A hearing aid is essentially an electro-acoustic amplifier. The acoustic signal is converted into an electrical signal by a microphone (microphone), and after being amplified by the amplifier, the electrical signal is restored to an acoustic signal by the receiver and transmitted to the human ear. Hearing aids are mainly composed of microphones, amplifiers, receivers, batteries, various volume and tone control knobs and other components. Hearing aids can be divided into box-type hearing aids, behind-the-ear hearing aids, in-ear hearing aids, glasses-type hearing aids and bone conduction hearing aids. Today, we first talk about the microphone, receiver and battery in the hearing aid components.
Microphone noise is composed of equipment noise and output noise. The microphone is the main component that generates the local noise of the hearing aid. The higher the sensitivity of the microphone is, the more difficult it is to achieve low noise.
Defects of the microphone:
1. It is easy to corrode. The microphone is a precision accessory, and it is easily corroded by long-term exposure to moisture, especially sweat;
2. Sensitive to vibration, such as the sound of rubbing hearing aids, direct body vibration (running on a hard surface), impact sound, etc. These vibrations will become a louder sound after being amplified;
3. It is susceptible to wind noise. When the wind hits the head, auricle or hearing aid, the patient may hear annoying low and medium frequency wind noise;
4. The frequency response range is easily affected. When the hearing aid collides with a hard object (such as falling on the ground, etc.), the distance between the microphone and the sound inlet will be changed, which will reduce the frequency response range of the hearing aid.
In order to solve the problem of the signal-to-noise ratio of hearing aids in noisy environments, directional microphone technology was born. Directional microphones are divided into omnidirectional microphones and adaptive directional microphones. The use of directional microphones enables hearing aids to amplify useful information and suppress noise, thereby improving speech intelligibility in noisy environments. Dual-microphone hearing aids have better noise reduction effects than single-microphone hearing aids.
The receiver is a device that converts electrical energy into sound energy. The frequency response of the BTE hearing aid receiver has three resonance peaks at 1KHz, 3KHz, and 5KHz. These resonance peaks are mainly caused by the microphone tube, including the short tube in the hearing aid, the ear hook and the flexible ear mold connected to the ear mold. Pipe: The frequency response of the ear canal hearing aid receiver has only two resonant peaks, one at 2.2~3KHZ and one at 5KHz. The different resonance frequencies of hearing aids can be adjusted by using different types of receivers.
Since the frequency response width of the amplifier and microphone of the current hearing aid can meet the requirements of 200Hz~8KHz, the frequency response width of the hearing aid mainly depends on the frequency response width of the receiver.
The receiver is the main component that produces the harmonic distortion of the hearing aid. As the output sound pressure level increases, the degree of distortion also increases, that is, the greater the loudness of the hearing aid, the greater the distortion. This is also the reason why many patients who wear extra-high-power hearing aids all year round often do not speak clearly, which is the “big tongue”.
The narrower the bandwidth of the receiver, the less likely it is for the hearing aid to produce acoustic feedback, so many hearing aids will deliberately limit the output bandwidth of the receiver, and the effective output bandwidth is usually 0.2~5.2KHz. With the development of digital hearing aid technology in recent years As a result, hearing aids with “super bandwidth” began to appear. Experiments have shown that hearing aid users with wider high-frequency compensation have higher resolution of consonants, so they believe that hearing aid users with a high-frequency compensation of 8KHz have better speech comprehension. But how to achieve the coexistence of super bandwidth and anti-feedback whistling, the technology needs to be improved.
At present, the most widely used in hearing aids is the zinc-air battery, which generates a 1.4V voltage through a long-lasting chemical reaction through oxygen in the air. Hearing aid batteries can be divided into four categories: A675, A13, A312, A10 batteries. Each battery can be used for a different length of time. A10 battery can be used for the shortest time, basically about 5 days; A312 battery can be used for 7-10 days, A13 battery can be used for 12-15 days, and A675 battery can be used for the longest time, which can be used for 20 days. -25 days; now there are some hearing aids that can be charged directly, using the matching charging base for charging, generally it can be used for 8-10 hours at a time.