A plane wave tube and design method for surface pulsating pressure measurement calibration

Abstract

The present invention discloses a plane wave tube and a design method for surface pulsating pressure measurement calibration. The plane wave tube includes a loudspeaker and a sound tube connected to the loudspeaker; the sound tube includes a continuously variable cross-section inner tube in a funnel shape, the large end of the continuously variable cross-section inner tube is connected to the sound-emitting end of the loudspeaker, and the small end is connected to an equal cross-section inner tube; along the axial direction of the inner wall of the equal cross-section inner tube, there are provided no less than two Helmholtz resonators; an equal cross-section branch pipe is installed between the outer tube and the equal cross-section inner tube, one end of the equal cross-section branch pipe leads to the inside of the equal cross-section inner tube, and the other end extends to the outer tube; and a microphone interface for connecting to a microphone is installed at the end of the equal cross-section branch pipe extending to the outer tube. The present invention can calibrate a surface pulsating pressure measurement system, making the measurement of the surface pulsating pressure by the measurement system more stable and accurate, and reducing measurement errors.

Description

Technical Field

[0001] The present invention belongs to the technical field of aerodynamic test, and particularly relates to a plane wave tube for surface pulsating pressure measurement calibration and a design method thereof. Background Art

[0002] The low-altitude economy drives the development of related industries with low-altitude flight activities as the core, covering many fields such as aircraft R & D and manufacturing, operation services, etc. The low-altitude airspace is usually below 1000 meters, and the application scenarios of the low-altitude economy mainly include emergency medical treatment, logistics, urban air traffic, fire rescue, etc. In order to reduce pollutant emissions and the requirements for runways, electric vertical takeoff and landing (EVTOL) multi-rotor unmanned aircraft are the mainstream development direction of low-altitude economy aircraft. Due to the relatively low flight altitude, the noise generated by the aircraft will have a serious impact on the work and life of urban residents. According to the theory of aeroacoustics, the noise generated by the rotor will increase with the increase of load and flight speed. Therefore, the noise problem needs to be focused on and solved during the aircraft design and R & D process.

[0003] During the movement of the aircraft, when the air flow passes through the surface of the airframe or rotor blades, complex flow phenomena such as flow separation and vortex shedding will occur, thus inducing surface pulsating pressure. Surface pulsating pressure is the main source of aircraft structural vibration and aeroacoustic noise. Due to the great difficulties in the numerical calculation and prediction of turbulence, accurately measuring the surface pulsating pressure through wind tunnel tests is the main means to deeply understand the generation mechanism of aircraft aeroacoustic noise and study and solve the noise problem.

[0004] The acoustic measurement method of aircraft surface pulsating pressure mainly uses the sound pressure measured by the microphone to reverse the surface pulsating pressure. At present, the microphone, which is the core component of the measurement system, has been completely domesticated. Arranging the microphone at the far end can improve the spatial resolution of the measurement point and is not restricted by the special shape of the object, etc. Therefore, the acoustic method for measuring surface pulsating pressure has the advantages of "low cost, high sensitivity, and convenient installation". Due to the existence of viscous heat dissipation in the waveguide of surface pulsating pressure acoustic measurement, the acoustic wave signal will produce phase distortion and amplitude distortion. In order to obtain accurate measurement results, the measurement system needs to be calibrated.

[0005] When calibrating the surface pulsating pressure measurement system, a pure plane wave needs to be provided as the physical quantity reference. However, the acoustic wave provided by the speaker will diffuse and attenuate and generate reflection interference, thus affecting the final calibration result. Summary of the Invention

[0006] The purpose of the present invention is to provide a plane wave tube for surface pulsating pressure measurement calibration and a design method thereof, which can partially solve or alleviate the above deficiencies in the prior art, and can generate a pure plane wave as the physical quantity reference to calibrate the surface pulsating pressure measurement system.

[0007] To solve the above-mentioned technical problems, the present invention specifically adopts the following technical solutions:

[0008] In a first aspect of the present invention, there is provided a plane wave tube for surface pulsating pressure measurement calibration, including a loudspeaker and a sound tube connected to the loudspeaker; the sound tube includes a funnel-shaped continuously variable cross-section inner tube, the large end of the continuously variable cross-section inner tube is connected to the sound-emitting end of the loudspeaker, and the small end is connected to an equal cross-section inner tube; the inner diameter of the equal cross-section inner tube is the same as the inner diameter of the opening of the small end of the continuously variable cross-section inner tube, and the loudspeaker, the continuously variable cross-section inner tube and the equal cross-section inner tube are coaxially installed; it further includes an outer tube sleeved outside the continuously variable cross-section inner tube and the equal cross-section inner tube, and the outer tube and the continuously variable cross-section inner tube and the equal cross-section inner tube enclose a cavity; at least two Helmholtz resonators are arranged along the axial direction of the inner wall of the equal cross-section inner tube; an equal cross-section branch pipe is installed between the outer tube and the equal cross-section inner tube, one end of the equal cross-section branch pipe leads to the inside of the equal cross-section inner tube, and the other end extends to the outer tube; and a microphone interface for connecting to a microphone is installed at the end of the equal cross-section branch pipe extending to the outer tube.

