Other types of hearing aids device
In addition to the types of hearing aids mentioned above, there are other types of hearing aids that are suitable for hearing loss in special cases. Compared to air-guided hearing aids, they compensate for hearing in other ways. This section simply addresses these types
Hearing aids are introduced.
Hearing aids with signal path
According to the position and number of microphones and receivers, the contralateral routing of signals (CROS) hearing aids can be divided into single-signal-path hearing aids, bilateral signal-pair hearing aids, and bilateral-signal cross-pair hearing aids (see figure). 3-14). The main disadvantage of all types of CROS hearing aids is that there must be a connection on both sides of the head. It is more troublesome to connect through wires. It is best to use an electrical connection for wireless signal transmission.
Single-signal paired hearing aid refers to the microphone and receiver are placed in the left and right ears respectively, which is suitable for patients with hearing loss in one ear. This kind of hearing loss generally refers to the serious hearing loss in one ear, normal hearing or mild to moderate hearing loss in the other ear. This type of hearing aid has the microphone in the poor ear and the receiver in the good ear. In this way, the signals reaching the poor ears can be heard by the good ears through amplification. Because of the long distance between the microphone and the receiver, it is not easy to generate feedback howling. A single signal is more useful for hearing aids when speech is on the side of the worse ear. But when the speech comes from a better ear direction, it is not good for the patient’s understanding of the speech. Therefore, some scholars believe that patients with moderate hearing loss in the better ears will be more likely to obtain satisfactory results.
Bilateral signal pairing hearing aid refers to placing a microphone in each of the left and right ears, and placing a receiver in the good ear, which is suitable for patients with asymmetric hearing loss in both ears. Hearing loss in one ear is heavy and hearing loss in the other ear Generally moderate to severe. The microphones in the left and right ears work like directional microphones, and the microphones collect sound signals
Figure 3-14 After the signal is transmitted to the hearing aid, it is transmitted to the receiver with a better ear.
A bilateral signal-crossing hearing aid is a microphone and a receiver in each of the left and right ears, but they are not connected to each other, but the microphone in the left ear is connected to the receiver in the right ear, and the microphone in the right ear is connected to the left ear. The receiver is connected and cross-connected, which is suitable for patients with low-frequency hearing near normal and high-frequency hearing loss, because it is not easy to generate feedback howling.
Although signal-to-hearing hearing aids have the above-mentioned advantages and corresponding applicable objects, in fact, the use of CROS hearing aids is not much. What is helpful to us is the concept of CROS, which may be applied to hearing aids of other signal processing methods with the advancement of technology, reflecting new value.
The built-in CROS hearing aid transmits sound signals from one side to the other through bone conduction. It is suitable for patients who have no useful hearing in one ear. Hearing aids are installed on non-functional ears, and the ~ side vibrations of the head are transmitted to the contralateral cochlea through the skull, which can reach 9 higher sensory levels in better ears. In such options, high-power hearing aids can be used. Vibration is transmitted to the human skull in two ways. One is that the earpiece of the hearing aid produces a relatively large gain sound in the ear canal volume of the useless ear, and this vibrating air causes the temporal bone to vibrate. The other is that the receiver of the hearing aid vibrates the wall of the ear canal. Hearing aids should be placed as deep as possible, making full contact with the bone wall of the ear canal.
3.7.2 Bone conduction hearing aid
General air-guided hearing aids convert amplified electrical signals into acoustic signals and pass them into the ear canal. Bone-conducted hearing aids convert amplified electrical signals into mechanical energy and transmit sound by vibrating the internal structure of the cochlea. The vibrator on the bone conductor is usually placed at the mastoid site.
The general types of bone conduction hearing aids are spectacles and head clips (see Figure 3-15), which are suitable for patients with atresia, narrowing of the external auditory canal, or suitable for congenital malformations in the middle ear, chronic suppurative otitis media, repeated suppuration, and conductive hearing loss Patients with a relatively large degree and general ineffective air conduction hearing aids. However, bone-conduction hearing aids are not comfortable to wear.
The skin becomes hard and painful, and the output has certain limitations, which affects the directionality of the sound, and it can provide a small amplification effect for sounds above 3000-4000Hz.
Implantable hearing aids
Bone conduction hearing aid
1. Bone fixation hearing aid
Bone-anchored hearing aid (BAHA) is a special type of bone-conduction hearing aid (see Figure 3-16).
It overcomes many of the disadvantages of bone conduction hearing aids. Similar to bone-conducting hearing aids, the output principle of BAHA is also the principle of vibration. The titanium screw ingot is used to transmit vibration to the skull at the mastoid portion. Titanium integrates the ingot with bone tissue. Acoustic energy is transmitted directly through the titanium screw ingot implanted in the mastoid through the electromagnetic conversion device, and is guided by the bone into the inner ear. In some countries, BAHA has largely replaced bone conduction hearing aids for patients who have a permanent need for bone vibration hearing aids. For example, for the BAHA300 model, the average bone conduction hearing thresholds of 500Hz, 1000Hz, 2000Hz, and 3000Hz for headphone hearing aid patients should not be greater than 45dB HL. If the hearing threshold is within 65dB HL, bone-fixed hearing aids worn on the body can be used. These judgment standards were proposed by Tj ellstrm and Hakansson. They suggested that patients should be selected strictly, and a BAHA sensor should be connected with a test rod before operation. For suspicious cases, it is recommended to try on bone conduction hearing aids for a few weeks to estimate the post-implantation effect. Studies have shown that, compared to a BAHA, bilateral implantation can provide better localization and better speech intelligibility in noisy and quiet environments. These advantages are mostly generated by the two-sided microphones, rather than the output vibrations on both sides.
