Hearing aids

Technical specifications of hearing aids

       Each model and type of hearing aid. There are specific characteristics of different machines, so it is important for the audiologist to know those characteristics of each machine. In order to be able to select a hearing aid that is appropriate to the level of hearing disability of each patient.

The specific features of hearing aids that you should be aware of are as follows.

       1. SSPL 90 (Saturation Sound Pressure Level 90) is the highest loudness level of a hearing aid in the frequency range 200 – 5 KHz when
       1.1 Input the audio signal into the hearing aid at 90 dBSPL.
       1.2 Adjust the accelerator button – reduce the sound at maximum volume (Full-on)
       1.3 The graph showing the loudness level at each frequency is called the Saturation curve ( SSPL 90 ).
       1.4 The average value of the Saturation curve at frequencies 1K, 1.6K and 2khz is called High.
       Frequency Average SSPL 90 is abbreviated as HFA SSPL 90.
In general, SSPL 90 should not exceed the loudness level that causes discomfort to the patient’s ears (Threshold of discomfort), and some hearing aid models may have a button to reduce SSPL 90.
       2. Acoustic Gain or Full-on Gain is the amplification power of the hearing aid equal to the difference in decibels between the amplified output sound signal of the hearing aid and the input sound signal. Measurement acoustic gain done by
       2.1 Adjust the volume up-down button at maximum volume (Full-on)
       2.2 Insert a sound signal into the hearing aid of 60 dBSPL, but in cases where the hearing aid has a special circuit to automatically control the amplification such as AGC, ARC, the loudness will be only 50 dBSPL.
       2.3 The average value of acoustic gain at frequencies 1K, 1.6K and 2.5khz is called High Frequency Average Full-on Gain. Selecting the gain size of the hearing aid will vary according to the level of hearing loss, for example, with less hearing loss, use gain. Little and more hearing loss will use higher gain.
       3. Frequency Response Curve is a graph showing the amplification power or gain of the hearing aid at each frequency. Different types and models of hearing aids provide different amplification power for each frequency. In the actual situation of using a hearing aid, the patient will not turn the volume up to maximum (Full-on). Therefore, the Frequency response curve test will adjust the volume up-down button at the Reference Test Gain (RTG) position, which is the loudness level. that is close to actual use
       4. Frequency Range is the frequency range that the hearing aid amplifies sound. Some hearing aid models may have a wide frequency range, some amplifying high frequencies and some amplifying low frequencies. To find the frequency range, consider the average frequency response curve in the high frequency range (HFA) subtracted by 20 dB and then draw a line parallel to the frequency axis. intersects with the frequency response curve 2 points at low frequencies and high frequencies The frequency range between these two points is called the Frequency Range.
       5. Reference Test Gain (RTG) is the reference amplification level obtained by releasing an input signal of 60 dBSPL and then adjusting the volume up-down button until getting a graph of the average loudness level at frequencies 1K, 1.6K and 2.5khz lower than HFA. SSPL 90 17 dB
       6. Harmonic Distortion is the sound signal that comes out of amplification (output) that has different characteristics from the sound signal before amplification (input). The value is equal to the number of overtones or harmonics of the fundamental frequency of the input sound signal. Harmonic distortion measurement is considered. It is the percentage at frequencies 500, 800 and 1600 Hz. If the percentage of distortion is high, it means The hearing aid has poor sound quality, and this distortion is generally less than 15%.
       7. Battery consumption (Battery current) Each hearing aid uses different amounts of electricity. Adjusting the volume too much will increase battery drain. To know how long your battery will last You will need to know how many milliampere hours (mAh) your battery has and how much power your hearing aid consumes. Testing how much power a hearing aid consumes This is done by using a hearing aid tester, setting the accelerating angle to reduce the loudness of the hearing aid at the reference test gain position, and using an audio input signal with a frequency of 1000 Hz, loudness of 65 dBSPL as usual. The average power consumption of the hearing aid is approximately 0.2 – 5.0. milliamps, but if it’s a box-type hearing aid, some models may consume 15 – 20 milliamps of power. When the power consumption rate of the hearing aid is divided by the electrical capacity of the battery, you will know the number of hours of use. of the battery, for example, the battery has an electrical capacity of 250 milliampere hours. And the power consumption rate of the hearing aid is more than 2 milliamps, so the number of hours of use of the battery will be equal to 125 hours.