[0009] As an improvement, the inner diameter of the equal cross-section inner tube is calculated by the formula:

[0010] Calculated;

[0011] wherein, d2 is the inner diameter of the equal cross-section inner tube, c0 is the speed of sound in air, f up The upper limit of the frequency range of the loudspeaker.

[0012] As an improvement, the curve between the large end opening and the small end opening of the continuously variable cross-section inner tube is an exponential smooth curve.

[0013] As an improvement, the area of any cross-section of the inner cavity of the continuously variable cross-section inner tube is calculated by the formula:

[0014] Calculated;

[0015] wherein, S(x) is the cross-sectional area at a distance x from the large end, x is the distance from the large end; S0 is the area of the large end opening of the continuously variable cross-section inner tube, and δ is the winding index of the continuously variable cross-section inner tube;

[0016] Using the formula:

[0017]

[0018] Calculate the winding index of the continuously variable cross-section inner tube;

[0019] where δ is the winding index of the continuously variable cross-section inner tube, L is the length of the continuously variable cross-section inner tube, d0 is the inner diameter of the large-end opening of the continuously variable cross-section inner tube, and d1 is the inner diameter of the small-end opening of the continuously variable cross-section inner tube;

[0020] Using the formula:

[0021]

[0022] Calculate the length of the continuously variable cross-section inner tube;

[0023] where L is the length of the continuously variable cross-section inner tube, d0 is the inner diameter of the large-end opening of the continuously variable cross-section inner tube, and d1 is the inner diameter of the small-end opening of the continuously variable cross-section inner tube (the same as the inner diameter of the equal cross-section tube); f low is the lower limit frequency of the surface pulsation pressure measurement system, and c0 is the speed of sound in air.

[0024] As an improvement, the Helmholtz resonator includes a cavity embedded in the inner wall of the equal cross-section inner tube. The cavity opening faces the inner side of the inner wall of the equal cross-section inner tube, and the width of the cavity opening is smaller than the width of the cavity.

[0025] As an improvement, the cavity of the Helmholtz resonator is a cube; obtain the acoustic capacitance of the cavity of the Helmholtz resonator and the acoustic mass of the cavity opening according to the acoustic cavity resonance frequency of the equal cross-section inner tube; and obtain the cavity depth of the Helmholtz resonator according to the acoustic capacitance of the cavity, and obtain the opening length and opening diameter according to the acoustic mass of the cavity opening.

[0026] As an improvement, the acoustic cavity resonance frequency of the equal cross-section inner tube is calculated using the formula:

[0027] Calculate;

[0028] where, is the acoustic cavity resonance frequency, c0 is the speed of sound in air, and d2 is the inner diameter of the equal cross-section inner tube;

[0029] The relationship between the acoustic capacitance of the cavity of the Helmholtz resonator and the acoustic mass of the cavity opening and the acoustic cavity resonance frequency of the equal cross-section inner tube is:

[0030] ;

[0031] where M b is the acoustic mass, C b is the cavity acoustic capacitance, f r is the acoustic cavity resonance frequency;

[0032] The cavity acoustic capacitance is calculated using the formula:

[0033] Calculate; where C bis the acoustic capacitance of the cavity, H is the cavity depth of the Helmholtz resonator, ρ0 is the density of air, and c0 is the speed of sound in air;

[0034] The acoustic mass of the opening of the Helmholtz resonator is calculated using the formula:

[0035] where M b is the acoustic mass, l is the length of the opening, d is the diameter of the opening, and ρ0 is the density of air.

[0036] As an improvement, there are three Helmholtz resonators arranged axially at equal intervals along the inner tube with a constant cross-section, and one Helmholtz resonator farthest from the continuously variable cross-section inner tube is aligned with the constant cross-section branch pipe.

[0037] As an improvement, the microphone interface includes a mounting seat connected to the constant cross-section branch pipe, and a sound guiding hole communicating with the constant cross-section branch pipe is opened on the mounting seat; a microphone adapter is arranged in the mounting seat.

[0038] The present invention also provides a design method for a plane wave tube for preparing the above-mentioned plane wave tube, including:

[0039] Select a loudspeaker according to the frequency range of the surface pulsating pressure measurement system to be calibrated;

[0040] Obtain the inner diameter of the cylinder body and the large end inner diameter of the continuously variable cross-section inner tube according to the outer diameter of the sound emitting end of the selected loudspeaker;

[0041] Obtain the height of the cylinder body, the winding index and height of the continuously variable cross-section inner tube, and the inner diameter and height of the constant cross-section inner tube according to the upper and lower limits of the frequency range of the surface pulsating pressure measurement system to be calibrated;

[0042] Obtain the acoustic capacitance of the cavity of the Helmholtz resonator and the acoustic mass of the cavity opening according to the acoustic cavity resonance frequency of the constant cross-section inner tube; and obtain the cavity depth of the Helmholtz resonator according to the acoustic capacitance of the cavity, and obtain the opening length and opening diameter according to the acoustic mass of the cavity opening.

