Introduction of the hottest hearing aid and its de

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Abstract: this application note will introduce the types of hearing aids, including BTE, ITe, ITC and CIC, and briefly summarize the analog and digital technologies used in hearing aids, discuss the importance of audio processing, as well as the functions and selection of key electronic components


off-site business sub hearing aids such as electric exhibition opening list serial exchange are small devices placed in or near the ear to improve the hearing of patients with hearing impairment. The basic unit of a hearing aid includes a microphone, a signal conditioning circuit, a receiver (also known as a speaker), and a battery. The microphone converts the acoustic signal into electrical signal, and the signal conditioning unit can be simple or complex. Simply, it only amplifies the audio signal according to a fixed proportion, and complex ones need to be balanced by digital signal processor. Speakers convert electrical signals into acoustic signals, while batteries provide power for electronic components


at present, there are mainly four types of hearing aids on the market, from large to small, including ear back type (BTE), inner ear type (ITE), ear canal type (ITC) and complete ear canal type (CIC). BTE is located behind the ear, and a hose is used to connect the ear mold in the ear to make sound. BTE also developed an open type (OTE) hearing aid. The ear mold is replaced by a small earplug, giving the human ear a more open feeling. Other variants include replacing the hose with a wire and moving the speaker from behind the ear to inside the ear. Ite puts the hearing aid into the outer ear and integrates it with the ear mold. This kind of hearing aid almost fills the outer ear and looks like a big piece. ITC fills the ear canal with hearing aids, which reduces the space occupied by the external ear, but it is still easy to be seen. CIC is the smallest of all types. The hearing aid is completely inserted into the ear canal and is almost invisible from the outside

back type (BTE), inner ear type (ITE), ear canal type (ITC) and full ear canal type (CIC) hearing aids, Starkey laboratories, Inc. licensed photos

technological evolution

hearing aids are basically divided into two types technically, analog hearing aids and digital hearing aids. The first is the analog hearing aid, which only processes electroacoustic signals in the analog domain, while the recently born digital hearing aid processes electroacoustic signals in the digital domain. The earliest analog hearing aids amplified both speech and noise, and they needed to be customized after testing the sensitivity of patients to specific frequencies. Later, some analog hearing aids can be programmed during the trial wearing process, and other hearing aid wearers can use a button to choose several preset different frequency responses. Digital hearing aids can also be programmed during the trial wearing process, and have a variety of optional frequency responses for the wearer. The technology of digitizing sound makes it possible to reduce noise, filter and control sound feedback (self excitation). Due to the significant improvement of the performance and flexibility of digital hearing aids compared with analog hearing aids, most of the hearing aids sold at present are digital

functional block diagram of digital hearing aid,

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current hearing aids have many new features, including volume adjustment, remote control, polar coil, direct audio input, FM reception, Bluetooth, directional microphone, compression, clamping, frequency shift, wind noise management, data recording, self-learning, moisture prevention, and ear mold ventilation. Some features cannot be realized in the narrow space inside the hearing aid and need to occupy the external space, while others can be realized inside the hearing aid

volume adjustment can be achieved through the keys or wheels on the hearing aid, while using the remote control can avoid installing keys or wheels on the narrow hearing aid, but it can still control all functions of the hearing aid. The pickup coil can be used to replace the microphone. The pickup coil can help the caller improve the call quality by picking up magnetic signals in the old-fashioned machine that used electromagnetic coil speakers in the past. Today's or other listening devices have built-in pickup coil function and are specifically pointed out to be compatible with hearing aids. Direct audio input and FM reception are the other two ways to input signals for hearing aids. The first one needs to be connected by wire, and the other is actually FM radio. Another new trend is to integrate Bluetooth functions so that signals can be received from or music players. Bluetooth can be integrated into the hearing aid or connected to the hearing aid as an accessory through the pickup coil or FM

hearing aids with directional microphones have two or more microphones facing different directions to receive signals, which can improve the signal-to-noise ratio (SNR) of listening in noisy environments, and the use of digital signal processing technology can further improve the voice quality. Compression and clamping reduce the volume when the volume is too high to make the ear feel more comfortable, but sometimes it will limit the volume. Frequency shift technology moves speech to low frequency through digital signal processing, which is very helpful for those with high-frequency hearing impairment. Wind noise management can detect wind noise and eliminate its feedback to prevent hearing aid wearers from hearing screams

the data recording function records the listening environment and how the hearing aid is used. Hearing experts can fine tune the hearing aid to improve its performance based on this information. The self-learning function can use the recorded data to automatically fine tune the hearing aid to improve performance. Moisture proof function can reduce the repair rate caused by humid environment. Ear mold ventilation can improve the comfort of human ears when wearing ear mold hearing aids

General requirements

the key components of hearing aids are located in the audio processing channel. It mainly includes one or more microphones, speakers, preamplifiers (if necessary) and Speaker Amplifiers. Compared with class AB power amplifier, class D power amplifier has low power consumption, small distortion and small size, so it is mostly used in today's hearing aids. In addition, whether the audio bandwidth is 20kHz or limited to 8kHz, the audio codec should have high SNR to ensure high playback sound quality

the core of digital hearing aids is digital signal processor (DSP), which is the key to realize all the advantages of digital hearing aids. Different manufacturers use different DSP technologies, but they usually include frequency division compression/amplification, positive feedback subtraction, noise reduction, speech enhancement, etc. DSP can also process direction information and adjust the characteristics of hearing aids to make them suitable for different wearers

power supply and battery management

some hearing aids have begun to use rechargeable single lithium ion (li+) batteries, but most of them still use disposable zinc air batteries. At present, five sizes of zinc air batteries are mainly used according to the type and size of hearing aids, circuit power consumption and requirements for service time. Table 1 compares the volume and size of the five most commonly used zinc air battery capacity measurement and control systems using high-precision digital servo valves, as well as their color codes and which hearing aids are usually used

the starting voltage of zinc air battery is 1.4V to solve and break through the long-standing conflicts and obstacles between sustainable development and wide-ranging production in the industry, and the ending voltage when the battery needs to be replaced is 1.0V or lower. If it is used for 16 hours per day, the battery life is between days and weeks, depending on the battery capacity and the design of hearing aids. The design with the highest battery efficiency directly supplies power from a single battery, and then uses the switch regulator boost circuit to raise the voltage to drive the 1.8V or 3.0V hearing aid circuit. The ideal power consumption of zinc air battery is between 1MW and 10MW. Hearing aids using rechargeable li+ batteries may require a linear or switching regulator to fully charge a 4.2V single li+ battery and drop it to a working voltage acceptable to the circuit. The battery charging voltage can also be limited to 3.3V, depending on the needs of the circuit. In order to prevent hearing impaired patients from running out of battery power when they need hearing aids, an accurate coulometer is needed to make a low battery warning in advance

table 1 Zinc air battery comparison

electrostatic protection

all hearing aids must meet IEC electrostatic protection (ESD) requirements. Using components with built-in electrostatic protection or adding ESD protectors to the leaked wiring can help meet these requirements. (end)

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