hearing aid

Abstract

With the aging population, sensorineural hearing loss is increasing among the elderly, and more people are using hearing aids as a means of compensating for this. However, it is estimated that only about 50% of people are satisfied with the improvement provided by hearing aids, and among those known to the inventor, the number is close to zero, indicating that many are dissatisfied in some way. This invention provides a hearing aid that can address sensorineural hearing loss and resolve the dissatisfaction of these elderly people. [Solution] In this invention, taking into account that the low-frequency portion of sound is conducted from outside the hearing aid's earphone, such as through skin conduction, bone conduction, and the sealing gap of the earphone, and also considering that the stapedius muscle, which supports the stapes bone in the middle ear that transmits sound to the cochlea, weakens due to aging, causing low-frequency emphasis and high-frequency reduction, a high-pass filter exhibiting the characteristics shown in Fig-5 is constructed, and the cutoff frequency is set to around 3kHz. This differentiates the sound waves in the main sound band around 500Hz to 2KHz, suppressing the high-frequency reduction included in the composite wave at the stapes bone, and compensating for the high-frequency reduction caused by the inner hair cells of the organ of Corti, thereby realizing a hearing aid with good clarity.

Description

Technical Field

[0001] The present invention relates to a hearing aid designed to compensate for the hearing of sensorineural hearing loss common in the elderly, the frequency characteristics of a sound collector, and the level control of the volume. Hereinafter, the hearing aid and the sound collector will be collectively referred to as the hearing aid.

Background Art

[0002] In recent years, the number of sensorineural hearing loss patients whose ears are less sensitive due to aging of the elderly has been increasing. However, current medical treatments such as prescribing medications are difficult to address, and the current situation is that some prescriptions encourage the use of hearing aids.

[0003] It is said that 500,000 to 600,000 of these hearing aids are shipped and sold every year. However, there is also a survey result that nearly 60% of the users of this hearing aid have some kind of dissatisfaction.

[0004] The following are cited as such dissatisfaction: (1) The noise becomes louder and tiresome. (2) Although the volume has increased, the words still cannot be distinguished or recognized. (3) Conversation is impossible or unusable in noisy places. (4) When something approaches the ear, such as a hand or a pillow, it causes howling and is annoying, etc.

[0005] Current hearing aids are inspected by professionals with national qualifications such as speech therapists and certified hearing aid technicians for the hearing-impaired, such as pure tone audiometry and speech audiometry. Based on the inspection results, a system for adjusting the frequency characteristics of the hearing aid is adopted. Regarding these dissatisfactions, both hearing aid manufacturers and doctors comment that since the hearing-impaired gradually progress in hearing loss and suddenly start hearing the surrounding sounds with a hearing aid, it is due to the problem of getting used to the audible environment or inappropriate adjustment of the hearing aid, and these problems can be improved if adjustments and guidance are provided frequently.

[0006] For example, Patent Documents 1 to 10 propose methods for how to reflect the results of pure-tone audiometry and speech audiometry tests for people with hearing loss in the electrical characteristics of hearing aids, and how to prevent feedback. Japanese Patent Application Publication No. 2023-035766 proposes providing a noise-free hearing aid by using digital technology to extract vowels and consonants from words, removing noise, and then synthesizing the words. Japanese Patent Application Publication No. 09-191498 provides a physical hearing aid that collects human voices as they are. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2001-157299 [Patent Document 2] Japanese Patent Publication No. 2024-066058 [Patent Document 3] Japanese Patent Publication No. 2024-021271 [Patent Document 4] Japanese Patent Publication No. 2024-004693 [Patent Document 5] Japanese Patent Publication No. 2023-155101 [Patent Document 6] Japanese Patent Publication No. 2023-138573 [Patent Document 7] Japanese Patent Publication No. 2023-035766 [Patent Document 8] Japanese Patent Publication No. 2023-0023451 [Patent Document 9] Japanese Patent Publication No. 2023-0023451 [Patent Document 10] Japanese Patent Publication No. 09-191498 [Overview of the project] [Problems that the invention aims to solve]

[0008] Even though manufacturers and doctors point to improper adjustments or issues with getting used to the device as the cause, as mentioned earlier, nearly 60% of people currently using hearing aids are not satisfied with their performance.

