A method for identifying wall hollowing based on laser sound vibration combined with mechanical impedance

Abstract

This invention discloses a method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, comprising the following steps: S1, setting a detection range and measurement point array on the wall surface to be tested, emitting acoustic wave signals to excite the wall surface to be tested, and measuring the magnitude of the excitation force of the acoustic waves reaching the wall surface to be tested; S2, collecting vibration velocity signals at each measurement point, performing Fourier transform on the vibration velocity signals at each point to obtain the vibration signals at each point in the frequency domain; S3, establishing a mechanical impedance model of the wall hollowness system, substituting the vibration signals in the frequency domain and the magnitude of the excitation force into the mechanical impedance formula, and fitting using the least squares method to obtain the identification parameters of the wall hollowness system; S4, based on the identification parameters and combined with preset material parameters, calculating the geometric area of ​​the hollowness, the thickness of the upper material of the hollowness, and the cavity depth of the hollowness. This invention can not only locate hollowness, but also accurately quantify its area, the thickness of the upper material, and the cavity depth of the hollowness, improving detection efficiency and identification accuracy.

Description

Technical Field

[0001] This invention relates to the field of laser detection technology, and in particular to a method for identifying hollow walls based on laser acoustic vibration combined with mechanical impedance. Background Technology

[0002] Hollow areas in walls are a common quality defect in construction projects. Their causes are complex, primarily stemming from violations of construction techniques, lack of standard implementation, mismatch between materials and design specifications, and material shrinkage and adhesive layer failure due to environmental temperature and humidity. Hollow areas not only weaken the aesthetics and durability of the wall but also exhibit significant progressive destructiveness. Over time, under the continuous influence of environmental loads and physical factors, initially localized hollow areas will continuously spread and expand, eventually easily inducing wall cracks, finish peeling, and other engineering accidents, seriously endangering structural stability and personnel safety. Therefore, employing efficient and accurate methods to detect and identify hollow areas in walls is of great significance for project quality assessment and subsequent maintenance.

[0003] Traditional tapping methods rely on human experience, resulting in low efficiency, high subjectivity, and limited detection range. While infrared thermal imaging technology can detect large areas quickly, its application is greatly limited by environmental conditions and the condition of the wall surface, and its recognition resolution is relatively low. Laser acoustic vibration technology, also a non-destructive testing method, determines the presence of hollow areas through sound wave excitation and laser vibration measurement; however, existing methods mostly rely on the "energy method"—that is, judging by detecting the strength of the feedback vibration signal. The energy method requires dense point sampling on the wall surface to accurately obtain the location and size of hollow areas, leading to low detection efficiency; reducing the number of sampling points makes it difficult to accurately quantify the geometric characteristics of hollow areas. Summary of the Invention

[0004] The purpose of this invention is to provide a method for identifying hollow areas in walls based on laser acoustic vibration combined with mechanical impedance. This invention can not only locate hollow areas with a smaller number of sampling points, but also accurately quantify their area, the thickness of the upper material, and the depth of the hollow cavity, thus improving detection efficiency and identification accuracy.

[0005] The technical solution of this invention: A method for identifying hollow walls based on laser acoustic vibration combined with mechanical impedance, comprising the following steps:

[0006] S1. Turn on the laser vibration measuring device and the excitation device, set the detection range and measurement point array on the wall to be tested, turn on the excitation device to emit sound wave signals to excite the wall to be tested, and measure the magnitude of the excitation force when the sound wave reaches the wall to be tested;

[0007] S2. Use laser vibration measurement equipment to collect vibration velocity signals at each measurement point, and perform Fourier transform on the vibration velocity signals at each point to obtain the vibration signals at each point in the frequency domain.

[0008] S3. Establish a mechanical impedance model for the wall hollowing system. Substitute the vibration signal and excitation force in the frequency domain into the mechanical impedance formula and fit it using the least squares method to obtain the identification parameters of the wall hollowing system.

[0009] S4. Based on the identification parameters and the preset material parameters, calculate the geometric area of ​​the hollow area, the thickness of the upper material of the hollow area, and the depth of the hollow cavity.

