The main components of a pocket hearing aid are a microphone, amplifier, and battery in a box. A small wire connects the body to the earphone (or a headphone attached to the ear mold).
Basic Structure of Pocket Hearing Aid
From the perspective of the external structure of the box-type hearing aid, it mainly consists of three main parts: the earphone, the lead and the body (take the JINGHAO box-type hearing aid as an example)


B- volume control knob
C-battery compartment
D-NH trim
E-MPO regulation
F Receiver connection socket
G-audio input socket
H-L plug
I-receiver connection
J-headphone plug
L-headphone connector

Figure 3-1 Structure of a cassette hearing aid

1. body
The external structure of the pocket hearing aid body is mainly composed of a switch, a volume control knob, a battery compartment, and various audio control knobs.
The interior is mainly composed of a microphone and an amplifier, and various hearing aid fine-tuning components.
(1) Switch
Common switches for cassette hearing aids are:
N-H-O switch. When the indicator knob points to the “O” position, the hearing aid is turned off; when it points to the “N” position, the hearing aid starts to work normally; when it points to the “H” position, the low-frequency attenuation function is enabled to reduce environmental noise to improve the use effect in a noisy environment.
M-T-O switch. The “O” position of the knob is equivalent to the function of the “O” position of the NHO switch; the “M” position of the knob has the same function as the “N” position of the NHO switch; a hearing aid with a T-position switch indicates that the hearing aid is equipped with an electric induction coil, which can Use it to answer a call with an electromagnetic induction coil, but it should be noted that when the switch is in the T position, the phone receiver needs to be placed on the hearing aid casing to enhance the answering effect.
(2) Volume control knob
Generally, the volume of the hearing aid volume control knob has a number indicating the size of the sound. The JINGHAO box hearing aid is indicated by 25 characters with different numbers from 1 to 9. The Danish Resound box hearing aid is generally 3, 6, 9, or 2, 4, 6. Numbers of 8, 8, and 10 indicate that small numbers indicate low volume and large numbers indicate high volume. In terms of regulation, it is generally believed that the volume control knob can be adjusted to the maximum position as indicated by the mark, but often when the position of the hearing aid volume control knob exceeds 2/3, the amplified sound will be distorted, resulting in speech that may be affected. When the intelligibility declines, sometimes the howling phenomenon will occur, which will affect the hearing aid effect. Therefore, when selecting a patient, if the hearing aid volume control knob exceeds 2/3 position and still cannot meet the hearing aid requirements, it should usually be considered. Replace the optional hearing aid with a higher power to improve the hearing effect.
(3) Fine tuning of hearing aids
Common trims in cassette hearing aids include high-frequency trim (NH trim), output-type automatic gain compression control trim (AGC-O trim), and so on.
The main function of NH trim is to reduce the low frequency during the hearing aid adjustment, and to retain the high frequency part of the gain. Patients with sensorineural hearing loss often experience high-frequency hearing loss, resulting in reduced speech intelligibility. This requires that the high-frequency gain be appropriately increased when the hearing aid compensates the hearing impaired. At this time, this button can be used. Make adjustments. The adjustment can be performed according to the hearing curve of the hearing impaired. For a flat hearing curve, the adjustment position can be placed in the middle of NH. Downhill hearing loss can point the knob to the high-frequency H end, and uphill hearing loss can point to the N position. It can also be adjusted according to the actual requirements of hearing impaired patients. If the hearing aid sounds noisy or confusing, adjust this button. Jinghao cassette hearing aids (see Figure 3-2) 176AO and 178PP-AO are adjusted counterclockwise for high-frequency adjustment. Resound cassette hearing aids commonly use the following (see Figure 3-3) marking knob, counterclockwise to the H position , The highest audio.

AGC-O trimming, also known as output-type automatic gain compression control, is a device for automatically adjusting the hearing aid gain of a pocket hearing aid, which can protect hearing. If the sound of the hearing aid is too noisy or the volume of the hearing aid needs to be adjusted frequently, slowly turn the knob to adjust the hearing aid (When the Jinghao pocket type hearing aid is adjusted counterclockwise, the output will change from large to small, see Figure 3-4) until you feel comfortable. This function can automatically adjust the maximum sound pressure level of the cassette hearing aid output.

