Nowadays, hearing aids are becoming more and more powerful, and the development of hearing aid technology can even be compared with chip and mobile phone technology. The development of technology has given the hope of hearing voices to children who were completely deaf in the past. All-digital hearing aids in hearing aids process sound signals in a new way, improving sound quality and reducing noise. After the microphone collects the sound signal, the analog signal is converted into a digital signal through an analog/digital converter, the sound signal is amplified, and finally the digital signal is converted into a sound output through the receiver.
Principle of digital hearing aids
1, acoustic pathways of hearing aids
For people wearing hearing aids, there are three ways for sound to enter the ear canal: The first way: the sound directly enters the ear canal through the vent hole or ear canal gap of the hearing aid or ear mold and reaches the eardrum.
The higher the degree of openness of the optional hearing aids and the lesser the degree of hearing loss, the more dominant this pathway is; the second pathway: when the wearer makes his own voice, the sound is transmitted to the person through the vibration of the skull The ear canal generates a certain sound pressure level at the tympanic membrane, which can only be felt when the person speaks; the third way: the sound enters the microphone of the hearing aid and is converted into electrical energy, which is amplified after calculation, and finally passes through the receiver. It is converted into sound energy and sent to the eardrum of the wearer.
It takes different times for the sound to travel through these paths to the eardrum before the wearer perceives the sound. When the wearer makes a sound, the sound reaches the eardrum quickly through the first two paths, so that it is first felt by the wearer, and when the sound passes through the third path, it needs to be processed by the hearing aid, which will cause a certain delay. When a certain level is reached, the wearer will notice a change in the sound quality.
Digital hearing aids need to go through complex digital operations, such as analog-to-digital/digital-to-analog conversion, filtering, and quantization of signals, so the resulting delay time is long. Over the past decade, the delay time caused by digital hearing aids has received increasing attention because it is much longer than that of analog hearing aids.
Although the delay time caused by digital hearing aids is only a few milliseconds, the sound quality of speech signals can still be affected. When the delay time reaches 10 ms, the sound directly entering the ear canal and the amplified sound will interfere with each other, which causes small fluctuations in the frequency spectrum and changes the sound quality. But this also changes the sound quality. But this is also conditional. Only when the two sounds are at similar intensities will small changes in the sound spectrum be caused.
That is, if the amplified sound energy is significantly greater than the direct sound energy, this change will not occur. When the delay reaches a certain time, the wearer will have a sense of “echo”. When the delay time reaches a certain level (more than 40ms), the phenomenon of hearing and visual asynchrony will occur, that is, “mismatch with the mouth”.
The delay effect of hearing aids has a lot to do with the degree of hearing loss: the worse the hearing, the more amplified sound the patient gets, the less sound goes directly into the ear canal, and the less noticeable the delay is ; Moreover, the heavier the hearing loss, the stronger the sealing between the hearing aid and the ear canal (such as the smaller the vent hole), resulting in a greater attenuation of the sound directly transmitted to the ear canal, making it difficult for people to perceive the occurrence of delay. Digital hearing aids inevitably introduce delays that cannot be eliminated at present, but the key is how long the delay can be accepted by most hearing aid wearers.
2, delay time for digital hearing aids
the acoustic signal from the incoming A certain delay time will be generated from amplifying the system to the output, which is usually called the processing delay time (PD). The delay time depends on the sample rate and algorithm of the system, in general, the higher the frequency resolution, the longer the delay time.
In the past ten years, Stone et al. have done a lot of research on the acceptable delay time of hearing aids, and their research results are summarized as follows: the delay time increases, the interference increases; the delay time of 20-30ms is Patients with mild to moderate hearing loss can feel the sound “distorted”; for normal people, if the delay time exceeds 30ms, their speech comprehension will be affected. In addition, some hearing aid researchers believe that delays up to 10 ms can affect sound quality.
The growing popularity of open-canal hearing aids in recent years has reignited concerns about hearing aid latency. From the naming point of view, “open” makes the seal of the earmold/earplug and the ear canal greatly reduced, which is very different from the traditional earmold or custom machine. Therefore, in the fitting of open-ear canal hearing aids, more sound is transmitted directly to the ear canal, and the wearer can easily feel this part of the sound. Moreover, people who choose open-canal hearing aids tend to have mild to moderate hearing loss, and they can feel the delay time of hearing aids more.
A recent study shows that if hearing loss first occurs at 2kHz and 1kHz, the delay time in open ear canal matching should not exceed 5ms and 6ms, respectively, otherwise the sound quality will be impaired. But the researchers also pointed out that the simulation method used in their experiments was mixed with many other factors, and the wearer could tolerate longer delays in practice.
3, Signal processing of hearing aids
In digital signal processing, each “characteristic” of a hearing aid contributes to the overall The group delay (GD) time increases. The higher the number of channels in the hearing aid, the more advanced the digital noise suppression function, the more complex the feedback processing system, and the longer the delay time it produces. The kind of signal processing is another factor to consider.