Types of hearing aids

       Each type of hearing aid has a different shape. But they have the same basic components and testing methods.
       1. Body hearing aids are hearing aids that look like a rectangular box, larger than other types of hearing aids. Inside the box are the microphone and amplifier parts together with an extension cord (Cord) to headphones (receiver). The hearing aid box can be worn in a shirt pocket. This type of hearing aid has high magnification power and is therefore suitable for children and the elderly. However, box-type hearing aids also have a disadvantage, namely that the microphone is not positioned to pick up sound naturally.
       2. Behind the ear hearing aids (BTE) are hearing aids that hang behind the ear. The device is smaller than a box. Inside the hearing aid is a microphone component. Amplifier and speakers combined together Uses a small battery From the end of the curved tube (ear hook) of the hearing aid. There is a plastic tube attached that carries sound into the ear canal through the ear mold. The advantage of this type of hearing aid is that the sound reception position is close to natural because there is a microphone above the ear.
       3. Eyeglass hearing aids are eyeglass hearing aids with a microphone, amplifier and speakers embedded in the temples. From the speakers there is a plastic tube that carries sound into the ear through the ear mold. This type of hearing aid is often used by patients who have vision problems and need to use glasses regularly. It can also be used for patients who need hearing aids in the CROS system.
       4. In the ear hearing aids (ITE) are small hearing aids that have a microphone, amplifier, and speaker all in the same part. The device is inserted into the ear (concha). It is considered a hearing aid with a very natural sound reception position.
       5. In the canal hearing aids (ITC) are smaller hearing aids than ITE. They have three basic components combined. and the device is inserted into the ear canal at the beginning
       6. Completely in the canal hearing aids (CIC) is the smallest hearing aid that is inserted deep into the ear canal.

       In addition, there are many other special types of hearing aids, such as:
CROS (Contralateral Routing of Signal) hearing aid system
       It is a hearing aid used for patients who have good hearing in one ear. The other ear cannot use a hearing aid. The technique of the CROS system is to use a microphone to pick up sound from the damaged ear. Then bring the sound to expand and
Output the speaker into the good ear. This hearing aid system can be BTE, Eyeglass, or ITE. Another form of the CROS system, BICROS, is used for patients with hearing problems in the good ear.
Therefore the sound in the good ear must be amplified as well.
Bone conduction hearing aids
       Used with patients who have conductive hearing loss, have fluid or pus flowing out of the ear all the time, or do not have an ear canal. There are two types of hearing aids that conduct sound into the ear through the bones: Body aids and Eyeglass aids.
Cochlear Implant
       It is different from normal hearing aids and is used for patients with Profound hearing loss who do not benefit from wearing conventional hearing aids. The doctor will surgically implant them.
An electrode is placed in the cochlea to directly stimulate the auditory nerve, causing awareness of sound to be heard.

Evolution of hearing aids

       Hearing problems Has created difficulties for humans for a long time. The ancients knew how to use their hands to protect behind their ears. To hear a loud sound But it only helps to hear the sound a little louder. Later, humans learned to use tools made from animal horns to help them hear better. By taking the horn of a cow or buffalo, drilling a small hole at the tip of the horn and inserting the small hole into the ear when listening to a voice or having a speaker speak at the base of the horn Which is larger to help hear louder sounds as well. This sound amplifying device made from horn is called an acoustic horn because the inside of the horn has rough, irregular conical holes. Causes the reflection of sound waves. Humans then thought of inventing a sound amplifier for their own use, made from metal in the beginning. Non-electrical hearing aids (Mechanical aids)

     Using the same principle as animal horns, which is to make them into a cone shape. There is a sound hole at the mouth of the trumpet. From the mouth of the trumpet the pipe gradually tapers down and the smallest end is the path for sound into the ear. The sound waves inside the pipe will roar, making the sound louder. At the same time, the pipe acts as a shield against environmental noise. This type of hearing aid is made in various shapes. To conceal or conceal a disability such as ear trumpet, acoustic chair, speaking tube, vase doubling and acoustic fan, etc.

