Propeller noise active noise reduction method based on anti-phase sound waves

Abstract

The invention belongs to the technical field of aviation science, and relates to a propeller noise active noise reduction method based on anti-phase sound waves. The main noise source of the propeller unmanned aerial vehicle is reduced in an active noise reduction mode, and the method has the advantages that whether active noise reduction is started or not is determined based on the actual use condition, and airborne energy of the aircraft is effectively saved; aerodynamic noise of the propeller unmanned aerial vehicle is reduced by adopting an inverse sound wave principle, and the method has the advantages that the propeller noise propagation direction is directionally reduced, and the application scene is more targeted; the process of firstly collecting and then sounding is adopted to reduce the noise of the propeller, and the method has the advantages that the noise in the specified direction is eliminated in a more targeted manner, and the noise reduction effect is improved.

Description

Technical Field

[0001] The invention belongs to the field of aviation science and technology, and relates to a method for actively reducing propeller noise based on anti-phase sound waves. Background Art

[0002] The low-altitude economy is a comprehensive economic model driven by various low-altitude flight activities of manned and unmanned aircraft, radiating and driving the integrated development of related sectors. It is widely reflected in the primary, secondary, and tertiary industries and plays an increasingly important role in promoting economic development, strengthening social security, and serving national defense. The low-altitude economy primarily relies on low-altitude aircraft, such as eVTOLs (electric vertical take-off and landing vehicles), drones (consumer and industrial), helicopters, and traditional fixed-wing aircraft, with propeller-propelled or vertical take-off and landing aircraft being the main products.

[0003] In future urban environments, low-altitude aircraft will be a common presence in people's lives. With the increasing number of aircraft, the noise generated by propellers will become increasingly noticeable, especially on city streets, where the noise transmitted between buildings will seriously affect the normal work, study, and daily lives of nearby residents. In addition to low-altitude economic operations, rotorcraft are also primarily used for close-range reconnaissance on battlefields. When flying close to the target, noise is one of the factors that determine their detection. Summary of the Invention

[0004] Purpose of the invention: The present invention adopts the principle of anti-phase cancellation and adopts active noise reduction means to reduce the noise of propellers and rotorcraft to expand their use in future low-altitude economic and military scenarios.

[0005] The technical solution of the present invention is as follows: 1. A method for actively reducing propeller noise based on inverse sound waves, which comprises installing three or more noise sound wave collection systems, inverse sound wave calculation systems and three or more active noise reduction wave sound generation systems at different positions on the fuselage or landing gear below a propeller aircraft and a rotorcraft; the noise sound wave collection system collects the noise field generated by the propeller of the aircraft during flight, and transmits it to the inverse sound wave calculation system through a signal line, and the inverse sound wave calculation system calculates the distribution of the noise field and calculates the inverse sound wave, and the active noise reduction wave sound generation system emits sound waves in reverse phase to the noise of the propeller aircraft and the rotorcraft, and controls the phase of the sound waves to offset or weaken the noise of the propeller aircraft and the rotorcraft.

[0006] Furthermore, the noise sound wave includes amplitude, phase, direction, and frequency information of the noise wave.

[0007] Furthermore, the noise sound wave wavelength calculation formula is:

[0008]

[0009] Where λ is the wavelength of the sound wave, f is the frequency, and v is the local speed of sound.

[0010] Furthermore, the frequency range of the propeller noise of the rotorcraft can be divided into the following intervals:

[0011] (1) Main rotor noise

[0012] Low frequency dominance: 20–100 Hz;

[0013] Corresponding wavelength: 3.4–17 meters;

[0014] (2) Tail rotor / ducted tail rotor noise

[0015] Mid-high frequency band: 100–2000Hz;

[0016] Corresponding wavelength: 0.17–3.4 m;

[0017] (3) Special operating noise

[0018] High-frequency noise: 1–5kHz;

[0019] Corresponding wavelength: 0.068–0.34 meters.

[0020] Furthermore, the noise sound wave collection system is mainly a miniature microphone array, which is installed on the fuselage or landing gear below the aircraft propeller; the miniature microphone array includes more than three miniature microphones, forming a surface in the direction of sound wave propagation.

[0021] Furthermore, the minimum spacing of microphone arrays on different aircraft is different and should be greater than the minimum sound wave wavelength.

[0022] Furthermore, the micro-microphone array converts the captured propeller noise into an electrical signal. According to the Nyquist theorem, the sampling rate must be ≥ 2 times the noise frequency.