[0043] Beneficial effects: In the present invention, the Helmholtz resonator is designed according to the acoustic wave resonance peak frequency of the constant cross-section inner tube, and its geometric parameters are determined by accurately calculating the acoustic capacitance and the acoustic mass of the cavity opening. It can accurately process sound waves of specific frequencies. When sound waves of this specific frequency appear in the plane wave tube, the Helmholtz resonator resonates, consumes the energy of the sound waves, effectively eliminates the resonance peak, makes the sound wave propagation in the tube more stable, and avoids resonance from interfering with the measurement calibration signal.

[0044] The solution to the resonance problem directly improves the accuracy of surface pulsating pressure measurement calibration. Since resonance can cause distortion of acoustic wave signals, resulting in deviation of measurement results. After the Helmholtz resonator eliminates resonance, the acoustic wave signals collected by the microphone can more truly reflect the actual surface pulsating pressure situation, providing a reliable basis for subsequent data analysis and research.

[0045] The special design of the continuously variable cross-section tube changes the propagation characteristics of acoustic waves in the tube. Its exponential smooth curve structure from the large end to the small end suppresses the high-order acoustic wave modes, reduces the reflection and scattering of acoustic waves, and makes the acoustic waves propagate closer to the ideal state of plane waves. This propagation method reduces the interaction with background noise, reduces the influence of background noise on the measurement signal, and thus improves the signal-to-noise ratio.

[0046] The improvement of the signal-to-noise ratio provides a stable pulsating pressure calibration source for the acoustic measurement system of surface pulsating pressure. A stable calibration source is the key to ensuring the accuracy and reliability of the measurement system. In the case of high signal-to-noise ratio, the measurement system is more stable and accurate in measuring the surface pulsating pressure, reducing measurement errors, which is conducive to in-depth research on the generation mechanism of aircraft aerodynamic noise and solving noise problems, providing strong support for the design and development of aircraft. Brief Description of the Drawings

[0047] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts do not necessarily draw according to the actual scale. Obviously, the following-described drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0048] Figure 1 It is a cross-sectional structure schematic diagram of Embodiment 1 of the present invention;

[0049] Figure 2 It is a three-dimensional structure schematic diagram of Embodiment 1 of the present invention;

[0050] Figure 3 It is a structure schematic diagram of the Helmholtz resonator;

[0051] Figure 4 It is a structure schematic diagram of the mounting seat;

[0052] Figure 5 It is a flowchart of Embodiment 2 of the present invention.

[0053] Summary of the identification of reference numerals: 1 is a loudspeaker; 2 is an outer cylinder; 3 is a continuously variable cross-section inner tube; 4 is an equal cross-section inner tube; 5 is an equal cross-section branch pipe; 6 is a mounting seat; 7 is a microphone adapter; 8 is a Helmholtz resonator; 9 is an annular cover plate. 61 is a mounting cavity; 62 is a sound guiding hole; 81 is an opening; 82 is a cavity. Detailed implementation manners

[0054] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0055] In this document, suffixes such as "module", "component" or "unit" used to represent elements are only for the convenience of describing the present invention, and have no specific meaning in themselves. Therefore, "module", "component" or "unit" can be used interchangeably.

[0056] In this document, the orientation or positional relationship indicated by terms such as "upper", "lower", "inner", "outer", "front", "rear", "one end", "the other end", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be construed as a limitation of the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0057] In this document, unless otherwise clearly defined and limited, terms such as "mount", "provided with", "connected", etc. shall be understood in a broad sense. For example, "connected" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, a direct connection, or an indirect connection through an intermediate medium, and can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0058] In this document, "and / or" includes any and all combinations of one or more of the listed related items.

[0059] In this document, "a plurality of" means two or more, that is, it includes two, three, four, five, etc.

[0060] Embodiment 1: As Figure 1As shown in the figure, this embodiment provides a plane wave tube for surface pulsating pressure measurement calibration, which includes a loudspeaker 1 and a sound tube connected to the loudspeaker 1; the sound tube includes a funnel-shaped continuously variable cross-section inner tube 3, the large end of the continuously variable cross-section inner tube 3 is connected to the sound-emitting end of the loudspeaker 1, and the small end is connected to an equal cross-section inner tube 4; the inner diameter of the equal cross-section inner tube 4 is the same as the inner diameter of the opening at the small end of the continuously variable cross-section inner tube 3, and the loudspeaker 1, the continuously variable cross-section inner tube 3 and the equal cross-section inner tube 4 are coaxially installed.

[0061] It further includes an outer cylinder 2 sleeved outside the continuously variable cross-section inner tube 3 and the equal cross-section inner tube 4, and the outer cylinder 2 and the continuously variable cross-section inner tube 3 and the equal cross-section inner tube 4 enclose a cavity; at least two Helmholtz resonators 8 are arranged along the axial direction of the inner wall of the equal cross-section inner tube 4.