[0009] Aside from hearing loss caused by certain diseases, the most common type of age-related hearing loss in the elderly is called sensorineural hearing loss. This type of hearing loss is caused by damage to the hair cells in the cochlea, an organ located behind the eardrum, due to aging. This damage reduces the function of converting sound waves transmitted from the ear canal into electrical signals and sending them to the brain, and can lead to nerve abnormalities. Symptoms include difficulty hearing high-pitched sounds and difficulty distinguishing words based on voice quality.

[0010] Current hearing aids are designed and systematized based on the above medical knowledge (pure tone, speech audiometry, certified hearing aid technicians), but there are many complaints. Some of these complaints stem from the fact that high-frequency sounds that have been gradually lost over a long period (everyday sounds, the sound of dishes clinking in the sink, the sound of water, the sound of metal clinking, etc.), commonly known as unpleasant squeaking sounds, suddenly return due to the hearing aid, and it takes time to get used to this. However, regarding complaints such as still having difficulty understanding speech, being unable to converse in noisy places or loud environments, and bothersome feedback, the inventor believes these issues are due to the design philosophy of the hearing aid.

[0011] In light of the current state of hearing aids as described above, this invention proposes and implements a hearing aid with a new design concept, thereby providing a hearing aid that can be comfortably used by elderly people suffering from sensorineural hearing loss. [Means for solving the problem]

[0012] When you ask people who own several types of hearing aids why they use them, the answers are still the same: it depends on who they are talking to, there are words that are difficult to hear, there is a lot of noise, they can't have a conversation in noisy places, etc. In short, it makes you feel that there is currently no hearing aid that is suitable for sensorineural hearing loss.

[0013] Sensorineural hearing loss is a medical term referring to hearing impairment caused by problems in the inner ear or auditory nerve pathway. It involves a decrease in the ability to properly transmit sound to the brain, particularly difficulty hearing high-frequency sounds, unclear speech even when sounds are heard, and difficulty hearing in noisy environments or with background noise. The cause is believed to be damage to the hair cells in the cochlea, located behind the eardrum, which cease to function. These hair cells are responsible for converting sound waves entering the ear into electrical signals and transmitting them to the brain. These hair cells tend to be damaged by aging, prolonged exposure to loud noises, side effects of certain medications, and illness.

[0014] While sensorineural hearing loss is generally incurable, it is believed that with the right hearing aids, individuals can regain their previous quality of life. Current hearing aids are designed and provided based on this medical finding.

[0015] Figure 1 shows the structure of the ear. In addition to the cochlea (5), it is important to remember that the function of hearing involves a small bone called the stapes (4), which transmits sound vibrations to the cochlea. This stapes (4) is supported by the stapedius muscle (3), and in addition to transmitting sound vibrations to the inner ear, it has the function of amplifying vibrations and preventing damage to the inner ear when loud noises enter. When a loud noise is transmitted, the stapedius muscle (3) reflexively contracts, suppressing the sound vibrations and reducing damage to the inner ear. It is important to understand that the logarithmic characteristic of human hearing is due to the function of this stapes (4) and stapedius muscle (3).

[0016] Furthermore, there is a disease called otosclerosis caused by the stapedius muscle 3 and stapedius bone 4. This is a hearing loss caused by the degeneration and hardening of the stapedius muscle, which prevents it from vibrating properly. It often develops during adolescence, progresses simultaneously in both ears, and the hearing loss progresses gradually, so the person may not even realize they have it. Interestingly, it is said that some people exhibit unique symptoms, such as being able to hear everything except human voices well, or even being able to hear human voices more easily in noisy places.

[0017] The inventor focused on the function of this stapedius muscle 3 and speculated that when the elderly lose muscle strength, the stapedius muscle may soften, leading to a disease called otosclerosis (a term coined by the author). Could it also cause sensorineural hearing loss due to this disease?

[0018] Fig-2 in Figure 2 is a diagram that simply models the functions of the stapes bone 4 and the stapedius muscle 3 in Fig-1 as the diaphragm and support mechanism of a microphone. Here, 9 is the support mechanism assuming the stapedius muscle, and its triangular shape (assuming thickness) is assumed to represent the muscle reflex reaction of the stapedius muscle 3. It is assumed to have the function of converting the linearly expanding vibration amplitude into a logarithmically expanding vibration amplitude. The diaphragm 8 is assumed to be the stapes bone 4 and is assumed to have the function of expanding the vibration amplitude. This diagram was devised to speculate and examine how the output 10, 11 changes for the pure sound vibration waveform (single waveform) 6 and the voice vibration waveform (composite waveform) 7 input to the diaphragm 8 when the hardness of the support mechanism 9 is changed using this model.