[0010] In the above-mentioned method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, in step S1, the acoustic signal input to the excitation device is a sweep frequency signal or a frequency comb signal; wherein, the sweep frequency signal is an audio signal whose frequency increases at equal intervals over time, and the frequency comb signal is a comb-shaped frequency function signal that divides a frequency signal within a certain range into equal parts with the same duration.

[0011] In the aforementioned method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, step S3 involves a mechanical impedance model that equates the wall hollowness system to a lumped parameter model consisting of a single-degree-of-freedom mass, spring, and damper. The identification parameters include the equivalent stiffness of the wall hollowness system. Equivalent quality and damping coefficient .

[0012] In the aforementioned method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, the mechanical impedance formula in step S3 is:

[0013] ;

[0014] In the formula, The vibration velocity of the wall hollowing system. For the magnitude of the incentive, Angular frequency, The imaginary unit, The equivalent stiffness of the wall hollowing system; The equivalent quality of the wall hollowing system. This is the damping coefficient of the wall hollowing system.

[0015] In the aforementioned method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, step S3, the least squares fitting method, refers to using the vibration signals in the frequency domain of each measurement point obtained in step S2 as observed values, substituting them into the mechanical impedance formula, and optimizing the solution using the least squares method to identify the equivalent stiffness that minimizes the sum of squared errors between the theoretically calculated value and the observed value. Equivalent quality and damping coefficient .

[0016] In the aforementioned method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, step S4, the relationship between the geometric area of ​​the hollow area, the thickness of the upper material of the hollow area, the depth of the hollow cavity, and the equivalent mass of the wall hollowness system satisfies:

[0017] ;

[0018] ;

[0019] In the formula, The density of the material layer, The thickness of the upper layer material of the hollow area. This represents the actual effective vibration area of ​​the wall hollowing system after being subjected to acoustic excitation. Let the area of ​​the hollow area be denoted by . These are correction coefficients based on the mode shape function;

[0020] By identifying equivalent quality Calculate the actual effective vibration area Then divide by the correction factor Obtain the geometric area of ​​the hollow area .

[0021] The aforementioned method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, wherein the correction coefficient Based on the theory of plate and shell vibration and the principle of energy equivalence, the correction coefficient is determined. Defined as the square of the mode shape function over the entire hollow geometric area The average integral value over, that is:

[0022] ;

[0023] In the formula, This is a hollow area. This is the mode shape function.

[0024] The aforementioned method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, the equivalent stiffness It consists of the bending stiffness of the material layer and the modified stiffness of the cavity structure;

[0025] The expression for the bending stiffness of the material layer is as follows:

[0026] ;

[0027] In the formula, The Young's modulus of the material layer. The thickness of the upper layer material of the hollow area. Let the area of ​​the hollow area be denoted by . These are constants related to boundary conditions and geometry; For the bending stiffness of the material layer;

[0028] The expression for the corrected stiffness of the cavity structure is:

[0029] ;

[0030] In the formula, Correcting the stiffness of the cavity structure; This is the correction factor for the stiffness of the cavity structure; The depth of the hollow cavity;

[0031] Based on the identified equivalent stiffness Combination , The expression is used to calculate the thickness of the upper layer material in the hollow area. and / or cavity depth .

[0032] In the aforementioned method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance, in step S1, the sound pressure of the sound wave reaching the wall surface to be tested is measured by a decibel meter, and the magnitude of the excitation force is calculated.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. This invention equates the wall hollowing system to a mechanical impedance model and uses the least squares method to fit the frequency domain vibration signal, which can accurately identify the equivalent stiffness, equivalent mass, and damping coefficient of the wall hollowing system. Thus, it can accurately obtain the geometric characteristics of the hollowing with fewer sampling points, effectively solving the problem of low detection efficiency caused by the reliance on dense sampling arrays in existing energy methods.

[0035] 2. This invention calculates the geometric area of ​​the hollow area by back-calculating the equivalent mass and introduces a correction coefficient based on the mode shape function, which fully considers the modal distribution characteristics of the vibration of the upper material of the hollow area and improves the calculation accuracy of the geometric area of ​​the hollow area.

[0036] 3. This invention decomposes the equivalent stiffness into the bending stiffness of the material layer and the corrected stiffness of the cavity structure. Through decoupled calculation, it achieves accurate quantification of the cavity depth of the hollow area, providing richer technical parameters for the comprehensive evaluation of wall hollowness. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of a laser acoustic vibration detection model.

