A method for measuring ship radiated noise power spectrum based on frequency difference beamforming

Abstract

The application discloses a ship radiated noise power spectrum measurement method based on frequency difference beam forming, comprising the following steps: obtaining a to-be-measured radiated noise signal received by a base array; constructing a difference frequency signal through the conjugate product of the to-be-measured radiated noise signal and a reference frequency signal; performing beam forming on the constructed difference frequency signal based on the difference frequency, and calculating the amplitude of the beam output signal of the difference frequency signal; correcting the amplitude of the beam output signal of the difference frequency signal by using the amplitude of the reference frequency signal to obtain the amplitude of the to-be-measured radiated noise signal, and calculating the power spectrum of the to-be-measured radiated noise signal. The application converts the signal outside the design frequency band of the base array to the inside of the design frequency band to perform beam forming, improves the array gain, better suppresses the environmental noise, improves the measurement performance of the base array on the radiated noise outside the design frequency band, greatly expands the frequency band width of the ship radiated noise measurement, and can realize the measurement of the radiated noise with a larger bandwidth by using a smaller aperture array.

Description

Technical Field

[0001] This invention relates to the field of ship radiated noise measurement technology, and in particular to a method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming. Background Technology

[0002] After a ship is built, its radiated noise needs to be measured to assess its noise emissions. Common ship radiated noise measurement systems primarily use single hydrophones, typically conducted in waters with low ambient noise. Accurate measurements are only possible when the ship's radiated noise level is significantly higher than the ambient noise level, resulting in a high signal-to-noise ratio (SNR) for the hydrophone. However, with advancements in vibration and noise reduction technologies, ship radiated noise is decreasing, while ambient noise in the ocean is rising annually due to new shipping lane development and increased merchant shipping. This leads to a decreasing SNR for the hydrophone, rendering single-hydrophone-based radiated noise measurement methods inadequate for meeting practical measurement accuracy requirements.

[0003] To address the issue of low measurement performance of single hydrophones under low signal-to-noise ratio conditions, Sun Guiqing (Sun Guiqing, Yang Desen, Zhang Lanyue. Research on measurement method of low-frequency radiated noise of underwater targets based on vector hydrophones [J]. Acta Acustica, 2002, 27(5):429-434.) and Fang Erzheng (Fang Erzheng, Wang Yan. Measurement method of radiated noise of underwater targets [J]. Journal of Nanjing University of Aeronautics and Astronautics, 2009, 41(2):238-242.) and others utilized the frequency-independent directivity of vector hydrophones for radiated noise measurement to improve the signal-to-noise ratio of the received signal, achieving higher spatial gain compared to traditional acoustic pressure hydrophones. Wu Guoqing (Wu Guoqing, Wang Meigang, Chen Shouhu, et al. Error analysis and correction method for measuring underwater target radiated noise using vertical array and single hydrophone [J]. Acta Acustica, 2007, 32(5):398-403.) et al. obtained higher spatial directivity and array gain through acoustic pressure hydrophone array, significantly reducing the influence of environmental noise on radiated noise measurement and improving measurement accuracy. Xiang Longfeng (Xiang Longfeng, Sun Chao, Liu Zongwei, et al. Measurement method of ship radiated noise level based on virtual time-reversal mirror [J]. Acta Acustica, 2013(1):57-64.) et al. used virtual time-reversal mirror technology for noise measurement, reducing the influence of multipath signals on the received signal and improving the measurement accuracy of ship radiated noise.

[0004] However, for broadband radiated noise measurement, the bandwidth that a fixed array can measure is always limited. When the signal frequency exceeds the designed bandwidth, it will affect the beamforming performance, leading to a decrease in the accuracy of radiated noise measurement. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming, which can measure radiated noise over a larger frequency band without changing the array.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming includes the following steps: acquiring the radiated noise signal to be measured received by an array and transforming the received radiated noise signal to the frequency domain; constructing a difference frequency signal by the conjugate product of the radiated noise signal to be measured and a reference frequency signal, wherein the difference between the reference frequency and the frequency of the radiated noise signal to be measured is the difference frequency; performing beamforming on the constructed difference frequency signal based on the difference frequency and calculating the amplitude of the beam output signal of the difference frequency signal; correcting the amplitude of the beam output signal of the difference frequency signal using the amplitude of the reference frequency signal to obtain the amplitude of the radiated noise signal to be measured; and calculating the power spectrum of the radiated noise signal to be measured based on the amplitude of the radiated noise signal to be measured.