Hearing aid manufacturers usually use two different methods for signal processing and frequency analysis: one is to process the sound signal according to the different frequencies through a filter, and the other is to use the fast Fourier transform (fast fourier transformation, FFT) processes all input signals simultaneously. Both methods follow the acoustic uncertainty principle or uncertainty principle, that is, both the frequency domain and the time domain can change, but the product of the frequency domain and the time domain is unchanged.
This shows that the time-domain resolution and frequency-domain resolution of the system restrict each other, that is, if the frequency-domain resolution increases, the time-domain resolution decreases, and vice versa. Therefore, the narrower the channel, the longer the group delay time (either filtering or fast Fourier transform). The fundamental difference between the two methods is that the bandwidth of each channel of the fast Fourier transform is the same, while the bandwidth of each channel of the filtering method can be different.
The filtering method simulates the logarithmic structure of the basilar membrane (“critical bandwidth”) in the cochlea, and the resolution of the basilar membrane in the low-frequency receptive area is higher than that in the high-frequency receptive area. According to the principle of acoustic uncertainty, the narrowband filters required at low frequencies induce longer delay times relative to high frequencies.
Studies have demonstrated that, if the frequency-dependent delay of hearing aids is too long, the listener will experience a delay when a consonant appears after a vowel in the original speech signal (i.e., “vowel ten consonant”). The presence of the consonant is felt before the vowel, i.e. “consonant ten vowels”, or both.
With the Fourier transform method, the signal envelope is analyzed uniformly, and the spectrum of each equal bandwidth is processed in the same way. Therefore, unlike the filtering method, the delay time caused by the Fourier transform is the same in each frequency band. , the above vowels and consonants will not be reversed one after the other. Regardless of the method used by the hearing aid manufacturer for signal processing, the delay time of hearing aids increases as the number of processors and algorithms through which the sound signal travels increases.
Features of digital hearing aids
All-digital hearing aids have one or more tiny chip processors that convert analog signals into a computer language of 0s and 1s . Sound waves in this format can be organized faster and more efficiently than analog sound waves. The digital algorithm can quickly analyze the sound intensity and sound frequency, and adjust the sound intensity and sound frequency to make the hearing aid suitable for different hearing losses.
Numerical algorithms continue to consult noise, and while the intensity of the loudness of a speech can vary rapidly within milliseconds, the noise remains stable for this relatively long time. After a period of use, digital technology can precisely reduce continuous noise such as traffic noise and household appliances. When short sounds such as speech are detected by the hearing aid, the Arabic numeral technology can immediately adjust the hearing aid to answer the sound to an amplified level.
Digital technology can continuously divide sound into different channels. For example, the hearing-impaired person can’t hear the bass well, but amplifying certain frequency bands, even a little, can overwhelm him. Hearing aids with an all-digital compression route are able to layer incoming sounds, detecting and emphasizing those sounds that require amplification.
The finely tuned performance of digital technology is also reflected in a tranquil environment. Hearing aids use a hearing technology called extension technology, a digital algorithm that can distinguish subtle ambient sounds from the noise of household appliances. Arabic numeral arithmetic can also reduce or eliminate acoustic reflections. Acoustic reflection is a major difficulty in hearing aid consumption. Numerical algorithms can monitor acoustic responses before they can be heard, and then reduce the noise in the frequency band to which the acoustic responses belong, without significantly affecting the perceptible noise level.
Difference between digital hearing aids and analog hearing aids
1. From the perspective of the movement, analog hearing aids are composed of a large number of electronic components, using analog hearing aids. The core of the route is the amplification of analog signals. Of course, the signals also use analog signals. The all-digital hearing aid chip uses a microcomputer chip, which is equivalent to a small computer and uses a logic circuit, that is, the input signal is output after high-precision calculation, and the signal is a digital signal.
2. In terms of audiology, traditional analog hearing aids use linear amplification to amplify all sounds to the same extent, so that relatively small sounds cannot be clearly heard, and excessive The sound is very uncomfortable, the sound quality is severely compromised, and the patient’s hearing may be further damaged. The all-digital hearing aids use nonlinear amplification, which can not only compress loud sounds and make small sounds louder, so that the processed sounds sound comfortable and clear, and can better protect residual hearing.
3. Functionally speaking, all-digital hearing aids also have many functions that analog hearing aids do not have. Change, it will automatically analyze the sound in the environment, if in a noisy environment, it will play an automatic noise reduction function, amplify the voice of the speech, and reduce the noise, thereby improving language comprehension.
4. Digital hearing aids also have the advantages of small size, good appearance, very stable, and anti-interference.
According to the above comparison, digital hearing aids are better than analog hearing aids in terms of performance and effect. Now most hearing patients will prefer to buy digital hearing aids. better experience.
