Author: Wei Kun
This is a noise-saving headset that is designed to operate. The so-called "noise reduction headset" is relatively quiet in the more noisy environment after wearing it.
The author introduces its working principle. We know that the sound is a mechanical wave. It is produced by mechanical vibration. We perceive the sound of sound waves into the tympanic membranes in the ear, depending on the principle of sound wave interference, if the two columns are the same, the opposite wave encounters. They will weaken each other, if the amplitude of the two waves is also equal, they will counteract each other. That is, if we can emit a sound wave that is the same as the frequency of noise sound waves, the phase is opposite and the amplitude is equal. Noise sound waves and headphones will cancel each other, theoretically no sound. However, it is difficult to do it. Although the noise-saving headset made in this principle cannot completely eliminate noise, it can greatly reduce noise, which also has achieved the purpose of design, and the external source is also designed in the circuit. Input port. Connected to the external source such as CD Walkman, MP3, can get relatively quiet to listen to music in the noise environment.
The principle of circuit is as shown above, the figure below. R1, R2, R3, C1 are the two channels of channels, C2 and C3 couple the sound signals picked up by the two MICs to op amp IC1-A and IC1-B, perform the first level as large, magnification ( Take IC1-A as an example:
A1 = (R8 + R6) / R6 = (33 + 1) / 1 = 34 times
The zoomed signal is divided into two roads, all the way straight through the phase switching switch S1, and the other will enter IC2-A and IC2-B inversion. The magnification is:
A2 = R12 / R10 = 10/10 = 1 times
The signal output from the inverter amplifier is only opposite, and the amplitude is not changed, and the phase switch is performed by S1-A and S1-B, and then enter the volume potentiometer R14, adjust the volume to make the sound wave amplitude emitted from the earphone and the noise wavelength. To achieve the best noise reduction effect, the signal output from the volume potentiometer enters the headphone drive amplifier composed of IC3-A and IC3-B, and the magnification is:
A3 = R18 / R14 = 100/10 = 10 times
The entire noise reverse magnifying magnification is A1A2A3 = 34 x 1 × 10 = 340 times. R23 is the volume potentiometer of an external sound source input signal. IC3 enlarges the signal input from the external source. Its magnification is:
A4 = R19 / R15 = 100/10 = 10 times
The above is a brief analysis of the circuit.
Because the frequency of the sound wave output is the same as the noise, the amplitude is the same as the noise, the MIC should be as close to the ear drum film, so the MIC is mounted on the headset, and MIC can be adhered to the housing of the earphone. That is, the left channel of the left sound is mounted on the left earphone, the MIC of the right channel is mounted on the right headset, and since the distance from the MIC and the ear drum film is closer. Therefore, the income error is relatively small, which can be ignored. The headphones are best used with headphones. Because this MIC is better installed.
The two-section 9V laminated battery for the power supply can also be used with the voltage regulator of Fig. 2, and the two channels of the sealing element error should be as small as possible. The characteristics of the two MICs are consistent, otherwise it will affect the noise reduction performance. Use NE5532 if you want to supply power with two 3.6V lithium batteries. Then the NE5532 is replaced with TDA1308, and performance is also quite good. Since the circuit is relatively simple, you can use the universal board to save the printed board.
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