[0062] An equal cross-section branch pipe 5 is installed between the outer cylinder 2 and the equal cross-section inner tube 4, one end of the equal cross-section branch pipe 5 leads to the inside of the equal cross-section inner tube 4, and the other end extends to the outer cylinder 2; and a microphone interface for connecting to a microphone is installed at the end of the equal cross-section branch pipe 5 extending to the outer cylinder 2.

[0063] The loudspeaker 1 serves as the sound source of the entire plane wave tube and can generate broadband acoustic wave signals. During the process of surface pulsating pressure measurement calibration, it provides the initial acoustic energy for the system, simulates acoustic waves of various frequencies that may be encountered in the actual measurement scenario, and is the basis for the entire measurement calibration work to be carried out.

[0064] The sound tube is the core functional module of the plane wave tube. Through structural optimization and resonance control, it provides a stable and broadband plane wave calibration signal for surface pulsating pressure measurement, and solves the problems of signal distortion and noise interference in traditional acoustic measurements.

[0065] Among them, the continuously variable cross-section inner tube 3 is funnel-shaped, including a large end and a small end. The large end is connected to the sound-emitting end of the loudspeaker 1, and the small end is connected to the equal cross-section inner tube 4. When the sound wave propagates from the large end to the small end, due to the gradually decreasing pipe diameter, the generation of high-order acoustic wave modes can be suppressed. In a traditional straight pipe, high-order modes may cause the propagation characteristics of the sound wave to become complex and affect the measurement accuracy. The design of the continuously variable cross-section inner tube enables the sound wave to propagate closer to the form of a plane wave, reducing the interference of high-order modes on the measurement results.

[0066] More specifically, the area between the large-end opening and the small-end opening of the continuously variable cross-section inner tube 3 is an exponential smooth curve, that is, the cross-sectional area changes gradually according to an exponential function. The advantages of such a setting are as follows: First, it can reduce the reflection and diffraction of sound waves in the tube, suppress the generation of higher-order modes (such as radial modes), make the sound waves propagate closer to the form of plane waves, and thus improve the purity of the calibration signal. Second, the mathematical continuity of the exponential gradual change curve reduces the phase distortion and amplitude attenuation of sound waves during propagation, ensuring the accuracy of the calibration signal. Third, the exponential gradual change curve enables a smooth transition at the connection between the continuously variable cross-section inner tube and the constant cross-section inner tube, reducing the risk of local turbulence and resonance. Finally, the exponential curve avoids sharp corners, reduces the generation of air flow separation and turbulent noise, and improves the signal-to-noise ratio.

[0067] In this embodiment, the area of any cross-section of the inner cavity of the continuously variable cross-section inner tube 3 is calculated using the formula:

[0068] where S(x) is the cross-sectional area at a distance x from the large end, x is the distance from the large end; S0 is the area of the large-end opening of the continuously variable cross-section inner tube, and δ is the winding index of the continuously variable cross-section inner tube;

[0069] Using the formula:

[0070]

[0071] calculate the winding index of the continuously variable cross-section inner tube 3; where δ is the winding index of the continuously variable cross-section inner tube 3, L is the length of the continuously variable cross-section inner tube, d0 is the inner diameter of the large-end opening of the continuously variable cross-section inner tube, and d1 is the inner diameter of the small-end opening of the continuously variable cross-section inner tube;

[0072] Using the formula:

[0073]

[0074] calculate the length of the continuously variable cross-section inner tube 3; where L is the length of the continuously variable cross-section inner tube 3, d0 is the inner diameter of the large-end opening of the continuously variable cross-section inner tube 3, d1 is the inner diameter of the small-end opening of the continuously variable cross-section inner tube 3, f low is the lower limit frequency of the surface pulsating pressure measurement system, and c0 is the speed of sound in air.

[0075] In addition, it can be foreseen that the area of the large-end opening of the continuously variable cross-section inner tube 3 can be calculated using the formula:

[0076] S0 = π d 0 2 / 4; where S0 is the area of the large-end opening of the continuously variable cross-section inner tube 3, d 0 is the inner diameter of the large-end opening of the continuously variable cross-section inner tube 3.

[0077] In this embodiment, the inner diameter of the constant cross-section inner tube 4 is the same as the inner diameter of the small-end opening of the continuously variable cross-section inner tube 3, and they are coaxially installed with the speaker 1 and the continuously variable cross-section inner tube 3. Its function is to provide a stable propagation channel for the sound waves processed by the continuously variable cross-section inner tube 3, ensure that the sound waves continue to propagate in a relatively stable state therein, reduce the scattering and reflection of the sound waves, and enable the sound waves to maintain good consistency and stability. The constant cross-section inner tube also provides space for the installation of the Helmholtz resonator, facilitating the processing of sound waves of specific frequencies by the resonator. In addition, the outlet end of the constant cross-section inner tube 4 also serves as the output port of the plane wave. During calibration, the reference microphone and the measuring microphone of the measurement system to be calibrated both monitor the plane wave through the output port.