”      However, the old hearing aids mentioned above There are no components working on sound amplification yet. Until the telephone was discovered Therefore, telephone microphones and headphones were used to assemble electric hearing aids in the first phase, which was a sound amplification system using carbon beads (carbon amplifier). The hearing aids received high amplification power. climb And if higher amplification is desired, several microphones can be connected in parallel. But this type of hearing aid has the disadvantage of having a lot of noise, making the sound not as clear as it should be.

​      When radio tubes or vacuum tubes were discovered. This allows the sound to be amplified from the microphone and output to speakers or headphones for better quality sound. Higher magnification But vacuum tube hearing aids require a lot of electrical power. make it smaller and can be assembled into a hearing aid with a battery box that is too large Until having to set it on the table and not being able to carry it. Until
Vacuum tubes have been developed to be smaller. and can be assembled into a box-style hearing aid But it’s still not as comfortable as it is today.

       The biggest change in hearing aids occurred after the discovery of the transistor, which is a semiconductor component, which resulted in smaller hearing aids. Has higher magnification power and consumes less electrical energy.
But transistor technology doesn’t stop there. Several transistors and various components were introduced. Abbreviated together on the same base, called an integrated circuit (integrated circuit or IC), which is used in today’s hearing aids.

Preparing hearing test information to consider hearing aids

       Hearing aids will be considered for patients with hearing loss in the better ear of 30 dB or more, and every patient who will wear a hearing aid should have a thorough hearing test as follows.
       1. Routine audiometry examination includes examination.
       1.1 Speech Reception Threshold ( SRT )
       1.2 Speech Discrimination ( SD )
       1.3 Puretone Air Conduction Audiometry
       1.4 Puretone Bone Conduction Audiometry
       2. Check Most Comfortable Level (MCL)
       3. Check Uncomfortable Level ( UCL )
       4. Consider Dynamic Range, which is the wide range between the patient’s beginning to hear the sound and reaching the loudness level at which hearing the sound causes discomfort to the ear. This value can be found from UCL – SRT.

Wearing hearing aids in both ears and one ear
       Hearing sound in both ears It is better than hearing only one ear. For this reason, patients with both ears are deaf. You will be advised to wear hearing aids bilaterally. And patients who are deaf in one ear and the other is normal will recommend it.
Put the hearing aid in the affected ear as well.

Advantages of wearing bilateral hearing aids
       Patients with deaf ears in both ears. Recommending wearing a hearing aid in two ears has the following advantages:
       1. Makes you feel like you can hear both sides equally (balauce hearing) and helps you know the direction of the sound.
       2. When in a noisy environment Wearing hearing aids in both ears helps to understand speech better.
       3. Makes you feel more relaxed than wearing a hearing aid on one side because the patient does not have to listen intently. Or turn the ear that wears the hearing aid towards the conversation partner.
       4. Wearing hearing aids on both sides will help you hear louder sounds than wearing them on one side. Patients do not need to adjust the volume knob as when wearing it on one side.
       5. Improve the quality of the sound. The patient will feel very close to nature.

Hearing aid testing for patients
       Choosing hearing aids for patients Audiologists must always remember that A hearing aid is only a tool to help you hear louder sounds. Is it not a device to help improve hearing?
Can be used instead of hearing at all Hearing aids will help deaf people hear more clearly. or can be used instead of hearing at all Hearing aids help deaf people hear more clearly and comfortably than ever before. But can’t help
       Allow hearing to return to normal. For this reason, the same hearing loss patient may need a hearing aid with different electroacoustic properties. For example, two patients with the same audiogram may
You can like different hearing aids.

