Hearing aid ear molds

What is Hearing aid ear molds?

Hearing aids are generally thought of as consisting of a microphone, an amplifier circuit, and a receiver. In fact, hearing aids also have an important component-earmolds. An ear mold is a device that transmits output sound from the earpiece of a hearing aid to the external auditory canal or eardrum.
The acoustic plug-in is part of a complete hearing aid system.

Why use ear molds

Before the emergence of ear molds, hearing aids were connected to the ear canal with different size ear plugs, and different size ear canals used different size ear plugs. However, because the earplugs and the ear canal are not tightly matched, when the volume of the hearing aid is large, the output sound of the hearing aid will leak to the microphone through the external ear canal, and acoustic feedback will occur. In this way, hearing aids cannot meet the needs of patients with severe and very severe deafness.
In addition, the earplugs are easy to fall off, which is not good for hearing aid fixation, especially for behind-the-ear hearing aids. Based on this, ear molds are produced, which are also two functions of ear molds.

Ear mold concept
Ear molds are acoustic inserts made according to the shape of the ear cavity and external auditory canal of the hearing aid user. It is placed in the ear cavity or ear canal, and is fixed in the ear using anatomical structures such as the ear wheel and the opposite ear wheel, the tragus and the opposite tragus, and the external ear canal.
The ear mold is an acoustic plug-in, which is part of the hearing aid system. It transmits the amplified sound of the hearing aid from the receiver of the hearing aid to the external ear canal or tympanic membrane. For in-ear, ear canal, and other customized hearing aids, the shell of the hearing aid is equivalent to an ear mold.
Standard ear mold as shown. This ear mold is a plug-type ear mold with a sound hole diameter of 2mm and no vent holes. Due to its better sealing in the ear, it is suitable for severe and very severe deafness. There is a small metal ring or a small ring of thermoplastic rubber on the outer end of the ear mold sound hole. The earphone of the box-type hearing aid is buckled on this ring, and the sound can be transmitted to the external ear canal through the sound hole. Therefore, standard ear molds are suitable for high-powered cassette hearing aids.

Standard ear mold composition
A-ear wheel B-ear screen
C-cut between the tragusD- pair of tragus
E-outer ear F-ear wheel

Figure 4-2 Anatomy of the auricle
Figure 4-3 Ear mold position

Hearing aid ear molds
With the development of hearing aid technology, the shape, material and manufacturing process of ear molds have changed greatly. People have gradually deepened their understanding of the acoustic characteristics of ear molds. Improve the sound quality after hearing aid, this
It is also the third role of the ear mold.
In addition, standard ear molds have good sealing properties, which can cause discomfort due to the plugging effect in the ears, resulting in various types of ear molds. Because the shape of the shell of the in-ear and in-the-ear hearing aids will not change much, the shape change of the ear mold is mainly used for the back of the ear
Hearing aid ear mold.

Ear mold style
There are various styles of ear molds, we need to choose the ear mold style according to the patient’s hearing and ear shape.
1. Ear plug
The ear plugs can be divided into three types: standard, shell and skeleton.
(1) Standard
The standard type is suitable for high-power box-type hearing aids, the diameter of the sound hole is 2mm, and there is no air hole. Because of the good sealing of the external ear, it is suitable for severe and extremely deaf use, as shown in Figure 4-11.
(2) Shell
The shell type is the “standard type” in the ear molds of a hearing aid. The ear mold occupies the entire ear nail cavity and seals well with the external auditory canal. It is mainly suitable for severe and extremely deaf patients, which can reduce howling.
(3) Skeleton
For people with severe deafness, if you want to wear comfortably, you can use skeleton ear molds.
Due to the uncomfortable wearing of the shell ear molds and from the perspective of aesthetics, half-shell, ear canal, and ear canal ear molds can also be used for light to moderate patients, as shown in Figure 4-5.

