Traditional Chinese Practical Noise Cancelling Hearing Aid: Circuit Design, Parts List & Workshop Prototype Guide

Premium 6,346 credits (=$63.46) · Priced by article length · permanent access after unlock · 1 $ = 100 credits

Noise suppression hearing aidNoise-suppression hearing aids have a strong ability to suppress environmental noise and can be used in situations where the noise is as high as 90dB. The voice is clear and not noisy, with only weak background noise in the headphones.The schematic diagram of the noise suppression hearing aid circuit is shown in Figure 111-1. The right side of the dotted line is the main circuit of the HA-5 hearing aid assembled with TDA2822M or D2822 integrated circuits that is currently cheaper and more popular. It is essentially a BTL power amplifier composed of TDA2822M integrated circuit. The left side of the dotted line is the additional "noise suppression preamplifier circuit". Only this part will be highlighted below.This part consists of a low-noise transistor VT as the core to form a preamplifier. RP is used to adjust the volume, and the noise suppression function is completed by two electret microphones connected in series. The electret microphone (BP) is composed of a condenser microphone capsule with polarization voltage provided by electret material and a special field effect tube. Its internal structure and external wiring diagram are shown in Figure 111-2. The capacitor film is connected between the gate G of the field effect transistor and the ground, and the signal is output by the field effect transistor. This kind of microphone has two types: source output mode and drain output mode. When the drain output mode is used, the phase of the output signal is opposite to that of the input signal; when the source output mode is used, the phase of the output signal is the same as the input signal. The design of this circuit cleverly takes advantage of this. alt="171.jpg (45744 bytes)">Figure 111-1In the circuit shown in Figure 111-1, BPA is the source output mode. At this time, BPB can be regarded as the source resistance of BPA. BPB is a drain output mode. In the same way, BPA can also be regarded as the drain resistance of BPB. If the same external signal acts on two electret microphones A and B at the same time, they will output two signals with the same waveform and opposite phases at the positive terminal of C1; thus "cancelled" and cannot be sent to the base of VT through C1, so the signal cannot emit sound from the headphones.Generally, hearing aids only have one microphone, while noise-cancelling hearing aids have two microphones. One microphone (BPA) is installed on the front of the hearing aid box, and the other microphone (BPB) is installed on the back of the box. It can be seen from practice that environmental noise can be considered to fill the entire room, so the intensity in front and behind the hearing aid box is almost equal, which makes its amplitude the same and opposite in phase and cancel it out. If the user points the BPA toward the speaker, then microphone A receives speech (stronger than BPB) plus noise, while microphone B still receives speech (but weaker than BPA) plus noise. The signal sent out at the positive end of C1 is only the voice strength value signal of BPA and BPB. As can be seen from the above, no matter how large the absolute value of the noise signals sent to microphones A and B is, their sum is always equal to zero. …

Sign in to unlock

Login / Register

One-time payment per article, billed in credits (1 $ = 100 credits). You will review the invoice and the non-refundable digital-asset policy before anything is charged. The PDF is generated on demand and embeds a licensing watermark.