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 PSAPs or other plain sound reinforcing systems cannot be sold as “hearing aids”.
Modern devices are computerized electroacoustic systems that transform environmental sound to make it audible, according to audio metrical and cognitive rules. Modern devices also utilize sophisticated digital signal processing to try and improve speech intelligibility and comfort for the user. Such signal processing includes feedback management, wide dynamic range compression, directionality, frequency lowering, and noise reduction.
Modern hearing aids require configuration to match the hearing loss, physical features, and lifestyle of the wearer. The hearing aid is fitted to the most recent audiogram and is programmed by frequency. This process is called “fitting” and is performed by a Doctor of Audiology, also called an audiologist (AuD), or by a Hearing Instrument Specialist (HIS). The amount of benefit a hearing aid delivers depends in large part on the quality of its fitting. Devices similar to hearing aids include the osseointegrated auditory prosthesis (formerly called the bone-anchored hearing aid) and cochlear implant.
Hearing aids are used for a variety of pathologies including sensorineural hearing loss, conductive hearing loss, and single-sided deafness. Hearing aid candidacy is typically determined by a Doctor of Audiology, who will also fit the device based on the nature and degree of the hearing loss being treated. The amount of benefit experienced by the user of the hearing aid is multi-factorial, depending on the type, severity, and etiology of the hearing loss, the technology and fitting of the device, and on the motivation, personality, lifestyle, and overall health of the user.
Hearing aids are incapable of truly correcting a hearing loss; they are an aid to make sounds more audible. The most common form of hearing loss for which hearing aids are sought is sensorineural, resulting from damage to the hair cells and synapses of the cochlea and auditory nerve. Sensorineural hearing loss reduces the sensitivity to sound, which a hearing aid can partially accommodate by making sound louder. Other decrements in auditory perception caused by sensorineural hearing loss, such as abnormal spectral and temporal processing, and which may negatively affect speech perception, are more difficult to compensate for using digital signal processing and in some cases may be exacerbated by the use of amplification. Conductive hearing losses, which do not involve damage to the cochlea, tend to be better treated by hearing aids; the hearing aid is able to sufficiently amplify sound to account for the attenuation caused by the conductive component. Once the sound is able to reach the cochlea at normal or near-normal levels, the cochlea and auditory nerve are able to transmit signals to the brain normally.
Common issues with hearing aid fitting and use are:
Once a common problem, feedback is generally now well-controlled through the use of feedback management algorithms.
Candidacy and acquisition
There are several ways of evaluating how well a hearing aid compensates for hearing loss. One approach is audiometry which measures a subject’s hearing levels in laboratory conditions. The threshold of audibility for various sounds and intensities is measured in a variety of conditions. Although audiometric tests may attempt to mimic real-world conditions, the patient’s own every day experiences may differ. An alternative approach is self-report assessment, where the patient reports their experience with the hearing aid.
Hearing aid outcome can be represented by three dimensions:
1. Hearing aid usage
2. Aided speech recognition
3. Benefit/satisfaction
The most reliable method for assessing the correct adjustment of a hearing aid is through real ear measurement. Real ear measurements (or probe microphone measurements) are an assessment of the characteristics of hearing aid amplification near the ear drum using a silicone probe tube microphone.
Current research is also pointing towards hearing aids and proper amplification as a treatment for tinnitus, a medical condition which manifests itself as a ringing or buzzing in the ears.
Types of Hearing Aids
There are many types of hearing aids (also known as hearing instruments), which vary in size, power and circuitry. Among the different sizes and models are:
1. Vacuum tube hearing aid, circa 1944
2. Transistor body-worn hearing aid.
3. Pair of BTE hearing aids with earmolds.
4. Receiver-in-the-canal hearing aids
5. In-the-ear hearing aid
6. In-the-canal hearing aid
7. Completely in the canal hearing aids
8. Bone anchored hearing aid
9. Hearing aid application
Behind the ear
A modern behind the ear hearing aid, the audio tube to the speaker is barely visible. A modern behind the ear hearing aid comes with a minicell battery.