[0019] Fig-3 in Figure 3 is a diagram that speculates and formulates how the main vibration frequency band behaves depending on the hardness of the support mechanism. A represents pure sound waves, where (a) indicates the low frequency range below approximately 500 Hz, (b) indicates the middle frequency range of approximately 500 Hz to 3 kHz, and (c) indicates the high frequency range above approximately 3 kHz. B represents the hardness of the support mechanism, assuming (i) a hard state, (ii) an optimal hardness, and (iii) a soft state. C, D, and E are the vibration results of the diaphragm corresponding to the hardness (i), (ii), and (iii) of the support mechanism B and correspond to the input sound wave vibrations (a), (b), and (c).

[0020] As a result of the mechanical consideration and speculation of Fig-3, as the hardness of the support mechanism B increases (B-i), as shown in C, the resonance moves to the high frequency range (C-c), and the conduction in the high frequency range becomes higher. However, as shown in the entire band (C-a), (C-b), (C-c), generally, the vibration is less transmitted, and the overall vibration becomes smaller. When the hardness is at the optimal value (B-o), as shown in D, it is faithful to the input sounds (A-a), (A-b), (A-c) in both the low (D-a), middle (D-b), and high (D-c) frequencies. When the hardness is soft (B-c), as shown in E, the resonance moves to the low frequency range (E-a), and the amplitude in the low frequency range becomes larger. However, the response in the middle frequency range (E-b) and especially in the high frequency range (E-c) becomes dull, and the high frequency range does not transmit.

[0021] Fig-4 in Figure 4 is a diagrammatic representation of speculation on how the hardness of the support mechanism B affects the case when a complex vibration waveform (F-ii) of voice waveforms F (a composite waveform of low, middle, and high frequency ranges), that is, a high frequency is superimposed on the low and middle frequency ranges, is input, similar to Fig-3.

[0022] Vibration waveforms such as (F-ii) are waveforms of voiced sounds and plosive sounds such as zu and tsu in speech. In terms of listening to words, it is speculated that if these waveforms are not transmitted correctly, there will be a lack of clarity and it may lead to a state of sensory hearing loss.

[0023] In Fig-4, B represents the hardness of the support mechanism, (B-i) represents a hard state, (B-o) represents the optimal hardness, and (B-c) represents a soft state. G, H, and J are the vibration propagation waveforms of the diaphragm corresponding to the states of the support mechanism (B-i), (B-o), and (B-c). (G-ii) slightly emphasizes the high frequency, but the overall sensitivity drops, and a state of hearing loss is assumed. (H-ii) is correctly amplified and transmitted, and a state of normal hearing is assumed. (J-ii) emphasizes the low frequency, and at the same time, the mixed high frequency is absorbed and attenuated by the vibration of the low frequency, and it is assumed that it becomes softer and swaying occurs, similar to the state where a low voice is emphasized and reverberation can be heard in a tunnel conversation.

[0024] In other words, if we assume that the general weakening of muscles in the elderly also affects the stapedius muscle 3, then we can consider this to be equivalent to the stiffness of the support mechanism 9 becoming softer. In this way, we thought that the symptoms described by people with sensorineural hearing loss, such as being unable to distinguish plosives and dull consonants, and hearing echoes like being in a tunnel, could be attributed to insufficient stiffness in the microphone diaphragm and the support mechanism that supports it, as shown in Fig-4 of Figure 4.

[0025] However, please understand that the above considerations are merely the inventor's mechanical considerations regarding the microphone mechanism and other mechanical support mechanisms and diaphragms, and have no medical basis whatsoever. They are simply the basic ideas that motivated the inventor when he devised this invention.

[0026] Based on the above considerations, this invention aims to provide a hearing aid that allows for speech discrimination and conversation in noisy environments, suitable for people with sensorineural hearing loss, which is common in age-related hearing loss.

[0027] Fig-5 in Figure 5 shows a high-pass filter with a cutoff frequency of 19 around 2-3 kHz. This is the basic technology of the present invention, and we believe it works to improve speech discrimination in sensorineural hearing loss. In other words, while conventional hearing aids address excesses or deficiencies in auditory frequencies based on the results of pure tone and speech hearing tests by adjusting an equalizer that divides the auditory frequency into several channels, the present invention is characterized by improving speech discrimination by suppressing low and mid-range components.