[0008] The beneficial technical effects of this invention are as follows: By constructing a difference frequency signal, this invention converts the signal outside the design frequency band of the array into the design frequency band for beamforming, thereby improving the array gain and better suppressing environmental noise. This improves the array's measurement performance for radiated noise outside the design frequency band, greatly expands the bandwidth of ship radiated noise measurement, and achieves the technical effect of measuring radiated noise on a larger frequency band without changing the array. Furthermore, it enables the measurement of radiated noise with a larger bandwidth using a smaller aperture array. Attached Figure Description

[0009] Figure 1 This is a flowchart illustrating the ship radiated noise power spectrum measurement method based on frequency difference beamforming according to the present invention.

[0010] Figure 2 The above are the waveform and spectrum diagrams of the ship radiated noise signal in the simulation of this invention.

[0011] Figure 3a , Figure 3b This refers to the single measurement results and measurement errors of the ship radiated noise signal in the simulation of this invention;

[0012] Figure 4 This represents the root mean square error corresponding to 1000 measurements of the ship's radiated noise signal in the simulation of this invention. Detailed Implementation

[0013] To enable those skilled in the art to more clearly understand the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0014] like Figure 1 As shown, in one embodiment of the present invention, the method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming includes steps S10 to S40.

[0015] S10. Acquire the radiation noise signal to be measured received by the array and transform the received radiation noise signal to be measured into the frequency domain.

[0016] In this embodiment, the array is an M-element uniform linear array, where M represents the number of array elements and can be any value as needed.

[0017] For the m-th element of the array, the frequency domain representation of the received radiated noise signal is as follows:

[0018] X m (f)=A(f)exp[-j2πf(m-1)dsinθ / c]

[0019] In the formula, f is the frequency of the radiated noise signal to be measured, A(f) is the amplitude of the radiated noise signal to be measured, θ is the direction of arrival of the radiated noise signal to be measured, d is the spacing between array elements, c is the speed of sound, j represents the imaginary unit, and m = 1, 2, ..., M.

[0020] S20. Construct a difference frequency signal by the conjugate product of the radiated noise signal to be measured and the reference frequency signal, wherein the difference between the reference frequency and the frequency of the radiated noise signal to be measured is the difference frequency.

[0021] The reference frequency signal can be measured using existing measurement methods within a frequency band where the radiated noise signal power is high. In this embodiment, the array uses an M-element uniform linear array, allowing the selection of a frequency within the array's design frequency band where the measurement results are more accurate as the reference frequency. For example, the radiated noise signal received at the center frequency of the array's design frequency band has high power; therefore, the radiated noise signal received at the center frequency of the array's design frequency band can be used as the reference frequency signal. Accordingly, the center frequency of the array's design frequency band is taken as the reference frequency, denoted as f. b .

[0022] For a difference frequency signal Y constructed by the conjugate product of the radiated noise signal to be measured and the reference frequency signal, the difference frequency signal corresponding to the m-th element of the array is expressed as:

[0023] Y m =X m (f) * X m (f b )

[0024] =A(f) * A(f b)exp[-j2πΔf(m-1)dsinθ / c]

[0025] In the formula, * represents conjugate, f b Let A(Δf) be the reference frequency, A(Δf) be the amplitude of the reference frequency signal, and Δf be the difference frequency. Δf = f b -f.

[0026] S30. Beamforming is performed on the differential frequency signal constructed based on the differential frequency pair, and the amplitude of the beam output signal of the differential frequency signal is calculated.

[0027] For the constructed difference-frequency signal Y, conventional beamforming can be performed on it according to the array manifold vector of the difference-frequency frequency Δf. Then, the normalized weighted representation corresponding to the m-th array element is:

[0028]

[0029] The amplitude of the beam output signal of the difference frequency signal is expressed as:

[0030]

[0031] S40. The amplitude of the beam output signal of the difference frequency signal is corrected by the amplitude of the reference frequency signal to obtain the amplitude of the radiated noise signal to be measured, and the power spectrum corresponding to the radiated noise signal to be measured is calculated based on the amplitude of the radiated noise signal to be measured.

[0032] Given the amplitude A(Δf) of the reference frequency signal, the amplitude of the radiated noise signal to be measured can be obtained by using it to correct the beam output signal of the difference frequency signal:

[0033]

[0034] Finally, the power spectrum of the radiated noise signal to be measured is expressed as:

[0035] p(f)=|A(f)| 2 .

[0036] The following numerical simulations using computers are used to verify the performance of the radiation noise power spectrum measurement method based on difference frequency beamforming proposed in this invention.

