TW201322782A - Implementation of active noise cancellation of exhaust hood using phase compensation technology - Google Patents

Abstract

Using the embedded systems or the analog circuits to reduce the noise in an open environment effectively, it is a difficult work until now. Especially for the hood, the wind noises are come into the 3D multi-directional hybrid frequency turbulence noises due to the influence of the structure of a hood system, and the phase offsets induced from the general noise cancellation circuits, speaker, microphone and noise reduction mechanism depended on the different frequencies. So, the effect of lower noise is not easy to control and to improve. In the study, the proposed method will focus on the research of active noise cancellation and implement the function of noise cancellation for the exhaust hood system. First, the method uses FFT to calculate the spectrum range of the noise sources. Second, in the whole noise cancellation system, they are divided into noise cancellation mechanisms and active noise cancellation circuits. It is useful to reduce the noises when equipping them with an exhaust hood. For the noise cancellation mechanisms, the structure transfers the 3D disturbed noise sources into 1D controllable noise sources. For the active noise cancellation circuits, first, the phase characteristics of the whole system are measured and then, the phase offsets for main frequencies are recorded into one table. After this, the corresponding phase compensation circuit according to the table is designed. When the active noise cancellation system starts to work, the 1D noises of the exhaust hood system are recorded by a microphone, and then the phases are compensated by the circuits. To compensate the phases, the signals with inverse phase are transmitted from an amplifier to the speaker to cancel the noise sources. Due to the inverse phase noise signals producing a destructive interference with the original noise sources, the goal to decrease the decibel value of the nose is achieved. The implementation of the present invention includes the following methods: 1. Using the noise cancellation mechanisms, the structure transfers the multi-directional hybrid frequency 3D turbulent noise sources into 1D controllable noise sources. 2. Improving the active noise cancellation circuits, first, build the table of phase offset corresponding to frequency by measuring the phase characteristics of the whole system and the available range of phase compensation for the circuit; then, the real-time corresponding phase compensation circuit according to the table and the range is designed.

Description

Realization of active noise reduction of range hood by phase compensation technology

The technique described in the present invention is embodied in an open environment in which a multi-directional three-dimensional spoiler noise source is converted into an easily controllable one-way one-dimensional noise source by using a noise reduction sound guiding area and a sound guiding tube mechanism. Using the fast Fourier transform FFT to measure the wind noise of the entire range of range hood systems and the complex noise phase-to-frequency characteristics generated by the phase delay of the circuit system, horn unit, microphone, noise reduction mechanism, etc., to establish the main frequency A table of phase offset records. Applying this table, using the active noise reduction circuit, the phase offset meter is used to implement the corresponding real-time phase compensation circuit to generate the anti-phase noise. After the inversion, the amplifier circuit transmits the noise to the horn, and the waveforms cancel each other out. Destructive interference with the noise source to reduce the noise decibel value.

In an open environment, it is still a difficult task to use a system such as an embedded circuit or an analog circuit to effectively reduce the decibel value of the spoiler noise. At present, the research and development of related products focuses on closed noise canceling headphones and open noise elimination with fixed noise sources, and some results have been achieved. The former noise canceling earphone is closed, the volume is low, and the system loop has a short phase delay. Therefore, the general wave type inversion or DSP technology can directly eliminate the effect; the latter has a fixed noise and a relatively low noise frequency. Focusing on certain frequency bands, the direction of noise progression is more predictable, and methods such as FXLMS have good noise reduction effects. However, the noise of the actual range hood is mainly wind-shearing, which is a multi-directional three-dimensional spoiler noise source, plus the phase delay of the circuit system, the horn unit, the microphone, the noise reduction mechanism, etc., and the actual noise reduction effect has been very difficult. British Silentium manufacturers have already applied active noise reduction technology to the range hood, although it has some effects, but it is different from the implementation methods 1, 2 mentioned in the Chinese abstract.

