Hearing Aid Main Performance Indicators
(A) Hearing Aid Test Standards
China's national standards provide for the measurement of hearing aids using the International Electrotechnical Commission publication of the IEC 118-7 or the IEC 118-0 requirements, according to the two standards can be carried out in the factory inspection of the product, product registration, quality sampling and so on. At present, in different parts of China is not the same standard, such as Shanghai using IEC118-0, while Beijing uses IEC118-7. The main difference between the two standards is that the coupling device used is different, in the IEC118-7 in the IEC126 2cm3 coupling cavity, in the IEC118-0 in the IEC711 plug ear simulator.
The IEC126 coupling cavity is a cylindrical steel body with an internal volume of 2 cm3 with an error of ±1%, and its bottom is equipped with a capacitive microphone that receives the sound pressure, and in the main frequency range of 0.2-5kHz in which hearing aids operate, the microphone's frequency response characteristics are flat. The coupling cavity has the advantage of simple structure, stable performance, conducive to reproduction and standardization; the disadvantage is that it fails to simulate the human ear ear canal softness of the acoustic resistance demonstrated by the role of the 2cm3 cavity than the actual wearing of hearing aids between the tip of the ear mold and the tympanic membrane formed by the volume is larger, so that the values measured with the IEC126 coupling cavity with actual use of a large discrepancy between the situation.
The IEC711 Plugged Ear Simulator consists of a main cavity, an acoustic load network, and a calibrated microphone. The volume of the main cavity is about 1.26 cm3, which simulates the average acoustic characteristics of a normal adult ear, and the test results are closer to actual use than those of the IEC126 coupling cavity.
(2) The main electroacoustic parameters and performance indicators of hearing aids
The main performance indicators of hearing aids are saturation sound pressure level, sound gain, frequency response curve, equivalent input noise, frequency range and distortion.
1. saturation sound pressure level (SSPL) saturation sound pressure level refers to the specified frequency point, the hearing aid in the plug ear simulator or coupling cavity may reach the highest sound pressure level. There are usually three methods used to describe the saturated sound pressure level.
(1) Take the average of three points on the saturation sound pressure level curve
at 1 kHz, 1.6 kHz, and 2.5 kHz for the average saturated sound pressure level.
The three frequencies are referred to in ANSI 3.22 as HFA (high frequency average).
(2) The peak on the saturation sound pressure level curve can be taken as the maximum saturation sound pressure level.
(3) You can choose one of the three frequencies, 1 kHz, 1.6 kHz, 2.5 kHz, and the sound pressure level corresponding to that frequency is the saturated sound pressure level, but the test frequency should be marked.
Measurement of the SSPL90 is performed at an input of 90 dB and at maximum gain. Under these test conditions, almost all hearing aids enter into a saturated operating state, so it is often equated with the measurement of the output sound pressure level (OSPL90) when the input sound pressure level is 90 dB, which is equivalent to the measurement of the SSPL90. It is important to note, however, that any amplification system can only provide a limited maximum output, beyond which the amplifier can no longer amplify and the receiver can no longer convert a larger signal. If the input increases beyond the saturation level, the output, instead of increasing, may drop and the signal will be distorted, so saturation sound pressure levels do not necessarily occur at the highest input sound pressure levels.
Knowing the saturation sound pressure level of a hearing aid is critical to the proper fitting of a hearing aid, as it ensures that: ①The maximum output produced by the hearing aid is above the user's threshold. ② The maximum output produced by the hearing aid will not exceed the user's discomfort threshold.
2. Full on acoustic gain (full on acoustic gain) at a specified frequency point or as a function of frequency, gain control at the maximum (full on), the other controllers in the specified position, the hearing aid is basically a linear input and output conditions of the measured acoustic gain. Full stop sound gain is mostly used to describe the maximum amplification capability of a hearing aid. When measuring, the volume control is placed at full, the input sound is 50 dB SPL or 60 dB SPL, and the frequency sweep is made in the range of 0.2~8 kHz, so as to measure the curve of the full-range sound gain.
When described as a numerical value, IEC 118-0 and ANSI S3.22 have different calculation methods. The former is described by the peak value on the full-range sound gain curve, called the maximum full-range sound gain. The latter takes HFA as the full-range sound gain value, or selects one of the three frequencies of HFA (usually 1.6kHz) as the nominal value of the full-range sound gain of the hearing aid.
3. Reference Test Frequency and Reference Test Gain
(1) Reference Test Frequency At this frequency point, adjust the position of the gain control to get the reference test position of the gain control related to the OSPL90. The reference test frequency is usually 1.6 kHz. 2.5 kHz may also be used as the reference test frequency for some hearing aids to reflect high frequency characteristics, but this needs to be noted in the test report.
(2) Reference test gain At the reference test frequency, when the input SPL is 60 dB SPL, adjust the gain control of the hearing aid so that the output SPL in the ear simulator is 15±1 dB lower than that of OSPL90, which is the gain position for the reference test gain control (or full gain control if the gain control is not up to this position). At this time, the gain of the hearing aid is extremely reference test gain.
