A hearing aid is an aid that is used to compensate for a functional deficit in the hearing organ and thus to improve speech comprehension and restore speech understanding and the social integration of the hearing impaired . Children with hearing loss should be able to acquire language through hearing aids, promote language development and enable them to attend school. Hearing aids are also used as part of tinnitus therapy. However, the effectiveness has not been clearly proven.
The first hearing aids were available as ear tubes in the 17th century . It was a funnel that amplified the sound. The effect was still quite modest, but an amplification of around 20 to 30 decibels was already achieved , a considerable improvement for someone who was hard of hearing at the time. Ludwig van Beethoven had such a hearing aid made by Johann Nepomuk Mälzel around 1813 .
It was only with the spread of telephone technology invented in 1876 that there was also a further development in hearing aids. Werner von Siemens invented a telephone receiver for the hard of hearing in 1878. In 1896, the Englishman Bertram Thornton invented a table-top hearing aid that had a carbon microphone. Based on this invention, the Acouphone Company started producing hearing aids from 1898 onwards.
In 1898 Miller Reese Hutchinson used carbon microphone technology to build the first portable hearing aid. In 1901, he registered a patent in New York for his "Acoustikon", which revolutionized what had previously been purely mechanical hearing aid technology. The twelve kilogram device consisted of a carbon microphone for sound recording, an amplifier and a loudspeaker that had to be held to the ear. In 1902, more manageable devices were invented in which the amplifier and batteries were hung around the neck and the microphone was held by hand.
From 1910, Siemens also offered devices that amplified not only the telephone, but also the ambient noise, but only for employees and their families. In 1913, now from Siemens & Halske , a revised model under the name Phonophor went on sale, consisting of a battery, microphone and receiver, these components being carried in a special handbag or suitcase. From 1914 the "phonophores" were equipped with a specially made insert earphone, known as an "earphone". This not only made the device less conspicuous, but also brought the sound amplification to the ear in a more targeted manner with this miniaturization.
In the 1920s, table-top tube devices were available. With these you could set the gain for different frequency ranges separately. The listener no longer perceived low tones as too loud and high tones no longer as too soft. The disadvantage was the size of the device and its connection to electricity from the socket, which ruled out mobile use. In 1920, shipbuilding engineers invented the more powerful “Vactuphone”, a hearing aid with electronic tubes that converted sound waves into electronic signals and amplified them. These unwieldy hearing aids were used until the 1950s.
In 1947, the invention of the transistor also changed hearing aid technology. With its space- and power-saving amplifier technology, it made it possible to improve the devices. The hearing aids soon became so small that they could no longer be carried in the hand but behind the ear.
By using tiny sub- miniature tubes as amplifier elements, the electronics could finally be miniaturized to such an extent that the hearing aids reached pocket size in the 1950s, which meant they could be carried around. Ordinary batteries provided electricity for their operation. These pocket devices were very expensive; only a small part of the people who were often affected by war-related hearing loss at the time could afford them.
On December 29, 1952, the Sonotone Corporation in Elmsford (New York) first offered transistor- based hearing aids . The sound was transported into the ear via a cable with a loudspeaker connected to it. The miniaturization of the devices took another step forward. They were now the size of a cigarette packet.
In the 1960s, the modern behind the ear hearing aids were developed. First came single-channel, behind-the-ear analog devices. These had relatively large batteries, some of which only lasted a day. In 1966, Siemens Audiologische Technik in Erlangen introduced the world's first in-the-ear hearing aid with the product name “Siretta 339”.
With the development of digital sound processing, the transition from analog to more efficient digital technology took place. Project Phoenix manufactured the world's first DSP ( digital signal processor chips) hearing aid in 1988. Modern digital hearing aids are equipped with a tiny programmable computer that amplifies sound signals at various frequencies and can also improve the hearing ability of people with severe hearing loss. With miniaturization, hearing aids disappear in the ear canal and obtain a more natural sound fidelity. The more powerful behind-the-ear hearing aids have also become smaller and more discreet. In the mid-1990s, the first fully digital behind-the-ear and in-the-ear hearing aids with an output of 40 million computing steps per second were mass-produced.
With digital technology , it was possible to differentiate between useful and interference noise and reduce the interference noise. While around ten transistors were built into a hearing aid around 1960, today (2017) there are around 20 million transistors.
From the 1960s onwards, the cochlear implant was developed as a new device concept (connection of telephone technology with speech processors) for people with impaired hearing, for whom even powerful hearing aids do not provide adequate care. It gives them access to the world of hearing.
Hearing damage can be fundamentally differentiated into damage to sound conduction (transmission of sound from the ambient air into the inner ear ) and damage to sound perception (perception of sound in the cochlea , stimulus transmission in the auditory nerve and to higher regions of the brain). Depending on the type and severity of the hearing impairment, affected people have a hearing threshold that has been shifted to higher sound levels and limited dynamics. The dynamic of healthy hearing covers a level range of approx. 100 dB (from “just audible” to “too loud”). In addition, in almost all cases the restrictions mentioned are strongly frequency-dependent, especially in the case of age-related hearing loss, which is by far the most common . For this reason, hearing aids work in principle with frequency-dependent amplification and dynamic compression in order to make inaudible tones audible for the hearing impaired on the one hand, and on the other hand not to generate unpleasantly loud levels.
The designs for the most common types of hearing loss are currently behind-the-ear and in-the -ear devices . Both forms can be further subdivided depending on the type of construction and the degree of miniaturization; In recent years, for example, a special design has developed in which the actual hearing aid sits behind the ear, but the loudspeaker is located in the ear canal. For certain cases of hearing loss, there are special forms based on bone conduction , for example , as well as implantable and partially implantable hearing systems.
Because hearing aids are usually perceived as conspicuous and stigmatizing, the inhibition threshold for use is relatively high. Manufacturers are trying to downsize their products, build the image of a lifestyle product, or to approximate the aesthetics of Bluetooth in-ear headsets , especially under this aspect and by using modern electronic components .