Choosing a hearing aid involves the following steps:
       1. Selecting the type of hearing aid.
Choosing the type of hearing aid There are several factors to consider, including the need for magnification and portability. Flexibility, price, beauty and user satisfaction ITC and ITE hearing aids are the most common and can be used for most hearing loss levels. However, some hearing aid users have special needs for certain devices, which may result in limitations in choosing a hearing aid model.

       2. Selecting characteristics electro-acoustic
At one time, hearing aid design was selected based on the circuit design and/or the model of hearing aid based on information obtained from a hearing test. Although some hearing aids have adjustment knobs, that can make the hearing aid highly flexible But these controls are adjustable within a limited range, which means hearing aid selection remains dependent on hearing loss.
       2.1 Selecting Frequency Response
The general principle is to choose the Frequency response of the hearing aid. Relate to the hearing loss frequency range.
       2.2 Selection of hearing aid circuit type
       2.2.1 Linear circuit
Selecting the appropriate circuit design Or the model of the hearing aid is considered very important in wearing the hearing aid. Most hearing aids use a linear circuit that provides constant amplification regardless of the loudness of the input signal. For example, if the input is 50 dB loud and the output is 80 dB, the hearing aid has 30 dB of amplification. The input signal changes to 60 dB but the amplification remains the same, resulting in an output signal of 90 dB. As in the example below, a Linear circuit with an amplification power of 30 dB (gain) will give a maximum loudness (maximum output) of 107 dBSPL hearing aid. Most use a Linear circuit, suitable for people with mild to severely deaf ears (mild to moderately severe) and if the patient does not have a problem with tolerance to loud noises. (recruitment) will be advised to use a Linear circuit.
       2.2.2 Compression circuit
Some hearing aid users with mild to moderately severe hearing loss have difficulty adjusting to loud sounds. Patients often complain that when their hearing aids are adjusted they can hear speech just right. But other sounds can be heard too loudly. Patients in this type should use hearing aids with a special compression circuit. This circuit is sometimes called Automatic Gain Control (AGC). The compression circuit reduces the loudness of the sound signal to match the sound. Loudness of the input signal: In an input compression circuit, there is a loudness level sensing device that controls the loudness of the input signal.
Hearing aid When the input signal is louder than the kneepoint, the gain will decrease. Adjusting the volume up-down button will make the output signal louder as well. In output compression circuits, a volume sensor controls the loudness of the hearing aid’s output.
       Compressing all or part of the input signal depends on the selection of Input or Output compression, the selection of kneepoint and the selection of compression ratio.
       2.2.3 Push-Pull circuit
Linear and compression circuits may provide sufficient amplification power for hearing aid users who are severely deaf (Severe to Profound). Patients with this level of hearing impairment must use a special Push-Pull circuit. In this special circuit, the signal is divided. leave are amplified and then put back together again. Push-Pull circuits cannot predict battery consumption because the speed of power consumption depends on the volume of the input signal. and adjusting the volume up-down buttons The amplification power of Push-Pull hearing aids is approximately 45 – 80 dB.
       2.2.4 Automatic Signal Processing (ASP) circuit
Some hearing aid users may complain that they cannot hear clearly in quiet places. But some situations such as Where there is noise, he wants to hear it more clearly than it is. Today’s hearing aids have various adjustment buttons. There is plenty for the audiologist to adjust as needed. And there may be some circuits that can be adjusted automatically in different environments. This type of hearing aid is called an Automatic Signal Processing circuit (ASP). This circuit is shown as shown in the picture as a low-frequency loudness reduction circuit. When the loudness of the input signal becomes stronger This is because the sounds in the environment are low frequency. The operation of this circuit helps hearing aid users hear more clearly in noisy environments.