Figure 4-4 Shell ear mold
(a) epiphysis (b) half-shell (c) ear canal (d) ear canal hook
Figure 4-5 Various skeleton ear molds
2. Non-plugging
For patients with mild deafness, non-plugging ear molds can also be used, such as signal-to-transmission (CROS), free-field, and Janssen, as shown in Figure 4-6. These ear molds are less used in China. First, because the Chinese pay more attention to aesthetics and mild ears.
Deaf patients basically use ear canal hearing aids when the ear canal allows it. Second, they are more complicated to make than ear plugs.
(a) CROSA (b) CROSB (c) CROSC (d) free-field
Figure 4-6 Various ear plugs

3． Special ear mould
Special types of earplugs include swimming earplugs and anti-noise earplugs, which are generally made of soft ear mold materials, as shown in Figure 4-7.

(a) Noise-proof earbuds (b) Swimming earbuds

Special type ear mold

4.1.4 Ear mold material
The materials used to make ear molds must be strong, non-toxic, and anti-allergic. Ear mold materials are mainly two kinds of hard materials and soft materials. Ear molds made of soft materials are comfortable to wear and have a good seal with the ear canal, making it less prone to howling. However, soft ear molds are prone to aging, have a short service life, and have poor breathability. In addition, they are more difficult to manufacture and repair than hard ear molds; they are mainly suitable for children, patients with severe and severe deafness. The connection between the sound guide tube and the soft earmould needs to be fixed with a metal ring to prevent weak adhesion.
The use of hard ear molds is more common. Ear molds made of hard materials have good durability and plasticity, can be easily made into various shapes, have easy acoustic characteristics, and have a long service life. But the sealing with the ear canal is poor, and the wearing comfort is not as good as the soft ear mold.
An ear mold made of a hard material in the front part of the ear canal and a soft material in the back part can take advantage of both, but because the connection between the soft and hard materials is easier to break, it is used less.
The ear molds are mostly colorless and pink. There are also various colored ear molds, as shown in Figure 4-8.

Various colored ear molds
4.1.5 Coupling system for ear molds
For ear molds to become plug-ins for hearing aid systems, ear hooks, sound pipes, and sound holes are required. If you want to improve the acoustics of hearing aids, you need to rely on the synergy of vent holes, dampers, and horn sound holes.
Acoustic signals entering the open ear canal will generate a resonance peak of about 2000 to 4000 Hz, which will result in a 10 to 15 dB gain at the intermediate frequency. The frequency of infant formants will be higher. However, when the ear mold is inserted into the external ear canal, the phenomenon of increasing the intermediate frequency gain will be reduced, which is called insertion loss.
This requires hearing aids to change the acoustic characteristics or ear molds to improve the additional hearing loss that results.

1. Ear hook
Ear hooks are devices that connect receivers and sound pipes. The behind-the-ear hearing aid products already include ear hooks, as shown in Figure 4-9. There are two types of ear hooks, soft and hard, which are crescent-shaped, and most of the connections with hearing aids are fixed and non-rotatable. However, this structure is easy to damage the ear hook when the hearing aid is worn and removed, because
Some hearing aid manufacturers use ear hooks that can rotate 360 ​​degrees.
Ear hooks can be divided into adult ear hooks and children’s ear hooks. In addition, the type of damper in the ear hooks determines the acoustic characteristics of the ear hooks.
Figure 4-9 Bent hearing aids already include ear hooks

2. Sound tube
The sound guide tube plays the role of connecting the ear mold and the ear hook, and the sound passes from the ear hook to the ear mold through the sound tube.
The length, aperture and wall thickness of the sound tube will affect its acoustic characteristics. Long sound pipes increase low-frequency gain and reduce high-frequency gain
The reduction of the aperture will increase the low-frequency gain and reduce the high-frequency gain; the thin tube wall is easy to cause howling.
The sound tube can be directly connected to the ear mold and plays the role of a sound hole. Soft ear molds generally use this connection method; they can also be connected to the ear mold through a horn adapter. In this method, the sound tube is connected to the ear mold. Strong connection, hard ear mold
This method is generally used (see Figure 4-10).
3． Sound hole
The sound hole is the sound hole in the ear canal of the ear mold. The diameter and shape of the sound hole will affect the acoustic characteristics, including the following aspects:
Direct connection horn adapter

