summary
As the wind speed increases, the contribution of wind noise gradually exceeds that of other noise sources, affecting comfort. First of all, according to the formation mechanism, acoustic analogy and pressure type of automobile wind noise, the automobile wind noise is classified in detail. Wind noise evaluation and development tools are then summarized. Finally, the characteristics and control methods of jitter noise generated by vehicle windows are discussed. Considering the appearance and field of view, it is difficult to passively control the jitter noise of the side windows. Therefore, the method of actively controlling the window size, actively opening multiple windows, and even releasing the inverted phase sound source proposed in this paper has a good application prospect based on the control logic.
introduction
Wind noise is one of the main sources of noise that affect user comfort. With the development of electric vehicles and the improvement of engine and tire noise control technology, the importance of wind noise is becoming more and more significant, and the pressure level of wind noise increases sharply with the increase of automobile speed.
In this paper, the classification of automotive wind noise sources is first analyzed from multiple angles, and the relevant theories of wind noise formation mechanism are summarized. Then, the evaluation indicators commonly used in the development of wind noise are introduced, and the perception of noise in different frequency bands by the human ear is considered by using psychological indicators. The test and simulation methods of automotive wind noise are summarized. Finally, the characteristics and control methods of automotive window noise are highlighted. This paper proposes an active control method based on control logic, which has broad application prospects.
1. Wind noise contribution
The influence of engine noise, tire rolling noise and wind noise on the interior noise can be studied through tests, and at low speeds of 50 km/h, the contribution of wind noise is small, and the internal noise is mainly based on engine noise and tire noise. At 90 km/h, wind noise contributes more than other noises above 2000 Hz in the sensitive band of the human ear. As the speed continues to increase to 160 km/h, wind noise dominates almost the entire frequency band, except for the greater contribution of engine noise at 160 Hz. Obviously, the higher the wind speed, the greater the contribution of wind noise.
2. Noise source classification
2.1. Distinguish the sealing leakage noise according to the original mechanism
Among the many wind noise sources in automobiles, leakage noise is usually the first consideration, leakage noise has medium and high frequency characteristics, and its contribution to internal noise is greater than the contribution to external shape noise, as shown in Figure 1. The most serious leaks occurred near doors, side windows, and side mirrors.
Figure 1 Comparison of leakage noise with shape noise spectrum
As shown in Figure 2, there are two mechanisms for the formation of leakage noise, the first is due to the pressure difference between the inside and outside of the car, causing the air flow through the sealing strip in and out of the car body, which will directly lead to noise generation, also known as sound absorption noise. The other is that due to the small sound loss of the sealing strip, the wind noise is transmitted directly from the outside of the car to the inside of the car through the seal, such as the cavity noise generated by the door gap.
Figure 2 Types of leakage noise mechanisms
The automotive flow field is a typical unsteady flow, especially in areas with large pressure fluctuations, such as the A-column position, where there is a non-steady mass flow at the sealing strip, forming a unipolar sound source. Airflow enters the vehicle through a gap, often forming a separation of airflows, interacting with surfaces such as door frames to form a dipole sound source. In addition, free jets in space can form quadrupole sources, although negligible in low Mach number flows.
The unsteady pressure pulsation outside the car stimulates the seal to vibrate and radiate noise inside the car. This is similar to the acoustic propagation mechanism of body panels and window glass, where the loss of acoustic propagation is related to the stiffness, quality and damping of the seal. Leakage noise can be effectively reduced by increasing the number of seals instead of increasing stiffness and damping. In addition, the stiffness of the door [8] affects the dynamic sealing performance of the door at high speed, which has a great impact on leakage noise.
Shape noise
As shown in Figure 3, there are many flow separations and vortex structures in the flow field near the vehicle, which will lead to unstable pressure pulsations, which in turn will produce a shape noise source.