Behind-the-ear devices (BTE)
Behind-the-ear devices (BTE) have a housing shape with which the device can be placed on top of the auricle , with most of the housing resting in the rear area of the auricle. The sound channel is led out at the front in the upper area of the housing. The sound, which is generated by the receiver built into the housing, is conducted into the ear through the sound channel, which consists of a hearing angle, sound tube and otoplastic (ear mold). Since BTE devices have enough space for batteries, electronics and sound transducers, a wide range of technical options and high amplification outputs can be implemented.
For severe hearing loss, the large distance between the hearing aid microphone and the sound outlet opening close to the eardrum enables higher amplifications, as this reduces the susceptibility to feedback . Acoustic feedback leads to annoying whistling, which occurs, for example, when the earmold is not properly seated , the ear canal is obstructed by cerumen or the sound tube is defective.
In the case of mild and moderate high-frequency hearing loss, it is possible to leave the ear canal as open as possible in order to continue to hear naturally the lower frequencies that are not or only slightly affected by hearing loss. This is achieved by using a thinner tube (“slim tube”) with an open end piece (“umbrella”) or an otoplastic with a construction that is as open as possible or the largest possible ventilation hole (venting). With an open fitting, the hearing aid wearer usually has a more pleasant, more natural hearing sensation, since the self- generated structure - borne sound is no longer reflected on the side of the otoplastic facing the eardrum . Disadvantages of an open supply are the increased tendency to feedback and an occasionally perceptible echo effect due to the time delay in the digital signal processing of the hearing aid.
The technical term “slim tube” has established itself for the thin sound tubes. The latter is often referred to as "bad tube" by experts, as the slim tube attenuates high frequencies too much for fluid mechanics reasons, so that they cannot get into the ear canal with sufficient sound pressure.
Ex handset devices (RIC)
RITE devices (also receiver-in-the-canal devices or RIC devices) are similarly shaped as BTE, but differ from them by an outsourced from the body ( ex ternal) transducer ( "listeners") at the end of a thin cable line (supply line) instead of a sound tube.
Advantages of the Ex receiver device are:
- Avoidance of unwanted resonances of the sound channel and thus a more natural sound,
- more compact design of the hearing aid,
- no acoustic resistance of a sound channel as with the BTE device, therefore stronger hearing damage can be treated,
- no acoustic resistance of a sound channel, thus lower power consumption of the listener and slightly lower energy consumption,
- appealing cosmetics due to the very thin cable line.
The receiver is guided and held in the ear canal through a custom-made otoplastic or a silicone end piece ("umbrella"). This means that an open supply is also possible with Ex earphones, as with BTE devices. As with BTE devices, an otoplastic enables better sealing of the auditory canal and an optimal fit and, associated with this, better reproduction of lower frequencies and more security against feedback. The decision for a BTE or RIC device also depends on the wearing sensation and shape of the ear canal of the user. The Ex earphones are used for different hearing losses, depending on the manufacturer, with different power levels, e.g. B. as S (Standard), M (Medium) and P (Power), offered.
Due to the design, there are also some disadvantages of a RIC device:
- more space required in the ear canal,
- higher cleaning and maintenance costs,
- Possibility of cable breakage and the associated higher repair costs,
- lower mechanical durability of the receiver and the supply line.
Due to the more sensitive components and the higher maintenance requirements of a RIC device, they are less suitable than BTE devices for people with motor, sensory and cognitive limitations.
In-the-ear devices (ITE)
These hearing aids are worn completely “in the ear”. The electronics are incorporated into a custom-made hollow shell, which is closed on the outward-facing side by a front panel. The front panel is the part of the ITE device that is visible from the outside and contains the battery flap, possibly operating elements and openings for the microphone (s) and the ventilation. In contrast to BTE devices, ITE hearing aids can use the acoustic advantages of the anatomy of the outer ear (auricle).
In-the-ear hearing systems are divided into the following subtypes:
- ITE: "In-The-Ear" The housing of the hearing system completely fills the auricle (concha). The system is very noticeable. For cosmetic reasons, the surface can also be adapted to the skin color and / or provided with fine lines ("small veins"). With the advancing miniaturization of hearing aid technology, ITE devices are only very rarely manufactured nowadays.
- ITC: “In-The-Canal” The housing of the hearing system is flush with the front edge of the ear canal. The front panel is tilted back as far as possible so that the device is visible to others from the smallest possible area. The auricle remains free. In the field of ITE devices, this design offers the best compromise between inconspicuousness, technical equipment (wireless and multi-microphone technology) and battery life .
- CIC: “Completely-in-the-Canal” The housing ends in the outer part of the ear canal and is therefore hardly visible from the outside. These devices usually have a nylon pull cord to pull them out of the ear canal. Relatively small batteries with a correspondingly short operating time are used. Because of their small size, CIC devices are built with only one microphone and mostly without a wireless antenna and therefore have to forego some of the functionality.
- IIC: “Invisible in the canal” This design sits deep in the ear canal in the area of the second curvature of the ear canal. It also has a nylon pull cord so that the device can be removed. These hearing aids are not visible from the outside even if you look closely. IIC devices only have a microphone and no wireless connection option (as of 2018).
- Pre- assembled ITE devices are ready-made ITE devices the size of CIC or IIC devices. These devices can be plugged into a shield that fits the ear canal and can be adapted immediately without a custom-made product. As the fit tends to be inaccurate, adjustments to pre-assembled ITEs are only useful if the anatomy of the auditory canal is advantageous and hearing loss is not too severe. In the case of less suitable anatomical conditions, these devices are ideally suited for short-term testing of a hearing aid fitting. The products quiX and Silk from the manufacturers AS Audio-Service and Signia (formerly Siemens ) from the Sivantos Group and the Step2Go from Interton ( GN Group ) are currently the only ready-made ITE hearing aids available (as of 2018).
Disadvantages of these designs are, on the one hand, the promotion of sweat and earwax formation, which can lead to a higher susceptibility to repair, and on the other hand, as with closed otoplastics or tightly fitting umbrellas, a sealing effect (occlusion) can occur. This effect can be counteracted with a ventilation (“vent”) in the housing of the hearing aid, through which part of the structure-borne sound is directed to the outside. Very deep-seated IIC devices do not have this disadvantage, since the device is located in the ear canal section behind the insertion of the lower jaw joint, which is largely responsible for the transmission of structure-borne noise into the ear canal.