In an open environment, the system uses an embedded circuit or an analog circuit to effectively reduce the noise decibel value of the range hood, overcome the wind-cut sound affected by the mechanism, generate a multi-directional mixed frequency spoiler noise source, and a general noise reduction circuit. The system, speaker unit, microphone, noise reduction mechanism, etc. will have different phase offsets for different frequencies. This study proposes a new active noise reduction technology for the range hood. The system can be divided into two parts: the noise reduction mechanism and the active noise reduction circuit. The noise reduction mechanism allows the multi-directional three-dimensional disturbance noise source to be transmitted in the closed pipeline space to form a one-dimensional noise source noise source that is easy to control; the active noise reduction circuit is responsible for completing Noise reduction work, noise reduction process is achieved in the entire system. The implementation method includes:

1. Use the flow-conduction noise reduction mechanism to convert the multi-directional three-dimensional spoiler noise source into an easy-to-control one-directional one-dimensional noise source.

2. Improve the active noise reduction circuit design method and design an adjustable real-time phase compensation circuit. include:

(1) Pre-measure the phase characteristics of the entire system and establish a phase offset table of the main frequency;

(2) Analyze the upper and lower range and the best effective frequency band of the compensable phase of the phase compensation circuit.

The corresponding phase compensation circuit is designed by the adjustment of (1) and (2).

Figure 1 is a schematic diagram of the system and function deployment diagram. The sub-items of the implementation method are described in detail as follows:

1. Use the flow-conduction noise reduction mechanism to convert the multi-directional three-dimensional spoiler noise source into an easy-to-control one-directional one-dimensional noise source.

The composition of the mechanism is composed of one-dimensional pipe, speaker unit, speaker, screw, microphone support frame and U-shaped iron frame. The structure diagram is shown in Figure 1. It is divided into vertical and side vertical. The upright function, structure, and entity diagram are as shown in the second diagram a, b, and c, and the side elevation is as shown in the first diagram. When the motor of the range hood is running, the disturbed multi-directional noise source is transmitted from the upper side of the noise reduction mechanism to the one-dimensional pipeline of the noise reduction mechanism, becoming a relatively simple one-way noise source, and the microphone is placed at the center of the pipeline, the horn Installed in the speaker, placed in the vicinity of the air outlet of the lower side or right side of the noise reduction mechanism, responsible for playing the anti-phase noise before the noise flows out of the noise reduction mechanism, and the microphone is placed in the position as shown in the figure, responsible for collecting noise data.

2. Improve the active noise reduction circuit design method and design an adjustable real-time phase compensation circuit.

The active noise reduction circuit design method includes:

(1) Pre-measure the phase characteristics of the entire system and establish a phase offset table of the main frequency;

(2) Analyze the phase characteristics of the entire system and the optimal effective area and upper and lower range of the phase compensation circuit.

(3) Design the corresponding active noise reduction phase compensation circuit based on the adjustment of (1) and (2).

During the analysis process, when the microphone is measured close to the fan and motor in the exhaust hood (Sakura brand, model R-7780BSHXL), the noise signal waveform is time-domain diagram as shown in the third figure, a spectrum from MATLAB. The analysis is mainly distributed in the low frequency region, and the energy at the high frequency is low, as shown in the figure b of the third figure. 0 to 300 Hz is the area where the noise energy distribution is the most concentrated, and the energy above 300 Hz is lower. The energy at a low rotational speed is only 3,000 or more and 4,000 or less, and the energy at a specific frequency of the medium rotational speed exceeds 4,000, and the energy at a specific frequency of the high rotational speed exceeds 5,000. In the figure c of the third figure, the spectrum of the microphone is measured at a position farther than the exhaust hood directly below the range hood. It should be noted that when the microphone measurement position is different or the microphone material is different, the time domain map and the spectrum energy map and the distribution map are different, but the active noise reduction has a falling effect.

The active noise reduction technology has been experimentally proved that the noise reduction effect in the lower frequency domain is better than the passive noise reduction technology, and the space volume of the whole system is also small. In passive noise reduction, the wavelength of the sound wave exceeds the thickness of the sound absorbing material, and the thickness must be increased to offset the low frequency noise, which is not suitable for low frequency noise reduction. Therefore, this study proposes a new phase compensation circuit to achieve active noise reduction and verify the method of reducing the noise of the range hood.