[0038] Figure 2 These are the prediction points marked on the wall to be tested.

[0039] Figure 3 It is the time-frequency diagram of the sweep signal in the excitation signal.

[0040] Figure 4 It is the spectrum diagram of the frequency comb signal in the excitation signal.

[0041] Figure 5 It is the time-domain signal of a certain point in the acquisition points.

[0042] Figure 6 It is the frequency domain signal of a certain point in the acquisition points.

[0043] Figure 7 It is a spring oscillator model for wall hollowing established by the mechanical impedance method.

[0044] Figure 8 It is a fitting curve of the frequency domain signal at a certain point in the acquisition point using mechanical impedance fitting.

[0045] Figure 9 The results are obtained using the energy method with M×N sampling points.

[0046] Figure 10 It is the result of the energy method detection using more sampling points.

[0047] Figure 11 The results are obtained using the mechanical impedance method with M×N sampling points. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0049] Example: A method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance. The detection system used in this method is a laser acoustic vibration detection system, such as... Figure 1 As shown, the core components include laser vibration measurement equipment, excitation equipment, sound pressure measurement equipment (decibels), and supporting control equipment. The functions and selection requirements of each piece of equipment are as follows:

[0050] Laser vibration measurement equipment: A laser vibration meter based on the Doppler frequency shift effect is used to collect vibration velocity signals at various measurement points on the wall surface to be tested. The equipment is required to be able to acquire multi-point array signals, store time-domain signals and perform preliminary analysis. The frequency shift detection accuracy of vertically irradiating the wall surface to be tested should meet the vibration measurement requirements under low-frequency acoustic excitation.

[0051] Excitation equipment: Select an acoustic excitation device (such as professional audio equipment or acoustic transducers) that can transmit sweep or comb signals. The signal frequency range must cover the resonant frequency range of the wall hollowing system. The signal type, frequency range and transmission power can be adjusted through the control equipment.

[0052] Sound pressure measurement equipment: A decibel meter with an accuracy of not less than 1dB is used to measure the sound pressure value when the excitation sound wave reaches the wall surface to be measured, and the magnitude of the excitation force is obtained through the sound pressure-excitation force conversion formula;

[0053] Control equipment: Used to synchronously control the opening / closing of the laser vibration measurement equipment and the excitation equipment, realize the setting of the measurement point array, the acquisition and storage of vibration signals, and support data processing such as Fourier transform of the acquired time-domain signals.

[0054] The working connection relationship of the detection system is as follows: the control equipment is connected to the laser vibration measuring equipment and the excitation equipment respectively. The excitation equipment is set facing the wall to be tested. The laser emitting end of the laser vibration measuring equipment is vertically irradiated into the measurement point array area of ​​the wall to be tested. The decibel meter is placed in the detection area between the wall to be tested and the excitation equipment for real-time measurement of sound pressure.

[0055] Preparations before testing include:

[0056] Pre-treatment of the wall surface to be tested: Clean the surface of the wall surface to be tested of floating dust, debris and protruding attachments to ensure that the wall surface is flat and to avoid obstruction of laser reflection or interference with vibration signal propagation;

[0057] Equipment setup and debugging: Place the excitation device at a reasonable distance in front of the wall to be tested (usually 0.5-2m, adjusted according to the power of the excitation device) to ensure that the sound wave signal uniformly covers the area to be tested; set up the laser vibration meter on a stable support, adjust the laser emission angle to vertically illuminate the wall to be tested, and complete the calibration and zero-point adjustment of the laser vibration meter; place the decibel meter at the center of the measurement area on the wall to be tested to complete the sound pressure measurement calibration;

[0058] Material parameter preset: By consulting engineering data or conducting material testing, the basic material parameters of the wall to be tested are obtained and preset into the control equipment, including the density of material layers. Young's modulus of material layer Poisson's ratio of materials The above parameters serve as the basis for subsequent calculations of hollow characteristic parameters.

[0059] The specific steps of the method in this embodiment are as follows:

[0060] S1. Turn on the laser vibration measuring device and the excitation device, set the detection range and measurement point array on the wall to be tested, turn on the excitation device to emit sound wave signals to excite the wall to be tested, and measure the magnitude of the excitation force when the sound wave reaches the wall to be tested.