A few years ago, in-the-ear devices were only possible for mild to moderate hearing losses. In the case of severe hearing loss, the tendency to feedback was too high or the output sound level of the listener was too weak. With the miniaturization of hearing aid components and further technical developments, severe hearing losses can now also be treated with ITE devices.
A development by the manufacturer Phonak since 2010 has been the Lyric , a device that is completely inserted very deep into the ear canal up to the eardrum and can remain there permanently for up to several months. Due to the close proximity to the eardrum, comparatively very little sound energy is required for auditory perception , which leads to a long service life for the permanently installed battery. Externally, the device consists of two articulated sleeves, in the front part (facing the eardrum) the loudspeaker and in the rear part the microphone, the electronic control and the battery. With the help of a specially shaped magnet that is held in the ear canal, the device can be switched on and off and the volume can be adjusted. Size adjustments are made with this system by selecting from several different sized sleeves. The adjustment is carried out by digital programming by the hearing aid acoustician . When the battery power drops, the entire device is removed from the ear canal and disposed of. Instead of a new battery as with "normal" hearing aids, a completely new device is used. According to the only provider to date (as of 2011), an annual subscription with a fixed cost per month and ear is provided for financing. Since the device does not comply with the current guidelines for aids due to its analog signal processing, costs are not covered by statutory health insurances in Germany.
Pocket hearing aids
The first design of electronic hearing aids were the so-called pocket hearing aids. These were still used frequently in the 1950s and 1960s, but due to the miniaturization of the components they were largely replaced by BTE and ITE devices. They are seldom used anymore in people with a hearing loss that is bordering on deafness or residual hearing loss. Therefore, the name is because the box-shaped controller usually in a pocket, on or under the clothing worn by the user. In the case of a pocket device, the earphone is on a twisted cable of sufficient length. An exchangeable custom-made otoplastic with a snap fastening similar to a push button is attached to the earphone capsule . A problem with pocket devices is often the rustling of clothes on the device's microphone , on the other hand, due to the relatively large distance, there is hardly any feedback between the receiver and microphone and the annoying whistling associated with it . This enables the high volumes with low residual hearing loss.
Hearing glasses are glasses in whose arms the hearing aid technology is housed or on whose arms a hearing aid is mounted. The earmold and battery compartment are often located on the back of the bracket. With the modern and fashionable spectacle frames that are now available, hearing glasses equipped with the latest digital hearing aid technology are attracting attention again. The front of the glasses can be changed at any time thanks to an easy-to-use plug connection. The disadvantage often assumed in the past that the wearer is not supplied with a repair of the glasses or the hearing aid no longer applies, since a replacement product can be fitted at any time through the plug connection.
Bone conduction hearing aids
Bone conduction hearing systems are used for special ear diseases. The sound is not transmitted through the air in the ear canal, but rather conducted through the bone to the inner ear. Use cases are e.g. B. a non-existent auditory canal with an otherwise normal structure of the hearing, a serious middle ear defect (e.g. a radical cavity ) or an ear that cannot be supplied with a BTE device / otoplasty due to secretion formation.
The sound transducer of these devices transmits the vibrations to the mastoid process behind the ear and thus sets the inner ear in vibrations that the hard of hearing can perceive as sound information. Bone conduction hearing aids are usually built into eyeglass temples. There is also the option of wearing a pocket hearing aid with a bone conduction receiver that is attached to a headband or headband.
Another variant of bone conduction hearing aids are bone anchored devices. The ENT doctor implants a titanium screw in the skull bone. The hearing aid is attached to this screw (BAHA = Bone Anchored Hearing Aid). Due to the direct coupling, BAHA devices transmit higher sound pressures and can therefore be used even for severe hearing impairments.
In addition to sound amplification, the technical development of hearing aid acoustics is also used for tinnitus therapy. The tinnitus masker used for this (also called intoxication device, tinnitus noiser, tinnitus control instrument) is externally and internally similar to a standard hearing aid, but has no microphone for sound transmission. Rather, the device generates a fixedly defined device noise in terms of frequency range and level , which is intended to mask the tinnitus.
Technically, there is no significant difference between the devices known as tinnitus maskers and tinnitus noisers. However, while a tinnitus masker is set in such a way that the device noise massively covers the tinnitus, a "noiser" is only set to an equally high level. The affected ear perceives the noise in the ear and the masking noise equally. This is supposed to put the actual tinnitus into the background in terms of perception.
For more information on the therapeutic approach see below
A tinnitus instrument is the combination of a hearing aid and a tinnitus noiser. In a noisy environment, the everyday noises amplified by the hearing aid mask the tinnitus; as the noise level decreases, the background noise of the noiser becomes increasingly audible.
Hearing aids with implants
Cochlear implants and brainstem implants are not hearing aids in the traditional sense that stimulate the inner ear via converted and amplified airborne or substrate sound. Separate articles are dedicated to them and their mode of action, see
In general, every hearing aid consists of at least one microphone , an analog amplifier or digital signal processor and a " loudspeaker " or earpiece that transmits sound signals to the ear. Further components are the energy source, the sound channel for acoustic coupling to the ear, operating elements (button, potentiometer , rocker, switch), the telephone coil, the audio input and a transmitter / receiver module for wireless signal transmission. Depending on the technical standard, design and size of the hearing aid, some of the components mentioned may be missing.
The devices are powered by a hearing aid battery or a rechargeable battery . The zinc-air batteries used today for hearing aids last for about three days to three weeks, depending on the built-in electronics, the battery size, the daily wearing time and the required amplification. Hearing aid batteries can either be exchanged ( NiMH or AgZn cells) or permanently installed in the hearing aid ( Li-ion batteries ). With daily use, due to the lower energy density of rechargeable battery cells, the hearing aids or batteries have to be charged daily. This usually takes place at night when the hearing aids are not being worn. According to the manufacturer Widex, the first hearing aid with a fuel cell power supply will be available from 2019.
The main technical challenges with hearing aids are directional hearing, understanding speech with background noises and in a very echoing environment, telephoning and watching television.