[0023] Furthermore, the anti-phase sound wave calculation system is installed in or on the aircraft body, receives the noise field collected by the noise sound wave collection system, decomposes the time domain signal into frequency domain components based on the fast Fourier transform (FFT), extracts the spectral characteristics of the noise, and dynamically generates a reverse signal with a phase difference of 180° from the noise through an adaptive algorithm according to the use environment and the position and orientation of the active noise reduction sound wave sound system, adjusts the amplitude matching, determines the direction of active noise reduction, solves the anti-phase sound field of the noise field, and transmits the sound signal of the active noise reduction field to the active noise reduction sound wave sound system.

[0024] Furthermore, the active noise reduction sound wave sounding system is composed of an array of multiple sound wave sounders, which are installed under the miniature microphone array. The installation requirements of the sound wave sounding array are the same as those of the miniature microphone array.

[0025] Technical effects of the present invention:

[0026] 1. Active noise reduction is used to reduce the main noise source of propeller drones. Its advantage is that it decides whether to activate active noise reduction based on actual usage, effectively saving the aircraft's onboard energy;

[0027] 2. The principle of anti-phase sound waves is used to reduce the aerodynamic noise of propeller drones. Its advantage is that it supports directional reduction of the propeller noise propagation direction, making the application scenario more targeted;

[0028] 3. The process of collecting first and then emitting sound is used to reduce propeller noise. Its advantage is that it can eliminate noise in a specified direction in a more targeted manner and improve the noise reduction effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Flowchart of the noise reduction method of the present invention;

[0030] Figure 2 Schematic diagram of the active noise reduction system for propeller noise based on anti-phase sound waves

[0031] Among them: 1--Noise sound wave acquisition system 2--Anti-phase sound wave calculation system 3--Active noise reduction sound wave system DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to the accompanying drawings:

[0033] like Figure 1 Figure 2 As shown, the present invention collects propeller noise by installing a sound wave collection device, a reverse sound wave intelligent computing module, and a sound wave generator on the propeller and the rotorcraft, calculates the reverse sound wave through the intelligent computing module, and emits a sound wave with a phase opposite to the propeller noise through the sound wave generator, thereby offsetting or weakening the impact of the propeller noise.

[0034] (1) Multi-point surface measurement

[0035] Noise fields diffuse in three-dimensional space, with information such as directionality, amplitude, frequency, and phase varying at every point and in every direction. Furthermore, the speed of noise propagation varies with the influence of propeller downwash, significantly complicating the construction of a complete noise field. The present invention employs three or more acoustic wave acquisition devices spaced apart on the aircraft fuselage or landing gear below the propellers to collect acoustic wave information at specific locations, including amplitude, phase, frequency, and direction. Using an intelligent computing module, the aircraft noise field is rapidly generated, and a digital simulation of noise in a specific direction is obtained, providing support for generating antiphase noise and controlling the sound field.

[0036] (2) Opposite phase cancellation

[0037] Based on the aircraft noise sound field obtained from multi-point surface measurements, the anti-phase sound field of the noise field is calculated by the anti-phase sound wave calculation system. Combined with the layout of the sound wave generating device, the aircraft operating speed, the propeller downwash airflow and other information, the anti-phase sound field is decomposed into corresponding sound wave generation instructions. The sound generating device quickly generates an anti-phase sound field, which is coupled with the original noise field to generate a synthetic sound field, thereby reducing the vibration between the peaks and troughs of the original sound field and achieving the function of active noise reduction.

[0038] (3) Synthetic steering

[0039] The principle behind the generation of a controllable sound field is to actively superimpose an anti-phase sound wave on the noise field's sound waves to cancel out the noise's amplitude. This can be achieved in principle by controlling the sound wave's phase, amplitude, and frequency. Given the limited onboard capabilities and computing resources of a propeller aircraft, acoustic wave generators are deployed at different locations. Based on the predicted noise field and the direction of the noise to be silenced, an anti-phase sound wave field is generated, canceling out noise to a certain extent and forming a synthetic controllable sound field.

[0040] The active noise reduction method for propeller noise based on inverse sound waves is achieved by installing more than three noise sound wave collection systems, inverse sound wave calculation systems and more than three active noise reduction wave sound generation systems at different positions of the fuselage or landing gear below the propeller aircraft and rotorcraft; the noise sound wave collection system collects the noise field generated by the propeller of the aircraft during flight and transmits it to the inverse sound wave calculation system through a signal line, the inverse sound wave calculation system calculates the distribution of the noise field and calculates the inverse sound wave, and the active noise reduction wave sound generation system emits sound waves that are in opposite phase to the noise of the propeller aircraft and rotorcraft, and controls the phase of the sound waves to offset or weaken the noise of the propeller aircraft and rotorcraft.

