Electrical audio signal processing systems and devices – Hearing aids – electrical – Noise compensation circuit
Reexamination Certificate
2000-09-14
2004-01-06
Kuntz, Curtis (Department: 2643)
Electrical audio signal processing systems and devices
Hearing aids, electrical
Noise compensation circuit
C381S320000
Reexamination Certificate
active
06674868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hearing aid that improves clarity by minimizing the sense that sounds instantly become louder, eliminating the metallic ring to sounds, and so forth.
2. Description of the Related Art
The process by which sound waves are recognized by our auditory system is generally considered to be extremely complex, but to summarize this process, sound waves travel through a conducting system consisting of the external ear canal, the eardrum, the auditory ossicle, the cochlea, hair cells, nerves, and brain cells, where the sound waves are recognized. Within this conducting system, the external ear canal and eardrum are called the outer ear, the eardrum and auditory ossicle are called the middle ear, and the cochlea and hair cells are called the inner ear.
A hearing impairment therefore occurs when any of the functions is diminished in this conducting system, and the symptoms will vary, as will the method of dealing with them, depending on which function is diminished and to what extent.
The typical form of senile deafness is an overall decrease in function, including brain function, making it difficult to hear weak sounds.
FIG. 7
is a graph of equisignal curves of the loudness of sound in humans with normal hearing. The horizontal axis is the frequency (Hz), and the vertical axis is the sound pressure level (dB). Sound pressure level will hereinafter be abbreviated as SPL.
The curves in the graph are known as Fletcher-Manson curves, and the hatched area in the figure indicates the distribution of acoustic energy in a typical conversation. The dashed line labeled “minimum audible level” is a curve corresponding to a human with normal hearing, but in the elderly this is higher on the graph, as with the curve indicated by the dashed line labeled “senile deafness minimum audible level.” This senile deafness minimum audible level varies from person to person, so the curve in the graph should be viewed as just an example.
As can be seen from the acoustic energy distribution in a typical conversation, a person with senile deafness is only able to hear about half of the sounds in the voice spectrum which a person with normal hearing is able to hear, so even though the sounds may be perceptible, the hearer cannot make out the words.
With the example shown in the graph, if the acoustic level is raised about 50 dB by a hearing aid, the voice spectrum of conversation will be more or less reach the audible level, allowing the wearer to understand the words, but sounds of, say, 80 dB, which are encountered on an everyday basis, become 130 dB, which is so loud as to be uncomfortable.
The highest level that a person with normal hearing is able to stand is about 130 dB, and is said to be between 120 and 130 dB for a person who is hard of hearing, which would seem to be about the same, but in fact the level is often much lower.
FIG. 8
is a graph of the formants of Japanese vowels. The horizontal axis is the first formant (kHz), and the vertical axis is the second formant (kHz) (see Rika Nenpyo, p. 491, published by Maruzen, Nov. 30, 1985).
What
FIG. 8
tells us is that for the Japanese vowels “A”, “I”, “U”, “E”, and “O” to be clearly distinguished, for example, the second formant must be reliably transmitted with respect to the first formant.
FIG. 9
is a table of typical values for various sounds and their corresponding formant frequencies. According to this table, the second formant frequency varies between 1.5 and 7.7 times with respect to the first formant frequency, but if it is not reliably transmitted, the hearer cannot distinguish between A, I, U, E, and O.
In general, the level of the second formant is about 20 to 40 dB lower than the level of the first formant, so even if the first formant can be heard, it is difficult to hear the second formant, and to make matters worse, there is usually a dramatic drop in the perception of high frequencies with a person with senile deafness, as indicated by the dashed line in
FIG. 7
, and this makes it even more difficult to hear the second formant, in which case even though the person may be able to hear the first formant, he does not understand what is being said.
Conventional Approach 1
Because of the above situation, one thing conventional hearing aids had in common was that they raised the level of the second formant high enough to be audible, but while employing this means does indeed work fairly well with mild deafness, with more severe deafness the level of the first formant often exceeds 100 dB, which sounds loud to the wearer.
Conventional Approach 2
Raising the degree of amplification of high frequencies has been accomplished by using a tone control circuit, and while this is effective with persons of mild deafness, with a more severe case of deafness, if the frequency of the first formant is high, the first formant level can rise over 100 dB and become painful, and as a result the wearer hears a so-called ringing noise.
Conventional Approach 3
Automatic volume adjusting circuits are frequently used to keep the volume below 100 dB by immediately lowering the gain if a loud sound over 100 dB should come in. Various methods have been developed for shielding the wearer from fluctuations in sound level by optimizing the attack time and release time, but if someone should suddenly shout during a conversation, the level is lowered to the point that it sounds as if the sound source is far away, and this is particularly undesirable when listening to sounds through a stereo audio device because the sensation of a fixed position is lost and the location of the sound source seems to float around.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a hearing aid which amplifies voices so that they can be clearly understood but do not sound overly loud.
The hearing aid of the present invention is designed so that the gain of the second formant is raised without raising the gain of the first formant, which keeps the clarity of voices high without their sounding too loud. A state in which even the first formant cannot be heard is not under discussion here, in which case it is necessary to perform overall amplification so that the first formant can be heard, and raise the gain of the second formant.
The level of the first formant in conversation is usually about 50 to 60 dB, which is high, and even people with mild to moderate deafness can still hear adequately, but because the level of the second formant is about 20 to 40 dB lower than that of the first formant, voices will not seem too loud even if the second formant is boosted to about this same level.
Therefore, not raising the gain of the first formant and raising the gain of the second formant makes voices become clear, and since the gain of the first formant does not change, the voices do not sound loud.
FIG. 1
consists of graphs of the operating condition settings of the hearing aid pertaining to the present invention. The horizontal axis is frequency, and the vertical axis is the SPL.
FIG. 1A
shows the frequency spectrum related to the vowel “I” seen in
FIG. 8
, and
FIG. 1B
shows the frequency spectrum related to the vowel “A” seen in FIG.
8
.
For example, if a person cannot hear sounds below an SPL of 50 dB, then, as is obvious from
FIG. 1A
, that person can only hear the first formant with the vowel “I” and cannot, tell which sound it is, further since he can faintly hear the second formant with the vowel “A” as shown in
FIG. 1B
, he can tell that the sound is “A”, although he will be uncertain if the voice is a little softer.
With the hearing aid pertaining to the present invention, as shown by the broken line in
FIG. 1A and 1B
, the first formant is not amplified, and just the second formant is amplified enough to reach the required level, thus bringing both the first formant and second formant within the audible range.
With the “I” sound in
FIG. 1A
, frequencies of the 350 Hz frequency of the first formant and higher are corrected by 6 dB/oct up to a maximum of 20 dB.
This co
Ensey Brian
Kuntz Curtis
Shoei Co., Ltd.
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