[0028] Furthermore, based on the results in Figure 4, Fig-4, if we consider that the loss of high-frequency components superimposed on the main speech (low-to-mid frequency) due to the softening of the stapedius muscle affects speech recognition, then we believe that suppressing the low-to-mid frequency vibration amplitude of sound waves entering the stapedius as much as possible, reducing the vibration amplitude of the stapedius, suppressing the attenuation of high-frequency components superimposed on the low-to-mid frequency components, and emphasizing the high-frequency components will compensate for the decrease in high-frequency components due to the deterioration of hair cells in the cochlea, leading to an improvement in speech recognition.

[0029] Figure 5 shows an example of the characteristics of a high-pass filter, and Figure 6 shows an example of the characteristics of filter 12, which is a high-pass filter with a cutoff frequency of around 2-3 kHz. When the waveform with high-frequency components superimposed on the main sound, Fig. 6 II, is passed through this filter 12, the main sound component is differentiated, and the rising edge becomes steep due to the harmonic components of the main sound, while the amplitude is suppressed and reduced, and the superimposed high-frequency components are emphasized, resulting in a waveform like the one shown in E.

[0030] When such a waveform is input to the stapedium, which is supported by a relaxed stapedius muscle, just as plucking a loosely strung string instrument forcefully does not produce the correct sound, but plucking it gently produces a relatively correct sound, it is hypothesized that when a vibration waveform with suppressed low and mid-range components, like Fig-6 E, is transmitted to the stapedium, which is supported by a relaxed stapedius muscle, the vibration of the stapedium will become closer to the vibration in Fig-6 D, and as a result, speech sound discrimination becomes possible.

[0031] There are concerns that the main low-to-mid-range sounds may not be reproduced, but in terms of vibration waveforms, the low-to-mid-range harmonics, although instantaneous and with small amplitude, do transmit vibrations to the stapes. Furthermore, the function of capturing human voice is transmitted through the sealed gap of the earphone, as well as through the skin, bones, etc., and is not lost. On the contrary, considering the unnatural lip-sync caused by the delayed sound due to the frequency correction technology of current digital hearing aids and the time difference between the sound transmitted through the sealed gap, skin, bones, etc., this invention provides better sound quality.

[0032] Furthermore, one of the current complaints about hearing aids is that they are ineffective in noisy environments. The inventor believes that the problem lies in the fact that many current hearing aids have an automatic level control function in the microphone amplifier. Figure 7 illustrates the difference between the automatic level control function and the limiter function adopted in this invention. Figure 8 illustrates the usefulness of the limiter function adopted in this invention as a function of the hearing aid.

[0033] Figure 8 shows a diagram illustrating the operation of the limiter function. The sound wave signals input to the limiter circuit 13, which is located before the output amplifier, are suppressed so that only the signal with the higher level does not exceed the limiting voltage of the limiter circuit 13, which is 20, and are level-limited as shown in Figure 8.

[0034] Following the explanation of the limiter function mentioned above, let's explain Figure 7 again. In Figure 7, K represents the ambient sound input to the hearing aid, L is the output sound of a hearing aid equipped with an auto-level control function in the microphone, and M is the output sound of the hearing aid of the present invention equipped with a limiter circuit before the output stage. This figure allows us to infer the reasons for dissatisfaction with current hearing aids, such as loud noise or ineffectiveness in noisy environments. In Figure 7, K represents the ambient sound waveform entering the hearing aid, (K-He) is the sound waveform of a quiet environment with slight noise, (K-To) is the waveform of an environment with sudden loud noises, and (K-Ch) and (K-Ri) are the waveforms of conversations in noisy environments. In the boundary sound waveforms, (K-chi) represents ambient noise, and (K-ri) represents conversation in a noisy environment. L is the auto-level control function of the microphone amplifier used in current hearing aids, which increases the gain when the microphone input is low and decreases the gain when a loud sound is input, resulting in waveforms that control the sound to a level that can always be heard by people with hearing loss, namely (L-he), (L-to), (L-chi, L-ri). M is the limiter function adopted in this invention, which normally ensures a gain appropriate to the degree of hearing loss of the hearing aid user, and only loud sounds have their amplitude limited, resulting in waveforms of (M-he), (M-to), (M-chi, M-ri).