4. Frequency Response Characteristics
(1) Comprehensive Frequency Response Curve The hearing aid in the entire operating range, the gain control is adjusted to the reference test gain control position, with a set of input sound pressure level, the measured frequency response curve of the input and output characteristics of the hearing aid family. The vertical coordinate is a dB linear scale, the horizontal coordinate is a logarithmic frequency scale, and the length of the 10-octave range on the horizontal coordinate is equal to the length corresponding to 50 dB on the vertical coordinate.
(2) Basic frequency response curve The frequency response curve measured at the reference test gain control position when the input sound pressure level is 60 dB SPL.
5. Frequency range On the basic frequency response curve, calculate the average value of the gain corresponding to the three frequencies of 1 kHz, 1.6 kHz, and 2.5 kHz, and make a horizontal line parallel to the horizontal axis by passing through the point of this value, and then move down by 20 dB to make another parallel line, and the two intersections of this line with the basic frequency response curve, that is, the starting and stopping frequency points of the frequency range of the hearing aid.
6. Steady-state input-output graph Under the conditions of the specified frequency and gain control position, the output sound pressure level as a function of the input sound pressure level are expressed in dB linear scale.
7. Hearing aid distortion When the output signal of a hearing aid differs from the characteristics of the original input signal, it becomes distortion, also called aberration. Hearing aid distortion includes harmonic distortion and intermodulation distortion. When the output signal produces an integer multiple of the original frequency, it is called harmonic distortion. For example: the original input signal frequency of 1 kHz, the output signal in addition to 1 kHz, but also contains 2 kHz, 3 kHz, 4 kHz and other frequency components, they are called harmonic distortion components. The original input and output signals 1 kHz, called the fundamental frequency or fundamental wave, while the output signal of 2 kHz called the second harmonic, 3 kHz called the third harmonic. The ratio of the output of each harmonic quantity to the output of the fundamental, i.e., the harmonic percentage; the sum of each harmonic is called the total harmonic distortion. We require the smaller the distortion the better, but completely free of harmonic distortion is unrealistic, less than 3% of the distortion, the human ear is not easy to recognize. Therefore, we usually use less than 3% as the harmonic distortion factory index requirements.
Intermodulation distortion refers to when the input signal is equal amplitude of the two signals, for example, 0.8 kHz and 1 kHz, the two signals through the hearing aid, in addition to the appearance of their respective harmonics 1.6 kHz, 2.4 kHz, 3.2 kHz and 2 kHz, 3 kHz, 4 kHz, there is a frequency of the two input signals and the difference between the frequency of the input signal, i.e., 0.8 kHz +1 kHz = 1.8 kHz. kHz=1.8 kHz and 1 kHz-0.8 kHz=0.2 kHz, this distortion is called intermodulation distortion. Hearing aids in the reception of speech, music and other complex frequency components, if the hearing aid itself is poor linear characteristics, often cause intermodulation distortion.
Harmonic distortion and intermodulation distortion are caused by the nonlinearity of the electroacoustic device, so they are sometimes referred to as nonlinear distortion. Corresponding to the linear distortion, it is the frequency response of the electroacoustic device is not flat, improve the signal amplitude of some frequency components, inhibit the signal amplitude of some other frequency components, so that the shape of the output frequency response caused by the shape of the input frequency response with the phenomenon of discrepancies.
8. Equivalent Input Noise It is an indicator for evaluating the inherent noise of a hearing aid. When testing, the intrinsic noise can be approximated as the equivalent of the input noise, the specific test method is: turn on the hearing aid and turn off the sound source, measure the output noise sound pressure level (dB value), and then subtract the reference test gain of the hearing aid (dB value), that is, to get the equivalent input noise level.
9. Battery Current The current strength in the hearing aid circuit measured at the reference test gain position when the input sound is 60dB SPL.
10. Induction pickup coil sensitivity The maximum sound pressure level measured in the plugged ear simulator or coupling cavity of a hearing aid at a specified input magnetic field strength and a specified frequency point, and under the condition of basic linearity of input-output characteristics. The test steps are: ① modulate the hearing aid to full gain, and the other controllers to their respective designated positions; ② adjust the magnetic field frequency to the reference test frequency; ③ adjust the magnetic field strength input to 0.01A/m; ④ direct the hearing aid towards the direction of the maximum pickup sensitivity, and measure the output sound pressure level in the acoustic coupling cavity; ⑤ express the maximum pickup coil sensitivity by the magnetic field strength of the 0.001A/m output sound pressure level; and ⑤ express the maximum pickup coil sensitivity by the magnetic field strength. sensitivity of the induction maximum pickup coil.
For hearing aids with a T-position, users can receive sound signals directly from FM systems or telephone handsets by using the inductive pickup coil in the hearing aid
The inductive pickup coil in the hearing aid can be used to receive sound signals directly from FM systems or telephone handsets.