DEVICE AND METHOD FOR EXAMINATION OF MATERIALS BY ACOUSTIC SPECTROSCOPY

Description

[0001] The invention relates to a device and a method for examining materials by acoustic spectroscopy according to the independent main claims 1 and 8, respectively.

[0002] A method for acoustic spectroscopy of glass materials is known, for example, from US 2006 / 028096 A1. According to this method, the attenuation and ultrasonic velocity are calculated based on the measurement of amplitude and phase shift, which allows conclusions to be drawn about the material properties of the glass. DE 10 2008 014300 A1 also discloses a generic method for determining the state of a mixture of substances by measuring the speed of sound and / or ultrasonic attenuation. In order to draw conclusions about the composition of the mixture of substances to be examined from the speed of sound and attenuation, the amplitudes of the transmitted and reflected sound waves are evaluated, and the propagation time of the individual sound waves is calculated from the squared signal amplitude of the transmitted and received signals.

[0003] In their publication entitled "Simultaneous measurement of longitudinal and transverse velocities, attenuation, density, and thickness of films by using point-focus ultrasonic spectroscopy" in the Journal of Applied Physics, Ju Bing-Feng et al. propose another generic method for determining the acoustic and geometric properties of a thin-film test material using spectroscopy. This method calculates the acoustic impedance, attenuation coefficient, and transit time of the focused ultrasonic waves from the reflection spectra of the ultrasonic waves.

[0004] For the continuous examination of a medium, a generic device is known from DE 20 2007 017911 U1, which generates a conical radiation characteristic in order to enable a condition assessment of the test material by comparing a reference pattern of phase shifts with the phase shifts occurring at a receiver.

[0005] US Pat. No. 5,433,112 A discloses another generic method and device for characterizing a polymer melt. This method allows the time shift between two consecutive ultrasonic echoes emerging from a polymer melt to be determined. The measuring path between an ultrasonic transmitter and an ultrasonic receiver is formed by two buffer rods and the centrally arranged polymer melt. This method incorporates the amplitude changes that occur to calculate the acoustic material properties.

[0006] US 2015 / 0059442 A1 proposes a method for acoustic spectroscopy using sound waves below the ultrasonic frequency range. The evaluation of the reflected acoustic spectra after interaction with the suspension to be characterized serves to determine the properties, content, and / or size of the particles in the suspension.

[0007] Another device for examining liquid or gaseous media using acoustic spectroscopy is known, for example, from DE 103 24 990 B2. This device includes a transmitting device for transmitting multiple transmitted signals of different frequencies and a receiving device for receiving corresponding received signals. A processing device can then determine the phase shift for each transmitted and received signal pair, and from this, a value qualifying the medium under investigation can be derived based on the phase shift. In the device described therein, signal packets of different frequencies are transmitted one after the other in direct succession. The frequencies preferably range between 1 and 15 MHz, with each signal packet preferably comprising at least 100 periods.For each signal packet, and thus for each frequency level, the specific received signals are recorded by the receiving device and then fed to a processing device. In the processing device, the phase shift angle between each frequency-specific transmitted and received signal pair caused by the passage through the medium under investigation is determined. Thus, this frequency-specific sampling of the medium ultimately records a multitude of different frequency-specific phase shift values, the number of which depends on the number of transmitted and received signal pairs. In this way, information about the behavior of the medium is obtained over a very wide frequency range defined by individual frequency levels. This information enables conclusions to be drawn about the properties of the medium under investigation.By frequency-specific sampling of the medium and the corresponding determination of the frequency-related phase angles, a much more specific investigation of the medium can be achieved, since a change in the medium has different effects on the respective frequency-specific transmit and receive signal pair.

[0008] A disadvantage of the device described in DE 103 24 990 B2 is that it is based on the evaluation of phase shift values, which are caused by the transmission signal passing through the feature under investigation and documented by the reception signal. This is because the evaluation of the phase shift between the transmission signal and the reception signal can lead to undesirable information losses, which significantly distort the evaluation result. This information loss is due to the fact that the phase shift value is limited to a range between 0 and 360°. However, if the phase shift between the transmission signal and the reception signal is greater than 360°, the method described in the document delivers a phase shift value that cannot be clearly assigned.For example, if the phase shift between the transmitted signal and the received signal is 400°, the evaluation device described in the document will produce a phase shift value of 40°. The resulting evaluation results are therefore significantly distorted and unusable, since the evaluation produces results that correspond to a phase shift of 40°, not a phase shift of 400° (= 360° + 40°).

[0009] To solve this problem of ambiguous results when evaluating the phase shift between the transmitted signal and the received signal, DE 198 41 154 A1 proposes generating transmitted signals with different frequencies. The different frequencies serve as a vernier scale for evaluating the phase shifts. This makes it possible to clearly assign the measured phase shift values to the actual phase shift between the transmitted signal and the received signal. However, this method has the disadvantage that the sound waves in the medium under investigation are transmitted at different speeds of sound depending on the respective transmission frequency. These different transmission speeds in the medium under investigation can lead to significant measurement errors. The dependence of the propagation speed of the transmitted signal on the respective transmission frequency is described by the Kramers-Kronig equation.In this respect, the measuring method according to DE 198 41 154 A1 has proven to be unsuitable, since the measurement errors cannot be taken into account due to the non-uniform propagation speed of the sound waves depending on the respective transmission frequency.