[0035] Comparing (K-He), (L-He), and (M-He), it becomes clear that with current hearing aids, in quiet, low-noise environments, sounds that are only faintly audible to people with normal hearing are amplified to the normal level for people with normal hearing in (L-He). As a result, sounds that are quiet to people with normal hearing sound normal to people with hearing loss who use hearing aids. This is the cause of dissatisfaction with current hearing aids, who find them noisy and tiring and that they do not provide sufficient quietness. On the other hand, the amplification level of the present invention is set to the same level as people with normal hearing, (M-He), and maintains the same level of quietness as people with normal hearing.

[0036] (K-T) represents the sound waveform in an environment with a sudden loud noise. However, as shown in (L-T), current hearing aids have an auto-level control function that reduces the gain at a certain time constant when a sudden loud noise enters the microphone, so the sound is momentarily louder, but then returns to the normal set volume level. On the other hand, as shown in (M-T), the hearing aid of the present invention limits the amplitude with a limiter circuit, so it does not produce loud noises even momentarily.

[0037] In Fig-7, (K-chi, K-ri) represents the sound waveform of a conversation in a noisy environment. (K-chi) represents the ambient noise, and (K-ri) represents the speech waveform of a conversation in a noisy environment. With current hearing aids, the gain of (L-chi) and (L-ri) is controlled by the microphone's auto-level control so that the maximum volume level (L-chi) reaches the set level. As a result, the speech sound (L-ri) does not receive enough gain and becomes quiet, leaving only the ambient noise audible. This is the source of dissatisfaction with current hearing aids, as it makes conversation impossible in noisy environments.

[0038] (M-Chi) and (M-Ri) are the results when using the limiter of the present invention. High-level ambient noise is amplitude-limited, and the level of conversation remains unchanged. As a result, you can enjoy conversations as before even in noisy environments, and consequently, the shortcomings of conventional hearing aids can be resolved.

[0039] Therefore, I believe you will understand that the hearing aid of the present invention, which uses a high-pass filter to limit the frequency range of the main sound domain and a limiter to limit the amplitude before the output, is a hearing aid that resolves the shortcomings of existing hearing aids and is a useful technology. Of course, in implementing the present invention, it is natural to use a low-pass filter to limit unnecessary high frequencies. [Effects of the Invention]

[0040] Among conventional hearing aid users, nearly 50% own multiple hearing aids (including expensive ones prescribed by medical institutions), or they buy them but find them unbearable due to noise, or they still can't distinguish speech depending on the person, resulting in them being left unused. Several people, including the inventor, who were dissatisfied with these issues, were lent prototypes of the present invention to test its effectiveness. The feedback was overwhelmingly positive, with comments such as: "It's so quiet, it's like a whole new world," "I can now understand what my wife is saying, which I couldn't do before," "I can now respond to my wife's speech and calls even in noisy environments like rooms with ventilation fans, and peace has returned to our home," and "Unlike any other hearing aid I've used before, this is the best hearing aid for speech discrimination." Based on these positive comments, we are confident that the present invention is an indispensable technology that can enhance the communication abilities of elderly people suffering from age-related hearing loss who need hearing aids, and indirectly help prevent the progression of dementia. [Brief explanation of the drawing]

[0041] [Figure 1] Diagram of the structure of the ear [Figure 2] A simplified model of the functions of the stapedius bone and stapedius muscle as the diaphragm and support mechanism of a microphone. [Figure 3] This diagram illustrates the predicted effects of varying support mechanisms and the vibration frequency ranges of pure tone waveforms. [Figure 4] This diagram predicts and illustrates how the speech waveform (a composite waveform of low, medium, and high frequencies)—that is, a composite vibration waveform in which high frequencies are superimposed on low and medium frequencies—will behave depending on the stiffness of the support mechanism. [Figure 5] Examples of characteristics of the high-pass filter used in the present invention [Figure 6]This diagram illustrates the waveform change when a speech waveform (a composite waveform of low, medium, and high frequencies) is passed through the high-pass filter of the present invention. [Figure 7] A diagram illustrating and comparing the output sound of a conventional hearing aid and the output sound of the present invention in relation to the sound environment in which the hearing aid user is placed. [Figure 8] A diagram illustrating how the limiter of this invention works in response to changes in ambient noise levels for hearing aid users. [Figure 9] Block diagram of an embodiment of the present invention [Modes for carrying out the invention] [Examples]

[0042] Hereinafter, embodiments of the present invention will be described with reference to the block diagram shown in Fig-9 of Figure 9 as an example.

