Method of calibrating hydrophones in field with continuous signal radiation in reverberating measurement pool

FIELD: physics.

SUBSTANCE: hydrophone is placed in a pool at a known distance from the radiator. The radiator is excited with by a linear frequency modulated signal (LFM signal) with known parametres. The hydrophone is exposed to the continuous signal from the radiator. Instantaneous current values of the radiator and output voltage of the hydrophone are then measured, from which the complex frequency dependency of the transient impedance (TI) of the radiator and the hydrophone in the reverberation field of the non-damped hydroacoustic pool is determined. The complex frequency dependency of the TI of the radiator and the hydrophone are then determined in conditions of a free field through sliding complex weighted averaging in the set frequency interval of the complex frequency dependency of the TI of the radiator and the hydrophone in the reverberating field using a weighting function which is given by signal time delays reflected by the measurement pool.

EFFECT: more accurate hydrophone calibration.

3 dwg

 

The invention relates to underwater acoustics and can be used for the calibration of hydrophones on the field in terms of the reverberation field produced by continuous radiation of sound waves in unkilled hydroacoustic the pool.

The term "calibration of the hydrophone on the field" involves determining the sensitivity of the hydrophone voltage free-field running sound waves.

Under free field understand the field of a spherical sound wave propagating in an isotropic homogeneous medium, which is in practice impossible to implement.

Therefore, calibration of the hydrophone on the field almost always done in terms of reflections (reverberation field).

In unkilled measuring the pool, the problem of calibration of the hydrophone on the field due to the need combat reflexes.

There is a method of calibration of the hydrophone on the field in the unkilled measuring the pool [Bobber R.J. Hydroacoustic measurements. / Lane. from English. edited Angelinaa. - M.: Mir. - 1974, CEI/IEC 60565:2006. Underwater acoustics - hydrophones - calibration in the frequency range 0.01 Hz to 1 MHz. The International Electrotechnical Commission. Geneva. Switzerland. - 2006], in which the direct signal emitter and the signal reflected measuring pool, divided by the method of time selection. At the same time as signal using the pitch-pulse sonar signal.

Not what Adami known method are the distortion of the tone pulse transients in narrowband measuring path, the decrease of efficiency of time selection with decreasing frequency and, consequently, the lower limit frequency calibration in the field, a great time detailed measurements of the frequency response, the need for coherent accumulation of tone pulses at the reception to improve the signal-to-noise.

There is a method of calibration of the hydrophone on the field in the continuous radiation is linearly frequency modulated signal is a chirp signal with known parameters in unkilled pool with frequency separation of the direct and reflected signals adopted for the prototype [Peder C. Pedersen, Peter A. Lewin, Leif Bjørnø, Application of time-delay spectrometry for calibration of ultrasonic transducers / IEEE transaction on ultrasonics, ferroelectric and frequency control. - March, 1988. - Vol. 35, No. 2. - P.185-205].

The prototype is based on the location in the measuring pool pairs of emitter-hydrophone at a known distance between the emitter and the hydrophone, determining for each pair of emitter-hydrophone time delays of the signals reflected at the reflecting surfaces of the measuring of the basin, the excitation of each pair of emitter-hydrophone linearly frequency modulated signal with known parameters, measuring the output voltage of the hydrophone and the current of the emitter, the determination of the frequency dependence of the transition impedance of a pair of emitter-hydrophone in the pool of reflections, the determination is received from the dependence of the transition impedance of a pair of emitter-hydrophone in free field conditions with subsequent determination obtained for each pair of emitter-receiver frequency dependencies the transition impedance conditions free field frequency dependence of the sensitivity of the hydrophone on the field.

The prototype is called the method of time delay spectrometry (SWW). The disadvantages of the method SWW are the errors of the calibration of the hydrophone due to a violation of free field conditions and distortions of the desired frequency response of the sensitivity of the hydrophone averaged, and the use of only one type continuous signal distributed in the frequency band power of the chirp signal.

The technical result from implementation of the invention is to improve the accuracy of the calibration of the hydrophone on the field in the unkilled measuring the pool (in terms of the reverberation field) by reducing the influence of reflections on the results of the calibration, noise reduction desired frequency response of the sensitivity of the hydrophone averaging.

This technical result is reached due to the fact that in the known method, which consists in the arrangement in the measuring pool pairs of emitter-hydrophone at a known distance between the emitter and the hydrophone, determining for each pair of emitter-hydrophone time delays of the signals reflected at the reflecting surfaces of the measuring of the basin, the excitation of each pair of emitter-hydrophone linearly frequency modulated signal with investigaparallel, measuring the output voltage of the hydrophone and the current of the emitter, the determination of the frequency dependence of the transition impedance of a pair of emitter-hydrophone in free field conditions with subsequent determination obtained for each pair of emitter-receiver frequency dependences of the transition impedance in free field conditions frequency dependence of the sensitivity of the hydrophone on the field for each pair of emitter-hydrophone measured instantaneous current of the emitter and the output voltage of the hydrophone, which determine the complex frequency dependence of the transition impedance of the pair in the pool of reflections, and the complex frequency dependence of the transition impedance of a pair of emitter-hydrophone in free field conditions determine a moving weighted complex averaging in the prescribed frequency range complex frequency dependence of the transition impedance of a pair of emitter-hydrophone in the pool with reflections using a weighing function that is based on values of the time delays of the signals reflected measuring pool.