Healthy hearing recognizes the direction of an acoustic signal on the basis of level and transit time differences between the two ears and spectral colors. While the differences in transit time are unchanged in the case of hearing aids supplied with both ears, the level difference and the spectral coloration can change so much with hearing aid supplies that directional hearing is initially very difficult for those who have just received hearing aid. The brain can relearn spectral coloring. The level differences can vary because hearing aids often work with level and frequency-dependent dynamic compression behavior. This can be remedied by the binaural coordination of the hearing aid pair, which ensures that the control behavior of both hearing aids is the same at all times. Depending on the technical equipment of the hearing aid, this can greatly improve directional hearing. Despite all the technical support, hearing training and learning about the changed sound patterns are primarily crucial for functional directional hearing with hearing aids.
The long reverberation time in large rooms proves itself for hearing aid wearers, u. a. due to dynamic compression, often as a hindrance to understanding language. This effect has a greater effect the further away the speaker stands in the room, e.g. B. the preacher in a church. Conservative options aim to make the following changes to hearing aid settings:
- Reduction of the bass gain (reverberation takes place more in the bass range than in the mid and high range),
- Activation of the directional microphone so that the reverberation is not picked up from all directions and the focus is on the speaker,
- Reduction of the compression in order to increase the (quieter) reverberation less; however, this means that lower speech intelligibility is accepted.
Further developed hearing aids offer the possibility of a digital reverberation blocker. This feature can recognize the original signal (the direct sound) and the corresponding reverberation and differentiate them from one another. Current methods work on the principle of spectral subtraction or an adaptive reduction in gain and compression in the frequency bands in which the reverberation outweighs the useful signal.
Speech understanding in background noise
Speech understanding in background noise could be improved with the increasing efficiency of digital signal processing, especially through the combination of several microphones. Two microphones pick up the sound, depending on its direction of incidence, slightly offset in time. This enables the hearing aid processor to recognize where the sound signal is coming from and to specifically attenuate unwanted (ambient) noises while the speech signal is amplified. In the case of particularly high-quality digital hearing aids, the directivity of the microphones is automatically fixed to the direction of the speech signal in order to further facilitate speech understanding. This directional microphone technology often works hand in hand with noise suppression algorithms that further improve the understanding of speech. The more bands the hearing aid has available for signal processing, the more precisely and finely this type of noise reduction works, since the hearing aid can distinguish between speech and noise in each of the bands and adjust the microphone characteristics accordingly.
High-quality hearing systems also have algorithms for situation recognition and analysis of the acoustic environment. The hearing aids automatically decide whether and to what extent technical features such as B. noise reduction, adaptive automatic microphone, gain, compression and expansion can be adjusted. This makes operation much easier for the user, since the appropriate hearing aid setting does not have to be set by hand.
Telephoning and watching TV
In the case of hearing aids with a built-in "telephone coil", the sound signals from telephone receivers with electrodynamically operating loudspeakers can be transmitted separately to the hearing aid without interference. This technology is hardly used today because the handset of modern telephones no longer generates a sufficiently strong alternating magnetic field. Instead, the following options are used:
- A permanent magnet attached to the telephone receiver activates a "telephone program" with appropriately equipped hearing aids, which optimizes the sound transmission for telephone calls and, if technically possible, can transmit the sound generated by the telephone receiver by radio technology, mostly near field magnet induction (NFMI) , to the opposite side. The latter enables a better understanding of the conversation partner even in noisy surroundings. This function is called Auto-Phone .
- Specially available telephones with integrated, manufacturer-specific radio transmitters enable wireless transmission of the telephone signal to the hearing aids. Widex and Phonak offer such solutions .
- Bluetooth-capable hearing aids can use the 2.4 GHz transmission technology to connect to a (mobile) telephone either directly or with manufacturer-specific additional devices (“streamers”).
- NFMI-enabled hearing aids can transmit phone calls via a phone line adapter or a Bluetooth streamer.
If these technical solutions are not feasible, hearing aid wearers are recommended to either use the hands-free function of the telephone to make calls or to hold the telephone receiver directly to the hearing aid microphones. The latter option requires some practice and can be optimized by switching the hearing aid to a hearing program for acoustic telephoning.
If you have poor speech understanding while watching TV, it is advantageous to first optimize the sound settings of the TV and the acoustic environment. Too much reverberation in the room has just as negative effects on speech understanding as a strongly low-pitched reproduction. Another disadvantage is that the surround sound effect is set too strongly in productions that do not have a separate center channel for voice reproduction. With the latter, at least in stereo productions, the signal component in the “middle” (speech) is reduced and the signal component “left” and “right” is amplified. Hearing impaired people often complain that the language in films is too quiet and incomprehensible. This is primarily due to the fact that post-dubbing is increasingly being dispensed with in film productions and music and noises are mixed in to underline the mood. These measures reduce the signal-to-noise ratio ( SNR ) considerably and make speech understanding more difficult. Simple and often helpful measures on the television set are:
- Reduction of the reproduction of low tones (below 500 Hz) in the sound settings; Attenuate the bass as much as possible,
- Stronger reproduction of the mid-high tones (500–4000 Hz),
- Reduction of a virtual room sound effect,
- If possible, select the "Language" preset in the sound settings. This setting optimizes the speech reproduction and amplifies the "middle" components of a stereo signal.
For an optimal understanding of the television sound , hearing aid manufacturers offer their own solutions that can transmit the sound of the television or other audio sources into the hearing aids via radio technology (often in the UHF or 2.4 GHz frequency band ). In many cases, the hearing aid microphones can be switched off if desired in order to block out disturbing ambient noises. In the recent past, a transmitter (on the television) and a receiver (worn around the neck) were necessary for this. Although these systems are still available today, the trend is clearly in the direction of Bluetooth low energy technology, which enables transmission without a separate receiver.