The phase compensation denoising method is based on the interference of sound waves. The interference phenomenon is based on the principle of linear addition. When two or more waves are added, in an ideal case, the amplitude of the synthesized wave is added to the amplitude of all the waves. . The destructive interference of sound waves means that when two sound waves meet and overlap, when the wave peak of one wave is exactly the wave of the other wave, the two waves are said to be inverted. When two inverted waves interfere, the amplitude is exactly the sum of the two wave amplitudes, which is the minimum amplitude that can be synthesized by the two waves, as shown in the fourth figure. The amplitude of the composite wave is typically less than the amplitude of its original individual wave. In the active noise reduction theory, the principle of reducing the noise decibel number is achieved by using the principle that noise and reverse noise cancel each other out.

A complete active noise reduction system, the necessary circuits include a microphone, preamplifier, horn amplifier and horn, and these circuits will have their own phase response and gain response, which will vary with frequency. In order for the entire noise reduction system to accurately play out the inverted sound waves, it is necessary to measure the phase gain characteristic map of the circuit. The phase compensator of the signal causes the phase of different frequencies to lead or fall behind. From the Bode diagram of the transfer function, the phase change is displayed, and the zero-point frequency causes the phase to drop or the phase rises, as shown in the figure b of the fifth figure. The phase compensator is generally an RC circuit composed of an operational amplifier, as shown in the a diagram of the fifth figure. From the S-plane of the time domain, the transfer function F(s) is expressed as a formula (1), consisting of a first-order pole and a zero point, K is DC gain, T is the time period. If a>1, the phase is advanced, otherwise the phase is backward.

T = C ‧ R ₂ (3)

The active noise reduction circuit is composed of a microphone receiving circuit, a first-order phase backward circuit and an amplifier circuit. After the noise is collected from the microphone, the signal is phase compensated by the first-order phase backward circuit, as shown in the figure a of the fifth figure, and finally the output of the amplifier circuit is output to the speaker to play the reverse noise. The flow of the actual active noise reduction circuit is as shown in the figure a of the sixth figure, the solid phase compensation circuit is as shown in the figure b of the sixth figure, the phase delay of the system is as shown in the figure c of the sixth figure, and the gain and phase diagram of the actual phase compensation circuit are as shown in the figure. Comparing the graphs d and d, comparing the graphs c and d, the phase is close to the inverse complement. In order to ensure the complementarity is effective, another experimental bit offsets and compensates the upper and lower bounds of the effective area. The actual measured phase shift of the system and the upper and lower bounds of the compensated effective area are as shown in the figure e of the sixth figure. The e-graph is analyzed, as shown in the sixth figure. In the black box, the range of 200Hz to 300Hz is the most phase compensation phase. The noise reduction effect is the best. The frequency compensation effect is obviously low at frequencies other than 200Hz to 300Hz.

The phase characteristics of the noise reduction system, the microphone, the amplifier circuit, the horn unit and the noise reduction mechanism all cause phase shift. How to measure these phases is the basic work, and the implementation method is proposed here. A noise source for broadcasting a single frequency wave is placed at the air outlet of the range hood, a speaker for transmitting reverse phase noise is placed in the pipeline, and a microphone is placed on the back of the reverse phase noise horn. The phase measurement uses a single-frequency wave delay designed by the FPGA to adjust the phase value of the single-frequency wave output from the FPGA. The decibel count value under the range hood is minimized, and the oscilloscope is viewed to record the delay phase required for the single frequency. The environment is as shown in the seventh figure.

