another frequency and intensity. the brain starts to perceive sounds amplified by the hearing aid immediately after the initial adjustment; however, it may not process them correctly right away. feeling the hearing aid in the ear may seem unusual. it also takes time to adapt to a new way of hearing. the ear has to be gradually adjusted to the new sound. the sound may seem unnatural, metallic, too loud or too quiet. a whistling sound may also appear, which can be unpleasant. hearing aids do not provide immediate improvement. the adjustment period can last from several hours to several months. patients are offered an initial schedule to wear their hearing aid, ensuring gradual adaptation to it. if the patient wears the hearing aid continually from the beginning, the unfamiliar sound may cause a headache, and as a result, the user may refuse to wear a hearing aid despite the fact that it helps. audiologists often run a quick preparation course for the patients. as a rule, users have inflated expectations of hearing aids. they expect that hearing aids will help them to hear in the same way as before hearing loss, but it is not the case. training sessions help hearing aid users to get accustomed to the feeling of new sounds. users are strongly recommended to regularly visit an audiologist, including for the purposes of additional hearing aid adjustment. hearing aid applications, in contrast to traditional hearing aids, allow the implementation of options such as a built-in adaptation course. the functions of the course may include: control of the amount of time spent on learning; control over the sequence of exercises; daily reminders to do the exercises. the goal of the course is to help a user adapt to using a 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 among background noise, etc. history madame de meuron with ear trumpet the first hearing aids were ear trumpets, and were created in the 17th century. some of the first hearing aids were external hearing aids. external hearing aids directed sounds in front of the ear and blocked all other noises. the apparatus would fit behind or in the ear. the movement toward modern hearing aids began with the creation of the telephone, and the first electric hearing aid, the "akouphone", was created about 1895 by miller reese hutchison. by the late 20th century, digital hearing aids were commercially available. the invention of the carbon microphone, transmitters, digital signal processing chip or dsp, and the development of computer technology helped transform the hearing aid to its present form. history of digital aids the history of dha can be divided into three stages. the first stage began in the 1960s with the widespread use of digital computers for simulation of audio processing and for the analysis of systems and algorithms. the work was conducted with the help of the very large digital computers of that era. these efforts were not actual digital hearing aids because the computers were not fast enough for audio processing in real time and their size prevented them from being described as wearable, but they allowed successful studies of the various hardware circuits and algorithms for digital processing of audio signals. the software package block of compiled diagrams (blodi) developed by kelly, lockbaum and vysotskiy in 1961 allowed simulation of any sound system that could be characterized in the form of a block diagram. a special phone was created so that a person with a hearing impairment could listen to the digitally processed signals, but not in real time. in 1967, harry levitt used blodi to simulate a hearing aid on a digital computer. almost ten years later the second stage began with the creation of the hybrid hearing aid, in which the analog components of a conventional hearing aid consisting of amplifiers, filters and signal limiting were combined with a separate digital programmable component in a conventional hearing aid case. the audio processing remained analog but it was controlled by the digital programmable component. the digital component could be programmed by connecting the device to an external computer in the laboratory then disconnected to allow the hybrid device to function as a conventional wearable hearing aid. the hybrid device was effective from a practical point of view because of the low power consumption and compact size. at that time, low-power analog amplifier technology was well developed in contrast to the available semiconductor chips able to process digital audio in real time. the combination of high performance analog components for real time audio processing and a separate low power digital programmable component only for controlling the analog signal led to the creation of several low power digital programmable components able to implement different types of control. a hybrid hearing aid was developed by etymotic design. a little later, mangold and lane created a programmable multi-channel hybrid hearing aid. graupe with co-authors developed a digital programmable component that implemented an adaptive noise filter. the third stage began in the early 1980s by a research group at central institute for the deaf headed up by faculty members at washington university in st. louis mo. this group created the first fully digital wearable hearing aid. they first conceived a complete, comprehensive full digital hearing aid, then designed and fabricated, miniaturized full digital computer chips using custom digital signal processing chips with low power and very large scale integrated (vlsi) chip technology able to process both the audio signal in real time and the control signals, yet able to be powered by a battery and be fully wearable as a full digital wearable hearing aid able to be actually used by individuals with hearing loss in real-world environments. engebretson, morley and popelka were the inventors of the first full digital hearing aid. their work resulted in us