Figure 4-10 Connection of sound guide tube
(l) The effect of the aperture diameter of the ear mold on the acoustic characteristics
Different calibers of ear molds have significant differences in deaf children’s high-frequency and low-frequency hearing compensation. An ear mold having a sound hole diameter of less than 2.0 mm is useful for amplifying low-frequency sounds, but not for amplifying high-frequency sounds. An ear mold with a sound hole diameter greater than or equal to 2.5 mm is advantageous
Amplify high-frequency sound, the difference between the two is about 15dB.
(2) Effect of the shape of the ear hole on the acoustic characteristics
The influence of the shape of the ear hole on the acoustic characteristics includes: ① parallel holes, which have the same amplification effect on high frequency, intermediate frequency and low frequency; ② horn sound holes, which have a horn effect, have a good effect on high frequency amplification.
4． Vent
Ventilation holes are small channels that open to the outside at the top of the ear mold or at the sound holes of the ear mold. The former are parallel vents, and the latter are called V-shaped vents because they form an acute angle with the channel (see Figure 4-11). The ear mold wearer, because the external ear canal is closed, thereby changing the acoustic structure of the external ear canal, will produce some side effects, these side effects are divided into sensory obstruction and acoustic obstruction. Feeling blocked is due to the external auditory canal being closed, such as bloating, foreign body sensation, discomfort and other feelings. The acoustic obstruction is mainly due to changes in the wearer’s pure tone bone conduction hearing threshold, including abnormal sound perception, mastication and swallowing. Specifically: 250Hz increases by about 15-25dB; 500Hz increases by about 10-20dB; 1000Hz increases by about 5-10dB; 2000dB is OdB. From this, we found that changes in bone conduction were mainly at low frequencies. As a result of the above two types of double blocking, ear mold wearers feel that the “sound quality” of the listening sound has changed significantly. If the ventilation hole is increased, the internal pressure of the external auditory canal is equal to the external pressure, which improves the feeling of poor ventilation and obstruction, and reduces the phenomenon of bone conduction and auditory changes caused by closed ear molds, so the patient consciously improves. Because when the diameter of the air hole is 1mm, it can have a significant effect on the frequency response of the hearing aid. In addition, a part of the sound pressure that reaches the eardrum after the hearing aid is amplified can be leaked out, because the air hole has low acoustic damping for low-frequency sound and high-frequency damping. Big so the leaked voice master
For the low frequency part, this reduces the masking of high frequencies by low frequencies.

Therefore, ear mold stomata are mostly used for people with mild, moderate, and moderately severe hearing loss, especially those with low-frequency hearing loss and downhill hearing loss. Ear mold stomata can be designed as parallel vents, V-shaped vents, external vents, and open ear molds depending on the hearing loss and the actual condition of the patient’s external ear canal. The diameter of the vent can also be changed.