[0061] In this step, the control equipment sets a measurement point array within the predicted detection range of the wall surface to be measured. The spacing between the points can be adjusted according to the required detection accuracy (this method does not require a dense point array; a standard point spacing of 5-10cm is sufficient for the required accuracy). The coordinate positions of each measurement point are recorded, such as... Figure 2 As shown. A sweep frequency signal or frequency comb signal is input to the excitation device via the control device, activating the excitation device to emit acoustic wave signals to excite the wall surface under test. Wherein:

[0062] Frequency sweep signal: An audio signal whose frequency increases at equal intervals over time, with a frequency range set to 20-1000Hz (to match the low-frequency resonance characteristics of wall hollowing systems). Figure 3 The time-frequency diagram of the swept frequency signal is shown.

[0063] Frequency comb signal: Divide the signal in the frequency range of 20-1000Hz into comb-shaped frequency function signals of equal length. The step size can be set to 5-10Hz to realize simultaneous excitation of multiple frequencies. Figure 4 The frequency comb signal spectrum is shown.

[0064] The sound pressure level at each measurement point on the wall surface is measured using a calibrated decibel meter. The magnitude of the excitation force is obtained based on the conversion formula between sound pressure and excitation force, and the data is stored in the control device. In this step, the measurement of the excitation force must be performed synchronously with the sound wave excitation to ensure the real-time performance and accuracy of the data.

[0065] S2. Use laser vibration measurement equipment to collect vibration velocity signals at each measurement point, and perform Fourier transform on the vibration velocity signals at each point to obtain the vibration signals at each point in the frequency domain.

[0066] In this step, while the excitation device emits acoustic signals, the laser vibration measuring device is activated by the control device to collect the time-domain signals of vibration velocity at various measurement points on the wall surface under test. The acquisition time is consistent with the transmission time of the excitation signal (usually 1-5s) to ensure that the vibration signal of each measurement point is completely acquired. Figure 5 The time-domain signal of a certain point in the data collection is displayed.

[0067] The vibration velocity time-domain signals collected at each measurement point are controlled by the equipment. Perform a Fast Fourier Transform (FFT) to convert the time-domain signal into a frequency-domain signal. ,in Let be the angular frequency, and ( (where ω is the frequency); after conversion, the vibration velocity amplitude at each measurement point at different angular frequencies is obtained, providing observation values ​​for subsequent mechanical impedance model fitting. Figure 6 The frequency domain signal at a specific point in the data collection system is displayed.

[0068] S3. Establish a mechanical impedance model for the wall hollowing system. Substitute the vibration signal and excitation force in the frequency domain into the mechanical impedance formula and fit it using the least squares method to obtain the identification parameters of the wall hollowing system.

[0069] This step is the core technical step of the present invention. By establishing a mechanical impedance model of the wall hollowing system, substituting the frequency domain vibration signal and excitation force into the model, and using the least squares method to fit and obtain the identification parameters of the wall hollowing system, including the equivalent stiffness. Equivalent quality Damping coefficient All operations are performed via control equipment to complete data fitting and calculations. Details are as follows:

[0070] S3.1 Establishing a mechanical impedance model for the wall hollowing system

[0071] like Figure 7 As shown, the wall hollowing system is equivalent to a single-degree-of-freedom mass-spring-damping lumped parameter model, that is, the hollow area is regarded as being composed of an equivalent mass... (Mass of the upper layer of hollow material), equivalent spring (Bending stiffness of material layer + modified stiffness of cavity structure), equivalent damping The single-degree-of-freedom vibration system (composed of energy loss during vibration) perfectly matches the low-frequency vibration characteristics of the wall hollowing system, can ignore unnecessary spatial distribution details, and lock in the key dynamic characteristics of the hollowing area.

[0072] S3.2, Substituting the mechanical resistance formula

[0073] Mechanical resistance of wall hollowing system Defined as the magnitude of the incentive force With vibration velocity ratio By combining the dynamic equations of a single-degree-of-freedom vibration system, the core formula for mechanical impedance is derived:

[0074] ;

[0075] ;

[0076] In the formula, The vibration velocity of the wall hollowing system. For the magnitude of the incentive, Angular frequency, The imaginary unit, The equivalent stiffness of the wall hollowing system; The equivalent quality of the wall hollowing system. This is the damping coefficient of the wall hollowing system.