[0078] More specifically, the inner diameter of the constant cross-section inner tube 4 is calculated using the formula:

[0079] where d2 is the inner diameter of the constant cross-section inner tube 4, c0 is the speed of sound in air, and generally c0 = 343 m / s, f up the upper limit of the frequency range of the speaker. Since the inner diameter of the constant cross-section inner tube 4 is the same as the inner diameter of the small-end opening of the continuously variable cross-section inner tube 3, in this embodiment, the inner diameter of the constant cross-section inner tube 4 can be determined first and then the inner diameter of the small-end opening of the continuously variable cross-section inner tube 3 can be determined.

[0080] The outer cylinder 2 is sleeved outside the continuously variable cross-section inner tube 3 and the constant cross-section inner tube 4, and encloses a cavity with the continuously variable cross-section inner tube 3 and the constant cross-section inner tube 4. In some embodiments, the shape of the outer cylinder 2 is preferably cylindrical. The purpose of setting the outer cylinder 2 is to provide physical protection for the internal continuously variable cross-section inner tube 3 and constant cross-section inner tube 4, prevent damage to the sound tube caused by external collisions, squeezes and other factors, and ensure the integrity and stability of the sound tube structure. In addition, the cavity can play a certain sound insulation role, reducing the interference of external environmental noise on the propagation of sound waves in the tube. At the same time, it can also buffer external vibrations to a certain extent and reduce the impact of vibrations on the measurement results.

[0081] In addition, in this embodiment, there are three Helmholtz resonators 8 arranged at equal intervals along the axial direction of the constant cross-section inner tube 4.

[0082] The Helmholtz resonator 8 is an acoustic element that selectively absorbs the energy of sound waves of specific frequencies through the matching of acoustic capacitance and acoustic mass, thereby eliminating resonance phenomena. In the present invention, it is integrated on the inner wall of the constant cross-section inner tube of the plane wave tube for calibrating the surface pulsation pressure measurement system.

[0083] When there are sound waves with the same resonance frequency as the resonator in the constant cross-section inner tube 4, the resonator will resonate with the sound waves, absorb the energy of the sound waves at that frequency, and thus eliminate or significantly weaken the resonance phenomenon at that frequency. Resonance may cause standing waves to form in the tube, affecting the accuracy of measurement. The Helmholtz resonator can effectively avoid this situation. By eliminating the resonance at specific frequencies, the frequency characteristics of the sound waves in the tube become smoother, improving the frequency response performance of the entire plane wave tube system and thus enhancing the accuracy of measurement calibration.

[0084] Specifically, the Helmholtz resonator 8 includes a cavity 82 embedded in the inner wall of the constant cross-section inner tube 4. The opening 81 of the cavity 82 faces the inner side of the inner wall of the constant cross-section inner tube 3, and the width of the opening 81 of the cavity 82 is smaller than the width of the cavity 82.

[0085] In this embodiment, the cavity 82 of the Helmholtz resonator 8 is a cube; the acoustic capacitance of the cavity 82 of the Helmholtz resonator 8 and the acoustic mass of the opening 81 are obtained according to the acoustic cavity resonance frequency of the constant cross-section inner tube 4; and the depth of the cavity 82 of the Helmholtz resonator 8 is obtained according to the acoustic capacitance of the cavity 82, and the length and diameter of the opening 81 are obtained according to the acoustic mass of the cavity opening 81. Specifically:

[0086] The acoustic cavity resonance frequency of the constant cross-section inner tube 4 is calculated using the formula:

[0087] where, is the acoustic cavity resonance frequency, c0 is the speed of sound in air, and d2 is the inner diameter of the constant cross-section inner tube 4;

[0088] The relationship between the acoustic capacitance of the cavity 82 of the Helmholtz resonator 8 and the acoustic mass of the opening 81 of the cavity 82 and the acoustic cavity resonance frequency of the constant cross-section inner tube 4 is:

[0089] ;

[0090] where, M b is the acoustic mass, C b is the acoustic capacitance of the cavity, f r is the acoustic cavity resonance frequency;

[0091] The acoustic capacitance of the cavity is calculated using the formula:

[0092] where, C b is the acoustic capacitance of the cavity, H is the depth of the cavity of the Helmholtz resonator 8, ρ0 is the density of air, and c0 is the speed of sound in air;

[0093] The acoustic mass of the opening 81 of the Helmholtz resonator 8 is calculated using the formula:

[0094] Calculation; where M b is the sound quality, l is the length of the opening, d is the diameter of the opening, and ρ0 is the density of air.

[0095] In this embodiment, an equal cross-section branch pipe 5 is further provided on the sound tube, which is used to guide the sound wave propagating in the equal cross-section inner tube 4 to the outside, facilitating docking with the microphone and realizing the acquisition of the sound wave signal.

[0096] In addition, one Helmholtz resonator 8 that is farthest from the continuously variable cross-section inner tube 3 in this embodiment is aligned with the equal cross-section branch pipe 5. The aligned layout can reduce the interference of the sound wave during propagation. If the positions of the Helmholtz resonator and the equal cross-section branch pipe are improper, it may cause additional reflection or interference phenomena when the sound wave propagates to the branch pipe.