[0041] The noise sound wave includes the amplitude, phase, direction, and frequency information of the noise wave.

[0042] The calculation formula for the wavelength of the noise sound wave is:

[0043]

[0044] Where λ is the wavelength of the sound wave, f is the frequency, and v is the local speed of sound.

[0045] In practical applications, it is necessary to consider that atmospheric conditions (temperature and humidity) cause the speed of sound to vary by approximately ±10 m / s and the wavelength to fluctuate by approximately ±3%.

[0046] According to existing data, the frequency range of propeller noise of rotorcraft (such as helicopters) can be divided into the following intervals:

[0047] (1) Main rotor noise

[0048] Low frequency dominance: 20–100 Hz (fundamental frequency and harmonics)

[0049] Corresponding wavelength: 3.4–17 meters (calculated based on the speed of sound of 340 m / s)

[0050] Characteristics: Mainly composed of blade rotation fundamental frequency and low-frequency harmonics, it has strong penetrating power and is easy to induce structural resonance.

[0051] (2) Tail rotor / ducted tail rotor noise

[0052] Mid-high frequency band: 100–2000Hz (typical peak 500–1000Hz)

[0053] Corresponding wavelength: 0.17–3.4 meters

[0054] Characteristics: The frequency is higher than that of the main rotor. Due to the small size and fast rotation speed of the tail rotor, the noise energy is concentrated in the narrowband harmonics.

[0055] (3) Special operating conditions noise (such as maneuvering flight, turbulence interference)

[0056] High-frequency noise: 1–5kHz (propeller vortex interference noise)

[0057] Corresponding wavelength: 0.068–0.34 m

[0058] Characteristics: Produced by the interaction between blades and turbulence, similar to the sound segment of a baby crying (100–5000Hz), with high perception sensitivity.

[0059]

[0060] The noise wavelength range of rotary-wing drones is mainly dominated by their high-frequency characteristics and can be divided into the following ranges:

[0061]

[0062]

[0063] The noise sound wave collection system is mainly composed of a micro-microphone array, which is installed on the fuselage or landing gear below the aircraft propeller. The microphone array should contain more than three micro-microphones, and the microphones cannot be distributed on the same straight line. At least one surface should be formed in the direction of sound wave propagation. The minimum spacing of microphone arrays on different aircraft varies and should be greater than the minimum sound wave wavelength. For example, the minimum spacing for eVTOL multi-rotor drones is 0.068m, and the minimum spacing for quad-rotor drones is 0.043m.

[0064] The micro-microphone array converts the captured propeller noise into an electrical signal. According to the Nyquist theorem, the sampling rate must be ≥ 2 times the noise frequency.

[0065] The anti-phase sound wave calculation system is installed in or on the aircraft body, receives the noise field collected by the noise sound wave collection system, decomposes the time domain signal into frequency domain components based on the fast Fourier transform (FFT) through the built-in intelligent algorithm, extracts the spectral characteristics (frequency, amplitude, phase) of the noise, and dynamically generates a reverse signal with a phase difference of 180° from the noise through an adaptive algorithm (such as the LMS algorithm) according to the use environment and the position and orientation of the active noise reduction sound wave sound system, adjusts the amplitude matching, determines the direction of active noise reduction, calculates the anti-phase sound field of the noise field, and transmits the sound signal of the active noise reduction field to the active noise reduction sound wave sound system.

[0066] The anti-phase acoustic wave calculation system uses the principle of destructive interference of acoustic waves, that is, destructive interference occurs when two acoustic waves meet the following conditions:

[0067] Same frequency: The vibration frequency of the noise and the anti-phase sound wave is consistent;

[0068] Phase difference of 180° (π radians): The peaks of the anti-phase sound waves align with the troughs of the noise;

[0069] Equal Amplitude: The two waves have the same amplitude of vibration.

[0070] Sound pressure amplitude after superposition:

[0071] A 总 =A 噪声 +A 反相 =0, (under ideal conditions).

[0072] The active noise reduction wave generator system consists of an array of multiple sound wave generators, installed below the micro-microphone array. The sound wave generator array requires the same installation requirements as the micro-microphone array. It receives the inverted signal of the active noise reduction field provided by the inverted sound wave calculation system and emits sound waves to form an active noise reduction wave field, achieving the active noise reduction effect.

