How Hearing Aids Work
There are many types of hearing aids, but all electronic hearing aids work the same way.
A hearing aid first converts an acoustic signal into an electrical signal, amplifies the electrical signal, and then converts it into an acoustic signal, thereby amplifying the sound. In the process of energy conversion, it is the microphone and the receiver that realize the function of the transducer.
First, a microphone is an input transducer that converts sound energy into electrical energy.
Second, the amplifier amplifies the weak voltage converted by the microphone.
Third, the receiver is another transducer, just opposite to the microphone, it converts the amplified electrical signal into an acoustic signal or mechanical vibration and transmits it to the ear canal. The receiver converted into acoustic signal is an air conduction receiver, and the receiver converted into mechanical vibration is a bone conduction receiver.
Fourth, the volume control is a variable resistor or potentiometer, used to adjust the current through the amplifier, the volume changes with the resistance of the electrical signal. Turning the volume up requires more current; turning it down reduces the current through the amplifier, making the sound softer.
5. Fine-tuning potentiometer In programmable hearing aids, various fine-tuning adjustments are carried out through computer programming, making the adjustment more precise and accurate, and can more finely compensate for hearing loss, including:
1. Tone control, change the frequency response of the hearing aid;
2. Peak clipping, can control the maximum output of the hearing aid;
3. Automatic gain compression control, control the sound at a comfortable level Within the loudness range;
4. Gain Control (GC): Adjust the hearing aid gain.
6. Batteries Generally speaking, the greater the gain and output of the hearing aid, the greater the battery energy required, and the larger the corresponding battery volume. If a battery has insufficient energy, it will limit the output sound pressure of the hearing aid.
The requirements for hearing aid batteries are: small size, constant voltage, reliable quality, long life, and harmless to the environment. Today’s hearing aid batteries are zinc-air batteries (button cells).
7. The accessories of hearing aids can include audio input and inductive coil:
1. Audio input: most hearing aids have contact sheets or jacks for audio input, which are mainly used for listening radio or watching TV. Because the audio signal comes directly from the sound source, without the conversion of sound-electricity, electricity-sound, the quality of the input signal is better than that of the signal converted by the microphone.
2. Inductive coil: The inductance is a magnetic induction coil, which can respond to the electromagnetic field leaked from the receiver on the telephone, convert it into an electrical signal and then amplify it, so that the hearing aid can be used to listen to the phone. Its advantages are that there will be no whistling, no interference, and a high signal-to-noise ratio in a noisy environment. The signal-to-noise ratio is the difference between the speech signal and the ambient noise. The higher the signal-to-noise ratio, the stronger the speech signal and easier to distinguish.
Eight, any hearing aid includes 6 basic structures:
1. The microphone (microphone or microphone) receives the sound and converts it into the form of radio waves, that is, converts the sound energy into electrical energy .
2. The amplifier amplifies the electrical signal (transistor amplifying circuit)
3. The earphone converts the electrical signal into an acoustic signal (ie, converts electrical energy into sound energy).
4. Insert the ear mold (ear plug) into the external auditory canal.
5. Volume control switch.
6. Dry battery for power supply for amplifier.
In addition to the above 6 parts, most models of hearing aids also have 3 accessories, or 3 additional circuits (tone control, induction coil, output limit control). A modern electronic hearing aid is an amplifier whose function is to increase the intensity of sound energy and deliver it into the ear as undistorted as possible. Since the sound energy of the sound cannot be directly amplified, it is necessary to convert it into an electrical signal, and then convert it into sound energy after amplification. The input transducer consists of a microphone (microphone or microphone), a magnetic induction coil and other parts. Its function is to convert the input sound energy into electrical energy and transmit it to the amplifier. The amplifier amplifies the input electrical signal, and then transmits it to the output transducer. The output transducer is composed of an earphone or a bone conduction vibrator, and its function is to convert the amplified signal from electrical energy into sound energy or kinetic energy for output. The power supply is an indispensable part for supplying the working energy of the hearing aid. In addition, there is a peak clipping (PC) or automatic gain control (AGC) device to suit the needs of patients with different degrees of deafness.