[0097] To facilitate the installation of the microphone, a microphone interface for connecting with the microphone is installed at one end of the equal cross-section branch pipe 5 extending to the outer cylinder 2. Specifically, the microphone interface includes a mounting seat 6 connected to the equal cross-section branch pipe 5, and a sound guiding hole 62 communicating with the equal cross-section branch pipe 5 is opened on the mounting seat 6; a microphone adapter 7 is arranged inside the mounting seat 6.

[0098] The sound guiding hole 62 on the mounting seat 6 provides a direct channel for the propagation of the sound wave, and its aperture is 1 mm - 3 mm, preferably 2 mm, ensuring that the sound wave in the equal cross-section branch pipe 5 can smoothly enter the inside of the microphone interface without obstruction. In actual measurement, the sizes and interface types of microphones may vary. The microphone adapter 7 can be designed specifically according to the specifications of the microphone. For example, in this embodiment, the microphone adapter 7 has two sizes of 1 / 2 inch and 1 / 4 inch, enabling the plane wave tube to be compatible with a variety of different specifications of microphones, improving the versatility and applicability of the plane wave tube, and meeting the connection requirements of different measurement scenarios and microphones.

[0099] More specifically, the mounting seat 6 is preferably made of resin material through 3D printing, and a rubber ring mounting groove is reserved in the center inside.

[0100] In this embodiment, the loudspeaker 1 selects the HiVi M3N full-range loudspeaker. The rated power of the loudspeaker is 15W, the frequency response curve is flat in the range of 100 Hz - 10000 Hz, the panel diameter is 90 mm, the opening size is 75 mm, the depth is 53 mm, and the total thickness of the loudspeaker is 57.4 mm.

[0101] The cylindrical outer cylinder 2 is made of 304 stainless steel, with an outer diameter of 100 mm, an inner diameter of 90 mm, and a thickness of 5 mm. Four M3×6 mm threaded holes are opened at the top of the outer cylinder 2. The speaker 1 is connected to the outer cylinder 2 by screws, and the opening edge of the speaker 1 and the outer cylinder 2 are sealed with a rubber ring gasket. A groove with a width of 30 mm, a height of 20 mm, and a depth of 20 mm is opened on the side of the outer cylinder 2 to accommodate the microphone interface, so that the microphone interface is flush with the outer surface of the outer cylinder 2, as Figure 2 shown.

[0102] The continuously variable cross-section inner tube 3 is made of 304 stainless steel, with a wall thickness of 2 mm, an inner diameter of the large end opening of 86 mm, an inner diameter of the small end opening of 12 mm. Between the large end and the small end openings is an exponential smooth curve S(x) = S0*exp(-50*x), where x is the distance from the large end, and the winding exponent δ = 50. Two M1.5×3 mm positioning holes are opened at the small end of the continuously variable cross-section inner tube, and a flange is provided at the large end. Four threaded holes with the same positions as the threaded holes of the outer cylinder are circumferentially opened on the flange.

[0103] The constant cross-section inner tube 4 is made of 304 stainless steel, with a wall thickness of 2 mm and a length of 80 mm. Two M1.5×3 mm positioning holes are opened at the same position at the top of the constant cross-section inner tube 4 and the bottom of the continuously variable cross-section inner tube 3. The two are connected by a pin, and the contact surface is welded by argon arc welding.

[0104] In order to achieve the closure of the bottom of the outer cylinder 2 and the constant cross-section inner tube 4, an annular cover plate 9 can be provided at their bottoms. Of course, the annular cover plate 9 can also be integrally formed with the outer cylinder.

[0105] As Figure 3 shown, a 2 mm single-sided through hole is opened 30 mm away from the bottom of the constant cross-section inner tube 4. A 2 mm through hole is opened opposite to this through hole, and a cavity 82 with an inner side length of 5 mm and a thickness of 2 mm is welded outside the through hole, thus forming a Helmholtz resonator 8. Three Helmholtz resonators 8 are distributed along the height direction of the constant cross-section inner tube, with a spacing of 10 mm.

[0106] As Figure 4As shown, the microphone mounting base 6 is made of resin material by 3D printing. Its external dimensions are 30 mm (width) × 20 mm (height) × 30 mm (depth). There is a microphone mounting cavity 61 with a diameter of 12.7 mm inside. A rubber ring is provided at the center of the mounting cavity 61. The depth of the mounting cavity 61 is 20 mm. A sound guiding hole 62 with a diameter of 2 mm and a depth of 10 mm is opened at the center of the bottom of the mounting cavity 61. The equal cross-section inner tube 4 is connected to the mounting base 6 through a stainless steel tube with a length of 20 mm, an outer diameter of 2 mm, and an inner diameter of 1.6 mm, that is, the equal cross-section branch pipe 5. The microphone adapter 7 is made of resin material by 3D printing. Its shape is a cylinder, with an outer diameter of 12.7 mm and an inner diameter of 6.35 mm.

[0107] Embodiment 2:

[0108] As Figure 5 shown, the present invention also provides a design method for a plane wave tube for preparing the above-mentioned plane wave tube, including:

[0109] S1 Select a loudspeaker according to the frequency range of the surface pulsating pressure measurement system to be calibrated.

