What are Assistive Listening Devices?

The hearing aid is a small loudspeaker that amplifies the sound that could not be heard, and then uses the residual hearing of the hearing impaired, so that the sound can be sent to the auditory center of the brain, and the sound is felt.

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Sound is generated by the vibration of an object. The object that is making a sound is called a sound source. Sound is just the movement of pressure waves through the air. Pressure waves vibrate the small bones (listening bones) of the inner ear. These vibrations are transformed into tiny electronic brain waves, which are the sounds we perceive. The inner ear uses the same principle as a microphone capturing sound waves or speaker sounds. It is the relationship between moving mechanical parts and pressure waves. Naturally, when the sonic tone is low, moving slowly, and large enough, we can actually "feel" that the air pressure waves vibrate the body.

Since the beginning of the 20th century, electronic hearing aids have been used, the structure of the hearing aid has not changed much, and it is still composed of a microphone, an amplifier, a receiver, and a power supply. However, the volume of each part of the hearing aid is gradually reduced, the sound quality is gradually improved, and there is more selective control.

1) Microphone: collects sound and converts it into radio waves.

2) Amplifier: Increase the intensity of the radio wave.

3) Receiver: Just the opposite of the microphone, turn the increased power back into sound waves.

4) Power: Small battery.

5) Shell: protects the internal structure.

Hearing aids are instruments that help to improve hearing. In fact, they are small semiconductor amplifiers. Their function can make relatively weak sounds to be amplified and transmitted to the earphones, so that the original hearing-degraded part can hear the sound through amplification.

Currently, there are the following types, but their functions and principles are the same.

(1) From the external structure, it can be divided into: box type, back-ear type, in-ear type, and in-ear type can be divided into ordinary in-ear type, ear canal type, and deep ear canal type.

(2) From the electronic principle, it can be divided into: analog hearing aids; programmable hearing aids, all digital hearing aids, wide dynamic language technology hearing aids, etc.

(3) Hearing aids are divided into air conduction hearing aids and bone conduction hearing aids according to the conduction mode. Box-type, behind-the-ear, and in-ear types are all air-guided hearing aids.

Cassette hearing aids are also called body-worn or pocket hearing aids. The shape is a rectangular box the size of a small radio. The microphone, amplifier and battery of the hearing aid are assembled in it, and a long wire is connected to the earphone and earplugs or a special ear mold. Usually placed in a clothes pocket or special pouch. Such hearing aids are large in size and suitable for use by the elderly and people with inconvenient finger movements. The distance between the microphone (microphone) and the receiver (headphone) of the hearing aid is not easy to produce acoustic feedback, so its maximum output limit is small, the power can be made large, and multiple manual adjustment knobs can be placed. Its low price and easy maintenance. It can use 5 or 7 batteries, and can also use rechargeable batteries. Body-worn hearing aids can also be made into binaural hearing aids, because such hearing aids are bulky and have poor concealment. Hearing aids use ordinary transistor components, which have a high background noise. In addition, the hearing aid itself and the friction between the wire and the clothes make the sound easily distorted and the sound quality reduced. At the same time, the earphone lead wire is easily damaged, which is not safe for children.

The in-the-ear hearing aid is a micro-shaped hearing aid, and the shell is customized according to the patient's ear. Amplifiers, microphones, and receivers are all housed in custom housings, all of which fit into the ear cavity. No external wires or hoses are required, which is more concealed and lighter. At present, medium-power, medium-power, and high-power in-the-ear hearing aids have come out. Because they need to be customized according to the individual ear cavity, the price is higher. Generally, middle-aged and young people use more, and those who are prone to sweat on their heads wear ear-hearing hearing aids that are susceptible to moisture damage. In-ear hearing aids can be considered. Deaf children in the development stage are not suitable because of the development of auricle wore.