Figure 3 Flow field around the car and some noise source areas
a柱
There is a strong a-post vortex between the front windshield and the side windows, which increases the noise inside the car due to the proximity to the human ear. The results showed that the unsteady pressure pulsation of the a-column vortex separation zone was significantly higher than that of the reattachment zone. When the car is yawning or has a crosswind, the a-column vortex increases on the leeward side, and the wind noise is larger. Increasing the turbulence can delay the separation of the boundary layer of the a-column and reduce the amount of a-column vortex, thereby reducing wind noise.
Rearview mirrors
As the main protrusion of the vehicle's exterior, the side mirror close to the side window is one of the main sources of shape noise. Studies have shown that increasing the gap between the mirror and the door can slow down the acceleration of local airflow and reduce wind noise. By changing the shape of the rearview mirrors, the wake is reduced, and the wind noise is significantly reduced. Both resonance of the mirror cavity and instability of the boundary layer produce unipolar noise. By changing the shape of the mirror trailing edge or turning the boundary layer into turbulence, the pole noise of the mirror can be eliminated. Current regulations require cars to be fitted with exterior mirrors, but some concept cars are designed to have no mirrors or smaller mirrors.
chassis
Since many components are exposed to airflow, a complex flow field is formed underneath the vehicle, which excites floor structure vibrations and radiates noise into the interior of the vehicle, mainly in the low frequency range, as shown in Figure 1, the skirt excludes the contribution of under-floor noise. In addition, the resonance of some of the lower cavities also forms a source of wind noise. Lower deflectors and dams deflect airflow from the bottom, reducing wind noise.
The rest of the shapes
Wind noise is more pronounced when the wiper located under the front windshield is exposed to the air. Raising the rear edge of the bonnet and hiding the wipers underneath can effectively reduce wind noise. There is a shedding vortex similar to von Carmen vortex at the tail of the antenna. Winding a spiral around the antenna reduces tonal wind noise. Reducing the coherent shedding vortex at the tail of the top plate with an irregular shape can also reduce wind noise. Although antennas and roof railings are less common in current passenger cars.
Cavity resonance noise
There are two main types of cavity resonance noise in automobiles, one is the jitter noise of large windows, and the other is the resonance noise of small cavities, such as door cracks and grille cavities. The study of cavity noise originated in the aviation industry. As shown in Figure 4, unlike wideband noise such as leakage noise and shape noise, cavity resonance noise is a kind of tone noise, usually with a fundamental frequency and a harmonic frequency.
Figure 4 Comparison of jitter spectrum with or without winding manifold sunroof
The window is vibrating
Due to the velocity gradient between the stationary air around the open window and the external airflow, the shear layer instability causes the vortex to fall off. The pressure pulsations of the shear vortex trigger the Helmholtz resonance in the cabin. The coupling of resonances to shear vortices produces frequency locking, often referred to as resonance or wind throb or booming. The more commonly used name now is buffeting. The current sunroof jitter noise is mainly eliminated by adding spoilers to reduce the excitation of shear vortexes on the sunroof. Considering the appearance and field of view of the side windows, it is difficult to control the jitter vibration of the side windows using a passive method.
Door slits and grilles
As air flows through the door panel gap, the pressure ripples within the boundary layer trigger cavity acoustic resonance, forming a wind noise source that can enter the cabin through the sealing strip. During the test, the door joint is usually sealed with tape to isolate this part of the sound source. In automotive design, assembly errors should be minimized so that door joints and chambers are as small as possible. In addition, hollow grilles can produce a similar sound source.
2.2. Distinguish by acoustic analogy
In classical acoustics, sound is produced by the compression and expansion of the surrounding air caused by vibrations on solid surfaces. Wind noise is generated by different mechanisms, but can be similar to unipolar, dipole, and quadrupole sound sources in classical acoustics. The power and radiation capacity of different sound sources are different. The unipolar source is caused by the continuous periodic compression and expansion of a pulsating sphere and radiates spherical waves toward the surroundings, as shown in Figure 5. The motion of the non-constant volume flow produces a unipolar source, a unipolar source around the seal as shown in Figure 2. Low Mach number (
ρ is the density, c is the speed of sound, and M is the ratio of flow rate to speed of sound.