Behind the ear hearing aids are one of two major classes of hearing aids – Behind the ear (BTE) and In the ear (ITE). These two classes are distinguished by where the hearing aid is worn. BTE hearing aids consist of a case which hangs 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 hearing aid 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 hearing loss. However, BTEs are very versatile and can be used for nearly any kind of hearing 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 have 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 hearing aid.
In the ear
In the ear aids (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 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. 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 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.
ITE hearing aids 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 hearing instrument.
Mini in canal (MIC) or completely in canal (CIC) aids
These aids are generally not visible unless the viewer looks 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 noticeable.
Completely-in-the-canal hearing aids fit tightly deep in the ear. It is barely visible. Being small, it will not have a directional microphone, and its small batteries will have 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 hearing aids are placed deep in the ear canal. They are barely visible. Larger versions of these can have 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 have the drawbacks of being rather small.
In-the-ear hearing aids 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 aid is assembled and shipped to the audiologist after a quality check.
Invisible in-canal hearing aids
Invisible in canal hearing aids (IIC) style of hearing aids fits inside the ear canal completely, leaving little to no trace of an installed hearing aid 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 aid is custom-made to the individual ear canal after taking a mold. Invisible hearing aid types use venting and their deep placement in the ear canal to give a more natural experience of hearing.
Unlike other hearing aid types, with the IIC aid 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 hearing. 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 hearing aids
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. 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 hearing 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 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. 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 hearing aid
A CROS hearing aid is a hearing aid that transmits auditory information from one side of the head to the other side of the head. Candidates include people who have poor word understanding on one side, no hearing on one side, or who are not benefiting from a hearing aid on one side. CROS hearing aids can appear very similar to behind the ear hearing aids. The CROS system can assist the patient in sound localization and understanding auditory information on their poor side.
Bone-anchored
A bone anchored hearing aid (BAHA) is a surgically implanted auditory prosthetic based on bone conduction. It is an option for patients without external ear canals, when conventional hearing aids with a mold in the ear cannot be used. 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 two (five in the USA) typically wear the BAHA device on a Softband. This can be worn from the age of one month as babies tend to tolerate this arrangement very well. When the child’s skull bone is sufficiently thick, 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.
The surgical procedure is simple both for the surgeon, involving very few risks for the experienced ear surgeon. For the patient, minimal discomfort and pain is reported. Patients may experience numbness of the area around the implant as small superficial nerves in the skin are sectioned during the procedure. This often disappears after some time. There is no risk of further hearing loss due to the surgery. One important feature of the Baha is that, if a patient for whatever reason does not want to continue with the arrangement, it takes the surgeon less than a minute to remove it. The Baha does not restrict the wearer from any activities such as outdoor life, sporting activities etc.
A BAHA can be connected to an FM system by attaching a miniaturized FM receiver to it. Two main brands manufacture BAHAs today – the original inventors Cochlear, and the hearing aid company Oticon.
Difference Between Digital and Analog Hearing Aids
Analogue hearing aids make louder all the sounds picked up by the microphone. For example, speech and ambient noise will be made louder together. On the other hand, digital hearing aid (DHA) technology processes the sound using digital technology. Before transmitting the sound to the speaker, the DHA microprocessor processes the digital signal received by the microphone according to a mathematical algorithm. This allows just making louder the sounds of certain frequency according to the individual user settings (personal audiogram), and automatically adjusting the work of DHA to various environments (noisy streets, quiet room, concert hall, etc.).
For users with varying degrees of hearing loss it is difficult to perceive the entire frequency range of external sounds. DHA with multi-channel digital processing allows a user to “compose” the output sound by fitting a whole spectrum of the input signal into it. This gives users with limited hearing abilities the opportunity to perceive the whole range of ambient sounds, despite the personal difficulties of perception of certain frequencies. Moreover, even in this “narrow” range the DHA microprocessor is able to emphasize the desired sounds (e.g. speech), weakening the unwanted loud, high etc. sounds at the same time.
Advantages of Digital Aids
1. According to researches DHA have a number of significant advantages (compared to analogue hearing aids).
2. Speech recognition – Can distinguish the speech signal from the overall spectrum of sounds which facilitates speech perception.
3. Noise reduction – Can reduce the background noise level to increase user comfort in noisy environments.
4. Flexibility in selective amplification – Can provide more flexibility in frequency specific amplification to match the individual hearing characteristics of the user.