The invention is illustrated by drawings. Figure 1 presents the scheme of implementation of the method; figure 2, 3 diagrams explaining the operation method.

Figure 1 presents the system emitter-receiver consisting of three cascaded l is nanih elements: emitter, the pool and the receiver. Each element of the system will be characterized by its transfer function. Emitter - sensitivity to radiation SP(ƒ), swimming pool - transfer function of the sound pressure HWT(ƒ), the receiver sensitivity for the reception of MH(ƒ). Input and output of the system are, respectively, the current transducer iP(ƒ) and the output voltage uH(ƒ).

Transfer function (PF) of the basin taking into account reflections HWT(*) we define as the ratio of the sound pressure at the point of reception: total sound pressure pΣ(ƒ) and the sound pressure p0(ƒ) in the direct wave HWT(ƒ)=pΣ(ƒ)/p0(ƒ). The calibration method of reciprocity in the field based on the measurement of the transition impedance (PI) system emitter-receiver in terms of the distribution of the direct wave. In this view, it corresponds to a single PF HWT(ƒ). Pose the problem: find conditions under which PF pool becomes close to one.

Assuming a finite number of significant reflections of the sound pressure at the point of reception pΣ(ƒ) imagine the amount of pressure in the direct wave p0(ƒ) and pressure pi(ƒ) in reflected waves:. PF pool record by reflections in the form:. It is obvious that the task can the be achieved, if the effect ofto reduce to zero.

The pressure in the reflected wave pi(ƒ) present via the pressure in the direct wave p0(ƒ) and phase delay for the difference between the progress of the direct and reflected wave Δri. To simplify records and without prejudice to the generality of the reasoning we assume that Δr1<Δr2<...<Δrnthe reflection coefficient is independent of frequency and equal to the unit, and the pressure amplitude in the reflected wave does not depend on the stroke difference Δr. Based on the assumptions pi(ƒ)=p0(ƒ)exp(-jk Δriandwhere k=2πƒ/C is the wave number.

Consider the result of a moving complex averaging function HWT(ƒ) in the frequency interval δ ƒ.

,

where εi(f,Δf)=ψ/(∆F,τi) exp(-jkΔri) is the residual effect of reflections number i, τi=Δri/s is the time delay of the reflected wave, thespecifies a relative decline in the influence of reflection caused by the averaging in the frequency interval δ ƒ, depending on the time delay τ reflection.

When averaging in the frequency interval δ ƒ1=1/τ1component due to the influence of the 1st reflection ε1(z, δ ƒ1) vanishes, and the influence of reflections εi(z, δ ƒi) (i>1), detainees at time τi1 reduced to not less than 5 times, since for τ>τ1the function ψ(δ ƒ1, τ) in absolute value does not exceed 0,21.

It is easy to show that the same considerations apply to any of the i-th reflection and each integrated moving average PF basin HWT(ƒ) in the frequency 1/τ1eliminates the influence of the reflection of the delayed time τiand leads to no less than 5-fold weakening of the influence of later reflections. In the General case, the total reduction of the influence of reflections in the m≤n consecutive averages in the frequency interval δ ƒ1=1/τ1, Δ ƒ2=1/τ2,..., Δ ƒm=1/τmsets the.

The above illustrates the dependencies presented in figure 2. Figure 2 panel a shows the frequency spacing δ ƒ1and δ ƒ2comprehensive moving averaging in which PF pool eliminates the influence of reflections, the detainees at time τ1=1/δ ƒ1and τ2=1,5τ1. The result of two consecutive averages can be achieved by a single averaging in the frequency interval δ ƒ1+Δ ƒ2using a weighing function h(ƒ). The weighing function is obtained by convolution of functions of two rectangular window of width δ ƒ1and δ ƒ2. The function h(ƒ) is depicted in figure 2 and has the d trapezoid with bases δ ƒ 1+Δ ƒ2and δ ƒ1-Δ ƒ2.