[0077] The vibration signals of each measurement point obtained in step S2 are analyzed in the frequency domain. Substituting the observed values ​​into the mechanical impedance formula, we obtain the observed mechanical impedance values ​​at different angular frequencies for each measurement point.

[0078] S3.3 Least squares method for fitting and identifying parameters

[0079] The frequency domain vibration signals at each measurement point are used as observed values, and the vibration signals calculated using the mechanical impedance formula are used as theoretical calculated values. The least squares method is then used for optimization to fit the equivalent stiffness that minimizes the sum of squared errors between the theoretical and observed values. Equivalent quality and damping coefficient This is the final identification parameter for the wall hollowing system. Figure 8 The figure shows the fitting curve of the frequency domain signal at a certain point in the acquisition point using the mechanical impedance model.

[0080] S4. Based on the identification parameters and the preset material parameters, calculate the geometric area of ​​the hollow area, the thickness of the upper material of the hollow area, and the depth of the hollow cavity.

[0081] In this step, the equivalent stiffness obtained from the fitting in step S3 is used. Equivalent quality Combined with the pre-set material basic parameters before testing ( , , By solving multiple formulas simultaneously, the geometric area of ​​the hollow wall can be calculated. Thickness of the upper layer material in the hollow area Hollow cavity depth Damping coefficient The parameters are used to assist in verifying the energy loss characteristics of the vibration system and to verify the effectiveness of the fitting results.

[0082] In this step, such as Figure 3 As shown, the hollow area (wall hollow system) is simulated as a lumped parameter model consisting of a mass, spring, and damper with reduced stiffness and a single degree of freedom. Through this simplified analysis based on lumped parameters, the vibration response change caused by the local decrease in stiffness can be quickly calculated.

[0083] The typical characteristic of hollow areas is the size of their geometric area. The depth of the upper surface of the cavity from the wall surface (i.e., the thickness of the upper layer of material in the hollow area). And the height of the cavity itself (i.e., the depth of the hollow cavity). When a wall surface containing voids is subjected to acoustic excitation, and the frequency of the wall void system matches the frequency of the acoustic excitation, the laser vibration meter will collect the frequency corresponding to the maximum amplitude of the wall void system. This frequency will then be calculated using the formula for resonant frequency. It can be seen that the resonant frequency is affected by the system's equivalent stiffness. and equivalent quality The impact.

[0084] Among them, equivalent mass This mainly originates from the material layer above the hollow areas in the wall hollowing system, and is represented as:

[0085] ;

[0086] In the formula, The density of the material layer; The thickness of the upper layer material of the hollow area. This represents the actual vibration area of ​​the wall hollowing system after it is subjected to acoustic excitation.

[0087] Due to nonlinear boundary conditions, the vibration of the surface of the finishing layer is not a uniform motion, but rather exhibits a modal distribution with "large amplitude at the center and zero amplitude at the edges." The area of ​​the hollow area is not equal to the actual vibration area after acoustic excitation. To obtain the true geometric area of ​​the hollow area... It is necessary to introduce a correction coefficient based on the mode shape function. Perform the reverse solution.

[0088] Specifically, the correction factor This is determined based on the theory of plate and shell vibration. It is assumed that the hollow region is a thin plate with fixed supports on all four sides, and its vibration follows a specific mode shape function. According to the principle of energy equivalence, the equivalent mass in the lumped parameter model... It is the integral of the physically distributed mass over the square of the mode shape function, that is:

[0089] ;

[0090] in, This is a hollow area.

[0091] Therefore, the actual effective vibration area of ​​the wall hollowing system after acoustic excitation Defined as the area of ​​the integral of the square of the mode shape function ( Correction factor That is, the square of the mode shape function over the entire geometric area of ​​the hollow area. Average integral value over:

[0092] ;

[0093] For a typical fundamental frequency vibration mode of a rectangular thin plate with fixed sides, the integral calculation yields... The theoretical value is typically between 0.25 and 0.35. Therefore, after experimentally measuring the equivalent mass and calculating... Then, divide by the corresponding mode shape correction factor. The geometric area of ​​the hollow area can then be obtained. .