5. Effective acoustic feedback reduction – The acoustic whistling common to all hearing aids can be adaptively controlled.
6. Effective use of directional microphones – Directional microphones can be adaptively controlled.
7. Extended frequency range – A larger range of frequencies can be implemented with frequency shifting.
8. “Self-learning” and adaptive adjustment – Can implement adaptive selection of amplification parameters and processing.
9. Improved connection to other devices – Connection to other devices such as smartphones, televisions, internet etc. are possible.
These advantages of DHA were confirmed by a number of studies, relating to the comparative analysis of digital hearing aids of second and first generations and analog hearing aids.
Hearing aid adaptation
It often happens that a person using a hearing aid for the first time cannot quickly make use of all its advantages. Structure and characteristics of hearing aids are thoroughly devised by specialists in order to make the period of adaptation to the hearing aid as simple and quick as possible. However, despite this, a beginning hearing aid user certainly needs time to get used to it.
The process of hearing prostheses consists of the following steps:
1. Initial adjustment of the device;
2. Adaptation to new sounding;
3. Fine adjustment.
Due to plasticity of central nervous system inactive hearing centers of the brain cortex switch over to processing of sound stimuli of another frequency and intensity. The brain start perceiving sounds amplified by the hearing aid right after the initial adjustment, however, it may not process them correctly at once.
Feeling the hearing aid in the user’s ear may seem unusual. It also takes time to adapt to the new way of hearing perception. The ear has to be gradually adjusted to the new sounding. The sound may seem unnatural, metallic, too loud or too quiet. Whistling sound may also appear, which is a rather unpleasant irritant. Hearing aid does not provide immediate improvement. The adaptation period can last from several hours to several months.
A patient is offered a schedule of wearing their hearing aid ensuring gradual adaptation to it. If the patient starts permanently wearing the hearing aid, unfamiliar sound may cause a headache, and as a result the user refuses to wear a hearing aid despite the fact that it helps.
Surdo-teachers often run a quick preparation course for the patients. As a rule, users have inflated expectations of using hearing aids. They expect that hearing aids will help them to hear in the same way as before hearing loss, but it is not like that. Conducted trainings help hearing aid users to get accustomed to new sound feelings. A user is strongly recommended to regularly visit a surdologist, including for the purposes of additional hearing aid adjustment.
Hearing aid application, in contrast to a traditional hearing aid, allows implementing nonspecific options, such as a built-in adaptation course.
The functions of the course may include:
1. control over the sequence of performed exercises according to the calendar;
2. control of the amount of time spent on learning (exceeding or lacking);
3. reminders of daily exercises and so on.
The goal of the course is to help a user adapt to hearing aid application. The adaptation course includes a certain number of stages, starting from listening to a set of low everyday sounds in a quiet environment, getting accustomed to one’s own speech and other people’s speech, getting accustomed to speech in the noise, etc.
According to researches DHA have a number of significant advantages (compared to analogue hearing aids):
1. Speech recognition – Can distinguish the speech signal from the overall spectrum of sounds which facilitates speech perception.
2. Noise reduction – Can reduce the background noise level to increase user comfort in noisy environments.
3. Flexibility in selective amplification – Can provide more flexibility in frequency specific amplification to match the individual hearing characteristics of the user.
4. Effective acoustic feedback reduction – The acoustic whistling common to all hearing aids can be adaptively controlled.
5. Effective use of directional microphones – Directional microphones can be adaptively controlled.
6. Extended frequency range – A larger range of frequencies can be implemented with frequency shifting.
7. “Self-learning” and adaptive adjustment – Can implement adaptive selection of amplification parameters and processing.
8. Improved connection to other devices – Connection to other devices such as smartphones, televisions, internet etc are possible.
These advantages of DHA were confirmed by a number of studies, relating to the comparative analysis of digital hearing aids of second and first generations and analog hearing aids.
Regulations on Hearing Aid In India
In India hearing aids of all kinds are easily available. Under Central and state government health services, the poor can often avail themselves of free hearing devices. However, market prices vary for others and can range from Rs 10,000 to Rs 275,000 per ear.
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