Hearing aid speaker (earpiece)
The first earphones or transducers were small electromagnets with a membrane, like those used in older headphones. They are still used today. In the 1930s and 1950s, the very small piezoelectric sound transducers (also known as “crystal headphones” or “ceramic speakers”) were used. In spite of the good sound quality, however, its performance coupling to the amplifier was so unsatisfactory and difficult due to the low operating voltage that this technology was abandoned again for standard hearing aids. The piezo concept is taken up again in implantable hearing aids, since the direct action on the auditory ossicles requires less power than the direct sound transmission to the eardrum. Today's hearing aid loudspeakers ("listeners") work almost without exception according to the electromagnetic principle, in which the flow of current through a coil inside a permanent magnet generates a force that moves a drive pin, which in turn stimulates the receiver membrane to vibrate through direct mechanical contact. This type of earphone offers a comparably high energy efficiency, but also a lower sound quality, which results from the non-linear signal conversion, the vibration properties of the components (high mass and high stiffness, therefore poor bass and treble reproduction) and the natural resonance of the construction.
Transmission range and quality
The physical properties of the electromagnetic hearing aid speakers used exclusively today also define the quality of the sound transmission. While the other components ( microphone , A / D converter ) can transmit a very wide frequency band with almost linear dynamics, the listener is the limiting component in the transmission chain. Frequencies from 1 ... 6 kHz can be reproduced very well; In the range of 2 ... 3 kHz, the listeners even have a desired resonance , since the greatest information content of human speech lies in this frequency range. Below 200 Hz and above 8000 Hz there is hardly any significant transmission power. Due to the possibilities of frequency transposition and compression, signals can also be made audible which are far beyond the transmission spectrum of the hearing aid loudspeaker or outside the hearing ability of the hearing aid wearer. Another disadvantage of hearing aid speakers is their high distortion factor . This increases significantly at higher output levels the closer the loudspeaker works to its performance limit. As a rule, however, this aspect is accepted, since the advantages of the electromagnetic receiver (size, energy efficiency) predominate and high sound quality is subordinate to the care objective (improved speech understanding).
Analog hearing aids
Hearing instruments, their amplification characteristics in principle infinitely " analogous " to the size of the input signal follows the first and long time single electronic hearing aids were. They are now referred to as analog hearing aids to distinguish them from the more modern, “ digitally ” adjustable in discrete steps and occasionally also program- controlled devices .
The basic setting based on the audiometrically determined hearing ability is usually made by the supplying hearing aid acoustician using miniature adjustment screws inside the device, while the user himself makes an individual temporary change to external adjustment wheels and switches (e.g. to activate the "telephone coil"). Here, too, there are features such as directional microphones, automatic or manual volume adjustment.
One disadvantage of the single-acting analog devices is that the physical characteristics of the electronic components used lead to distortions and superimposition of interference signals, especially at the border areas of the transmitted audio signals . On the other hand, analog devices do not have the noticeable processing delays that "digital" devices do because of the timing of the program control steps. Since “digitally” adjustable and working hearing aids are now available in the same price range, purely “analog” devices are practically no longer offered in Europe. In Germany, according to the currently valid guidelines for aids, analog hearing aids are no longer allowed to be given to end customers.
Digital hearing aids
The signal processing and the setting of the hearing aid take place here exclusively digitally via Fourier filters . Separate filter "channels" are usually used for different frequency ranges. Digital hearing aids are offered with at least two to four channels. Current hearing systems work, depending on the manufacturer, with at least four to nine, higher-quality devices with up to 48 channels (as of 2015). With regard to the number of channels that can be set separately, the scientific point out that with more than four channels there is no longer any notable increase in speech intelligibility. From this it is deduced that "three to four compression channels provide sufficient flexibility to accommodate the vast majority of audiometric configurations found in a clinic". In the case of audiometric steep drops or hearing losses that fluctuate strongly over the entire frequency range, however, greater channeling can have a positive effect on hearing comfort. Furthermore, feedback and noise management systems benefit from a higher number of channels. In Germany, hearing aids subsidized by health insurers and employers' liability insurance associations must have at least four adjustable channels.
Two megabytes of on-chip flash memory are state-of-the-art and allow several hearing programs and complex processing algorithms. As with analog devices, the volume can usually be adjusted manually. Highly optimized integrated circuits in CMOS technology and low operating voltages allow relatively long battery runtimes.
Almost all higher-quality digital hearing aids are equipped with radio technology, with which the two devices communicate and coordinate with each other in a binaural supply. This ensures that both devices are always set the same when e.g. B. on one side the hearing program is changed or the volume is varied. The control of so-called adaptive parameters, i.e. algorithms for the detection of interfering noises or the adaptation of the characteristics of directional microphones, is synchronized in both hearing systems and thus increases the localization ability. There are now hearing aids that can transmit the microphone signal in real time to the hearing aid on the other side in order, for example, to optimize the hearing situation recognition, to refine the focus of the directional microphone system or to facilitate speech understanding in the case of strong noise on one side (e.g. wind noise).
In the meantime, so-called non - linear techniques are being pursued almost exclusively . In order to adapt to the loudness perception in a noisy environment, the gain is automatically reduced ("AGC circuit"). For this purpose, different circuits compare the received sound level and the sound level emitted from the hearing aid amplifier and attenuate the amplification or the output level from an individually defined and set threshold. This automatic gain control (technical term "Automatic Gain Control" or "AGC") is necessary to protect the inner ear from overload caused by so-called recruitment .
The feedback compensation can automatically adapt to changing acoustic conditions, for example when wearing headgear or when hugging. Years ago, a notch filter was placed in the amplifier circuit, which statically and selectively attenuated the frequency that was particularly susceptible to feedback. Nowadays, these feedback-suppressing algorithms usually work dynamically by phase shifting, phase reversal or frequency transposition in order to attenuate the coupling signal. The hearing aid detects feedback, depending on the manufacturer, for example through an inaudible marking of the output signal or through binaural comparison (“Is it whistling on the other side too?”).
In the human ear, the lateral exposure leads to level and transit time differences in the auricle and on the head. With hearing aids, the phase information is not lost, but it should be noted that digital devices first record the sound for a few milliseconds, then process it and then pass it on to the ear. H. A good digital hearing aid should have the lowest possible cycle delay (a few milliseconds) in order to preserve spatial hearing as much as possible. This is particularly important in the case of one-sided care. The same throughput time in both ears balances out again. This problem is less pronounced with analog devices; unnatural, lower level differences between the two ears result from the gain control (AGC).