[0110] Assume that the frequency range of the surface pulsating pressure measurement system to be calibrated is 100 Hz - 10000 Hz. Accordingly, the model of the full-frequency loudspeaker selected is: HiVi M3N. The rated power of the loudspeaker is 15 W. The frequency response curve is flat in the range of 100 Hz - 10000 Hz. The panel diameter is 90 mm, the opening size is 75 mm, the depth is 53 mm, and the total thickness of the loudspeaker is 57.4 mm.

[0111] S2 Obtain the inner diameter of the cylinder and the large-end inner diameter of the continuously variable cross-section inner tube according to the outer diameter of the sound-emitting end of the selected loudspeaker.

[0112] In step S1, the panel diameter of the selected full-frequency loudspeaker is 90 mm. The outer diameter of the designed cylindrical outer cylinder is 100 mm, the inner diameter d0 = 90 mm, and the thickness t = 5 mm. The top inner diameter of the continuously variable cross-section inner tube is 90 mm. The height of the cylindrical outer cylinder, the height of the continuously variable cross-section inner tube, the bottom inner diameter, the inner diameter and height of the equal cross-section tube are to be determined.

[0113] S3 Obtain the height of the cylinder, the winding index and height of the continuously variable cross-section inner tube, and the inner diameter and height of the equal cross-section inner tube according to the upper and lower limits of the frequency range of the surface pulsating pressure measurement system to be calibrated.

[0114] S31 The upper frequency limit of the surface pulsating pressure measurement system to be calibrated is 10000 Hz. According to the formula:

[0115]

[0116] It is concluded that the inner diameter d2 of the equal-cross-section inner tube should be less than 17 mm, and the preferred value is d2 = 12 mm, so that the acoustic cavity resonance frequency is outside the upper frequency limit of the measurement system. d2 is the inner diameter of the equal-cross-section inner tube, and c0 is the speed of sound in air. f up The upper frequency limit of the frequency range of the loudspeaker.

[0117] S32 According to the lower frequency limit of 100 Hz of the surface pulsation pressure measurement system, according to the formula

[0118] ,

[0119] ,

[0120] the length L and the meandering index δ of the continuously variable cross-section inner tube are obtained.

[0121] According to the convenience of calibration, the length of the equal-cross-section inner tube is reasonably selected to be 200 mm, and thus the height of the cylindrical outer cylinder is obtained as 1099 mm + 200 mm = 1299 mm.

[0122] S4 Obtain the acoustic capacitance of the Helmholtz resonator cavity and the acoustic mass of the cavity opening according to the acoustic cavity resonance frequency of the equal-cross-section inner tube; and obtain the cavity depth of the Helmholtz resonator according to the acoustic capacitance of the cavity, and obtain the opening length and opening diameter according to the acoustic mass of the cavity opening.

[0123] Specifically, the acoustic cavity resonance frequency of the equal-cross-section inner tube is:

[0124] ;

[0125] The relationship between the acoustic capacitance of the Helmholtz resonator cavity and the acoustic mass of the cavity opening and the acoustic cavity resonance frequency of the equal-cross-section inner tube is:

[0126] ;

[0127] where M b is the acoustic mass, and C b is the cavity acoustic capacitance;

[0128] The cavity acoustic capacitance is:

[0129]

[0130] where C b is the cavity acoustic capacitance;

[0131] The acoustic mass of the Helmholtz resonator opening is:

[0132]

[0133] where Mb For sound quality, l where \(l\) is the length of the opening, \(d\) is the diameter of the opening, and \(\rho_0\) is the density of air.

[0134] Then,

[0135] .

[0136] After the parameters of the plane wave tube are determined, it can be prepared according to the parameters.

[0137] It should be noted that in this article, the terms "including", "comprising" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element in the process, method, article or device including that element.

[0138] The embodiments of the present invention have been described above in conjunction with the accompanying drawings. However, the present invention is not limited to the above specific embodiments. The above specific embodiments are merely illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can also make many forms without departing from the spirit and scope protected by the claims of the present invention. These all fall within the protection scope of the present invention.

