CN109238441B - Acoustic covering layer echo reduction measurement method based on optimal space-time focusing technology - Google Patents
Acoustic covering layer echo reduction measurement method based on optimal space-time focusing technology Download PDFInfo
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Abstract
An acoustic coating echo reduction measurement method based on an optimal space-time focusing technology comprises the following steps: 1) generating an optimal space-time focusing transmitting signal; 2) acquiring a direct signal and an echo signal under the condition of a sample; 3) calculation of echo reduction measurements. The invention utilizes the channel coherent information between each transmitting array element and each receiving array element, obtains the optimal focusing transmitting signal of each transmitting array element by an objective function optimization method, realizes the optimal space-time focusing at the preset hydrophone by synchronous transmission, achieves the purposes of low-frequency high resolution and reverberation suppression in echo reduction measurement, and is particularly suitable for the measurement of echo reduction of low-frequency band and multi-layer shell models; meanwhile, pressure tank measurement experiments prove the effectiveness of the invention in acoustic overburden echo reduction measurements.
Description
Technical Field
The invention discloses an underwater acoustic covering layer, which is an underwater part widely used and crucial in underwater acoustic engineering, and relates to a low-frequency measurement method for echo reduction of a large sample of the underwater acoustic covering layer, wherein the echo reduction measurement of the large sample under a laboratory condition is an important means for evaluating the echo reduction performance of an acoustic covering layer sample.
Background
The acoustic covering layer refers to special functional acoustic materials and structures laid on underwater parts, and is an acoustic protection system mainly composed of series products with different acoustic functions, such as an anechoic tile, an acoustic isolation tile, a vibration suppression tile, a decoupling tile, an array silencer and the like. The acoustic covering layer can absorb active detection sound waves to reduce the strength of an acoustic target of the underwater structure and can be used as a material for inhibiting the self radiation noise of the structure.
The strength of the echo signal is closely related to the reflection characteristic of the target, which is an important index for measuring the sound absorption performance of the underwater acoustic covering layer, and the target sound reflection capacity is described by the target strength in engineering. Under the condition of a limited space near field, the target intensity of the sample cannot be directly measured, but the suppression effect of the acoustic coating on the target intensity of the model can be measured by measuring the variation (namely echo reduction) of the model echo before/after the acoustic coating is laid under the same incident sound wave condition.
Existing measurement methods for echo reduction of acoustic coatings include spatial fourier transform methods, wideband pulse compression methods, time reversal focusing methods, multichannel space-time inverse filtering methods, and the like. In a limited space environment, reverberation and multipath effects are serious, and the precise decomposition of plane waves cannot be realized, so that the application of a space Fourier transform method is limited; the broadband pulse compression method can realize time domain pulse signal waveform focusing, but under the condition of low frequency, the single-channel emission characteristic of the broadband pulse compression method causes poor directivity and serious diffraction and reverberation interference; the time reversal focusing method can realize space-time focusing, but because the time reversal focusing method does not counteract the influence of channel amplitude, the pulse width of a focused time domain signal is limited, the optimal focusing effect cannot be realized, and the measurement of echo reduction under the low-frequency condition is not facilitated; the multichannel space-time inverse filtering technology does not use coherent information between channel transfer functions of array elements of a receiving array and a transmitting array, and does not realize optimal space-time focusing for a measuring system with a plurality of receiving array elements. In addition, for a multilayer shell model, the echo performance of each layer of sample is usually evaluated to realize the optimal comprehensive sound absorption performance evaluation, so that not only the direct wave and the echo need to be separated, but also the echoes of each layer of shell need to be separated, and especially, as the frequency is reduced, the measurement has higher requirements on the time domain waveform focusing of echo signals.
Disclosure of Invention
In order to overcome the defects of the prior art, realize separation of reflected echoes and direct waves of each sound absorption layer and multi-path interference suppression in the acoustic covering layer large sample echo reduction measurement under the conditions of low frequency and limited space, and further improve the precision of the measurement result of the existing acoustic covering layer echo reduction measurement technology in low frequency band and multilayer shell model tests, the invention provides an acoustic covering layer large sample echo reduction measurement method based on an optimal space-time focusing technology. The method is suitable for low frequency bands, effectively reduces measurement errors, improves measurement accuracy, and reduces requirements of measurement experiments on sample sizes and experiment spaces. Pressure tank measurement experiments demonstrate the effectiveness of the present invention in acoustic overburden echo reduction measurements.