[0043] Figure 9 shows 14, a condenser microphone used to pick up ambient sounds, and 15, a microphone amplifier, ensuring a gain of approximately 20-30 dB.

[0044] Element 12 is a high-pass filter with a cutoff frequency (-3dB) of 2KHz to 3KHz according to the present invention. It suppresses the main sound and emphasizes high frequencies, and consists of two CR filters and one active filter with a gain of approximately 40dB.

[0045] 13 is a limiter that clips sounds louder than the level at which normal conversation can take place. The limiter's clipping level is set so that the level of your voice is roughly the same as the level of the conversation of the person you are speaking to, as picked up by the microphone.

[0046] Component 16 is a level controller that adjusts the output level of the output amplifier according to the level of hearing loss of the person with hearing loss. It can be a variable resistor or an electronic VR, but it is essential that the setting is done after the limiter circuit.

[0047] 17. The output amplifier has a gain of about 1-12 dB, and rather than amplification, it is sufficient if it has the ability to drive earphones. In this embodiment, a capacitance is loaded into the negative feedback resistor, which also serves to remove unnecessary high frequencies.

[0048] 18 is an earplug that is inserted into the ear canal and should have a moderate degree of seal to prevent feedback. [Industrial applicability]

[0049] As the population ages and an increase in dementia and other related conditions is predicted, and given that communication is considered effective in preventing dementia, hearing aids are important as a tool to improve communication deficiencies caused by age-related hearing loss, and we believe that the technology of the hearing aid of the present invention will play a part in this. [Explanation of Symbols]

[0050] 1 Ear canal 2 Eardrum 3. Stapedis muscle 4. Stapes 5 Cochlea 6. Pure tone air vibration waveforms 7. Speech sound air vibration waveform (composite wave) 8. Mechanical vibrator designed to simulate the stapes bone. 9. Mechanical support mechanism based on the stapedius muscle. 10. Vibration output of a mechanical diaphragm corresponding to a pure tone air vibration waveform input. 11. Vibration output of a mechanical diaphragm corresponding to speech sound air vibration waveform input. 12. High-pass filter having the characteristics of the present invention 13 Limiter used in the present invention 14 Mike 15 Microphone Amplifier 16 Level Adjustment 17 Output amplifier 18 Earphones 19. Examples of cutoff frequencies for high-pass filters 20 Examples of limiter levels A Pure sound waveform I. Low-frequency components below approximately 500Hz R. Mid-range frequency components around 500Hz to 3KHz High-frequency components of approximately 3kHz or higher B. Hardness of the support material of the support mechanism hard Moderate It is soft C, D, E Output changes corresponding to the input of a pure tone waveform (A) according to the stiffness of the support mechanism F word sound form 2. A composite waveform in which high-frequency components are superimposed on low-mid frequency components. Output changes corresponding to speech waveform (F) input according to the stiffness of the support mechanism G, H, J The waveform after passing the composite waveform of (H) and (D) through the high-pass filter 12. K. Sounds of the surrounding environment, A quiet, faint ambient sound can be heard. A sudden loud noise can be heard in the environment. Noisy ambient sounds Noise in a noisy environment Waveform of a situation where a sudden loud noise is heard in a normal conversation environment. The waveform of the sound of the situation in Lunu, suppressed at level 20 by 13 limiters.

Claims

1. A hearing aid and sound amplifier characterized by using a high-pass filter with a cutoff frequency (-3 dB) of 1.5 to 3 kHz to create a characteristic that blocks low and mid-range frequencies.

2. A high-pass filter for use in a hearing aid and sound amplifier as described in claim 1, wherein the filter is composed of an active filter including L, C, R, and digital, has a cutoff frequency in the vicinity of 1.5 to 3 kHz, and has a blocking characteristic of at least second order, thereby blocking low and mid-range frequencies.

3. A hearing aid and a sound amplifier according to claim 1 and claim 2, characterized in that they use a limiter circuit that suppresses the output voltage to a predetermined level after the frequency characteristics have been formed.

4. Hearing aids and sound amplifiers according to claim 1, claim 2, and claim 3, characterized in that the level of suppression by the limiter is adjusted to match the conversation level of the person closest to you and your own conversation level.

5. Hearing aids and sound amplifiers according to claim 1, claim 2, claim 3, and claim 4, characterized in that a means for setting the volume is placed after the limiter circuit, and the device has a function for adjusting the volume according to the level of hearing loss of the person.

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