(a) Parallel vents (b) V-shaped vents (c) External vents
Figure 4-11 Vent holes for ear molds of various shapes
5. Damper
Hearing loss is different, and the range and degree of audio compensation are different. Especially for deaf patients with revitalization, the hearing area is narrow, and it is difficult to choose suitable hearing aids. This kind of patients often feel that they can’t hear sound without hearing aids, and feel that the sounds are “piercing” or “ear-shaking”, especially for those with severe hearing loss. Due to the influence of acoustic feedback, sometimes the stomatal shape cannot be processed as desired. Moreover, the AGC or PC adjustment of the hearing aid itself cannot be achieved. What should we do? We can adjust it with the help of damping. Damping is mainly used to smooth the resonance peaks in the frequency range of 1000-3000Hz, and has basically no effect on other frequencies, and the harmonic distortion is small. It is a debugging method for selecting hearing aids for deaf patients with revitalization. The damper can be in the sound hole or in the earhook or the sound tube. The closer to the eardrum, the more obvious the damping effect is.
6. Effect of ear mold length on acoustic characteristics
Different degrees of deafness have different requirements for ear mold length. The long ear mold is generally suitable for severe deafness and extremely severe deafness. It reduces the volume of the external auditory canal and reaches the isthmus of the external auditory canal. In the case of short ear molds with a diameter of 2.5 mm in the case of sound holes, there is a good amplification effect on high frequencies. If the sound hole of the short ear mold is enlarged to a horn shape with a diameter of 3. Omm, the high-frequency amplification is more obvious, and the frequency response curve of 3000 to 4000 Hz is significantly shifted up. Due to the short ear mold’s closed outer ear canal, the ear Modulus is suitable for moderately deaf, moderately deaf and partially severe deaf.
7. The influence of the combination of the acoustic components of the ear mold on the acoustics
①Ten short ear molds with parallel sound holes. This type can attenuate low frequency, good high frequency characteristics, ventilation, and comfortable wearing. Suitable for light to moderate hearing loss.
②Y-shaped sound hole ten short ear mold. This type is mostly used when it is difficult to process parallel sound holes. Compared with parallel sound hole ear molds, its high-frequency frequency response is not good.
③Ten long ear mold with open sound hole. This model has the characteristics of the sound hole and the ear canal part of the ear mold is slightly longer, and can hardly be amplified below 1000 Hz. Suitable for patients with mild high-frequency hearing loss.
④ Ten short ear mold with open sound hole. Compared with the above-mentioned long ear mold, the sound hole diameter is large, and the external auditory meatus portion is also short. Therefore, this type is used for patients with mild high-frequency hearing loss.
With the increase of hearing aid power, adjustable items and adaptive functions, the acoustic effect of ear molds has gradually weakened.

Methods of taking ear impressions

4.2.1 Preparations for Ear Impressions
1. Tools and materials for ear impression
Tools for taking ear impressions include otoscopes, otoscopes, tweezers, syringes, etc. These tools should be disinfected before taking ear impressions.
The ear impression materials are mainly impression A and impression B. When used, the two materials are mixed in equal amounts to become the ear impression material; Used to prevent eardrum material from damaging the eardrum.
2. Ask a medical history
Check whether gill patients have ear infections or surgery in the last three months.
3． Check ear canal
First clean the inner canal of the ear canal, and then check the ear canal with an electric otoscope, an ear lamp, etc. Look inside the ear canal: ① Whether there is any pus or inflammation. ② Whether there is enlarged eardrum or perforated eardrum. ③ Whether there is deformity. ④ What is the size, curvature, and direction of the ear canal to determine where the ear block is placed. ⑤ Have you ever done radical mastoidectomy? In patients who have undergone radical mastoidectomy, the interior of the ear canal will be very wide. Therefore, do not take very deep ear prints and place appropriate ear barriers to avoid difficulties when removing them.
For ear pus and inflammation, less than three months after intra-ear surgery, and sudden deafness, do not take ear impressions. Instead, patients should be advised to go to the hospital for treatment. After their condition is stable, an optional device can be used .
4.2.2 Steps for Ear Impression
Ear molds need to be made according to the shape of the patient’s ear canal, so we must take a model of the patient’s ear canal. Taking the impression is the first step in making an ear mold. The quality of the impression directly affects the effect of making the ear mold.
前 Before taking the impression, you should prepare tools and materials, and clean the head of the electric otoscope, ear lamp, and syringe with a sterile cotton ball.
The steps for taking impressions are as follows:
① Place the patient. The patient should sit in a moderately high chair.
② Obtain patient cooperation. Explain to the patient or his family the procedures you will perform, tell the patient how they feel when taking the impression, and what aspects of their cooperation are needed to eliminate their tension.
③ Check the ear canal. Sit on the side of the patient, adjust the light so that the light shines into the ears and ear canals, pull the auricle (adults pull upwards and outwards; children pull backwards and downwards) to straighten the ear canal, pay attention to observe Whether there is maggot, if it affects taking the impression, it should be taken out and observed.
④ Place a cotton swab with thread or other types of ear plugs. Take medical cotton and cut it into cotton pieces, use a 20cm long cotton thread, fold it in the center of the cotton piece, fold it in half, trim the cotton, make the corners of the piece round, and decompose the cotton piece into a fluffy ball. The size of the cotton ball should be similar to the size of the second curve of the ear canal.
⑤ Plug the ear canal. Stand up, pull out, and back, so that the earball is in the same position as the ear canal, check the ear canal, bend the auricle toward the ear canal, and make the cotton roll up, down, left, and right. Push to the second corner.
If you do n’t know if the position of the cotton ball is correct, measure the distance to the second bend with the ear light and mark the ear light before putting the cotton ball. When putting the cotton ball, just push the cotton ball to the ear The marking depth of the lamp is sufficient.