The technical scheme adopted by the invention to solve the technical problems is as follows.
An acoustic covering layer echo reduction measurement method based on an optimal space-time focusing technology comprises the following steps:
1) generating an optimal space-time focusing transmitting signal: under the condition of no sample, each transmitting transducer sequentially transmits an initial guide signal, each hydrophone sequentially receives the signals, a channel response function of each channel is obtained by using a least square method, and secondary transmitting signals of each transmitting channel are obtained by using an objective function optimization method;
2) and (3) acquiring direct signals and echo signals under the condition of a sample: placing a sample with an acoustic covering layer in a test environment, synchronously transmitting the transmitting signals obtained by calculation in the step 1) by each transducer, generating waveforms of which the time and space domains are all Dirac functions at the position of a preset hydrophone according to the principle of the objective function optimization method by the transmitting signals, and recording direct signals p of the sample by the hydrophoneiAnd echo signals p reflected by nearby samplesr；
3) Calculation of echo reduction measurements: the calculation formula of echo reduction can be obtained
Wherein ErIndicating the echo reduction value.
The technical conception of the invention is as follows: the method comprises the steps of obtaining receiving signals containing circuit channel and underwater sound channel information through initial guide signal transmitting and hydrophone receiving one by one, further obtaining secondary transmitting signals of each array element of a transducer array through a target function optimization method, then synchronously transmitting by each transducer to obtain optimal focusing signals at a preset hydrophone, and accordingly optimally achieving space focusing and time domain pulse compression of measuring signals and achieving the purpose of suppressing low-frequency reverberation.
Compared with the existing echo reduction measurement method, the invention has the technical advantages that: the method comprises the steps of obtaining optimal focusing transmitting signals of all transmitting array elements by utilizing channel coherent information between all transmitting array elements and all receiving array elements through an objective function optimization method, achieving optimal space-time focusing at a preset hydrophone position through synchronous transmitting, achieving the purposes of low-frequency high resolution and reverberation suppression in echo reduction measurement, and being particularly suitable for measurement of echo reduction of low-frequency-band and multi-layer shell models.
Drawings
FIG. 1 is a schematic view of the overall measurement system of the method of the present invention.
FIG. 2 is a comparison graph of the echo reduction theory and the measurement test result of a steel plate sample with the thickness of 5mm under the pressure tank environment.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
Referring to fig. 1-2, an acoustic coating echo reduction measurement method based on an optimal space-time focusing technique is used for measuring large sample echo reduction of an acoustic coating in a limited space. The technical scheme of the whole measuring method is as follows:
1) generating an optimal space-time focusing transmitting signal;
firstly, estimating channel response by using a least square algorithm; in the absence of a sample, each transmitting transducer transmits a pilot signal in the frequency domain denoted as enAnd (omega), N is 1,2, …, N, is transmitted through the circuit channel and the underwater sound channel and then is received by the mth hydrophone, and the output signal is expressed as y (omega, r)n；rm) M is 1,2, …, M; wherein N, M is the number of transmitting transducers and hydrophones respectively, ω represents angular frequency, rn、rmRespectively representing the nth transmitting transducerAnd the location of the mth hydrophone; the process in the frequency domain can be represented as
y(ω,rn；rm)＝en(ω)·G(ω,rn；rm) (2)
Wherein G (omega, r)n；rm) Is a channel response function between the nth transmit channel and the mth receive channel;
defining an objective function according to the least squares principle
P(G)＝(Y-EG)H(Y-EG) (3)
Wherein the superscript H represents conjugate transpose, and G represents matrix composed of response functions of different transceiving channels
Y is a matrix of output signals
E is a diagonal matrix composed of the input signals, i.e.
E＝diag[e1(ω),e2(ω),…,eN(ω)](6)
Where diag is expressed as a vector e1(ω),e2(ω),…,eN(ω)]A matrix of diagonal elements;
to minimize P (G), the partial derivative of G is made equal to zero, i.e.