In the case of laterally incident sound events, there is the problem that the hearing aid on the opposite side compensates for the level difference between the two ears using AGCi. This worsens the wearer's directional hearing. In a noisy environment, this can also lead to impaired speech intelligibility. In hearing aids with binaural coordination, both hearing aids communicate with each other. Data is exchanged between the two devices via a radio link, which means that the use of the control circuits can be synchronized. To put it simply: if one device regulates, the other also regulates in the same or appropriate manner.
Modern hearing aids recognize interference and wind noise and in this case reduce the gain in the relevant frequency bands. Music, especially classical music, is very different from noise, recognizable by sharp peaks in the frequency spectrum. The hearing aid then selects a program with a linear frequency response, lots of dynamics and omnidirectional reception. Speech is recognized by the dynamic range in the range of seconds and a hearing program with suppressed bass, strong dynamic compression and focus on the speaker - or in the case of several speakers on the speaker in front of you - is selected. In the case of speech in background noise, on the other hand, less compressive signal processing is advantageous for speech understanding.
Ability to learn
Another advance through digital technology is the introduction of adaptive algorithms. The hearing aid can recognize in which situation (in the car, on the street, in a quiet room, while listening to music, etc.) the wearer is predominantly. In devices with a volume control, the hearing aid can “remember” the most frequently selected volume setting to a certain extent and automatically adjust its amplification strategy to these values in the future.
The manufacturer Widex provides an app-supported algorithm that uses the computing power of the smartphone to adapt to the user's sound preferences in particularly special listening situations. In order to use this function, the user has to compare different sound settings presented and as a result receives a sound image calculated by machine learning . This sound setting can be saved in the app and called up again if necessary for later use.
Digitally programmable analog hearing aids
The signal processing is analog, only the setting of the hearing aid takes place digitally (usually via a programming interface with a PC, in individual cases with manufacturer-specific programming devices).
This technology also allows the use of a directional microphone, automatic or manual volume adjustment, automatic or manual hearing program selection, remote control, etc. The main advantage of this technology over analog hearing aids is that a theoretically unlimited number of virtual controls can be accommodated in the system. This increases the accuracy of fitting to the wearer's individual hearing loss. With this technology, it was possible for the first time in the early 1990s to set several - separate - amplifier channels on a clearly laid out platform.
However, functions such as noise detection and cancellation or speech recognition cannot yet be implemented. Since the development of fully digital hearing aids with a different number of performance features in different price categories continues, digitally programmable hearing aid amplifiers are losing more and more importance today. Hearing aid supplies in Germany are practically only provided with fully digital hearing systems, as the cost bearers stipulate this.
Fully digital hearing aids with trimmers
These hearing aids, which are especially located in the lower middle class, have fully digital signal processing. However, the frequency and dynamics are not adjusted via the PC, but - as with purely analog devices - via trimmers in the device. Due to the size of the housing, a maximum of four trimmers can be arranged, while devices with a programming interface can change up to a hundred parameters. The advantage is that a computer-independent setting of the hearing aid is possible at any location. There are only trimmers for frequency response adjustment and one trimmer each for automatic volume control and output limitation. Complex algorithms for situation analysis, such as interference noise suppression or music recognition, are not implemented in these digital hearing aid amplifiers.
Extension of the area of application through additional external systems
With additionally connected transmission systems, the range of use of hearing aids can be expanded and speech intelligibility can be increased through selective transmission. The following systems exist for this:
Bluetooth transmission: Hearing aids in medium and higher price ranges are available with Bluetooth receivers. This allows the signals from cell phones and audio devices to be selectively transmitted to the hearing aid without any interference from the environment. The direct coupling with a smartphone also allows many additional options, e.g. B .:
- Remote adjustment of the hearing aids by the hearing care professional
- Finding lost hearing aids
- Use of the smartphone as an external hearing aid microphone
- NFMI technology : inductive transmission ( near field magnetic induction , acronym: NFMI), the hearing aid can exchange and process information with a streaming device worn around the neck, or streamer for short . The streamers offered by the manufacturer are equipped with a Bluetooth module and mostly with an analog audio input and are mainly used for direct sound transmission from external sound sources (e.g. television, music system, telephone) into the hearing aids.
- FM system for wireless signal transmission e.g. B. in conference or classrooms and meetingplaceswith frequency-modulated radio signals (FM), so that speech can be selectively transmitted in noisy surroundings, poor acoustics or a great distance. The transmitted FM signals are transmitted to the hearing aid using an audio shoe attached to the behind-the-ear hearing aid, which either contains receiver electronics or a cable connection to an FM receiver that is also worn on the body. Transmission from the additional FM receiver via an induction loop is also possible, depending on the system. FM systems are mainly used in educational institutions for the hearing impaired.
- Inductive hearing system for the interference-free reception of audio signals such as music in cinemas and theaters, spoken contributions at events and lectures, e.g. in churches etc. The hearing aids of the system users must be equipped with a "telecoil" for use, but this is the case with almost all modern Hearing aids is the case. Depending on the hearing aid, the activation of this coil must be carried out by the hearing care professional.
- Infrared hearing aids can transmit sound from audio devices or a microphone for the hard of hearing . The signal transmission between the device components takes place with infrared light from a transmitter to a receiver, whichconvertsthe infraredsignalback into sound waves directly at the earortransmits it to the hearing aidwith an additional induction loop around the neck, which must be equipped with a "telephone coil" .
- Additional microphones , especially table and directional microphones ; the hearing aid must be Bluetooth-enabled or equipped with an audio connection or a compatible streaming device (“streamer”) to which the microphone can be connected.
Assumption of costs
Hearing aids are considered medical devices in Germany and are assigned to risk class IIa according to the Medical Devices Act . This classification takes into account the characteristic of “moderately invasive and short-term use in the body”.
The statutory health insurance covers the costs for basic care in the form of a financial subsidy based on this allocation and according to a prescription by the ENT doctor. The medical aids guidelines of the statutory health insurance provide differentiated provisions in § 18 to § 31 regarding the assumption of costs for various types of hearing impairment.