patent 4,548,082, "hearing aids, signal supplying apparatus, systems for compensating hearing deficiencies, and methods" by a maynard engebretson, robert e morley, jr. and gerald r popelka, filed in 1984 and issued in 1985. this full digital wearable hearing aid also included many additional features now used in all contemporary full digital hearing aids including a bidirectional interface with an external computer, self-calibration, self-adjustment, wide bandwidth, digital programmability, a fitting algorithm based on audibility, internal storage of digital programs, and fully digital multichannel amplitude compression and output limiting. this group created several of these full digital hearing aids and used them for research on hearing impaired people as they wore them in the same manner as conventional hearing aids in real-world situations. in this first full dha all stages of sound processing and control were carried out in binary form. the external sound was picked up by a microphone positioned in an ite ear module to take advantage of the acoustic effects of the pinna, then converted into binary code, digitally processed and digitally controlled in real time, then converted back to an analog signal sent to two miniature loudspeakers positioned in the same ite ear module. the ite module also contained an inward facing microphone to measure the sound actually generated in the ear canal, a precursor to separate probe tube measures now routinely used for hearing aid fitting. the necessary electronic components, including batteries, to support this arrangement were situated in a bte module that could be supplemented with a body worn module. these specialized hearing aid chips continued to become smaller, increase in computational ability and require even less power. now, virtually all commercial hearing aids are fully digital and their digital signal processing capability has significantly increased. very small and very low power specialized digital hearing aid chips are now used in all hearing aids manufactured worldwide. many additional new features also have been added with various on-board advanced wireless technology. regulation canada hearing aids are class ii regulated medical devices under canada’s food and drugs act. under health canada, the medical devices directorate (mdd) regulates the safety, quality, and effectiveness of hearing aids. all hearing aids imported and sold in canada are subject to a pre-market review. post-market, health canada monitors the performance of the hearing aid and any consumer complaints. hearing aid financial assistance is available at both the federal and provincial level. provincial hearing aid assistance and coverage can vary widely depending on the province and territory. in canada, a prescription is required to purchase hearing aids. only licensed audiologists, ear, nose and throat (ent) doctors, hearing instrument practitioners, and audioprothesistes (in quebec) can prescribe hearing aids. over-the-counter (otc) hearing aids are currently not available for sale in canada. canadian taxpayers can claim tax relief for hearing aids as a medical expense. ireland like much of the irish health care system, hearing aid provision is a mixture of public and private. hearing aids are provided by the state to children, oaps and to people whose income is at or below that of the state pension. the irish state hearing aid provision is extremely poor; people often have to wait for two years for an appointment. it is estimated that the total cost to the state of supplying one hearing aid exceeds €2,000. hearing aids are also available privately, and there is grant assistance available for insured workers. for the fiscal year ending 2016, the grant stands at a maximum of €500 per ear. irish taxpayers can also claim tax relief at the standard rate as hearing aids are recognised as a medical device. hearing aids in the republic of ireland are exempt from vat. hearing aid providers in ireland mostly belong to the irish society of hearing aid audiologists. united states ordinary hearing aids are class i regulated medical devices under federal food and drug administration (fda) rules. a 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". inconsistent state regulation is preempted under the federal law. in the late 1970s, 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. the over-the-counter hearing aid act (otc act) was passed under the fda reauthorization act of 2017, creating a class of hearing aids regulated by the fda available directly to consumers without involvement from a licensed professional. this law's provisions are expected to go into effect in 2020. in august 2022, the fda issued a final rule to improve access to hearing aids. the action establishes a new category of over-the-counter (otc) hearing aids, enabling consumers with perceived mild to moderate hearing impairment to purchase hearing aids directly from stores or online retailers without the need for a medical exam, prescription or a fitting adjustment by an audiologist. the fda action amends existing rules that apply to prescription hearing aids for consistency with the new otc category, it repeals the conditions for sale for hearing aids, and it includes provisions that address some of the effects of the fda otc hearing aid regulations on state regulation of hearing aids. the fda also issued the final guidance, regulatory requirements for hearing aid devices and personal sound amplification products (psaps), to clarify the differences between hearing aids, which are medical devices, and psaps, consumer products that help people with normal hearing amplify sounds. cost hearing aid shop, dublin, ireland several industrialized countries supply free or heavily discounted hearing aids through their publicly funded health care system. australia 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. canada 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