Patients with different hearing loss show a wide range of hearing impairments. Increasing the patient’s hearing threshold, reducing the dynamic audible range of hearing, abnormal sensitivity to loudness, and reduced speech discrimination are all problems caused by hearing impairment.
How to choose a suitable hearing aid and how to adjust the performance parameters of the hearing aid to match the hearing impairment of each patient in order to achieve the best hearing compensation effect is the most fundamental issue and the basis for the realization of hearing and speech rehabilitation.
This article introduces the selection of hearing aids in detail from the selection formula of hearing aids, the selection, selection and adjustment of hearing aids.

Hearing aid fitting selection formula

Audiology professionals have been working to find a regular parameter between hearing loss and the amplification characteristics of the hearing aid, so that the output of the hearing aid can achieve the best hearing compensation and produce the best communication effect. These parameters constitute an optional match. formula. Optimizing patient compensation seems like a simple problem, but decades of research have found that finding a simple relationship between hearing loss and gain is not easy. Because the optimal compensation effect is related to the degree of hearing loss and listening experience of the patient, it is also related to many aspects such as the strength of the speech signal, the patient’s loudness feeling, and the ability to judge the frequency. It has long been recognized that there are two different auditory characteristics that can be used as the basis for the matching formula: one is to measure the hearing threshold; the other is to measure the perception of loudness above the threshold, such as the optimal threshold and loudness classification. These two methods are not completely independent. The formula is introduced here to let the reader understand the principle and purpose of the formula, and does not introduce the detailed calculation method.

Generally, the current matching formulas are included in the software of the computer. As long as the user inputs the necessary data of the patient (such as audiogram or loudness of the test), the computer will automatically calculate the characteristics that need to be amplified. The software of the real ear analyzer also contains many well-known formulas, so that you can understand the amplification characteristics and amplification goals of the hearing aids intuitively, adjust the performance parameters of the hearing aids, and also see the differences between the different formulas and their respective frequencies.响 CHARACTERISTICS.
The gain of the optional formula generally refers to the real ear gain, including the real ear hearing aid gain and the real ear insertion gain. In linear hearing aids, more research has been done on gain than on maximum output. The general requirement for maximum output is to avoid discomfort and damage to residual hearing without reducing the effectiveness of the hearing aid. Non-linear hearing aids mainly study the frequency response of different input intensities. The maximum output can be regarded as one of the frequency response curves, and also includes parameters such as compression threshold and compression ratio. A non-linear hearing aid can be seen as the sum of several linear hearing aids, which respond differently to the input strength of different frequency bands.