Claims

1. A plane wave tube for surface pulsating pressure measurement calibration, characterized in that: It comprises a loudspeaker and a sound tube connected to the loudspeaker; the sound tube comprises a funnel-shaped continuously variable cross-section inner tube, the large end of the continuously variable cross-section inner tube is connected to the sound-generating end of the loudspeaker, and the small end is connected to the uniform cross-section inner tube; the inner diameter of the uniform cross-section inner tube is consistent with the inner diameter of the opening of the small end of the continuous variable cross-section inner tube, and the loudspeaker, the continuously variable cross-section inner tube and the uniform cross-section inner tube are coaxially installed; it also comprises an outer tube sleeved outside the continuous variable cross-section inner tube and the uniform cross-section inner tube, the outer tube and the continuous variable cross-section inner tube and the uniform cross-section inner tube form a cavity; the inner wall of the uniform cross-section inner tube is provided with no less than two Hertz resonators along its axial direction; a uniform cross-section branch tube is installed between the outer tube and the uniform cross-section inner tube, one end of the uniform cross-section branch tube reaches the interior of the uniform cross-section inner tube, and the other end extends to the outer tube; and a microphone interface for connecting to a microphone is installed at one end of the uniform cross-section branch tube extending to the outer tube; An exponential smooth curve is formed between the large end opening and the small end opening of the continuously variable cross-section inner tube; the Helmholtz resonator comprises a cavity embedded in the inner wall of the inner tube of equal cross-section, the cavity opening faces the inner side of the inner wall of the inner tube of equal cross-section, and the cavity opening width is smaller than the cavity width; the cavity of the Helmholtz resonator is a cube; the acoustic capacity of the Helmholtz resonator cavity and the acoustic mass of the cavity opening are obtained according to the acoustic cavity resonance frequency of the inner tube of equal cross-section; and the cavity depth of the Helmholtz resonator is obtained according to the acoustic capacity of the cavity, and the opening length and opening diameter are obtained according to the acoustic mass of the cavity opening; The area of ​​any cross section of the inner cavity of the continuously variable cross-section inner tube is calculated using the formula: Calculation; where S(x) is the cross-sectional area at a distance x from the large end, x is the distance from the large end; S0 is the opening area of the large end of the continuously variable cross-section inner tube, and δ is the winding index of the continuously variable cross-section inner tube; Using the formula: Calculate the winding index of the continuously variable cross-section inner tube; where δ is the winding index of the continuously variable cross-section inner tube, L is the length of the continuously variable cross-section inner tube, d0 is the inner diameter of the large end opening of the continuously variable cross-section inner tube, and d1 is the inner diameter of the small end opening of the continuously variable cross-section inner tube; Using the formula: Calculate the length of the continuously variable cross-section inner tube; where L is the length of the continuously variable cross-section inner tube, d0 is the inner diameter of the large-end opening of the continuously variable cross-section inner tube, and d1 is the inner diameter of the small-end opening of the continuously variable cross-section inner tube; f low is the lower limit frequency of the surface pulsating pressure measurement system, and c0 is the speed of sound in air.

2. The plane wave tube for surface pulsating pressure measurement calibration according to claim 1, characterized in that The inner diameter of the uniform cross-section inner tube is calculated using the formula: Calculation; wherein, d2 is the inner diameter of the equal cross-section inner tube, c0 is the speed of sound in air, f up The upper limit of the frequency range of the loudspeaker.

3. A plane wave tube for surface pulsating pressure measurement calibration according to claim 1, characterized in that The acoustic cavity resonance frequency of the equal cross-section inner tube is calculated using the formula: Calculation; wherein, is the resonance frequency of the sound cavity, c0 is the speed of sound in air, and d2 is the inner diameter of the equal cross-section inner tube; The relationship between the acoustic capacity of the Helmholtz resonator cavity and the acoustic mass of the cavity opening and the acoustic cavity resonance frequency of the inner tube of equal cross section is: ； Among them, M b is the sound quality, C b is the acoustic capacitance of the cavity, f r is the resonance frequency of the sound cavity; The cavity acoustic capacity uses the formula: Calculation; wherein, C b is the cavity acoustic capacitance, H is the cavity depth of the Helmholtz resonator, ρ0 is the density of air, and c0 is the speed of sound in air; The acoustic mass of the Helmholtz resonator opening is calculated using the formula: Calculation; wherein, M b is the sound quality, l is the length of the opening, d is the diameter of the opening, and ρ0 is the density of air.

4. A plane wave tube for surface pulsating pressure measurement calibration according to claim 1, characterized in that: The three Helmholtz resonators are arranged at equal intervals along the axial direction of the inner tube with a uniform cross-section, and the Helmholtz resonator farthest from the inner tube with a continuously variable cross-section is directly opposite to the branch tube with a uniform cross-section.

5. A plane wave tube for surface pulsating pressure measurement calibration according to claim 1, characterized in that: The microphone interface comprises a mounting seat connected to a branch pipe of equal cross section, and a sound guide hole communicating with the branch pipe of equal cross section is opened on the mounting seat; a microphone adapter is arranged in the mounting seat.

6. A design method of a plane wave tube for preparing the plane wave tube according to any one of claims 1 to 5, characterized in that include: Select the loudspeaker according to the frequency range of the surface pulsation pressure measurement system to be calibrated; According to the outer diameter of the sound-emitting end of the selected speaker, the inner diameter of the sound tube and the inner diameter of the large end of the continuously variable cross-section inner tube are obtained; According to the upper and lower limits of the frequency range of the surface pulsating pressure measurement system to be calibrated, the height of the sound cylinder, the winding index and height of the inner tube with continuously variable cross-section, and the inner diameter and height of the inner tube with constant cross-section are obtained; Obtain the acoustic capacitance of the Helmholtz resonator cavity and the acoustic mass of the cavity opening according to the acoustic cavity resonance frequency of the equal cross-section inner tube; and obtain the cavity depth of the Helmholtz resonator according to the acoustic capacitance of the cavity, and obtain the opening length and opening diameter according to the acoustic mass of the cavity opening.

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