[0094] Equivalent stiffness of wall hollowing system It consists of a material layer for bending stiffness and a cavity structure for correcting stiffness. The material layer is considered as a fixed structure with an area of... Thickness is The bending equation of the thin plate is as follows:

[0095] ;

[0096] In the formula, The bending stiffness of the plate; Let be the deflection of the plate; This represents the uniformly distributed load acting on the plate. It is the fourth spatial derivative.

[0097] The bending stiffness of the thin plate is as follows:

[0098] ;

[0099] In the formula, The Young's modulus of the material. Let be the Poisson's ratio of the material.

[0100] Based on the boundary conditions and load conditions of the thin plate, the deflection at the center of the plate can be obtained as follows:

[0101] ;

[0102] In the formula, These are constants related to boundary conditions and geometry.

[0103] Substitute the above formula into In this process, the equivalent stiffness (i.e., the bending stiffness of the material layer) within the central region of the thin plate is obtained:

[0104] .

[0105] The stiffness correction for the cavity structure mainly comes from the stiffness enhancement / empirical stiffness correction resulting from geometric and boundary coupling. Unlike enclosed spaces, cavities allow for gas exchange, so the stiffness change of the material plate due to acoustic-structure interaction between the cavity and the material layer differs. Furthermore, based on experimental fitting results, a stiffness coefficient related to the cavity depth is proposed. The relevant second-order correction term describes the final change in stiffness:

[0106] ;

[0107] In the formula, This is a correction factor for the stiffness of the cavity structure. By conducting preliminary tests on standard specimens with known cavity depths, and establishing stiffness-height characteristic curves, the results were obtained using least squares regression analysis.

[0108] In routine wall plaster layer hollow detection, The value of is usually determined as a constant through experiments, and it usually changes with the material of the wall to compensate for the increase in equivalent dynamic stiffness caused by air damping.

[0109] Therefore, the equivalent stiffness of the wall hollowing system is obtained. Afterwards, the alliance , and the geometric area of ​​the hollow area The calculation formula can be used to determine the depth of the hollow cavity. And the thickness of the upper layer material of the hollow area .

[0110] Furthermore, through the damping coefficient To verify the fitting effect of the mechanical impedance model, if If the value is within the range of the conventional damping coefficient of wall materials (the damping coefficient of non-metallic building wall materials is 10-100 N·s / m), it indicates that the model fit is effective and the calculated hollow characteristic parameters are reliable; if If the deviation deviates from the normal range, repeat steps S1-S3 for fitting calculations until the hollow characteristic parameters are reliable. Furthermore, to further improve detection accuracy, the spacing between the measurement points can be appropriately reduced, and the above steps can be repeated for retesting. The average of the multiple test results is taken as the final result.

[0111] Example 2: This example details the results of using a laser vibration measuring device to select the detection range and measurement points M×N in Example 1, comparing the energy method with the impedance method of this invention. The result of using the energy method to determine hollow areas is shown in the figure below. Figure 9 As shown, the approximate location of the hollow areas is accurate, but the size of each hollow area is unclear, and the number of hollow areas cannot be determined. Increasing the number of sampling points would increase the sampling time. Figure 10 As shown, the number of sampling points increased significantly, and the results clearly showed the size and number of voids, but the detection efficiency decreased significantly.

[0112] The results of using the impedance method of this invention are shown in the figure below. Figure 11As shown, when the number of measurement points is M×N, the location of the hollow areas and the attribution of the hollow areas to the measurement points can be clearly observed. Simultaneously, the approximate area of ​​the hollow areas can be determined based on the calculation results. It is evident that the method of this invention can quantify hollow areas within the wall without increasing the number of sampling points. Therefore, compared to the energy method, the method of this invention can significantly improve sampling efficiency while ensuring the accuracy of various hollow area information, including the geometric area of ​​the hollow areas, the thickness of the upper material of the hollow areas, and the depth of the hollow cavities.