Fig. 5 Unipolar sound sources radiate sound waves
As shown in Figure 6, a dipole source consists of a pair of unipolar sources with opposite phases. Unsteady pressure fluctuations on the body surface produce dipole sources. It comes from reaction forces on solid surfaces. Its sound power is proportional to the sixth power of the flow rate u
Fig. 6 Dipole sound source radiates sound waves
As shown in Figure 7, a quadrupole source consists of a pair of dipole sources that are very close but in opposite phases. Two fluids collide, creating unstable internal stresses that form a quadrupole source. It exists in an unstable shear turbulent layer. Its sound power is proportional to the 8th power of the flow rate u
Figure 7 Quadrupole sound sources radiate sound waves
In supersonic flow, the quadrupole source is the highest. In the flow around a low Mach aircraft, the unipolar source is the largest when there is leakage noise, otherwise, the dipole source is the largest.
2.3. Differentiation by type of volatility
The hydrodynamic component of pressure fluctuations is also known as turbulent pressure or convection pressure. It relies on the movement of the fluid mass and propagates in the direction of convection. Sound pressure relies on the compression and expansion of the air in all directions, it is 2 to 3 orders of magnitude lower than convection pressure, but it has a great impact on the noise inside the car. The frequency f of a pressure wave is related to the wave velocity v and wavelength λ
It is also usually expressed in wavenumber k[46].
The wave velocity of hydrodynamic pressure is determined by the speed of the car, which is much smaller than the speed of sound. That is to say, compared with sound waves, hydrodynamic pressure has a short wavelength and a large number of waves. Therefore, the coupling between hydrodynamic pressure and glass vibration bending wave is weak, and the transmission efficiency is low. Near the window glass coincidence frequency, the sound pressure can be effectively transmitted, which has a great impact on indoor noise.
3. Evaluation and development methods
3.1. Noise Evaluation
Sound pressure level and spectral component
Sound pressure is a pressure fluctuation p' due to the propagation of sound waves relative to the average pressure p in a stationary medium, in pascals (Pa).
Since the auditory range of the human ear spans several orders of magnitude, from 2 × 10 – 5 pa (audible) to 200 Pa (pain), sound is measured in logarithmic sound pressure decibels (dB) for convenience. The relationship between sound pressure level and root mean square sound pressure and reference sound pressure is as follows
Among them, Pref = 2e 5pa is the smallest sound pressure that the human ear can hear, and its frequency is 1000 Hz. Since the sound pressure stage is logarithmic, when there are multiple sound sources, it cannot be simply added linearly.
Time-domain pressure fluctuations are represented by the superposition of sound waves of different frequencies after the Fourier transform. The frequency resolution of the narrowband spectrum is equal to the ratio of the sample frequency to the number of samples used for FFT. For wideband noise, such as wind noise, local frequency information in the narrowband spectrum is usually not important. Especially when comparing experimental results with simulation results, it is difficult to obtain the same frequency resolution due to the limitations of simulation time and other factors. At this point, multiple noises can be compared using a third spectrum and an overall sound pressure level without emphasizing the frequency resolution of the narrowband spectrum. For tonal noise such as jitter, frequency resolution of the narrowband spectrum is necessary.
Loudness and A-Weighted SPL
Loudness is a psychological evaluation index that considers the human ear's perception of sound in different frequency bands. The loudness level is defined as the sound pressure level of pure sound at 1000hz, expressed in Phon. Figure 8 shows the relationship between loudness, loudness level, and sound pressure level, with the dotted line representing the lower audible limit of the human ear. Below 100hz, the isosity curve is denser, indicating that loudness is more sensitive to changes in sound pressure level at low frequencies. It is clear that due to the resonance of the ear canal, the human ear is most sensitive at 4000 hertz. The unit of loudness N is Sone, and the relationship with loudness level LN is as follows
That is, 40 "Phon" equals 1 "Sone", and for every 10 "Phon" increase in "loudness", "loudness" doubles. In the frequency domain, loudness is more sensitive to changes in low-frequency sound.