Sometimes some patients will have a slight discomfort and some may have a reflex cough.
In addition, no matter if the cotton ball is too large or too small, it should be trimmed or redone. Repeat as many times as necessary until it fits well into the ear canal.
⑥ mixed impression materials. Measure and mix impression materials as required. The mixing method is as follows: Take a flat spoon of impression cream A and put it on your hand. Take a piece of impression paste B equal to the white impression paste, and immediately knead the two in the shortest time, knead them into a strip, put it in the syringe as soon as possible, and push it to the mouth of the syringe with the insert of the syringe Remove air bubbles, if there is still air, hold the syringe flat and squeeze out about 0. Scm long impression paste, discard it and start injection.
⑦ Fills the ear canal. Pull the pinna up, out, and back again to straighten the ear canal. Put the tip of the syringe about 0.5cm into the ear canal. Pay attention to leave a small gap between the mouth end and the ear canal to prevent damage to the eardrum due to pressure transmission. Carefully squeeze the impression paste against a cotton ball to quickly complete the ear canal. Fill and then exit to the external ear canal, but the tip of the syringe is still buried in the impression paste and continues to be injected at an angle to avoid air bubbles on the edge of the ear wheel. Gently inject the impression paste back and down into the ear cavity, the edge of the ear wheel, the notch between the tragus, and the tragus. Fill up the auricle and go up to the triangular fossa. Be careful not to leave bubbles in the impression paste. Fill up to the outer edge of the ear wheel and print the outer structure of the ear. After the injection is completed, it does not need to be used as a press outside to avoid the model being too large. When making some parts, it will make the patient feel pain and discomfort. You can make the patient make a few mouth opening and closing motions and turn the head forward, backward, left and right, so that the ear canal tissue can move normally and the impression material is fully filled, Avoid hearing aids when the patient is doing an action in the future
Howling occurs.
⑧ wait. Generally, the stamp paste can be completely solidified in about 5-6 minutes. At this time, the hardness of the stamp should be tested. When the indentation can be restored after pressing the material with your fingers, you can remove the impression. Do not take it when it is very soft, then the impression paste is easily deformed; do not take it when it is very hard, so that the ear canal part is easily broken.
⑨Remove. Usually, the impression paste will stick to the skin of the patient’s ear, so before removing the impression, you need to gently pull the impression away from the auricle to allow air to enter the ear to loosen the seal.
Grasp the edge of the impression with your fingers, gently separate the area at the top of the triangular fossa, then tighten the auricle, gently raise the impression, move the impression forward and backward, and gently rotate up, backward, and outward , Can make the ear canal part of the impression smoothly, the cotton ball should be attached to the end of the ear canal of the impression when removed.
⑩ Examine the ear. Make sure nothing is left in the ear canal.
⑨Check the ear prints. Eligible impressions should meet the following criteria: a complete impression must include the ear canal, ear canal opening, ear wheel, tragus, and ear cavity cavity; the ear canal is complete with a second curve (deep ear canal hearing aids, must pass the first 2 bends 3mm); the ear canal is complete, the element is defective; the ear wheel is complete, including all the curves;