       2.3 Selecting the magnification power (GAIN)
Choosing a hearing aid’s magnification depends on a number of elements. It’s a straightforward consideration that if the ear is impaired by 50 dB, it should also use 50 dB of amplification. But wearing a hearing aid is not as easy as it seems. Because it was found that the most comfortable listening level requires only 1/2 to 1/3 of the amplification power of the handicap. Although almost all hearing aids have a volume knob to control the gain, the purpose of selecting the power is for the hearing aid user to adjust the volume knob by about 1/2 in order to be able to adjust the volume. Increase or decrease the volume in different environments. There are several methods for determining amplification including National Acoustic Labs (NAL), Prescription of Gain and Output (POGO), Berger, and Libby. These methods are based on hearing level data used to determine the amplification power of the hearing aid. Adjust the magnification until all speech can be heard comfortably. The NAL and Libby formulas used a magnification of approximately 1/3, while the POGO and Berger formulas used the 1/2 magnification rule.
       2.4 Selecting the maximum loudness (Maximum Output)
The selection of maximum loudness (maximum output or SSPL 90) was based on UCL data with the aim that Hearing aids will not amplify sounds beyond the patient’s UCL. SSPL 90 is calculated from UCL measured in dBHL to dBSPL using the following formula: dBHL for speech +20 = dBSPL. For example, a patient has a UCL of 90 dBHL, so the maximum loudness Should not exceed 110 dBSPL.
When considering UCL vs. SSPL 90, one thing the audiologist must keep in mind is that maximum loudness and amplification must be consistent. This is because it is impossible that a 45 dB amplification power would produce a maximum loudness of only 100 dBSPL. Since the maximum loudness measurement uses an input signal of 60 dB, the output signal would be 60 + 45 = 105 dB, which exceeds the 100 dB SSPL 90 by 5 dB. which if desired
Limit the maximum loudness to 100 dB. You may need to consider using a special compression circuit.
       3. Selecting options
Some hearing aid users require a hearing aid with a special circuit, such as a Telecoil, by turning the switch to T so that the hearing aid can directly receive electromagnetic signals, allowing the user
The hearing aid can receive signals directly from the phone without interference from environmental noise and without feedback from the hearing aid. Some hearing aids have a switch that allows the microphone and telecoil to work at the same time (MT). Telecoils are not commonly found in ITE and ITC hearing aids.
Another option is Direct audio input, which looks like an outlet. For receiving audio signals via a cable connected to a hearing aid, such as from a hearing trainer. Tape player or television It will help hearing aid users hear more clearly. Some hearing aid users may be allergic to ear prints. Allergic reactions may include ear pain, swollen red ears, drainage, and/or scaly skin in the concha. Patients with this type may need to use substances. hypo-allergenic material to make ear prints instead
There may also be adjustment buttons on the hearing aid. for fine tuning of hearing aids These adjustment buttons include maximum output, gain, low tone, high tone, etc. BTE hearing aids, Eyeglasses and Body aids have standard adjustment buttons. But in ITE and ITC hearing aids, additional information must be sent specifically. In the event that the document showing the specific characteristics of the hearing aid provides information about the “fitting range” (Fitting range), it will make selecting the hearing aid more convenient by…
       1. Select a hearing aid with a fitting range appropriate to the patient’s level of hearing loss.
       2. Select a hearing aid that provides SSPL 90 and does not exceed the patient’s UCL.
       3. If the patient has problems with recruitment, they should choose a hearing aid with a compression circuit.
       4. Other options should be selected as appropriate as mentioned above.