In this study, the actual exhaust hood noise reduction measurement environment is as shown in the eighth figure. The microphone measurement position is on the back of the horn. The hood is placed at about 147 cm from the ground and erected 10 cm below the hood. In decibels, the actual entity is as shown in the ninth figure. The measurement results, the comparison of the spectrum db decibel before and after noise reduction, as shown in Table 1, the results of noise reduction are reduced by 4.4dB in low speed, 4.4dB in medium speed and 4.2dB in high speed. The range hood is reduced by an average of 4.3dB. In order to understand the noise band actually eliminated, a microphone is installed in advance 10 cm below the range hood to record the sound of the three speeds of the range hood. The spectrum of the three speed sounds is analyzed by MATLAB, and the noise reduction of the range hood is analyzed. The main noise spectrum energy distribution is eliminated before and after. The result is mainly distributed in the low frequency section, concentrated between 200Hz and 400Hz, and the energy below 200Hz and above 400Hz is low. As shown in the tenth figure, the red (upper signal) is the spectrum before noise reduction, and the blue (lower signal) is the spectrum after noise reduction. After the overlap comparison, the energy drop in the range of 200 Hz to 400 Hz is the most.

The first picture: system and function deployment diagram a. Upright horn noise reduction system b. side horn noise reduction system.

Figure 2: Vertical vertical drainage pipe diagram: a. Vertical body flow diagram b. Vertical vertical noise reduction horn system horizontal structure diagram c. One-dimensional pipeline and horn loudspeaker entity diagram.

The third picture: noise signal a. The time domain of the low-speed, medium-speed and high-speed noise signals of the cherry blossom hood (the microphone measurement position is close to the fan and motor in the tube) b. The low speed of the cherry blossom hood , medium speed and high speed noise signal spectrum energy map (microphone measurement position close to the fan and motor) c. cherry blossom range hood low speed, medium speed and high speed noise signal spectrum energy map (microphone measurement position in The hood is far below the exhaust hood).

Figure 4: Interference of noise sound waves.

Figure 5: Phase Compensator a. RC circuit of the operational amplifier of the first-order phase compensator b. Bode plot and phase diagram of the transfer function.

Figure 6: Actual active noise reduction circuit system a. Flow reduction process b. Actual compensation circuit c. Actual system phase offset d. Actual phase compensation circuit gain, phase diagram e. Actual measured system phase offset And the upper and lower bounds of the compensation effective area f. The effective compensation interval for noise reduction.

Figure 7: Measurement environment for the phase characteristics of the noise reduction system.

Figure 8: Experimental environment diagram of actual noise reduction measurement.

Figure IX: Upright combination of the actual range hood and noise reduction mechanism.

Figure 10: Comparison of the decibel spectrum before and after noise reduction (the microphone measurement position is 10 cm directly below the range hood).

Table 1: Actual noise reduction data of the range hood.

10. . . Phase compensation table: measuring the phase characteristics of the entire system after fast Fourier transform (FFT), establishing a record of the phase shift and frequency relationship of the main frequency, as the phase compensation of the noise canceling circuit

11. . . Phase compensation: phase compensation for noise cancellation circuit based on phase compensation table

12. . . Amplifier: Elimination of noise circuit amplification

20. . . Exhaust motor / fan: noise source

twenty one. . . Ventilator

30. . . Microphone: pick up the noise source signal

31. . . Pilot zone: a sound guide that converts multi-directional three-dimensional spoiler noise into one-way one-dimensional noise

32. . . Speaker: Sending a sound wave with reversed noise

40. . . Db decibel meter: noise meter

50. . . Guide tube: a sound tube that converts a multi-directional three-dimensional noise source into a one-directional one-dimensional noise source

Claims (2)

A new noise reduction and guiding mechanism and concept to convert multi-directional three-dimensional spoiler noise sources into one-way one-way noise sources that are easy to control A design method for improving an active noise reduction circuit, and designing an adjustable phase compensation phase compensation circuit, comprising: (1) measuring the phase characteristics of the entire system in advance, and establishing a phase offset recording table of the main frequency. (2) Analyze the phase characteristics of the whole system and the phase compensation circuit to establish the best effective area and upper and lower range. (3) Design the corresponding phase compensation circuit by adjusting (1) and (2).