Hearing aidsMain technical indicators
To understand the acoustics of hearing aids, first of all The technical indicators of the characteristics are analyzed. The main technical indicators include gain, frequency response, maximum sound output, distortion, level of input noise and dynamic range, etc. These technical indicators can be measured by hearing aid analyzers.
1. Acoustic gain
The amplification rate of a hearing aid is expressed by gain, that is, the difference between the output sound pressure level of the hearing aid earphone and the input sound pressure level of the microphone . For example: input 60dB output 130dB, gain = 130-60 = 70dB. Gain will change with the volume control.
Second, frequency response (ferquencyrange)
The relationship curve of the change of the output gain of the hearing aid with the change of the frequency of the input signal is called the frequency response curve. This frequency response curve is not based on maximum gain. If the ordinate is changed to the output sound pressure level, the resulting curve is the frequency response curve. The hearing range of the human ear is 20-20000Hz, and the speech frequency range is 500-2000Hz. Experiments have shown that low frequencies mainly provide the energy of language, and high-frequency hearing compensation is of great significance to the clarity of language, so hearing aids range from 250-4000Hz. The gain value of the sound curve is very important for the selection of hearing aids.
Three, the maximum sound output (output sateretion sound pressure level)
When the external signal gradually increases from 60dB to 90dB input, the output signal, but When the input signal is greater than or equal to 90dB, the output signal is no longer correspondingly increased, and the output at this time is the maximum sound output. Acoustic output = input + gain.
Dynamic range (dynamic range)
The dynamic range is the difference between the maximum output of the hearing aid and the gain. The dynamic range can be changed with the adjustment of the volume control.
V. Total hamonic distortion
When the external sound is amplified by the hearing aid, except for the amplification of the amplitude, any other noise on the The change ratio is distortion. For example, harmonic distortion is the most common (eg 5-17). Cassette hearing aids should be ≤5%.
6. Equivalent input noise level
When the input signal is 0, the inherent noise output of the machine is called equivalent noise , it is required to be below 30dB, the smaller the value, the better.
7. Induction pick-up coil sensitivity
When the input field strength is 10Ma/mfeild, the output sound pressure level of the hearing aid will be higher. larger, the higher the sensitivity.
Eight, Acoustic Feedback
In an amplification loop, the amplified sound is picked up by the microphone and amplified again, and the screaming sound is called feedback. Acoustic feedback is a common phenomenon in common audio amplifier systems. If the amount of acoustic feedback at a certain frequency reaches a certain level, the circuit will become an oscillation circuit for the frequency signal, the hearing aid will generate a strong oscillation signal, and the external signal will be “submerged” in the oscillation signal. That’s why hearing aids tend to scream. Different hearing aid peaks have different frequencies and produce different levels of feedback.
This process is a single loop between the source of the sound leakage and the microphone. The amplified sound escape is picked up by the microphone and then amplified. In low- and medium-gain hearing aids, the leaked sound has lost a lot of energy by the time it reaches the microphone, so it doesn’t cause a big problem. High-strength signals from high-power hearing aids can reach the microphone to form a re-amplification loop process. Repeatedly, a signal is amplified again and again to produce audio oscillation, that is, howling. At this time, the hearing aid is already in a useless state, especially users of high-power hearing aids are often disturbed by this annoying harsh feedback, and have to turn down the volume of the hearing aid, so that they cannot get greater audio gain. If the system has a fairly flat frequency response and lacks formants, acoustic feedback is unlikely to occur. No such system exists in hearing aids, however, and there are always peaking and feedback issues.
The length and diameter of the in-ear or spectacle hearing aid tube can affect the formants of the system, increasing or decreasing the amplitude and frequency of oscillations and the pitch of the feedback. To some extent, the length and diameter of the microphone tube determines the presence of silent feedback.
Acoustic feedback can come from sound propagation through the air between the receiver and the ear mold coupled to the microphone. This type of feedback can occur in the following situations: ① the amplified sound leaks directly from the inner side of the ear mold to the outer side; ② the receiver and microphone sound insulation is not enough.