1.1 Selection formula based on hearing threshold
There are NAL, NAL-R, NAL-RP, NAL-NLI, Berger, POGO, POGOII, FIG6, MSU, DSL [i / o], Libby, etc. based on the threshold of gain matching methods. They are generally based on the listening thresholds of different frequencies, calculate the required amplification at different frequencies, and make different corrections based on experience, or experimental data, or the perception of loudness.
In 1944, Lybarger proposed the principle of one-half gain, which is the basis of several current matching methods. He found that the amount of gain required by the patient was about half of his hearing loss. Moreover, the formula also includes consideration of the optimal threshold, which is suitable for patients with mild to moderate sensorineural hearing loss.
1. Linearly amplified frequency response formula
Linear hearing aids produce the same gain-frequency response to all input strengths, unless the output exceeds a defined strength. Here are two formulas applied to sensorineural hearing loss.
(I) NAL formula
In 1976, the National Acoustics Laboratory of Australia proposed the NAL formula based on the principle of half gain. In 1986, in consideration of the characteristics of the slope-shaped hearing loss, corrections to the average hearing thresholds of 500 Hz, 1000 Hz, and 2000 Hz were added, and the NAL formula was modified, called the NAL-R formula. This formula is suitable for speech with mild to moderate hearing loss at the comfort level. In 1990, based on the original, a very severe deaf correction factor was added, called the NAL-RP formula, which increased the low frequency response and reduced the high frequency gain, making it more suitable for patients with severe and very severe deafness. Some researchers believe that when hearing loss exceeds 60dB HL, even if high-frequency components in speech can be heard, the usefulness of high-frequency speech signals decreases significantly.
This may be related to the dead zone in the cochlea-although residual hearing sensitivity is also reflected in the hearing threshold, there are no hair cells or neurons in the cochlea that respond to this frequency. A damaged cochlea is like a bottleneck. It can only send limited information. If there is too much information, the cochlea cannot process all the received content. The result is not as good as sending only a small amount of information to the cochlea.
The starting point of the NAL formula is to assume that all speech bands through the hearing aid output make the wearer have the same loudness experience (such as loudness), which is about 60% of normal hearing. The goal is to maximize hearing comprehension of the hearing aid wearer under speech listening intensity. The type of gain described in the NAL response is insertion gain (also called functional gain).
(2) DSL formula
In 1985, Seewald, Ross, and Spiro proposed the DSL formula. The original intention of the designers was to select hearing aids for children with speech disabilities. Based on the study of speech perception of children with sensorineural hearing loss, it is found that the basis for generating the greatest speech intelligibility is that the speech signal is amplified to a sufficient sensory level, which gradually decreases with the increase of hearing loss. The DSL formula is an optional formula for calculating the target sensory level based on different levels of hearing loss. The goal is to provide hearing aid users with optimal hearing and comfort at every frequency.
Earlier versions of DSL attempted to compare different data for adults and children, and converted all data into dB SPL. This attention to acoustics has continued throughout the development of DSL. Future versions have gradually reflected their understanding of the real-ear test system’s detection of microphones and the use of plug-in headphones to evaluate hearing, especially the application of the real-ear coupling difference test is considered to be the best way to convert the measured data to the coupled or real-ear data. The first method, these studies integrate the DSL formula with the test method, which is convenient for infants and children. In 1991, a version of DSL 3.1, which was implemented with the help of a computer, was proposed to make the loudness of speech sound comfortable, such as setting the target to the optimal threshold. The purpose of the formula is to calculate the real-ear hearing gain, not the real-ear insertion gain. The formula also includes the 2CC coupling target and the real-ear hearing target. The average correction factor is not required. The test volume at the eardrum can be continuously used in order to Make the most accurate comparison of speech intensity and hearing aid hearing threshold, suitable for use in children.
2. Nonlinear amplification frequency response formula
Non-linear options can be understood as different gain-frequency responses to different input intensities. The study of linear amplification provides amplification requirements for medium-intensity input, which also applies to non-linear hearing aids.
(I) NAL-NLl formula
The NALNL1 (nonlinear, version 1) formula differs from the already described NAL formula in that it does not attempt to make the output of the hearing aid produce the same loudness at each frequency, and its purpose is to enable the wearer to produce speech of different intensities Maximum intelligibility. Generally, computer software calculates the amplification parameters that produce the maximum speech intelligibility according to the requirements of the designer based on the patient’s audiogram.

(2) FIG6 formula
In 1993, Killion & FikretPasa first proposed the calculation outline of the FIG6 formula. In 1995, the FIG6 formula was officially proposed by Killion. The purpose of the FIG6 formula is to make the loudness of the hearing aid output of different frequencies and different input intensities in the wearer as much as possible for normal hearing persons, that is, to achieve normalization of loudness. It is based on the average loudness data of a large sample of people with the same hearing loss as the standard, not based on the individual loudness test, it calculates the required gain based on the hearing threshold. For the input intensity of 40dB SPL, 65dB SPL, and 95dB SPL, there are direct gain formulas, which are related amplification parameters. Similar to the DSL formulas, these formulas contain three input strength real ear insertion targets and 2CC coupling targets, which can also be accomplished by software.

(3) DSL [i / o] formula
Corresponding to the non-linear amplification circuit, in March 1995, Cornelisse, Seewald, Jamieson proposed the “input / output formula [i / o]”, the purpose of which is to control the output of the hearing aid between the patient’s dynamic range and the amplified Speech is accepted as much as possible by hearing aid users. Due to the different compression types, there are two versions, called DSL [i / o] linear and DSL [i / o] curve, the former is suitable for hearing aids with a fixed compression ratio, and the latter is suitable for hearing aids with a changing compression ratio. The input-output function diagram is not a straight line in the compression envelope. DSL [i / o] also requires entering the patient’s loudness discomfort intensity. If not entered, the system will use the preset intensity. Researchers still recommend the same sensory level as DSL 3.1. The purpose of the amplification is the same as that of FIG6, in an attempt to achieve normalization of loudness.

1.2 Matching formula based on loudness

Gain options based on loudness (optimal valve, discomfort threshold, loudness scale) include: Shapiro, LGOB, IHAFF, ScaIAdapt, etc. These formulas are based on loudness.
The design of hearing aid output, whether it is normalization or equalization, is a non-linear amplification. Several formulas are described below.

