What is hearing aid ear mold

Ear mold is an English word composed of two English words, “ear” and “mold” (sometimes written as mouse), which can be directly translated into the meaning of ear model. According to the Chinese English Dictionary of audiology, its complete definition is “coupler formed to fit into the auricle that channels sound from the earhook of a hearing aid into the ear canal.” In the dictionary, I translated it as “a coupling device that fits the auricle in size and shape and leads sound from the ear hook of the hearing aid into the ear canal”. This is a technical term. In fact, the ear mold is a customized earplug connecting the hearing aid and the ear However, the role of ear mold cannot be ignored. The quality and design of ear mold customization directly affect the use of hearing aids, especially for high-power analog hearing aids.

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The history of earmold can be traced back to the early 20th century, but from the literature, it should start in the 1920s of the 20th century. Western Electronics laboratories applied for the first patent on ear mold around 1920, but the ear mold at that time was not customized. Like the standard earplug used today, the customized ear mold should have been invented by a dental student in New York at that time. Since then, ear mold began to develop rapidly. In 1962, the National Association of earmold laboratories (nael) was established in the United States. Its main responsibility is to standardize the definition of ear mold components, materials and shapes. By 1970, all the basic technical indicators of ear mold used today had been standardized and unified.

In terms of the function of the ear module, it is a sound transmission system that connects the ear back type or box type hearing aid to the human ear. From the point of view of the specifications of ear molds, traditionally, ear molds include ear molds and acoustic tubes. Now, academic circles tend to include ear hooks of hearing aids in this system. For example, the “pediatric amplification guidelines” issued by the American hearing society in 2003 refers to ear hooks and acoustic tubes as “sound channel”, which is called “sound channel” in English The meaning of the paper is “the sound channel concerns of the earhook and tube that leads through the earmold and sends sound into the ear canal”. Obviously, the material making and design of every part of the whole sound transmission system of the ear mold will affect the sound amplification effect. Most problems in the use of the ear back machine are related to this, such as howling, insufficient gain, unclear, wearing pain, discomfort, etc., which are caused by the improper principle and use of the ear mold.

Ear mold is generally made of two materials, namely hard ear mold and soft ear mold. Hard ear mold is made of special chemical materials, adding vinyl acid or poly vinyl acid, which is harmless to human body; while soft ear mold is made of silica gel, which are both certified harmless to human body. The manufacturing process of the hard ear mold is simple, but it is uncomfortable to wear; the soft ear mold is comfortable to wear, easy to insert into each part of the ear canal, with good sealing type, suitable for high-power hearing aids. The disadvantage of soft ear mold is that it is not easy to modify. Generally speaking, the use of ear mold materials can be based on the characteristics of human skin, age and regional, for example, the south is humid and hot, the soft ear mold is easy to sweat, and the north is relatively better.

The ear mold is very important for sound transmission. It involves quality problems such as technical manufacturing and ear mold design. For example, the quality of ear mold may be reduced due to improper sampling of ear mold, or the lack of technology and experience of technicians during processing, which will eventually affect the use of hearing aids. Therefore, the key to improve the quality of ear mold is to understand the acoustic characteristics of ear mold and to design it scientifically. In addition, it should be noted that the part of the ear mold inserted into the ear canal, known as the ear mold sound channel mouth, whose length, thickness and a series of technical processing involve complex physical acoustic and mathematical principles, such as the size of the ear mold, the length of the ear mold sound channel, and the diameter of the opening of the ear mold sound channel directly affect the acoustic performance of the sound transmission process. Generally speaking, the inner diameter of the middle length ear mode channel is 3 mm and the length is 18 mm. In the transmission, the sound quality and frequency response can be kept flat until 3400 Hz. If the channel is longer than 23.4 mm, the high frequency may be affected. Sometimes this length can lead to an increase or decrease of several decibels.

At present, most of the patients with hearing impairment in China use high gain and high-power hearing aids, and general ear models should be used. However, in practice, we often find that many patients use framework or other types of ear models, resulting in the volume switch of hearing aids can only be maintained at 1 / 4 on-off position, greatly reducing the actual use power of hearing aids, affecting the normal hearing training of users 。

In clinic, the influence of acoustic tube on the process of sound transmission is mainly its inner diameter, thickness and length of the tube wall. When the diameter of the tube is smaller, the power of sound transmission can be saturated, and the gain can be reduced. The smaller the inner diameter of the acoustic tube is, the closer the frequency peak wave is to the low frequency; otherwise, the closer it is to the high frequency. The length of the acoustic tube itself also has an impact on sound transmission. The shorter the acoustic tube is, the higher the frequency amplification peak wave is. If the length of the acoustic tube is reduced from 55 mm to 30 mm, the peak wave can be increased from 850 Hz to 1100 Hz. As early as 1979, the parameters of the sound tube have reached the standardization, which can be divided into 9-16 sound tubes with different labels. Generally speaking, the smaller the number is, the larger the inner and outer diameter is. For example, the inner diameter of 9 sound tube is 3 mm, while that of 16 sound tube is only 1.35 mm. The commonly used acoustic tubes are No. 13 standard type and thickened type, with inner diameter of 1.93mm, but the outer diameter is different. The outer diameter of thickened type is 0.35mm thicker than that of standard type. In terms of acoustics, the sound transmission effect of No. 13 is better than that of other models. Its peak value is 1000 Hz, and the sound tube with small inner diameter is not conducive to high gain and high power hearing aids. One is that it may generate sound feedback, and the other is to reduce the output power.

Another problem that affects the effect of the ear mold is that the acoustic feedback is related to the fact that the acoustic tube does not connect multiple sections of the acoustic tube according to the correct method. From the ear mold to the ear hook, the acoustic tube is connected by several plastic tubes with different diameters. This kind of connection is not wrong. Lybarqer (1972) did a lot of research on it as early as the 1970s, and invented the libar acoustic tube named after it, that is, using two different diameter acoustic tubes to improve the frequency response curve of the eyeglass hearing aid can increase 5-7 dB. However, the diameter of the two sections of acoustic tubes must be made and connected according to the specified size, rather than at will. After that, killiam (1981) developed a variety of dual acoustic tubes with different inner diameters, and confirmed the different effects of these channels on the acoustic characteristics of the ear model through experiments, which can help clinical audiologists adjust the hearing aids according to the needs of patients and achieve the best effect.

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