Calculating the channel response
G＝(EHE)-1EHY (8)
After obtaining the channel response, in order to solve the optimized secondary transmission signal, the following objective function is established
J(s1(ω),s2(ω),…,sN(ω))＝∫W|Φ(ω,r)-exp(-iωT)δ(r-rc)|2dr (9)
Where W is the set of hydrophone positions. Is additionally provided with sn(ω) corresponds to a time domain signal sn(t), N ═ 1,2, …, N, t denotes time; when s isn(t) transmitting at time t-0, transmitting through channel, and setting at preset focusing position rcFocusing at T moment, and to realize the space-time optimal focusing, the theoretical value of the focusing signal at the preset position on the frequency domain can be expressed as exp (-i omega T) delta (r-r)c) I.e. the space time is all dirac functions; let phi (omega, r)m) M is 1,2, …, M, which is the received signal at the mth hydrophone element, and can be expressed as
Wherein x isn(ω,rm) M is 1,2, …, M, the received signal at the mth hydrophone position, denoted as
xn(ω,rm)＝sn(ω)·G(ω,rn；rm)(11)
Will phi (omega, r)m) The optimal secondary emission signal can be obtained by bringing in an objective function and carrying out minimum solution
Γ(ω)s(ω)＝exp(-iωT)g*(ω) (12)
Wherein g is*(ω) represents the conjugate function of the function g (ω) and the constituent element of the Γ matrix is
Γnm(ω)＝∫WG(ω,rn；r)G*(ω,rm；r)dr (13)
The constituent elements of the vector g (ω) are
gn(ω)＝G(ω,rn；rc),n＝1,2,…,N (14)
Gamma is the cross-correlation representation of each channel response function, and g (omega) is a vector formed by the response functions between each transmitting array element and a preset focusing position; the optimal secondary transmission signal can be expressed as
Wherein
And a pseudo inverse matrix representing the matrix gamma, and performing inverse Fourier transform on the signal to obtain a transmitting signal s (t) with optimal time-domain space-time focusing.
2) Acquiring a direct signal and an echo signal under the condition of a sample;
placing a sample with an acoustic covering layer in a test environment, as shown in figure 1, controlling a multichannel independent control signal generator to generate an optimal secondary emission signal generated in the step 1) through a computer-aided processing system, amplifying the optimal secondary emission signal by a multichannel power amplifier, synchronously emitting the optimal secondary emission signal by each emission transducer, generating an optimal space-time focus at a preset position by the emission signal, controlling an acquisition device to record a signal when the sample is stored through the computer-aided processing system, and intercepting a direct wave signal p from the recorded signal according to a path difference relationiAnd echo signals p reflected by nearby samplesr。
3) Calculating an echo reduction measurement;
using the sample direct wave signal p collected in 2)iAnd echo signal prThe echo reduction measurement is calculated from equation (1).
Examples illustrate that: in order to verify the effectiveness of the invention in the measurement of the acoustic covering layer echo reduction, test verification in the environment of the pressure muffling water tank is carried out under laboratory conditions. In the experiment, the distance between the ternary transmitting array and a test sample is 4.5m, and 3 circular transmitting transducer array elements are uniformly distributed on a circle with the radius of 1.5 m. The test specimens are steel plates with a geometry of 1.1 m.times.1.0 m.times.5 mm and a density of 7.84X 103kg/m3The sound velocity is 5470m/s, and the distance between the five-element cross hydrophone array and the surface of the steel plate is about 0.25 m. The experiment carried out echo reduction data acquisition and processing at frequencies of 0.5kHz to 5 kHz. From fig. 2, it can be seen that the measurement result is substantially consistent with the theoretical value, and the error is less than 1dB, and it can be seen that the method of the present invention is effective in the acoustic coating echo reduction measurement. Furthermore, the figures show that the echo reduction at low frequencies is slightly more measurement error, mainly due to the performance of the transmitting transducer of the measurement system in the low frequency regionThe drop is obvious, which causes the distortion of signal waveform and the reduction of signal-to-noise ratio, thereby affecting the measurement result.
The embodiments described in this specification are merely illustrative of implementations of the inventive concept and the scope of the present invention should not be considered limited to the specific forms set forth in the embodiments but rather by the equivalents thereof as may occur to those skilled in the art upon consideration of the present inventive concept.