[0073] The calculation method of the anti-phase sound wave is combined with information such as the layout of the sound wave generating device, the aircraft operating speed, and the propeller downwash airflow. The active noise reduction wave sound generation system quickly generates a sound field with an anti-phase phase, decomposes the anti-phase sound field into corresponding sound wave generation instructions, and couples it with the original noise field to generate a synthetic sound field, thereby reducing the vibration between the peaks and troughs of the original sound field and achieving the function of active noise reduction.

[0074] (1) Influence of wind speed on sound wave propagation

[0075] The propagation of sound waves relies on the vibration of molecules in the air medium. In a static environment, the vibration of sound waves can be easily simulated and reproduced. However, in a flowing medium, the propagation of sound waves can be decomposed into the superposition of two motions: the flow of air and the motion of the sound waves relative to the air. When the aircraft's flight speed fluctuates and the propeller downwash flows at high speed, the propagation of sound waves is difficult to simply analyze and model.

[0076] A simple analysis is as follows: Let the wind speed be u, the speed of sound waves in the air reference frame be v, and the sound wave originates from point S. Consider two observers, T1 and T2, located downwind and upwind, respectively, at a distance L from point S. After the sound originates from the source, it propagates in the air reference frame as a spherical wave.

[0077] In the air reference frame, the equivalent distance a sound wave travels from the source to the receiver is

[0078]

[0079] Therefore, when the wind is blowing in the right direction, the energy surface density received by the receiver is:

[0080]

[0081] When facing against the wind, the energy surface density received by the receiver is:

[0082]

[0083] From the ground reference frame, the listener receives more energy when the wind is blowing downwind.

[0084] Aircraft noise is caused by the interaction between high-speed propellers and the air. As the propellers move, they rapidly push the air in contact downward, causing downwash. This causes the normal vibrations of the noise-transmitting air molecules to be superimposed on particle motion, making the simulation and construction of the noise field even more difficult. Furthermore, during flight, the aircraft also experiences three-dimensional motion, which adds to the influence of the aircraft's motion on the noise-transmitting molecules. This coupling of multiple factors makes noise field modeling and prediction extremely complex. However, onboard computing and acquisition resources are limited. Obtaining a relatively accurate noise field prediction within these extremely limited resources is crucial to the successful synthesis of a controllable sound field.

[0085] (2) Briefly analyze the noise spectrum analysis process using Fourier transform

[0086] 1) Sampling: Discretize the continuous sound wave into a digital signal to satisfy the Nyquist theorem.

[0087] 2) Framing: The long signal is divided into short time segments, and each frame is filtered through a window function (such as Hanning window) to reduce spectrum leakage.

[0088] 3) Perform FFT on each frame of signal to obtain the complex spectrum result:

[0089] ●Amplitude spectrum: The energy intensity of each frequency component (|FFT|).

[0090] ●Phase spectrum: The time offset of the frequency components (usually negligible in noise analysis).

[0091] 4) Energy distribution identification: Noise presents a specific pattern in the frequency domain:

[0092] ●White noise: evenly distributed across the entire frequency band.

[0093] ●Periodic noise: specific frequency spikes (such as 50Hz power frequency interference).

[0094] ● Colored noise: The energy is concentrated in a certain frequency band (such as the low-frequency roar of a fan).

[0095] 5) Statistical comparison: Establish a baseline spectrum through background noise samples, compare it with the current signal spectrum, and locate abnormal frequency bands.

[0096] Fourier transform spectrum analysis can intuitively locate the interference source and directly observe the frequency where energy is concentrated, which facilitates filter design. In addition, the FFT algorithm has low complexity and is suitable for real-time noise analysis.

[0097] (3) Generation of anti-phase noise field

[0098] The anti-phase sound wave calculation system converts the time domain noise signal into frequency domain components based on Fourier transform, identifies the main frequency of the noise, and calculates the phase information of the noise signal to provide a benchmark for generating anti-phase sound waves.

[0099] The anti-phase sound wave calculation system performs a 180° phase shift on the original noise signal to generate an anti-phase sound wave (same amplitude, opposite phase).

[0100] The LMS / RLS algorithm is used to dynamically optimize the anti-phase sound wave parameters to adapt to noise changes, such as noise fluctuations caused by wind speed changes, the array arrangement of active noise reduction wave sound systems, and directional noise reduction requirements.

[0101] After the inverted noise field is generated, it is converted into an electrical signal that can be executed by the active noise reduction wave sound system through signal transformation. The inverted sound wave is output through the speaker array and superimposed with the original noise in space to achieve destructive interference.