[0113] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for identifying hollow walls based on laser acoustic vibration combined with mechanical impedance, characterized in that: Includes the following steps: S1. Turn on the laser vibration measuring device and the excitation device, set the detection range and measurement point array on the wall to be tested, turn on the excitation device to emit sound wave signals to excite the wall to be tested, and measure the magnitude of the excitation force when the sound wave reaches the wall to be tested; S2. Use laser vibration measurement equipment to collect vibration velocity signals at each measurement point, and perform Fourier transform on the vibration velocity signals at each point to obtain the vibration signals at each point in the frequency domain. S3. Establish a mechanical impedance model for the wall hollowing system. Substitute the vibration signal and excitation force in the frequency domain into the mechanical impedance formula and fit it using the least squares method to obtain the identification parameters of the wall hollowing system. S4. Based on the identification parameters and the preset material parameters, calculate the geometric area of ​​the hollow area, the thickness of the upper material of the hollow area, and the depth of the hollow cavity.

2. The method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance according to claim 1, characterized in that: In step S1, the acoustic wave signal input to the excitation device is a sweep frequency signal or a frequency comb signal; wherein, the sweep frequency signal is an audio signal whose frequency increases at equal intervals over time, and the frequency comb signal is a comb-shaped frequency function signal that divides a frequency signal within a certain range into equal parts with the same duration.

3. The method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance according to claim 1, characterized in that: In step S3, the mechanical impedance model is a lumped parameter model that equates the wall hollowing system to a single-degree-of-freedom mass, spring, and damper. The identification parameters include the equivalent stiffness of the wall hollowing system. Equivalent quality and damping coefficient .

4. The method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance according to claim 1, characterized in that: In step S3, the mechanical impedance formula is: ； In the formula, The vibration velocity of the wall hollowing system. For the magnitude of the incentive, Angular frequency, The imaginary unit, The equivalent stiffness of the wall hollowing system; The equivalent quality of the wall hollowing system. This is the damping coefficient of the wall hollowing system.

5. The method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance according to claim 4, characterized in that: In step S3, the least squares fitting method refers to taking the vibration signals in the frequency domain of each measurement point obtained in step S2 as observed values, substituting them into the mechanical impedance formula, and optimizing the solution using the least squares method to identify the equivalent stiffness that minimizes the sum of squared errors between the theoretically calculated value and the observed value. Equivalent quality and damping coefficient .

6. The method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance according to claim 4, characterized in that: In step S4, the relationship between the geometric area of ​​the hollow area, the thickness of the upper material of the hollow area, the depth of the hollow cavity, and the equivalent mass of the wall hollow system satisfies: ； ； In the formula, The density of the material layer, The thickness of the upper layer material of the hollow area. This represents the actual effective vibration area of ​​the wall hollowing system after being subjected to acoustic excitation. Let the area of ​​the hollow area be denoted by . These are correction coefficients based on the mode shape function; By identifying equivalent quality Calculate the actual effective vibration area Then divide by the correction factor Obtain the geometric area of ​​the hollow area .

7. The method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance according to claim 6, characterized in that: The correction coefficient Based on the theory of plate and shell vibration and the principle of energy equivalence, the correction coefficient is determined. Defined as the square of the mode shape function over the entire hollow geometric area The average integral value over, that is: ； In the formula, This is a hollow area. This is the mode shape function.

8. The method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance according to claim 4, characterized in that: The equivalent stiffness It consists of the bending stiffness of the material layer and the modified stiffness of the cavity structure; The expression for the bending stiffness of the material layer is as follows: ； In the formula, The Young's modulus of the material layer. The thickness of the upper layer material of the hollow area. Let the area of ​​the hollow area be denoted by . These are constants related to boundary conditions and geometry; For the bending stiffness of the material layer; The expression for the corrected stiffness of the cavity structure is: ； In the formula, Correcting the stiffness of the cavity structure; This is the correction factor for the stiffness of the cavity structure; The depth of the hollow cavity; Based on the identified equivalent stiffness Combination , The expression is used to calculate the thickness of the upper layer material in the hollow area. and / or cavity depth .

9. The method for identifying wall hollowness based on laser acoustic vibration combined with mechanical impedance according to any one of claims 1-8, characterized in that: In step S1, the sound pressure of the sound wave reaching the wall to be tested is measured by a decibel meter, and the magnitude of the excitation force is calculated.

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