There are many types of<unk> s (also known as<unk>  instruments), which vary in size, power and circuitry. Among the different sizes and models are: File:Vintage Zenith Radionic 3-Vacuum Tube (Body)<unk> , Model-A3A, Pastel Coralite Case, Bone-Air, Original Cost = 50.00 USD, Circa 1944 (10840966755).Vacuum tube<unk> , circa 1944 File:Vintage Telex Transistor (Body)<unk> , Model 70A, Made in the USA (12483173304).Transistor body-worn<unk> . File:BTEhearingaids.Pair of BTE<unk> s with earmolds. File:Hinter-Ohr-Hörgeräte.Receiver-in-the-canal<unk> s File:HearingAid ITE.In-the-ear<unk>  File:Hearing<unk>  cic.In-the-canal<unk>  File:Lyric - Ear.Completely in the canal<unk> s File:Baha user sound processor behind ear.Woman<unk>  a bone anchored<unk>  File:Hearing<unk>  application.Hearing<unk>  application Body-worn Body worn<unk> s were the first portable electronic<unk> s, and were invented by<unk> rvey Fletcher while working at Bell Laboratories. Body<unk> s consist of a case and an earmold, attached by a wire. The case contains the electronic amplifier components, controls and battery, while the earmold typically contains a miniature loudspeaker. The case is typically about the size of a pack of playing cards and is carried in a pocket or on a belt. Without the size constraints of smaller<unk>  devices, body worn<unk>  designs can provide large amplification and long battery life at a lower cost. Body<unk> s are still used in emerging markets because of their relatively low cost. Behind the ear A modern behind the ear<unk> . The audio tube to the speaker is barely visible. A modern behind the ear<unk>  with a minicell battery Behind the ear<unk> s are one of two major classes of<unk> s – behind the ear (BTE) and in the ear (ITE). These two classes are distinguished by where the<unk>  is worn. BTE<unk> s consist of a case which<unk> ngs behind the pinna. The case is attached to an earmold or dome tip by a traditional tube, slim tube, or wire. The tube or wire courses from the superior-ventral portion of the pinna to the concha, where the ear mold or dome tip inserts into the external auditory canal. The case contains the electronics, controls, battery, and microphone(s).The loudspeaker, or receiver, may be housed in the case (traditional BTE) or in the earmold or dome tip (receiver-in-the-canal, or RIC). The RIC style of BTE<unk>  is often smaller than a traditional BTE and more commonly used in more active populations. BTEs are generally capable of providing more output and may therefore be indicated for more severe degrees of<unk>  loss. However, BTEs are very versatile and can be used for nearly any kind of<unk>  loss. BTEs come in a variety of sizes, ranging from a small, "mini BTE", to larger, ultra-power devices. Size typically depends on the output level needed, the location of the receiver, and the presence or absence of a telecoil. BTEs are durable, easy to repair, and often<unk> ve controls and battery doors that are easier to manipulate. BTEs are also easily connected to assistive listening devices, such as FM systems and induction loops. BTEs are commonly worn by children who need a durable type of<unk> . In the ear In the ear<unk> s (ITE) devices fit in the outer ear bowl (called the concha). Being larger, these are easier to insert and can hold extra features. They are sometimes visible when standing face to face with someone. ITE<unk> s are custom made to fit each individual's ear. They can be used in mild to some severe<unk>  losses. Feedback, a squealing/whistling caused by sound (particularly high frequency sound) leaking and being amplified again, may be a problem for severe<unk>  losses. Some modern circuits are able to provide feedback regulation or cancellation to assist with this. Venting may also cause feedback. A vent is a tube primarily placed to offer pressure equalization. However, different vent styles and sizes can be used to influence and prevent feedback. Traditionally, ITEs<unk> ve not been recommended for young children because their fit could not be as easily modified as the earmold for a BTE, and thus the<unk> <unk> d 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. ITE<unk> s can be connected wirelessly to FM systems, for instance with a body-worn FM receiver with induction neck-loop which transmits the audio signal from the FM transmitter inductively to the telecoil inside the<unk>  instrument. Mini in canal (MIC) or completely in canal (CIC)<unk> s are generally not visible unless the viewer looks directly into the wearer's ear. These<unk> s are intended for mild to moderately severe losses. CICs are usually not recommended for people with good low-frequency<unk> , as the occlusion effect is much more noticeable. Completely-in-the-canal<unk> s fit tightly deep in the ear. It is barely visible. Being small, it will not<unk> ve a directional microphone, and its small batteries will<unk> ve a short life, and the batteries and controls may be difficult to manage. Its position in the ear prevents wind noise and makes it easier to use phones without feedback. In-the-canal<unk> s are placed deep in the ear canal. They are barely visible. Larger versions of these can<unk> ve directional microphones. Being in the canal, they are less likely to cause a plugged feeling. These models are easier to manipulate than the smaller completely in-the-canal models but still<unk> ve the drawbacks of being rather small. In-the-ear<unk> s are typically more expensive than behind-the-ear counterparts of equal functionality, because they are custom fitted to the patient's ear. In fitting, the audiologist takes a physical impression (mold) of the ear. The mold is scanned by a specialized CAD system, resulting in a 3D model of the outer ear. During modeling, the venting tube is inserted. The digitally modeled shell is printed using a rapid prototyping technique such as stereolithography. Finally, the<unk>  is assembled and shipped to the audiologist after a quality check. Invisible-in-canal<unk> s Invisible-in-canal<unk> s (IIC) style of<unk> s fits inside the ear canal completely, leaving little to no trace of an installed<unk>  visible. This is because it fits deeper in the canal than other types, so that it is out of view even when looking directly into the ear bowl (concha). A comfortable fit is achieved because the shell of the<unk>  is custom-made to the individual ear canal after taking a mold. Invisible<unk>  types use venting and their deep placement in the ear canal to give a more natural experience of<unk> . Unlike other<unk>  types, with the IIC<unk>  the majority of the ear is not blocked (occluded) by a large plastic shell. This means that sound can be collected more naturally by the shape of the ear, and can travel down into the ear canal as it would with unassisted<unk> . Depending on their size, some models allow the wearer to use a mobile phone as a remote control to alter memory and volume settings, instead of taking the IIC out to do this. IIC types are most suitable for users up to middle age, but are not suitable for more elderly people. Extended wear<unk> s Extended wear<unk> s are<unk>  devices that are non-surgically placed in the ear canal by a<unk>  professional. The extended wear<unk>  represents the first "invisible"<unk>  device. These devices are worn for 1–3 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<unk>  loss. 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<unk> s require daily insertion and removal, extended wear<unk> s are worn continuously and then replaced with a new device. Users can change volume and settings without the<unk>  of a<unk>  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<unk> s are popular with those who are self-conscious about the aesthetics of BTE or ITC<unk>  models. As with other<unk>  devices, compatibility is based on an individual's<unk>  loss, ear size and shape, medical conditions, and lifestyle. The disadvantages include regular removal and reinsertion of the device when the battery dies, inability to go underwater, earplugs when showering, and for some discomfort with the fit since it is inserted deeply in the ear canal, the only part of the body where skin rests directly on top of bone. CROS<unk>  A CROS<unk>  is a<unk>  that transmits auditory information from one side of the head to the other side of the head. Candidates include people who<unk> ve poor word
Hearing aid
Vision therapy
Voice prosthesis
Craniofacial prosthesis
Vision rehabilitation
Visual prosthesis
Audio therapy
Aural rehabilitation