Fig. 8 Loudness profile
As shown in Figure 9, the a-weighted filter is similar to the 40 Phon isoreoffic curve spectrum, except that the filter does not capture several resonant peaks of the isoreoffic curve. Before the advent of the overall loudness calculation method, the overall sound pressure level using a weight was widely used due to the convenience of measuring the total sound energy due to the consideration of the actual perception of the human ear. Generally, the a weight is selected in the range of 24 Phon ~ 55 Phon.
Figure 9a - Weighted frequency response curve
Clarity index
The Articulation index (AI) represents the speech intelligibility of passengers in noisy environments. 100% means that the speech intelligibility is perfect, and 0% means that the noise completely masks the voice. The relationship between the upper limit of the voice H(f), the upper limit of the noise UL(f) and the lower limit of the noise LL(f) is
That is, when the noise is 12 decibels higher, the passenger's voice is completely inaudible. When the noise falls below the upper limit of 30db, the sound is completely clear. The relationship between AI and the weight factor W(f) is
thereinto
The values of the weight factor, upper bound, and lower bound values are shown in Figures 10 and 11.
Figure 10 Clarity indicator weight coefficient curve
Figure 11 Noise upper and lower limit curves
3.2. TestIng Methods
Wind tunnel and road testing
Wind noise test methods mainly include wind tunnel test and road test. Since wind speed and environmental parameters can be controlled in the wind tunnel, it is more repeatable. On the contrary, road tests are susceptible to changes in weather factors such as crosswinds, temperature, and humidity. Wind tunnel tests eliminate the effects of background noise sources such as tires, drivetrains, engines, etc., which often occur in road tests. Road tests have the advantage of low cost.
Microphone test
Automotive wind noise collection typically includes exterior surface noise, far-field noise, and in-vehicle noise. Phase microphone arrays based on beamforming techniques consist of a set of microphones with known spatial locations. Calculate the direction of propagation of the waves by analyzing the difference in phase and position of the sound waves that reach each microphone (Figure 12). It is placed outside the flow field perpendicular to the vehicle side to study the distribution of wind noise sources on the outer surface of the vehicle. Spherical microphone arrays can also be used for wind noise distribution acquisition in the cabin.
Figure 12 Array of phase microphones in a wind tunnel
Surface microphones or flat-top microphones are often used to measure pressure fluctuations at specific locations on the surface of the window. The use of a flat-top microphone requires punching holes in the surface of the vehicle, and the microphone is placed in a hole flush with the surface of the vehicle, placed outwards to avoid exposure to the flow field, which may introduce additional noise (Figure 13).
Figure 13 Embedded microphone array and surface microphone
Cabin wind noise reflects the occupant's perception of wind noise and can be captured using a normal acoustic microphone or an artificial head (Figure 14). The artificial head simulates the shape of the passenger's head and ear canal, so noise collection is directional
Figure 14 Artificial head in a car
Other tests
Seal testing
The first task of acoustic packaging is to solve the sealing problem, which determines the leakage noise. Commonly used test methods are smoke measurement, ultrasonic measurement and airtightness measurement. The air tightness measurement uses a blower to inflate the body, and when the airflow of the blower is stable, the leakage area can be calculated according to the flow rate Q.
In the formula, aD is the flow coefficient, A is the leakage area, Pi and P0 are the internal and external pressures, and ρ0 is the gas density.
Acoustic transmission loss test
The ability of shape noise to enter the cockpit through the body structure and glass is mainly related to the loss of sound propagation. The transmission loss of most body structures is much greater than that of window glass, which is the main sound transmission component. As shown in Figure 15, as the frequency increases, the transmission loss of the single-layer structure gradually increases. In the frequency domain, it can be divided into three areas: stiffness control, quality control, and coincidence effect control. In the quality control area, the slope is 6 dB per octave, and the transmission loss increases by 6 dB when the mass is doubled.
Figure 15 Loss of sound transmission of a single-layer board in different frequency bands.