Claims (1)
1. An acoustic coating echo reduction measurement method based on an optimal space-time focusing technology comprises the following steps:
1) generating an optimal space-time focusing transmitting signal: under the condition of no sample, each transmitting transducer sequentially transmits an initial guide signal, each hydrophone sequentially receives the signals, a channel response function of each channel is obtained by using a least square method, and secondary transmitting signals of each transmitting channel are obtained by using an objective function optimization method; the process of generating the optimal space-time focusing transmission signal is as follows:
firstly, estimating channel response by using a least square algorithm; in the absence of a sample, each transmitting transducer transmits a pilot signal in the frequency domain denoted as enAnd (omega), N is 1,2, …, N, is transmitted through the circuit channel and the underwater sound channel and then is received by the mth hydrophone, and the output signal is expressed as y (omega, r)n；rm) Where M is 1,2, …, M, where N, M is the number of transmitting transducers and hydrophones, ω represents the angular frequency, rn、rmRespectively representing the positions of the nth transmitting transducer and the mth hydrophone; the process in the frequency domain can be represented as
y(ω,rn；rm)＝en(ω)·G(ω,rn；rm) (2)
Wherein G (omega, r)n；rm) Is a channel response function between the nth transmit channel and the mth receive channel;
defining an objective function according to the least squares principle
P(G)＝(Y-EG)H(Y-EG) (3)
Wherein the superscript H represents conjugate transpose, and G represents matrix composed of response functions of different transceiving channels
Y is a matrix of output signals
E is a diagonal matrix composed of the input signals, i.e.
E＝diag[e1(ω),e2(ω),…,eN(ω)](6)
Where diag is expressed as a vector e1(ω),e2(ω),…,eN(ω)]A matrix of diagonal elements;
to minimize P (G), the partial derivative of G is made equal to zero, i.e.
Calculating the channel response
G＝(EHE)-1EHY (8)
After obtaining the channel response, in order to solve the optimized secondary transmission signal, the following objective function is established
J(s1(ω),s2(ω),…,sN(ω))＝∫W|Φ(ω,r)-exp(-iωT)δ(r-rc)|2dr (9)
Wherein W is the position set of the hydrophones; is additionally provided with sn(ω) corresponds to a time domain signal sn(t), N ═ 1,2, …, N, t denotes time; when s isn(t) transmitting at time t-0, transmitting through channel, and setting at preset focusing position rcFocusing at T moment, and to realize the space-time optimal focusing, the theoretical value of the focusing signal at the preset position on the frequency domain can be expressed as exp (-i omega T) delta (r-r)c) I.e. the space time is all dirac functions; let phi (omega, r)m) M is 1,2, …, M is the received signal at the mth hydrophone element, and mayIs shown as
Wherein x isn(ω,rm) M is the received signal transmitted by the nth transmit channel and at the mth hydrophone position, denoted as 1,2, …, M
xn(ω,rm)＝sn(ω)·G(ω,rn；rm) (11)
Will phi (omega, r)m) The optimal secondary emission signal can be obtained by bringing in an objective function and carrying out minimum solution
Γ(ω)s(ω)＝exp(-iωT)g*(ω) (12)
Wherein g is*(ω) represents the conjugate function of the function g (ω) and the constituent element of the Γ matrix is
Γnm(ω)＝∫WG(ω,rn；r)G*(ω,rm；r)dr (13)
The constituent elements of the vector g (ω) are
gn(ω)＝G(ω,rn；rc),n＝1,2,…,N (14)
Gamma is the cross-correlation representation of each channel response function, and g (omega) is a vector formed by the channel response functions between each transmitting array element and a preset focusing position; the optimal secondary transmission signal can be expressed as
Wherein
A pseudo-inverse matrix representing the matrix gamma is used for carrying out inverse Fourier transform on the signal to obtain an emission signal s (t) with optimal time-domain space-time focusing;
2) and (3) acquiring direct signals and echo signals under the condition of a sample: placing the sample with the acoustic covering layer in a test environment, and synchronously transmitting the transmission signals obtained by calculation in the step 1) by each transducer according to the purposeThe principle of a calibration function optimization method is that a transmitting signal generates a waveform with all time-space domains being Dirac functions at a preset hydrophone position, and the hydrophone records a direct signal p of a sampleiAnd echo signals p reflected by nearby samplesr；
3) Calculation of echo reduction measurements: the calculation formula of echo reduction can be obtained
Wherein ErIndicating the echo reduction value.
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