Figure 3 2 Jinghao cassette hearing aid NH fine-tuning


Figure 3-3 AAC fine tuning for hearing aid 107-6PP

Figure 3-4 Siemens 176AO hearing aid AGC-O fine-tuning

    (4) Audio input socket
Siemens 176AO and 178PP-AO hearing aids have audio input sockets, which can be connected to external sound sources such as televisions, radios, and recorders via wires. Because the audio signals come directly from the sound source, there is no sound-electricity, electricity-acoustic conversion, so the input The signal quality is better than the signal converted by the microphone. You can hear clearer sound directly without interference from background noise.
2. wire
The wire is composed of a body plug, a connection line and a headphone plug, and is a transmission path for sound and electric signals after being amplified by a microphone and an amplifier. Generally, the body plug is L-shaped, and the body plug and the headphone plug core are of different sizes and diameters. When connecting, pay attention to inserting the plug corresponding to the size of the hearing aid body and the headphone jack. At present, there are two types of two-core and three-core types of cassette hearing aids. Most cassette hearing aids use two-core wires. Some super-powered cassette hearing aids such as Siemens 178PP-AO and AAC 107-6PP use three-core wires. The hearing aid wire itself is relatively thin. In addition to its own use characteristics, it often causes friction or even traction with other objects, such as clothing, which breaks or has poor contact at the connection, resulting in silence or intermittent sound in the hearing aid box. Continued phenomenon, at this time you should go to the hearing center or designated place for update.

3. Earplugs
The earplug is a key part that converts sound electrical signals into sound signals, and is mainly composed of earplugs, a headphone connector, and a receiver.
(1) Earplug
Earplugs are a type of plug used to connect the earphones to the external ear canal. The shape of the earplugs is generally mushroom-shaped, the texture is soft, and there are sound holes in them. They are usually made of silicone. Generally, the color and shape of the earplugs produced by various hearing aid manufacturers can be slightly different, but their diameter can be basically divided into three types: large, medium and small, to suit different sizes of ear canals for the hearing impaired.
(2) Headphone connector
The earphone connector is white and transparent, and plays the role of connecting the earphone to the earphone. When using, the connector should be fixed to the earphone end before connecting with the earphone.
(3) Receiver
The receiver is an electro-acoustic conversion part of the pocket hearing aid. Its main function is to convert the amplified electric signal into an acoustic signal and transmit it through the earplugs.
The above mainly introduces the main structure and simple use method of the box hearing aid. The box hearing aid is by far the most powerful hearing aid. Therefore, when some super-power behind-the-ear hearing aids cannot meet the hearing compensation needs of hearing impaired patients, the box hearing aid is still Can play a role. When the pocket hearing aid is selected, in order to further improve its hearing aid effect, an ear mold can be customized at the optional hearing center.
3.1.2 Advantages of Hearing Aid
Hearing aids include:
① High power. Compared with other types of hearing aids, cassette hearing aids are the most powerful hearing aids. Because of their large size, such hearing aids are more suitable for people with severe deafness and elderly with inflexible hand movements.
② The feedback is small. Hearing aid earphones are far away from the microphone that receives the sound, which makes it difficult to generate feedback.
③ Easy to adjust. Its switch / volume potentiometer knob is large, easy to use and easy to adjust.
④ Battery saving. The battery it uses is usually an AA battery, which has a long life and is cheap.
⑤ The price is low. Hearing aids are low in cost, so they are inexpensive and have relatively low maintenance costs.
3.1.3 Disadvantages of Hearing Aids
Disadvantages of cassette hearing aids include:
1. Noise from friction with clothing
pocket hearing aids are mainly placed in the user’s pocket or worn on clothing. When a person is exercising, clothes will rub against the hearing aid to generate noise, which will affect the effective hearing of the deaf.
2. Increase unnecessary low frequency gain
Because the pocket hearing aid is placed on the human body, the human body is equivalent to a barrier, and external sound waves are transmitted to the body. The sound of the low-frequency part can be reflected to the microphone of the hearing aid, thereby increasing the low-frequency amplification of the hearing aid. For most deaf people, too much low-frequency gain is not helpful. One is that the residual hearing of most deaf people is better than low-frequency, so I hope hearing aids have better hearing compensation at high-frequency. The low-frequency part requires relatively little compensation. In addition, the low-frequency reflection gain effect of the pocket-type hearing aid will have an adverse masking effect on the high-frequency amplification of the hearing aid, thereby reducing the adaptability of the pocket hearing aid. Second, because most of the environmental noise belongs to the low-frequency range, the low-frequency body reflection gain of the pocket hearing aid will significantly amplify the environmental noise, which will make the signal-to-noise ratio of the pocket hearing aid smaller, and the user’s speech intelligibility decline.

3. Not beautiful
Most adult deaf people are reluctant to let others find themselves deaf, and are unwilling to choose a hearing aid for reasons of self-esteem. Deaf children’s self-esteem increases with age, and they wear pocket hearing aids with wires, making them obviously feel different from other children. Some children who are in good recovery, after entering ordinary elementary and middle schools, would rather not hear or wear hearing aids, which is inseparable from the appearance of the cassette hearing aids that impair their self-esteem.

4. Inconvenient to wear
Hearing aids must be worn on clothing. When they are too bulky, they are very inconvenient when the clothes are thin or they are involved in sports.
3.1.4 Adaptation range of cassette hearing aids
Hearing aids are generally more powerful, about 120-140dB. Because of its high power, low price, and large size, it is generally used for people with severe or severe hearing loss, the elderly, and people with relatively poor economic conditions. It is not suitable for people with mild to moderate hearing loss.