In addition, plate damping can also affect transmission losses. At conforming frequencies, the transmission loss of glass is lower than in other frequency ranges. Laminated glass has a high damping, increasing the loss of acoustic propagation near the coincident frequency. The coincidence frequency of a single-layer uniform siding can be calculated according to the formula
Due to the density ρ, Poisson's ratio μ and Young's modulus E, the frequency range of glass is 4.2 to 2.1 kHz, and the glass thickness is 3 to 6 mm. Studies have shown that the sound insulation performance of laminated side window glass is improved by an average of 4db in the 2khz to 6khz frequency band, while the same thickness as tempered glass, compared to traditional tempered glass, weight reduction of 12%. Laminated glass also improves safety and resistance to intrusion.
As shown in Figure 16, the sample glass to be measured is placed between the reverberation chamber and the anechoic chamber, the reverberation chamber is placed with a spherical sound source and microphone, and the anechoic chamber is placed with a sound intensity probe. The incident sound power and transmitted sound power of the sample are as follows
thereinto
And I are the average sound pressure and average sound intensity, respectively, and S is the sample area. The acoustic propagation loss of the sample STL can be expressed as
,
Represents the average sound pressure level of the reverberation chamber and the average sound intensity level of the anechoic chamber, respectively.
Figure 16 Sound transmission loss test
Damping tests are usually performed using the attenuation rate method. As shown in Figure 17, the frequency response function of a randomly distributed accelerometer under hammering or exciter action is obtained. Through Fourier inversion and filtering, the frequency response signal is converted into a time domain signal of different frequencies. Using Schroeder's integral, the attenuation rate RD is calculated based on the slope of the attenuation plot, as shown in Figure 18. The damping loss factor for each frequency fn η calculated as follows
Figure 17 Damping loss factor test
Figure 18 Vibration attenuation plot
Sound absorption test
The sound absorption coefficient and cavity damping reflect the sound absorption capacity of the material inside the cabin. As shown in Figure 19, the material sample is placed in a reverberation chamber and its absorption coefficient and damping are calculated by reverberation time testing. When the sound source stops, the energy generated in the reverberation chamber is uniform. Under the action of specimen sound absorption, the sound pressure level is reduced by 60 dB over a period of time in T60, as shown in Figure 20. The relationship between reverberation chamber volume V, sampling area S, absorption coefficient α is
The relationship between the damping loss factor η and the reverberation time T60 and frequency f is:
The sound absorption and damping characteristics of the material can be calculated by equation 18 and equation 19 and used as input parameters such as vehicle wind noise simulation.
Figure 20 Sound pressure level attenuation diagram
3.3. Simulation Methods
Leakage noise can be studied and optimized by means such as wind tunnel tests. At present, it is difficult to directly simulate leakage noise using flow simulation. By simulating the degree of deformation of the door under pressure load in the time-sharing field, the stiffness of the door structure is improved for indirect control.
Shape noise radiates into the interior of the car through vibrations from the window glass and body panels. It is difficult and expensive to optimize the deformation of the real vehicle during the test, while the wind noise prediction and shape optimization through simulation have the advantages of high efficiency and low cost.
Cabin wind noise simulation processes typically involve obtaining external sound sources and simulating the propagation of sound through windows and inside the vehicle. External sound source simulation includes two methods: Computational Aeroacoustics and Hybrid CAA.
Direct CAA uses a compressible gas as a medium and calculates both fluid dynamic and sound pressure in the flow field, taking into account the coupling effects of the two. Traditional computational fluid dynamics (CFD) tools are mostly based on the finite volume method, which causes numerical errors due to the dispersion of time and space when solving the Navigat-Stokes partial differential equation system. In the simulation of the sound propagation process, the sound pressure is orders of magnitude smaller than the hydrodynamic pressure. Therefore, direct CAA requires high time and space discrete accuracy, large mesh size, and computing resources. The lattice Boltzmann method (LBM), which solves mesoscopic-scale molecular dynamics equations, has the advantages of high parallelism and low dissipation. After obtaining the sound source on the surface of the window, the sound pressure and hydrodynamic pressure are obtained by wave number decomposition, respectively.