Bone fixation hearing aid
2. Middle ear implanted hearing aid
Traditional hearing aids amplify the sound and pass it along the path of the middle and inner ears through ear molds or cases placed on the external ear canal. Middle-ear hearing aids are surgically implanted with some devices in the middle ear. After receiving these sound signals, these devices will drive the middle ear to listen to the vibrations of the small bones and pass the sound directly to the inner ear or directly to the inner ear . An example is shown in Figure 3-17, which we use to explain the middle ear implanted hearing aid. Its advantage is that it will not have ear plugging effect and sound feedback effect, and it is more effective in transmitting energy than traditional ones. If it is a fully implanted hearing aid, it will be more beautiful in appearance. However, it requires surgical implantation, and some operations require damage to the ossicles, making recovery difficult; and if there is a problem with the hearing aid, it needs to be re-operated to replace and repair it.
Figure 3-17 Middle-ear implanted hearing aid
Middle ear implanted hearing aids use piezoelectric or electromagnetic output sensors. There are many ways to amplify electrical signals into the sensors. Piezoelectric sensors use ceramic materials, which change shape when the voltage changes. One end of the sensor is fixed to the skull, and the free end (the other end) is connected to the ossicular chain, transmitting sound to the cochlea. The free end of some implant systems is placed at the junction between the anvil and the sacrum. In some systems, piezoelectric sensors are fixed in the mastoid cavity and transmit vibration to the anvil via a coupling chain. The chain made a hole in the anvil with a laser. Electromagnetic sensors include permanent magnets that are placed in a magnetic field generated by a coil. When a current is passed through the coil, the magnet moves. This is the same principle as the receiver used in speakers and hearing aids. In most middle ear implant systems, the magnet is tightly fixed to the ossicular chain, allowing vibrations to be transmitted directly from the magnet to the middle ear system. The mounting position of the magnet can be the tympanic membrane, anvil bone, anvil joint or round window. The coil that drives the magnet can be in a hearing aid outside the ear, or in a custom housing, or in the middle ear cavity.
Another electromagnetic sensor that works on the same principle is a bone guide. The coil is connected to the anvil, and the magnet is loosely suspended in the coil. When the fluctuating magnetic field vibrates the magnet, the inertia of the magnet keeps it moving. As a result, inertial forces are generated on the coils and sensors, causing them to move, transmitting vibrations to the anvil.
Microphones implanted or partially implanted in hearing aids can be placed outside the skin, or under the skin (such as the back wall of the ear canal wall), and in the middle ear cavity. The main problem they need to overcome is the attenuation caused by the skin, and the increased internal noise of the hearing aid, which is caused by the internal noise of the microphone and the perception of the self-noise.
The battery can be used as an external component or it can be implanted. If implanted, minor surgery is required every 3 to 5 years to replace the battery. If the microphone and battery are not implanted, an external system is needed to transmit the signal to the vibrating sensor. Most dogs use cochlear implants with two similar components. External coils transmit magnetic fields or electromagnetic emissions, which can be induced by implanting coils or antennas. The signal picked up by the implanted induction coil is obtained by the piezoelectric sensor or the implanted driving coil through the implanted wire. Some systems transmit signals directly, such as when an external coil generates a magnetic field that directly drives the implanted magnet.
Compared with bone conduction and air conduction hearing aids, the advantages of fully implanted hearing aids are less visibility, convenience, less feedback, and less ear plugging. The signal quality may be better.
Software Hearing Aid
Since the appearance of custom hearing aids in the 1960s, the traditional dental material acrylic has been used as the housing material. The casing is rigid and hollow, which provides suitable placement space for hearing aid components, but also has many limitations. Its biggest weakness is hard, so it is difficult to comfortably cope with the dynamic changes of the human ear canal. Therefore, the hearing aid feels good when first worn, and problems can occur once the wearer speaks, chews, or moves his jaw. The most common are feedback howling and discomfort, resulting in failed fitting or reduced satisfaction.
In order to solve the above problems, people have been looking for better alternative materials for many years. Early trials of ethylene polymers, soft acrylics, etc. have all failed due to faster hardening, breakage and discoloration. However, the software technology developed by Xinxing can successfully solve these problems. This soft material is an elastic implantable medical silicone with a hardness of only 12 (acrylic case has a hardness of 90). This new material has many advantages. First, it will not harden over time, as was previously the case with soft materials; second, the flexibility of the new material can well adapt to the dynamic changes of the ear canal caused by mandibular movement, improving the acoustic seal; finally, the soft shell is Solid rather than hollow, hearing aid components are embedded in it without being surrounded by air, eliminating internal vibrations and howling, and also protecting the components from vibration and moisture damage, thereby extending the life of the hearing aid. The birth of software hearing aids marks a landmark revolution in hearing aid technology. This material has passed a series of tests for comfort, durability and safety and reliability, and has been developed into a “soft” series of hearing aids.
As a new type of hearing aid, software hearing aids have the following characteristics:
① Created new comfort standards and improved patient satisfaction. Related studies show that patients are 87% satisfied with “software”, 11% with software plus hardware (acrylic), and 2% with hardware (acrylic).
② Reduced repair rate. The main cause of hearing aid failure is damage caused by moisture, vibration or dropping of electronic components.
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