1. LGOB formula
The purpose of the LGOB formula is to achieve normalization of loudness, which is also the first formula to be applied clinically. Proposed by Allen, Hall, and Jeng in 1990. In this method, they divide the loudness into 7 levels. The stimulus used is narrow-band noise. The hearing loss and the normal 7 kinds of loudness heard have corresponding average intensities. The difference between the two intensities can be understood as Insert the gain so that for each input intensity you can get a gain value that produces normal loudness. This method can be performed using specific software and hardware.

2. IHAFF formula
In 1993, some researchers believed that there is an urgent need for formulas that can be applied to adjustable wide dynamic range compression hearing aids. They formed the Independent Hearing Aid Selection Forum (IHAFF). The loudness of loud speech sounds amplified by hearing aids is also light, comfortable, and loud for hearing-impaired people, that is, normalization of speech loudness. This particular loudness scale method is called the Contour test. The IHAFF formula requires at least two frequency loudness tests, such as a low frequency 500Hz and a high frequency 3000Hz test. After completing the loudness test with cymbal sound, input the obtained data into the calculation software VIOLA of IHAFF, and calculate the result of normalizing the loudness of the three points that represent lightness, comfort, and loudness in the input and output function chart at each frequency. These three points represent the output intensity required for 1 / 3-octave speech loudness to reach normalization. The three-point input is when the normal hearing person classifies the loudness of the three complete speech signals of different strengths into three categories: light, comfortable, and loud. The intensity produced by the time.
The difference between the two intensities constitutes the insertion gain at each frequency, as well as the gain targets for different input intensities. The formula also proposes other amplification parameters. In terms of required hearing data, it differs from the FIG6 formula in that the IHAFF formula measures the loudness scale of 108 test individuals, while the FIG6 formula uses average loudness data.

Conductive hearing loss requires a greater gain than sensorineural hearing loss. It is generally believed that the additional amount of mixed hearing loss is 1/4 or 1/5 of the air-bone conduction gap. The gain required for pure conductive hearing loss is about 3/4 of the hearing loss.
There are many specific formulas, but almost all formulas so far have focused on the purpose of normalizing loudness, or equalizing loudness, or maximizing speech intelligibility for hearing aid users. The same audiogram and different formulas may cause the optional hearing aids to produce different average gains and frequency responses. The same formula also produces different gain and frequency response in hearing aids with different lines. In the past, the amplification characteristics of linear hearing aids had certain limitations, which made the differences between different formulas in hearing aids very small. But things are different now, with the progress and development of hearing aids, differences between the formulas appear.
There are differences between the formulas, but for any kind of hearing loss, it cannot be said that this or that formula must be applicable. For patients who can express their feelings, for example, when debugging hearing aid selection formulas, be sure to ask and respect their feelings after hearing aids. Audiology professionals should correctly recognize that even with the same hearing loss and the same hearing loss sickness, the same amplification parameters of the same hearing aid from different patients will still show large individual differences, so many experienced audiologists The formula is often regarded as a basis for matching and debugging, and the corresponding adjustment is made according to the patient’s hearing assistance feelings and individual differences. For patients who cannot express their feelings, it is recommended to choose the most suitable hearing loss characteristics and hearing aids Formulas of characteristics, and carefully do stage evaluation and follow-up.

From the above introduction, we know that the above formulas have been developed on the basis of a large number of basic research and clinical experiments by foreign audiologists and research institutions. China was in the mid-to-late 1990s. With the input of foreign advanced hearing aids and matching technologies, domestic professionals gradually accepted the professional matching formula. Unfortunately, even Chinese and foreigners are of the same nature and degree. Hearing loss will still show a certain difference as a whole. This difference is recognized by domestic audio professionals, just like there are differences in the choice of hearing aids between adults and children. Different matching formulas for adults and children can be found in the matching software. Even in the same matching software, the software design also considers the age to modify the hearing aid formula. However, there is currently no matching formula or modified prescription that suits the differences of the Chinese . Therefore, I look forward to Chinese audiology professionals through continuous efforts and on the basis of a large number of basic research, can develop matching formulas suitable for Chinese people, even at this stage, can find the laws of differences between overseas and foreign matching formulas Sex or correction value is also a great improvement of Chinese audiology.

Even abroad, there are still many unresolved problems in the formulation of non-linear hearing aids. Such as compression problems, although formulas such as IHAFF, FIG6, DSL, and NAL also provide specific compression parameters, how many compression thresholds, compression ratios, and how to choose fast compression and slow compression are still to be studied.

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