounted hearing aids through their publicly funded health care system.
The Australian Department of Health and Ageing provides eligible Australian citizens and residents with a basic hearing aid free-of-charge, though recipients can pay a "top up" charge if they wish to upgrade to a hearing aid with more or better features. Maintenance of these hearing aids and a regular supply of batteries is also provided, on payment of a small annual maintenance fee.
In Canada, health care is a responsibility of the provinces. In the province of Ontario, the price of hearing aids is partially reimbursed through the Assistive Devices Program of the Ministry of Health and Long-Term Care, up to $500 for each hearing aid, every three years. Like eye appointments, audiological appointments are no longer covered through the provincial public health plan. Audiometric testing can still easily be obtained, often free of charge, in private sector hearing aid clinics and some ear, nose and throat doctors offices. Hearing aids are often covered to some extent by private insurance or in some cases through government programs such as Department of Veteran Affairs or Workplace Safety & Insurance Board.
Social Insurance pays a one time fee of ISK 30.000 for any kind of hearing aid. However, the rules are complicated and require that both ears have a significant hearing loss in order to qualify for reimbursement. BTE hearing aids range from ISK 60.000 ISK to 300.000 ISK.
Within the UK, the NHS provides BTE hearing aids to NHS patients, on long-term loan, free of charge. BTEs are often the only style available, and private purchases may be necessary if a user desires a different style. Batteries are free. Private purchases can cost between £600 to £3,500.
Most private US health care providers do not provide coverage for hearing aids, so all costs are usually borne by the recipient. The cost for a single hearing aid can vary between $500 to $6,000 or more, depending on the level of technology and whether the clinician bundles fitting fees into the cost of the hearing aid. Though if an adult has a hearing loss which substantially limits major life activities, some state-run vocational rehabilitation programs can provide upwards of full financial assistance. Severe and profound hearing loss often falls within the "substantially limiting" category. Less expensive hearing aids can be found on the internet or mail order catalogs but many of these are poorly made sound amplifiers that do not amplify sounds associated with the human voice. One notable exception is the Acoustitone, an analog BTE hearing aid that costs under $200  
While there are some rare instances that a hearing aid uses a rechargeable battery or a long-life disposable battery, the majority of hearing aids use one of five standard Button Cell Zinc-air batteries. These types are typically referred to by their common number name or the color of their packaging.
These batteries all operate at 1.35 Volts.
The type of battery a specific hearing aid utilizes depends on the physical size allowable and the desired lifetime of the battery, which is in turn determined by the power draw of the hearing aid device. Typical battery lifetimes run between 1 and 14 days (assuming 16 hour days).
|Type/ Color Code||Dimensions (Diameter×Height)||Common Uses||Standard Names||Misc Names|
|675||11.6 mm × 5.4 mm||High-Power BTEs, Cochlear Implants||IEC: PR44, ANSI: 7003ZD||675, 675A, 675AE, 675AP, 675CA, 675CP, 675HP, 675HPX, 675 Implant Plus, 675P (HP), 675PA, 675SA, 675SP, A675, A675P, AC675, AC675E, AC675E/EZ, AC675EZ, AC-675E, AP675, B675PA, B6754, B900PA, C675, DA675, DA675H, DA675H/N, DA675N, DA675X, H675AE, L675ZA, ME9Z, P675, P675i+, PR44, PR44P, PR675, PR675H, PR675P, PR-675PA, PZ675, PZA675, R675ZA, S675A, V675, V675A, V675AT, VT675, XL675, Z675PX, ZA675, ZA675HP|
|13||7.9 mm × 5.4 mm||BTEs, ITEs||IEC: PR48, ANSI: 7000ZD||13, 13A, 13AE, 13AP, 13HP, 13HPX, 13P, 13PA, 13SA, 13ZA, A13, AC13, AC13E, AC13E/EZ, AC13EZ, AC-13E, AP13, B13BA, B0134, B26PA, CP48, DA13, DA13H, DA13H/N, DA13N, DA13X, E13E, L13ZA, ME8Z, P13, PR13, PR13H,PR-13PA, PZ13, PZA13, R13ZA, S13A, V13A, VT13, V13AT, W13ZA, XL13, ZA13|
|312||7.9 mm × 3.6 mm||miniBTEs, RICs, ITCs||IEC: PR41, ANSI: 7002ZD||312, 312A, 312AE, 312AP, 312HP, 312HPX, 312P, 312PA, 312SA, 312ZA, AC312, AC312E, AC312E/EZ, AC312EZ, AC-312E, AP312, B312BA, B3124, B347PA, CP41, DA312, DA312H, DA312H/N, DA312N, DA312X, E312E, H312AE, L312ZA, ME7Z, P312, PR312, PR312H, PR-312PA, PZ312, PZA312, R312ZA, S312A, V312A, V312AT, VT312, W312ZA, XL312, ZA312|
|10||5.8 mm × 3.6 mm||CICs, RICs||IEC: PR70, ANSI: 7005ZD||10, 10A, 10AE, 10AP, 10DS, 10HP, 10HPX, 10SA, 10UP, 20PA, 230, 230E, 230EZ, 230HPX, AC10, AC10EZ, AC10/230, AC10/230E, AC10/230EZ, AC230, AC230E, AC230E/EZ, AC230EZ, AC-230E, AP10, B0104, B20BA, B20PA, CP35, DA10, DA10H, DA10H/N, DA10N, DA230, DA230/10, L10ZA, ME10Z, P10, PR10, PR10H, PR230H, PR536, PR-10PA,PR-230PA, PZA230, R10ZA, S10A, V10, VT10, V10AT, V10HP, V230AT, W10ZA, XL10, ZA10|
|5||5.8 mm × 2.1 mm||CICs||IEC: PR63, ANSI: 7012ZD||5A, 5AE, 5HPX, 5SA, AC5, AC5E, AP5, B7PA, CP63, CP521, L5ZA, ME5Z, P5, PR5H, PR-5PA, PR521, R5ZA, S5A, V5AT, VT5, XL5, ZA5|