The role and debugging of the hearing aid fine-tuning controller

The role and debugging of the hearing aid fine-tuning controller

The role of the trimmer controller is to adjust the parameters of the amplifier so that the final output sound meets the hearing loss of each hearing impaired person.It includes switches, volume control, various fine-tuning, and more.The controller can be a mechanical potentiometer or a register that can be adjusted via a computer.
1) Switch
Traditional ear-back and box-type hearing aids generally use an “OTM” switch, and the switch and volume control of the custom machine are integrated.The “OTM” switch of the behind-the-ear hearing aid is very susceptible to moisture and causes the whole machine to malfunction. Therefore, the new design uses the battery door as a switch.
2) Volume control
The volume control is used to allow the user to adjust the volume themselves.Some digital hearing aids have a volume adjustment range that can be set by software, and the fitter can set the appropriate adjustment range for the user.
3) Fine tuning
The trimmer potentiometer is used to adjust the output signal of the hearing aid, which is generally set by the fitter based on the hearing loss of the hearing impaired.There are many types of fine-tuning. The most common one is the frequency response regulator. The frequency characteristics of the hearing aid are determined by the filter. The filtering characteristics are related to the resistance and the capacitance. In order to change the frequency response of the hearing aid, a small variable potentiometer is used to adjust. Generally, the effect of increasing the high frequency (such as NH fine tuning) is achieved by attenuating a part of the low frequency component, and the effect of increasing the low frequency (such as NL fine tuning) is achieved by attenuating a part of the high frequency component.
1. NH-Low frequency regulation: The effect of increasing the high frequency is achieved by attenuating a part of the low frequency components, and the main effect is the decrease of the low frequency noise.
2. NL-high frequency regulation: The effect of improving low frequency is achieved by attenuating a part of high frequency components. The main effect is the improvement of speech intelligibility and the elimination of acoustic feedback.
3. MPO-Maximum output limit: In order to prevent the sound output from being too high beyond the discomfort threshold of the hearing impaired, most hearing aids have maximum sound output limiting devices, such as Peak clipping (PC), automatic gain control (automatic gain control). , AGC) circuits, etc.The peak clipping circuit is to cut off the peak portion of the signal that exceeds a certain limit, but the output is distorted after clipping, and the signal-to-noise ratio is small.The AGC circuit is a more advanced limiting control circuit that compresses the output sound larger than the uncomfortable threshold and outputs it, so there is no distortion, but the compression is time-responsive. For particularly sensitive patients, sometimes the peak is cut. safer.
4. TK-compression inflection point/CR–compression ratio: It is a unique fine-tuning of the compression line.Automatic Gain Compression Control (AGC) is a feedback device that automatically reduces the gain of the hearing aid without causing distortion, thereby limiting the hearing aid output.Once the automatic gain compression control is active, a new linear relationship is established between the input and output.The compression inflection point is the minimum input sound pressure level required for compression. The higher the inflection point is, the larger the hearing aid gain is. The compression ratio is the ratio of the input variation to the output variation after the compression is started. The absolute value of the compression ratio is higher. Large, indicating that the gain is smaller, when the compression ratio is equal to 1:1, it means linear amplification.
 5. Gain control (GC) and VC are used to adjust the main sound gain of the whole frequency band. The wearer feels the change of the overall loudness. Generally, the adjustment range of VC is smaller than GC.

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