Deep ear canal hearing aids are currently the smallest and most concealed hearing aids. Customized according to the ear canal of the user, the sound amplification performance is closer to that of a normal person. It is placed deep in the ear canal and is almost invisible on the appearance, which greatly meets the user's requirements for his appearance and image. In addition to the advantages of ordinary ear canal hearing aids, more importantly, the deep ear canal hearing aids are placed in the ear canal, which can make full use of the structure of the human ear canal to enhance the sound, especially the high-frequency part, and make the hearing more clear and natural. Because the sound output port of the hearing aid is close to the eardrum of the human ear, the sound output by the hearing aid can immediately reach the eardrum, so that the output gain and waveform are not attenuated, and sufficient sound energy and sound quality fidelity are guaranteed. The use range is narrow, only suitable for people with mild to moderate deafness. Hearing loss is best within 65dB. Children's ear canals are immature and unsuitable for wearing.

Behind-the-ear hearing aids are suitable for all types of hearing loss and are a type of hearing aid widely used at present. Microphones, amplifiers, batteries, volume adjustments, receivers, etc. are all housed in small hook-shaped boxes, slim in shape, relying on a hard plastic earhook bent into a semi-circular shape to hang behind the ear, and the shell can borrow skin Or the color of the hair can be masked, and the amplified sound is passed through the ear hook through a plastic tube into the sound hole of the ear mold. Divided into different types of low power, medium power, high power, high power, low noise, small distortion, easy to wear. The disadvantage is that the hearing aid will be wet due to sweating of the user, which accelerates the aging of the components.

The ear canal hearing aid is suitable for patients with mild to moderate deafness and is a high-precision personal hearing aid. The hearing aid housing is specially designed and manufactured according to the shape of the patient's ear, which is very snug and tight when worn, ensuring the sound tightness and comfortable wearing when used; the ear canal hearing aid is smaller than the in-ear hearing aid and has no external leads, which can maximize To a limited extent imitate the process of sound transmission by the human ear. Because the ear canal hearing aid is placed in the ear canal, the wearing is hidden, and it is not easy to be noticed, which meets the user's image requirements for wearing the hearing aid. Children's ear canals are immature and unsuitable for wearing; ear canal deformities, stenosis and ulcers should not be worn.

Under normal conditions, we listen to sound in at least two ways: bone conduction and air conduction. The so-called bone conduction method is that the sound signal vibrates the skull and is not directly transmitted to the inner ear through the outer ear and the middle ear. The air conduction method refers to the sound transmitted to the inner ear through the outer ear and the middle ear. The latter of these two methods has the absolute advantage.

Under normal circumstances, what we call the box type, behind the ear type and in the ear type are all air-conducting hearing aids. Air-guided hearing aids transmit amplified sound signals into the ear canal. Bone conduction hearing aids are different. Strictly speaking, what the bone conduction hearing aid produces is not a sound signal but a vibration signal. Bone conduction hearing aids do not have so-called "headphones" or earplugs. Instead, they are a vibrator capable of generating vibration signals. The vibrator is pressed against the protruding mastoid bone behind the ear, and the vibration of the vibrator will cause the skull to oscillate and signal across the outer and middle ears-the advantage of bone conduction hearing aids is passed directly to the inner ear . Due to this advantage, in theory, bone conduction hearing aids are particularly suitable for patients with conductive deafness (that is, patients with problems in the outer and middle ears). However, this is not the case. Bone conduction hearing aids are far less effective than imagined. The reasons are as follows: first, the bone conduction hearing method is far less effective than the air conduction method; second, the output device of the bone conduction hearing aid-the oscillator, is not only extremely unsightly, but also extremely uncomfortable and difficult to be accepted by the user.

Generally speaking, as long as air conduction hearing aids can be used, air conduction hearing aids should be selected first. Patients who can't wear earplugs, ear molds, or in-ear hearing aids after external ear canal deformities or external and middle ear infections have frequent pus discharge. Consider using bone conduction hearing aids.

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