[0102] The noise field of propeller drones has a certain directionality in its impact on the surrounding environment, causing interference only to nearby users. Or during close reconnaissance, the propeller noise only needs to be silenced in the direction or object being reconnaissanced. Therefore, active noise reduction can be performed only on the noise field within a certain angle around it, so that the noise in a limited direction can be reduced. This can not only reduce unnecessary energy consumption, but also reduce the computational burden of the intelligent computing module. When calculating the generation of the anti-phase sound wave, by adjusting the noise phase, active noise reduction measures can be implemented only in the specified direction. The basic calculation formula for the synthetic sound wave phase is:

[0103] A 总 =A 噪声 sin(ωt)+A 反相 sin(ωt+φ) (φ is the phase difference)

[0104] The inverse sound wave calculation system controls the sound wave phase through FIR / IIR filters or time domain delays (such as sampling point offsets).

Claims

1. A propeller noise active noise reduction method based on anti-phase sound waves, characterized in that: By installing more than three noise sound wave collection systems, inverse sound wave calculation systems and more than three active noise reduction wave sound generation systems at different positions on the fuselage or landing gear below the propeller aircraft and rotorcraft; the noise sound wave collection system collects the noise field generated by the propeller of the aircraft during flight, and transmits it to the inverse sound wave calculation system through a signal line. The inverse sound wave calculation system calculates the distribution of the noise field and calculates the inverse sound wave. The active noise reduction wave sound generation system emits sound waves that are in opposite phase to the noise of the propeller aircraft and rotorcraft, and controls the phase of the sound waves to offset or weaken the noise of the propeller aircraft and rotorcraft.

2. The active noise reduction method for propeller noise based on anti-phase sound waves according to claim 1, characterized in that: The noise sound wave includes the amplitude, phase, direction, and frequency information of the noise wave.

3. The active noise reduction method for propeller noise based on anti-phase sound waves according to claim 1, characterized in that: The calculation formula for the wavelength of the noise sound wave is: Where λ is the wavelength of the sound wave, f is the frequency, and v is the local speed of sound.

4. The active noise reduction method for propeller noise based on anti-phase sound waves according to claim 1, characterized in that: The frequency range of the rotorcraft propeller noise can be divided into the following intervals: (1) Main rotor noise Low frequency dominance: 20–100 Hz; Corresponding wavelength: 3.4–17 meters; (2) Tail rotor / ducted tail rotor noise Mid-high frequency band: 100–2000Hz; Corresponding wavelength: 0.17–3.4 m; (3) Special operating noise High-frequency noise: 1–5kHz; Corresponding wavelength: 0.068–0.34 meters.

5. The active noise reduction method for propeller noise based on anti-phase sound waves according to claim 1, characterized in that: The noise sound wave collection system is mainly a micro-microphone array, which is installed on the fuselage or landing gear below the aircraft propeller; the micro-microphone array includes more than three micro-microphones, forming a surface in the direction of sound wave propagation.

6. The active noise reduction method for propeller noise based on anti-phase sound waves according to claim 1, characterized in that: The minimum spacing of microphone arrays on different aircraft varies and should be greater than the minimum sound wave wavelength.

7. The active noise reduction method for propeller noise based on anti-phase sound waves according to claim 1, characterized in that: The micro-microphone array converts the captured propeller noise into an electrical signal. According to the Nyquist theorem, the sampling rate must be ≥ 2 times the noise frequency.

8. The propeller noise active noise reduction method based on anti-phase sound waves according to claim 1, characterized in that: The anti-phase sound wave calculation system is installed in or on the aircraft body, receives the noise field collected by the noise sound wave collection system, decomposes the time domain signal into frequency domain components based on the fast Fourier transform (FFT), extracts the spectral characteristics of the noise, and dynamically generates a reverse signal with a phase difference of 180° with the noise through an adaptive algorithm based on the use environment and the position and orientation of the active noise reduction sound wave sound system, adjusts the amplitude matching, determines the direction of active noise reduction, calculates the anti-phase sound field of the noise field, and transmits the sound signal of the active noise reduction field to the active noise reduction sound wave sound system.

9. The propeller noise active noise reduction method based on anti-phase sound waves according to claim 1, characterized in that: The active noise reduction sound wave system is composed of an array of multiple sound wave generators, which are installed below the miniature microphone array. The installation requirements of the sound wave array are the same as those of the miniature microphone array.

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