Mixed CAA uses an incompressible gas as a medium, combines traditional CFD simulation with acoustic simulation, decouples the convection field from the acoustic field, and calculates the fluid dynamics and sound pressure in turn. The convection information obtained by the incompressible CFD simulation is used as the input to calculate the sound information, ignoring the influence of the sound field on the convection. Major hybrid CAA methods include lighthill acoustic analogy, Ffowcs Williams and hawkins (FW-H) integral method, acoustic perturbation equation (APE), and random noise generation and radiation (SNGR) method. Sometimes the incompressible CFD simulation is combined with the finite element acoustic simulation to obtain an external flow field sound source. When fluid software exports volumetric sound sources (convection velocity, pressure), hundreds of gigabytes of storage space are typically required. Automotive wind noise mainly involves dipole sound sources, which can be obtained through fluid software as a surface sound source, and data storage is greatly reduced.
Acoustic transmission of the window and interior can be calculated by SEA (Statistical Energy Analysis), FEM (Finite Element Method) or BEM (Boundary Element Method).
Window vibration modes below 5000 Hz are hundreds of orders, and internal acoustic response modes below 3000hz are tens of thousands of orders. The higher the frequency, the smaller the curved wavelength, the more finite element meshes are required, and the amount of computation increases dramatically. SEA is more efficient in solving for high-frequency wind noise and less accurate in low-frequency regions with lower modal density. The advantage of the finite element method is that it is possible to predict the acoustic response of a particular measurement point in the cockpit, but sometimes it is more important to quickly achieve relative optimization of wind noise. SEA is used to quickly predict the average response of a vehicle, for example in the early stages of vehicle development.
4. Jitter characteristics and control
Side window jitter is mainly related to the development of shear vortex and resonance phenomena in the cabin. This can be similar to the flow of air through a simplified cavity opening, where there are three forms of oscillation: hydrodynamic oscillation caused by instability of the velocity shear layer, fluid resonance oscillation caused by compressed expansion of air in the cavity, and hydroelastic oscillation caused by elastic displacement of the cavity solid wall, as shown in Figure 21. CFD software-based vehicle simulations typically ignore the effects of interior vibration displacement and sound absorption. Combined with CAE software, the influence of the boundary impedance in the cabin can be considered to improve the simulation confidence.
As mentioned earlier, when the shear vortex frequency coincides with the natural frequency of the cavity, the Helmholtz resonance occurs, and the jitter noise in the cavity reaches more than 100 dB, and the jitter frequency (
Fig. 21 Flow oscillation, fluid resonance, elastic cavity pressure oscillation
4.1. Jitter characteristics
Speed
As the speed increases, the shear vortex moves towards the trailing edge of the window and breaks in a shorter time, increasing the jitter frequency of the sunroof and side windows. When the jitter frequency is close to the natural frequency of the cockpit, the stronger the Helmholtz resonance, the stronger the jitter. Therefore, as the speed increases, the jitter usually increases first and then decreases, while the jitter frequency increases continuously, as shown in Figure 22.
Figure 22 Effect of wind speed on sunroof peak sound pressure level (top) and jitter frequency (bottom).
Window opening
The reduced window opening flow length shortens the motion path of the shedding vortex. At the same speed, the shedding cycle is shortened, increasing the frequency of sunroof shake. Since the flow direction length does not change much when the side window is opened, the jitter frequency remains unchanged. In addition, the smaller the window opening, the weaker the general jitter. The sunroof jitter intensity is more sensitive to the size of the opening.
Ventilation and yaw
Due to the pressure ventilation effect, the jitter vibration is weaker when multiple windows are opened at the same time than when they are opened separately. For the left rear window, the jitter is minimal when the right front window is opened at the same time. When there is a crosswind, i.e. yaw, the shaking increases on the windward side and decreases on the leeward side. When the window is closed, the broadband wind noise is the opposite of the pattern when yawing.