On figb curves "1st", "2nd" and "1-e + 2-e shows the function ψ(δ ƒ1, τ) and ψ(δ ƒ2(τ)corresponding to ravnovzveshennuju averaging in the frequency intervals δ ƒ1and δ ƒ2and the function ψΣ(Δ ƒ1, Δ ƒ2, τ)=ψ(δ ƒ1, τ)ψ(δ ƒ2(τ), the corresponding averaging using a weighing function h(ƒ). The time delay τ in the graphs are expressed in units relative to the delay of the first reflection τ1. Given the equivalence relations time delays τ/τ1and the relationship of the distances Δr/Δr1(Δr=SG, Δr11for figb be useful to consider how the characteristics of spatial filters, which are implemented by averaging PF basin in the frequency interval. Since ψ(δ ƒ1τ1)=ψ(δ ƒ2τ2)=0 the function ψ(δ ƒ1, τ) and ψ(δ ƒ2(τ) can be considered as characteristics of the transmission registeruser spatial filter configured to reflect delayed by τ1and τ2respectively. Slow damping of oscillations of the function ψ(δ ƒ1(τ) when τ > τ1and the function ψ(δ ƒ2(τ) when τ > τ2shows a significant parasite transmission, the spatial filter (up 21%)implemented ravnovzveshennuju averaged PF pool. The combined use of rejectio the existing filters of the first and second reflections gives a much better characteristic of the transmission. Moving the integrated weighted averaging (SCWO) in the frequency interval δ ƒ=δ ƒ1+Δ ƒ2leads to a complete suppression of the influence of the first and second reflections, and the influence of reflections, the detainees at time τ>τ2reduces at least 98,3%. In practice, this may be sufficient. The transmission characteristic of the spatial filter can be improved by the use of the third averaging. When this happens it is preferable to apply a filter that is configured to the maximum of the oscillations of the function ψΣ(Δ ƒ1, Δ ƒ2(τ) when τ > τ2and not a spatial filter that is configured on the third reflection. The result of applying such a filter is illustrated in figb dependence "1 e+a 2 e + a 3 e oscillations which for τ>τ2the absolute value does not exceed 0,005. This is enough to apply the SQUAW in the standards for the calibration of hydrophones on the field.

Thus, applying to PF pool SQUAW in the frequency interval δ ƒ equal to the sum of the frequency intervals registeruser filters, with weight function h(ƒ)obtained by the convolution of the respective rectangular frequency Windows, get:

Frequency averaging interval δ ƒ, which is achieved by the relation (1)will be called the frequency interval of the pool.

Assessment PI emitter-the receiver in free field according to the measurement results PEE in the pool with reflections based on the property PF the pool, expressed by the formula (1). Returning to the system depicted in figure 1, PI emitter-receiver Z'PH(ƒ)measured in the field, the distorted reflected waves can be written as product of PI emitter-receiver in free-field ZPH(ƒ)=SP(ƒ)MH(ƒ) on PF basin: Z'PH(ƒ)=ZPH(ƒ)HWT(ƒ) and consider the result of a moving weighted complex averaging PI emitter-receiver Z'PH(ƒ) in the frequency interval δ ƒ basin:

,

whereand

.

In the past it is easy to see, avoiding rigorous mathematical proof and based on the typical assumption about the frequency dependence of ZPH(ƒ): frequency range oscillations PI emitter-receiver in a free field is usually greatly exceeds the frequency range of oscillations PI Z'PH(ƒ)measured with the reflection.

On figa and b rows 1 presents the frequency dependence of PI emitter-receiver in the pool with reflections obtained in the frequency interval from to 17,8 22,2 kHz when the distance between emitter and receiver 0.83 and 4.3 m, respectively, and converted (as shown) to the distance of 1 m Rows 2 and 3 shows the frequency dependence of the module shows the transition impedance (PPI), respectively after the first and the who ravnovzveshennuju moving averaging of the complex frequency dependence of PPI emitter-receiver. The frequency interval of the first averaging selected to eliminate the effect of the first reflection, the influence of the other reflections are suppressed enough. The remaining distortion is almost completely eliminated the second complex averaging that demonstrate the series 3. Series 3, obtained when the distances are 0.83 and 4.3 m, almost the same (differences not exceed 0.5%), which confirms the execution is inversely proportional law of change of sound pressure with distance between transmitter and receiver with sufficient accuracy for the reference measurements.

The method of calibration of the hydrophone on the field when a CW signal in the measuring pool with reflection, which consists in the arrangement in the measuring pool pairs of emitter-hydrophone at a known distance between the emitter and the hydrophone, determining for each pair of emitter-hydrophone time delays of the signals reflected at the reflecting surfaces of the measuring of the basin, the excitation of each pair of emitter-hydrophone linearly frequency-modulated signal with known parameters, measuring the output voltage of the hydrophone and the current of the emitter, the determination of the frequency dependence of the transition impedance of a pair of emitter-hydrophone in the pool with reflections, determining based on the received frequency dependence perehodnik the impedance of a pair of emitter-hydrophone in free field conditions with subsequent determination obtained for each pair of emitter-receiver frequency dependences of the transition impedance in free field conditions frequency according to the sensitivity of the hydrophone on the field, characterized in that for each pair of emitter-hydrophone measured instantaneous current of the emitter and the output voltage of the hydrophone, which determine the complex frequency dependence of the transition impedance of the pair in the pool of reflections, and the complex frequency dependence of the transition impedance of a pair of emitter-hydrophone in free field conditions determine a moving weighted complex averaging in the prescribed frequency range complex frequency dependence of the transition impedance of a pair of emitter-hydrophone in the pool with reflections using a weighing function, which is based on the values of the time delays of the signals reflected measuring pool.



 

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