Hearing aids fitting True ear analysis

True ear analysis

True ear analysis refers to the process of making acoustic measurements in the human ear. The acoustic measurements referred to here are related to hearing aids. The amplification parameters seen in the hearing aid instructions are their test results under standard requirements. Because hearing aids are an electroacoustic device, a quantitative analysis of performance is required. The test is generally performed in a standard coupler and an ear simulator. The data of the electroacoustic performance test are of comparative value. However, standard couplers and ear simulators are, after all, different from our real ears.

Hearing aids also produce different effects in couplers and ear simulators than they do on the wearer. The actual amplification characteristics of hearing aids in different individual ears, especially in children, will be significantly different from the data measured in coupled cavity ear simulators, and the actual amplification performance of hearing aids in human ears is our real Question of interest. Real-ear analysis can objectively evaluate the amplification performance of hearing aids in actual use, and clarify how the hearing aid processes sound in the individual ear.

This section aims to introduce some technical terms used in true ear analysis, as well as the specific methods and precautions for operation. At the same time, understanding the conversion between real-ear analysis data and standard test data will help to understand the hearing aid formula and hearing aid debugging. True-ear analysis is usually performed on a real-ear analyzer, and sometimes it can be operated in the sound field.

2.1 Terms
1. Common terms in true-ear analyzers
(l) Reference microphone
The reference microphone is also called the control microphone. The test location is near the ear. Its role is to detect the signal strength generated by the real-ear analyzer and form a feedback path until the generated signal strength is the target intensity.
(2) Test microphone
The test microphone has a self-calibration function and is connected to the detection hose. The hose has a certain elasticity and is generally not completely crushed by the ear mold or the hearing aid. The measured data near the eardrum of the test microphone is the actual sound pressure level of the external ear canal. There are markers on the hose. You can determine the length of the ear canal as needed and adjust the position of the markers for positioning.
2. Common ear analysis terms
(1) Real-ear hearing aid gain
Real-ear hearing gain is also called true-ear hearing response, or in-situ gain, which refers to the sound pressure level near the eardrum minus the sound pressure level recorded by the reference microphone at the near external ear canal mouth when the hearing aid is working. (ANSI S 3.46) history of gain or response.
The position of the reference microphone is the reference test point. Different hearing aids have different test methods, and the position of the reference test point will also be different.
(2) Real ear hearing aid gain
The true ear non-hearing gain is also called the true ear non-hearing response, which refers to the response of the ear canal to the sound source when the hearing aid is not worn, which reflects the natural amplification characteristics of the external ear canal, which varies greatly from individual to individual. Its measurement is that when the ear canal is completely open, a probe tube is inserted near the eardrum. A speaker at a certain distance from the ear canal produces pure sounds with different frequencies and the same sound intensity. The probe tube detects the frequencies in the ear canal. The sound intensity at the point is subtracted from the sound source sound pressure level recorded by the reference microphone near the mouth of the external ear canal, and the difference between the two at each frequency point.
(3) Real ear intervention gain
The true ear intervention gain is also called the true ear intervention response, which refers to the difference in sound pressure levels obtained by the ear canal near the eardrum with or without hearing aid, that is, the true ear hearing gain minus the true ear unassisted Listen to the data obtained by gain. The difference between it and true-ear hearing aid is that intervention gain takes into account the natural change effect of the ear canal on the characteristics of the sound source, while true-ear hearing aid does not consider this factor.
(4) True ear coupling difference
The true ear coupling difference is the difference between the hearing aid’s response near the eardrum and its response in a 2CC coupler (IEC 126). It reflects the difference between the frequency response to the hearing aid output caused by the skull, auricle, ear canal, etc. after the hearing aid is worn with the 2CC coupled cavity simulated ear test and the real ear test after hearing aid.
This difference comes from many aspects: ① First, the acoustic resistance caused by the elasticity and friction coefficient of the human external auditory meatus skin, pitting tissue, and tympanic membrane is different from the 2CC metal coupling cavity, which makes the two test results different. The latter test curve will be sharp and uneven. ② Hearing aids are worn behind the human ear. The ear canal hearing aid or ear mold reduces the volume of the external auditory canal. The volume of the external auditory canal after such real-ear hearing aids is often less than 2CC, so the frequency response gain after real-ear hearing aids is often greater than Measured result of 2CC coupling cavity. ③ The 2CC coupling cavity is different from the external auditory meatus of the human ear. It is a regular cylinder. There are two external auditory meatus of the human ear.
Physiological bending, the diameter of the external auditory canal also varies irregularly, so the 2CC coupling cavity test is different from the true ear test.