Wearing hearing aids for patients

       At one time, perfect fitting of a hearing aid was all about selecting the best circuit design and selecting the option that provided the most benefit. But sometimes the most important indication of successful hearing aid selection relies on reports or feedback from the hearing aid user. For testing with Real Ear by adjusting the hearing aid correctly with Target, some patients may complain that the tone quality is still not satisfactory. In such cases, the examiner should adjust the hearing aid as best as possible, which is better than doing nothing at all. Because it is believed that Real ear gain gives correct results.

Testing hearing aids with Sound field
       Testing hearing aids using the sound field method requires a pure tone autometer that produces warble tones and a speaker by having the patient sit at a point that has already been calibrated and testing for hearing levels as well. Warble tones are produced through loudspeakers while the patient is wearing a hearing aid. This test is called the “Functional gain test” and the results are recorded on an audiogram table, as in the example showing the functional gain of the left ear. When considering each frequency, the functional gain has the following values:

20 dB at 250 Hz
45 dB at 500 Hz
55 dB at 1000 Hz
45 dB at 2000 Hz
and 25 dB at 4000 Hz

       Often, audiologists test speech discrimination with and without sound field hearing aids to show the patient the benefits he or she receives from wearing a hearing aid. Normally, the test will be done with speech at a normal conversation loudness level of about 50 dBHL, with noise that is less than 5 -10 dB as in this example. Patients cannot hear speech without wearing a hearing aid. But you can hear it when you wear a hearing aid.

Hearing aid testing using Real Ear measurement
       The best way to choose a hearing aid is Real Ear Testing Normally all hearing aid qualification testing is done on the 2 cc coupler, which is not the same as how the hearing aid works in the actual ear. In reality, no coupler can provide a hearing aid response like the real ear. Because each person’s ears are different. The Real Ear Test method allows the examiner to see the maximum magnification and loudness of the hearing aid when testing it in the patient’s ear. which records work via a small microphone inserted into the ear canal It is a test that gives better results than functional gain testing in many ways, such as not requiring a response from the patient. Therefore, it can be tested on young children or people who do not respond to sound. PureTone can detect slight changes in loudness of 2 -10 dB, which Behavior testing cannot, and Real Ear testing can detect it. Fast and accurate results

       Real ear testing consists of a series of curves. The first graph is called Real Ear.
Unaided Response (REUR) is a measurement of the change in sound signals that occur within the ear canal when a hearing aid is not worn by emitting sound field sound to the measurement point near the eardrum. From the picture you can see the characteristic of the sound reverberation in the concha and in the ear canal, causing an increase in SPL.
Frequency 2500 to 3000 Hz REUR measurement serves two purposes: to calculate Insertiongain and to predict the desired Frequency Response and gain.

       The next stage of testing is the Real Ear Occluded Response (REOR) examination.
Displaying changes in REUR when the ear canal is closed with a hearing aid or REOR ear print.
You can find this by wearing your hearing aid but turning it off. Notice the graph in comparison to the REUR graph.
REOR is lower especially above 2000 Hz. The difference between REUR and
REOR is “Insertion Loss,” which is hearing loss caused by wearing a hearing aid.

       The next important piece of information obtained from the Real Ear test is Real Ear Aided Response (REAR), which refers to the hearing aid loudness (dBSPL) measured in the ear canal when the hearing aid is worn and the hearing aid is on, sometimes referred to as “ Aided Response ” This test is similar to the loudness measured by a 2 cc coupler, except it uses the patient’s ear instead of the REAR coupler. It is most often used to predict the loudness of a hearing aid in the ear for normal listening situations. And another benefit of REAR is used to calculate Real Ear Insertion Res-ponse (REIR), which is the difference between REAR and REUR.

       Real Ear Insertion Response This last graph is sometimes called “Insertion gain.” This test is similar to the Functional gain test, but REIR is the difference between SPL in the ear when the patient is wearing a hearing aid and not, which is calculated by REAR – REUR
REIR is useful in determining whether Does the hearing aid provide amplification power appropriate to the patient’s level of hearing loss? The REIR is compared with the patient’s Target calculated from the formula. As shown in the table, REIR compares closely with Target, with an acceptable deviation of 5 dB between the two values. However, some hearing aid users may not like a hearing aid that fits Target because Target is only a point. Begin to adjust your hearing aid only. It is not the final step in wearing a hearing aid.

       Real Ear Tests are most meaningful when considered together as a set of More than just looking at each graph in isolation, REUR alone is not enough to predict desired magnification and loudness, and REIR alone is not enough to tell whether a hearing aid is right for a patient. Therefore, the Real Ear test can be a device.
A valuable hearing aid fitting tool can be used in conjunction with patient reporting of hearing aid fitting results.