[0037] 1) Set array parameters and acquire received signals

[0038] In the simulation, the ship's radiated noise is represented by a broadband random signal with a power spectrum of 70 dB, a bandwidth of [10, 2000] Hz, and a signal arrival direction of 30°. Figure 2As shown. The array uses a uniform linear array of M=5 elements with an element spacing of 0.5 meters. The speed of sound in water is 1500 m / s, corresponding to a center frequency of approximately 1500 Hz. The time-domain received signal duration is 1 second, and the sampling frequency is 10 kHz, resulting in 10000 time-domain snapshots. The received signal is then transformed to the frequency domain using a Fourier transform. The background noise is Gaussian white noise with a power spectrum of 60 dB, resulting in a signal-to-noise ratio of 10 dB for the received signal.

[0039] 2) Construct a difference frequency signal from the received signal and perform beamforming.

[0040] In this simulation, the reference frequency is assumed to be 1500Hz. For the received signal in the [10,100]Hz frequency band, the difference frequency signal corresponding to different frequency points is calculated by multiplying it with the reference frequency signal by its conjugate. Then, conventional beamforming is performed on the constructed difference frequency signal according to the array manifold vector corresponding to the difference frequency to obtain the output signal.

[0041] 3) Calculation of signal power spectrum

[0042] Assuming the amplitude of the reference frequency signal is accurately known, it is used to correct the amplitude of the difference frequency beam output signal, thus obtaining the amplitude of the signal at the measured frequency. The power spectrum of the radiated noise signal at the corresponding frequency can then be calculated, as shown below. Figure 3a , Figure 3b As shown. To reduce the impact of randomness, the root mean square error calculated from 1000 Monte Carlo simulations is shown below. Figure 4 As shown.

[0043] The simulation results show that the ship radiated noise power spectrum measurement method based on frequency difference beamforming of the present invention can accurately measure the power spectrum of signals outside the array design frequency band, thereby achieving the purpose of expanding the radiated noise measurement frequency band.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Those skilled in the art can make various equivalent changes and improvements based on the above embodiments, and all equivalent variations or modifications made within the scope of the claims should fall within the protection scope of the present invention.

Claims

1. A method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming, characterized in that, Includes the following steps: S10. Acquire the radiation noise signal to be measured received by the array, and transform the received radiation noise signal to be measured into the frequency domain. S20. Construct a difference frequency signal by the conjugate product of the radiated noise signal to be measured and the reference frequency signal, wherein the difference between the reference frequency and the frequency of the radiated noise signal to be measured is the difference frequency. S30. Beamforming is performed on the difference frequency signal constructed based on the difference frequency pair, and the amplitude of the beam output signal of the difference frequency signal is calculated. S40. The amplitude of the beam output signal of the difference frequency signal is corrected by using the amplitude of the reference frequency signal to obtain the amplitude of the radiated noise signal to be measured, and the power spectrum of the radiated noise signal to be measured is calculated based on the amplitude of the radiated noise signal to be measured.

2. The method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming as described in claim 1, characterized in that, The array is an M-element uniform linear array, the first... m The frequency domain representation of the radiated noise signal received by each array element is as follows: In the formula, f The frequency of the radiated noise signal to be measured is... A ( f The amplitude of the radiated noise signal to be measured is denoted as . θ The direction of arrival of the radiated noise signal to be measured. d For the spacing between array elements, c For the speed of sound, j The imaginary unit is represented by M, which represents the number of array elements and can take any value as needed. m =1, 2...M.

3. The method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming as described in claim 2, characterized in that, No. m The difference frequency signal corresponding to each array element is represented as follows: In the formula, * represents conjugate. f b As the reference frequency, The amplitude of the reference frequency signal, Δ f The difference frequency, .

4. The method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming as described in claim 3, characterized in that, Beamforming is performed on the constructed difference frequency signal using the array manifold vector of the difference frequency, to obtain the amplitude Z of the beam output signal of the difference frequency signal: in, For the first m Normalized weighted average of each array element.

5. The method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming as described in claim 4, characterized in that, The amplitude of the beam output signal of the difference frequency signal is corrected using the following formula to obtain the amplitude of the radiated noise signal under test. A ( f ): ； The power spectrum of the radiated noise signal to be measured is calculated using the following formula. p ( f ): 。 6. The method for measuring the power spectrum of ship radiated noise based on frequency difference beamforming as described in claim 1, characterized in that, The center frequency of the array's design frequency band is used as the reference frequency, and the radiated noise signal received by the array at the reference frequency is used as the reference frequency signal.

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