4.2. Jitter control mode
The current jitter control is mainly achieved through flow control, which is mainly divided into active control and passive control according to whether there is additional energy input.
skylight
Passive mode
As shown in Figure 23, the spoiler at the leading edge of the skylight will shear the vortex up to avoid collision with the trailing edge of the skylight. This reduces stress excitation on the passenger compartment and effectively controls leakage from the sunroof. The result is figure 23, the window is fully open, and at 60 km/h, the noise reduction rate of the spoiler reaches 32.6 dB.
Figure 23 The sunroof winds the airflow plate
Proactive approach
Consumers are increasingly favoring large sunroofs, and traditional fixed spoilers are difficult to meet the requirements of jitter control, and active deflection spoilers based on closed-loop control logic are more effective [95]. The dividing rod [96] interferes with the development of the shedding vortex, and through multi-parameter optimization of optimal shape and position, the skylight shake can be completely eliminated.
Side windows
Passive mode
For side window jitter, many passive control methods have been tried in previous studies. The convection length of the a-column vortex is relatively uncontrolled compared to the b-column vortex, which results in the jitter of the front window being usually lower than that of the rear window. At the same time, the rearview mirror wake interferes with the shear vortex, and the front window shake is significantly higher when there are no rearview mirrors. Therefore, the control is mainly concentrated in the rear window, where the situation is more serious. As shown in Figure 24, adding a cavity at the trailing edge of the b-column prevents the shear layer from falling vortex into the crew compartment, reducing jitter by more than 6 dB. A divider bar is added in the middle of the rear window, effectively reducing shake. Considering the appearance and appearance of the side windows, it is feasible to use passive control methods such as spoilers.
Figure 24 Passive mode of rear window jitter control
Proactive approach
For active control, the b-column jet acts similarly to a deflector and can effectively control rear window flutter. However, there are also obvious drawbacks, such as large energy input, complex structural arrangement, and increased costs. Noise active control releases a sound wave source with the opposite phase and equal amplitude to suppress the noise wave from entering the human ear. This method is widely used in the control of low- and medium-frequency noise. Combined with the side window jitter characteristics, this paper proposes a side window jitter active noise control method, and the control logic is shown in Figure 25.
Fig. 25 Active control logic diagram of rear window opening size
Using the left rear window as an example, when a passenger opens the window, the logic automatically resizes the opening to a comfortable (no jittering) size. If the passenger continues to open the window until jitter occurs, the logic will automatically open the right front window to ventilate and suppress jitter. When the left front seat automatically closes the window, turn on the Jitter Active Noise Control (BANC) system, as shown in Figure 26.
Figure 26 Inverted sound source active control schematic
5. Conclusion
In the classification of automobile wind noise, noise is divided into leakage noise, shape noise and cavity resonance noise according to the mechanism; According to acoustic analogy, sound sources are divided into unipolar, dipole and quadrupole; According to the type of fluctuation, it is divided into sound pressure and kinetic water pressure. The unipolar sound source caused by the leak contributes the most, followed by the dipole sound source that passes through the glass. The amplitude of the sound pressure fluctuations is less variable relative to the hydrodynamic pressure, but contributes more to the noise in the cabin.
In the noise evaluation, psychological indicators such as loudness, a-weighted sound pressure level, and speech intelligibility are introduced. In terms of test methods, the wind tunnel test has good repeatability and reliability, and introduces the test methods of air tightness, sound insulation and sound absorption performance. In external wind noise source calculations, wind noise source calculations for direct CAA require an accurate discrete format and more computing resources than hybrid CAA methods. In the calculation of the wind noise response in the car, SEA can quickly calculate the middle and high frequency bands.
This article discusses the variation characteristics of window jitter with vehicle speed and opening size. The application of passive control methods is limited by requirements such as side window views. It also proposes to adjust the opening size and position of the window based on the control logic, and combine the BANC system to realize the active control of side window jitter. This method has broad application prospects.
文章来源：Wang, Q., Chen, X., and Zhang, Y., "An Overview of Automotive Wind Noise and Buffeting Active Control,"SAE Int. J. Veh. Dyn., Stab., and NVH5(4):443-458, 2021, https://doi.org/10.4271/10-05-04-0030.