(5) Functional gain
Test in the sound field to get hearing aid and non-hearing thresholds for hearing aids. The difference between the thresholds is called the functional gain.
2.2 True ear test
There are alternative methods, comparison methods, and sound pressure method for the real ear test. The sound pressure method is the most commonly used method. The IEC 118 standard specifies that the test microphone probe tube is located in the external ear canal, and the reference microphone is located in the external ear canal. Reference point maintained after calibration
Constant.
1. Test Conditions
(l) Environmental requirements
Although the requirements for ambient sound do not require low noise intensity as in hearing evaluation, if the low-amplification of non-linear hearing aids needs to be evaluated, the environment must meet certain requirements, of course the quieter the better. Patients are also required to remain as quiet as possible during the test.
(2) Cleaning the external ear canal
Although a small amount of radon has a small effect on the real ear test, if the radon plugs the tube of the detection microphone, the effect on the test will be greater. Therefore, please clean the external ear canal before testing.
(3) Orientation of the speaker
Generally, the plane of the sound source and the test ear is 45. , There is a suggestion of 30 for directional microphones. The distance between the tester and the speaker is Im. If you are testing interventional gains, such as a complete ear canal hearing aid, a true microphone without real hearing aid and a true microphone
The wind has the same directionality, because both pick up the sound after it enters the ear canal. Regardless of the azimuth chosen, assuming no head movement during the test, the intervention gain will not be affected. For other hearing aids, the microphone is installed outside the ear canal, and the intervention gain is related to the azimuth, but it is lower than 5000Hz, and the hearing aid is from front to 45. Within the range, the azimuth angle has a great influence on the intervention gain.
If you are testing true-ear hearing gain, the gain-frequency response curve is related to the azimuth.

(4) Test signal
Choose the signal with the largest bandwidth possible during the test, because although the test equipment uses a reference microphone, it only controls the strength of the location of the reference microphone. Controlling the stability of the test signal around the location of the reference microphone has a positive significance for ensuring the accuracy of the test results. The control and stability of the test signal strength around the auricle is related to the test signal bandwidth and the test environment. If the reflections (house boundaries, nearby objects, subject’s shoulders) cause the signal to generate standing waves near the head, the sound pressure will change dramatically in a small range, especially for low-frequency sounds. Pure tones are prone to standing waves, because when reflected waves cancel out direct waves, the smallest nodes can occur. With a wideband test signal, because the signal contains multiple frequencies, it is impossible to produce the same node at the same location in space, and the sound field becomes uniform. The test signal can be selected from chirp, narrowband noise, and wideband noise.

(5) Test microphone placement
The position of the test probe is related to the test item. The position of the Yu-in gain test probe is not as strict as the real-ear hearing gain. In general, the head of the probe should be 5mm beyond the hearing aid or the inner end of the ear mold, because within 5mm, the sound pressure is less affected by the conversion of sound waves from a narrow sound hole to a wide ear canal. The placement should be as deep as possible, but do not touch the eardrum, because to avoid the existence of standing wave phenomenon, if you want to perform a very accurate test, you should consider the relationship between the probe position and the standing wave error.

2. Real ear hearing gain (REAG) test
Real-ear hearing gain is a necessary step in testing intervention gain. As mentioned before, the ear canal should be cleaned, the equipment should be calibrated, the probe should be placed according to the patient’s hearing aid or ear mold, and the angle and distance between the speaker and the patient should be adjusted. Put the person to detect the tube, observe with an otoscope to ensure that the tube is in the correct position, wear the patient’s hearing aid, make it in working condition, select the test signal and the appropriate strength to test. The data obtained at this time is the real-ear hearing aid gain. If the test hearing aid is programmable, compare the real-ear hearing aid gain and the amplification target curve, and adjust the parameters of the hearing aid through programming software until the required
Response; non-programmable hearing aid, adjust various fine-tuning directly on the hearing aid, change the frequency response curve of the hearing aid until it is close to the target
Subscript curve.
3． Real Ear Intervention Gain (REIG) Test
The real ear intervention gain is equal to the real ear hearing aid minus the real ear unhearing gain. Need to obtain two data, have already obtained the real-ear hearing aid gain data, and also need to obtain the real-ear hearing aid gain data, the position of the two test probes should be the same position. Put the marked tube directly into the external ear canal, adjust the position of the speaker and the patient, select the test signal and the appropriate intensity for testing, and the data obtained is the real ear unhearing gain.
4． True ear coupling difference (RECD) test
The schematic diagram of the true ear coupling difference test is shown in Figure 7-1. One end of the 2CC coupling cavity is connected to the microphone and one end is connected to the sound generator. The sound is picked up by the microphone through the coupling cavity to obtain the sound pressure in the 2CC coupling cavity.
Put the tube of the detection microphone into the ear canal, wear an ear mold or ear plug, and connect one end of the ear mold or ear plug to the sound generator, so that the detection microphone can pick up the sound pressure generated in the residual ear canal after being inserted into the ear mold. The data obtained in the real ear minus the data of the 2CC coupling cavity is the individual true ear coupling difference.