The actual supply, device selection and adjustment is usually carried out by the hearing aid acoustician , whose field of work is the interactions between the hearing aid technology and the hearing impression ( audiology ). In some cases, hearing aids are set directly by the hearing aid manufacturer according to the information and diagnosis provided by the ENT doctor and then delivered via the ENT doctor. This is known as a “direct” or “shortened supply route”.
In the Federal Republic of Germany, the participating public health insurance with appropriate medical prescription to the cost of hearing aids ( hearing aids ) at the level of the Central Federal Association of Health Insurance according to § 36 Fifth Social Code Book (SGB V) set fixed amounts , one since 1 November 2013 The amount is set at 733.59 euros. Hearing aids that are given to the insured at a fixed price must be fully digital, have at least three hearing programs, have at least four adjustable frequency channels and have multi-microphone technology, noise and feedback suppression. For hearing aids with equipment features that exceed the statutory health insurances' obligation to provide benefits, the difference to the insurance amount must be borne by the hearing aid user as a contribution. Further fixed amounts are specified in this catalog for different types of hearing aids and accessories, such as earmolds. For the second device with two-ear supply, discounts are set on the fixed amount. The costs for the batteries are covered for insured persons up to the age of 18, in some cases this is done as part of flat-rate repairs according to the specifications of individual health insurance companies. If the insured person opts for a hearing aid without paying an additional fee, the costs will be settled directly between the hearing aid acoustician and the health insurance company. The hearing aid acoustician receives a flat rate supply from the health insurance company. In addition to the hearing aid, this flat rate also includes the earmold and all necessary repairs as well as replacement earmolds. This also includes aftercare, i.e. checking and maintaining the hearing aid for a period of six years.
For all hearing aids, the insured person must pay a statutory additional payment of ten euros per device, unless the insured person is exempt from such additional payments for the current year. For adults, the batteries required to power the hearing aids, as well as cleaning and care products, are not included in the health insurance benefits. Likewise, all additional costs for repairs resulting from the insured's choice of a higher-quality hearing aid must be borne by the insured.
In Germany, in addition to the health insurance companies, other service providers can cover the costs of hearing aids in full (or in part). For working people, public bodies such as the employment agency , pension providers , civil servants ' welfare, accident insurance providers or state welfare associations can finance a hearing aid as a work aid or to maintain the workforce and as a service for participation .
In Switzerland , the disability insurance (IV) covers the costs of hearing aids, but only after medical and social clarifications. As a rule, people who are hearing impaired at birth or who become so at a young age are covered in full. The remuneration is based on indication levels. The indication level is calculated according to hearing ability or language understanding, occupation and social necessity.
The IV only pays for “simple and practical” care. Depending on the case, it takes on both monaural and binaural supplies. The IV goes according to the motto "integration before retirement". The old-age and survivors' insurance (AHV) is responsible for the first adjustment after reaching retirement age. In any case, she only pays monaural benefits and of this only 75% of the IV remuneration. In addition, the ongoing consultation appointments and adjustments by the hearing aid acousticians are paid for by the IV.
An hearing test (audiometry) is carried out by the ENT doctor to establish the indication. If a hearing impairment is determined, the patient can be provided with a hearing aid. The hearing aid supply is usually carried out by a hearing care professional in a specialist company. Alternatively, the device can be supplied by the ENT doctor using the so-called shortened supply route.
Adaptation in adults is different from that in infants and children. While adults can provide feedback about their hearing impression, attention must be paid to the smallest behavioral and movement reactions in children. The fitting formulas used to set the amplification and compression of the hearing aid also differ between children and adults. Adults usually only need adequate amplification of speech signals. In the case of children and especially small children, care is taken to amplify all audible noises as far as possible, including those that are disturbing.
By creating an audiogram , the hearing performance of the patient without a hearing aid and with a hearing aid (inflation curve in children and people without knowledge of German, free-field language test in adults) is recorded. Scaling the subjective hearing perception is another way of checking and further adjusting the fitting of hearing aids. The customer is shown various sound images that he has to evaluate. By using surround examples, the adjustment can be shortened today, because here concrete listening situations can be tried out with the hearing aid wearer. Furthermore, the subjective hearing ability of a patient is determined with a standardized questionnaire (e.g. the APHAB ).
The sound and speech audiogram forms the starting point for the selection of the hearing aid type and the setting of the hearing aid in the delivery state. Before hiring, it must also be clarified what the person concerned would like to use it for and what requirements he / she has. For the presetting of the hearing aids, the stored customer data (sound and speech audiogram, age, gender, language, etc.) can be used to obtain rough guide values for frequency and .alpha. Using various fitting formulas (e.g. NAL-NL2, DSL v5, manufacturer's own fitting formulas) Determine level-dependent gain and for the maximum output sound pressure level (so-called frequency and dynamic adjustment). However, these values should only be viewed as a basic setting, as the subjective hearing experience of the hearing aid wearer must be given priority. As a further control instrument, the hearing aid acoustician has a measuring box (with various couplers that simulate the volume of the ear canal) that can carry out acoustic measurements on hearing aids; There is also a so-called in-situ measuring system, which uses a tiny tube to measure the level in the ear canal in front of the hearing aid wearer's eardrum. This makes sense in many cases due to the different volumes of the ear canal and different earmolds, which cause considerable frequency-dependent level changes.
Since around the mid-2010s, almost all current hearing aids can be measured for the hearing threshold directly on the customer's ear at the beginning of the fitting process. In the wearing position, the hearing threshold of the hearing aid wearer is determined directly via the hearing aid loudspeaker, similar to tone audiometry. The audio audiogram hearing threshold stored in the customer profile is largely ignored; only information on the dynamics (discomfort limit and bone conduction hearing threshold) is generally taken into account to a certain extent and is included in the calculation of the fitting formula. The advantage of this measurement is that the individual acoustic properties of the customer's ear and the otoplastic are automatically taken into account and the hearing aid “knows” exactly where the hearing threshold of the hearing aid wearer is. In the case of conventional adaptation, this information is only an estimated value that must be compared through in-situ measurements and manual corrections of the reinforcement behavior. Hearing aid manufacturers use terms such as “in-situ audiometry”, “audiogram-direct”, “sensogram” or similar terms to describe this integrated fitting measurement. In addition, the transmission behavior of the hearing systems is further optimized with an automated feedback measurement that determines the acoustic leakage of the hearing aid supply. In the wearing position, the hearing aid loudspeaker emits clearly defined signals (usually white or pink noise, also sinus sweeps or random narrowband noise), while the hearing aid microphones measure the sound emerging from the ear.
In the case of children, the adaptation takes place in the open field with what is known as play audiometry. Only BTE devices are suitable for children up to about 14 years of age, as their ear canal is still growing. The children regularly need new ear tips to ensure that the growing ear canal is sealed. The children's earmolds are mainly made of soft materials. The hearing aids falling out during rapid body movements (e.g. when romping around) is less likely with soft materials, since soft earmolds are more “supple”. In addition, the risk of injury (acrylic earmolds can break) is significantly reduced.
In the selection process, at least three different devices should also be tested under everyday conditions. It can therefore often take a few weeks to months to find the most suitable device and the best setting. The currently valid contracts between hearing aid acousticians and statutory health insurances only require a comparative adjustment with a device that does not have to be paid for. The actual duration of the trial and the number of hearing aids compared is not precisely prescribed.
Acceptable Noise Level
The noise level that has just been accepted is also known as the Acceptable Noise Level (ANL) and can be measured when the hearing aid is being fitted. Noise not only has an impact on speech understanding, but also has a very strong effect on wearing habits. Dr. Nabelek at the University of Tennessee around 1990 and published an article in 2006 in the Journal of the American Academy of Audiology (JAAA).
During the measurement, the hearing aid wearer is first presented with a binaural or monaural speech signal. The test person must then indicate when the signal is perceived as pleasant. The level is maintained while broadband noise is slowly faded in until it is just accepted. The ratio of useful to interfering noise is referred to as ANL. For example, if the speech is 63 dB and the noise is 58 dB, the ANL is 5 dB.
The ANL is divided into 3 classes: <8 dB, 8 dB - 13 dB and> 13 dB. According to the study, people with a small ANL are more likely to accept hearing aid coverage. The chances of success in these three classes are: 85%, 50% and 10%. The ANL is hardly used in today's practice by hearing aid acousticians.
- Claudia Czmok: The importance of the health reform 2006/2007 for the area of aids using the example of hearing aids. Diplomica Verlag, 2008, ISBN 978-3-8366-6388-5 .
- Rainer Hüls: The hand on the ear. A little history of hearing aids. Innocentia Verlag, 2009, ISBN 978-3-9808107-3-9 .
- Rainer Hüls: The history of hearing acoustics. 2000 years of hearing and hearing aids. Median-Verlag 1999, ISBN 3-922766-66-8 .
- ↑ Roland Gööck: The great inventions: Agriculture, food, medicine . Sigloch Edition, Stürtz Verlag, 1986, ISBN 3-8003-0237-3 , p. 262.
- ^ Siemens hearing aids, history, HTML document ( Memento from October 31, 2010 in the Internet Archive )
- ↑ Siemens Hearing Aids, History, PDF ( Memento from November 5, 2010 in the Internet Archive )
- ↑ December 29, 1952 - Market premiere for the first transistor hearing aid
- ↑ Augsburger Allgemeine. December 29, 2007.
- ^ Siemens Audiologische Technik, Geschichte, year 1966 ( memo of November 18, 2012 in the Internet Archive ), accessed on November 9, 2012.
- ↑ Cochlear Blog December 22, 2017: The Historical Development of Hearing Aids
- ^ Andi Vonlanthen: Processor in the ear - hearing aids from Phonak. In: Franz Betschon , Stefan Betschon, Willy Schlachter (eds.): Engineers build Switzerland. First-hand history of technology. Volume 2, Verlag Neue Zürcher Zeitung, Zurich 2014, ISBN 978-3-03823-912-3 , pp. 445–458.
- ↑ Product information Phonak Lyric ( Memento from December 12, 2013 in the Internet Archive ) (PDF; 476 kB).
- ↑ Illustrations of BAHA
- ↑ Fuel cell technology in hearing aids Website of the hearing aid manufacturer Widex. Retrieved March 8, 2019.
- ↑ Jürgen Kießling: Supply and rehabilitation with technical hearing aids. S. 189. (medi.uni-oldenburg.de , PDF; 372 kB).
- ^ Structure of the listener website of the German Hearing Aid Institute. Retrieved March 6, 2019.
- ↑ Fraunhofer Institute, piezoelectric sound transducer for an implantable hearing aid
- ↑ Image of a sound transducer for an implantable hearing aid (Image source: Natural Science and Medical Institute at the University of Tübingen) ( Memento from February 10, 2014 in the Internet Archive )
- ↑ Processor concept for hearing aids Article on elektroniknet.de . Retrieved March 6, 2019.
- ↑ MEMS microphone technology article on itwissen.info . Retrieved March 6, 2019.
- ^ Structure of the listener website of the German Hearing Aid Institute. Retrieved March 6, 2019.
- ↑ Features of modern hearing systems - frequency lowering Technical article by Jürgen Kießling in a publication by HÖREX Hör-Akustik eG
- ↑ Starkey "Digital hearing aid development - facts against fantasy" (PDF; 161 kB)
- ↑ Signia hearing aids, technology brochure, explanation of binaural data transmission (page 14). Retrieved March 8, 2019.
- ↑ Widex hearing aids - machine learning in the hearing aid weblog of the manufacturer Widex. Retrieved March 8, 2019.
- ↑ Wirtschaftswoche, high-end hearing aids with Bluetooth (from February 11, 2009)
- ↑ Bluetooth for hearing aids ( Memento from August 14, 2011 in the Internet Archive )
- ↑ Medical aids guidelines of the statutory health insurance (PDF; 182 kB)
- ↑ North Rhine Association of Statutory Health Insurance Physicians "Aids: Guideline revised" ( Memento from October 8, 2014 in the Internet Archive )
- ↑ gkv-spitzenverband.de (PDF; 17 kB)
- ↑ Assumption of costs for hearing aids by the health insurance companies , accessed on January 17, 2014.
- ^ Rehabilitation Engineering Research Center