[0010] The principles of acoustic spectroscopy underlying the device according to the invention are described, for example, but by no means exclusively, in the following specialist books: Technical book 1: Molecular acoustics, Werner Schaaffs, Springer Verlag, 1963 (ISBN-10:3642491413, ISBN-13:978-3642491412) Technical book 2: Molecular Acustics / Molekularakustik, K.-H. Hellwege, A.M. Hellwege, W. Schaaffs, Springer Verlag, 1967 (ISBN-10:3540038973, ISBN-13:978-3540038979) Textbook 3: Molecular Acoustics, A.J. Matheson, John Willy & Sons Verlag, 1971 (ISBN-10:1861561857, ISBN-13:978-1861561855) Textbook 4: Materials Testing with Ultrasound, Josef Krautkrämer, Herbert Krautkrämer, Springer-Verlag, 1975

[0011] Based on this state of the art, a device and method for examining test materials using acoustic spectroscopy are proposed. In particular, a method and device are proposed for online investigation of the material properties of transformer oils.

[0012] Transformers convert an input alternating voltage into an output alternating voltage, whereby the electrical energy must be transferred between two coils via magnetic fields. For very high input or output alternating voltages, transformers are usually used whose housings are filled with transformer oil to ensure sufficient insulation and cooling between the transmission coils. Because transformer oil is highly hygroscopic, it tends to absorb water over time, which is then converted into acids in the transformer oil. The water or acid in the transformer oil increasingly limits the functionality of the transformer over time, as the water or acid reduces the electrical insulation provided by the transformer oil, which can be indicated, for example, by the maximum breakdown voltage.For conventional transformers, it is therefore common practice to replace or regenerate the transformer oil during regular maintenance cycles to ensure reliable functionality. However, this regular replacement of the transformer oil is extremely time-consuming and expensive, as the transformer oil contains highly toxic components and therefore requires complex disposal or recycling. Furthermore, if maintenance cycles are set too long to minimize the cost of replacing the transformer oil, this can lead to total transformer failure.

[0013] Known methods for testing transformer oil require opening the transformer and taking a sample. However, opening the transformer for sampling is highly problematic because transformer oil is highly hygroscopic, and when the transformer housing is opened, a relatively large amount of water molecules from the ambient air migrate into the transformer oil.

[0014] The object of the present invention is therefore to propose a device and a method for examining materials by acoustic spectroscopy, in which the disadvantages mentioned above are avoided.

[0015] This object is achieved by a device and a method according to the teaching of the independent main claims 1 and 8, respectively.

[0016] Advantageous embodiments of the invention are the subject of the subclaims.

[0017] The device according to the invention comprises a measuring section consisting of a reference material and the test material to be examined. The objective of the examination is to determine the acoustic material properties of the test material. The acoustic material properties of the reference material for different frequencies, i.e., the density, the sound transmission velocity, the acoustic damping coefficient, and / or the acoustic impedance of the reference material, are known. Furthermore, the geometry of the measuring section determines the length of the reference material and the length of the test material through which the ultrasound signals travel during the examination.

[0018] The device comprises an ultrasonic transmitter for transmitting ultrasonic signals with an initial amplitude A 0 through the measuring section. Furthermore, the device comprises a first ultrasonic receiver, with which the transmitted ultrasonic signals can be received and measured after passing through the measuring section. In addition to the first ultrasonic receiver, the device comprises a second ultrasonic receiver for receiving the ultrasonic signals reflected at the interface between the test material and the reference material after passing through the reference material or test material twice. The second ultrasonic receiver can also be implemented by appropriately switching the ultrasonic transmitter.

[0019] Both the transmitting device and the two receiving devices are suitable for transmitting or receiving ultrasonic signals of different frequencies in order to be able to carry out a multispectral ultrasonic examination of the test material at different transmitting or receiving frequencies.

[0020] In contrast to the teaching of DE 103 24 990 B3, however, the phase shift for each transmit and receive signal pair is not determined during signal evaluation. Instead, according to the invention, the transit time of the associated transmitted ultrasonic receive signal is measured for each transmit signal in a first processing device after it has completely passed through the measuring section. Furthermore, according to the invention, a second processing device determines the amplitude of the associated transmitted ultrasonic receive signal for each transmit signal after it has passed through the measuring section, and a third processing device determines the amplitude of the associated ultrasonic receive signal reflected at the interface between the test material and the reference material after it has passed through the reference material or test material twice.

[0021] According to the invention, the acoustic material properties of the test material for different frequencies, i.e., the density, the sound transmission velocity, the acoustic attenuation value, and / or the acoustic impedance of the test material, are then calculated in an evaluation device from the three measured values determined by the processing devices, namely the transit time, the amplitude of the transmitted ultrasonic received signal, and / or the amplitude of the reflected ultrasonic received signal. Depending on the acoustic material property, not all measured values are included in the calculation. The amplitude of the transmitted ultrasonic received signal or the amplitude of the reflected ultrasonic received signal can be measured directly or determined by determining other measured values, for example, the signal attenuation, and subsequently deriving the amplitude values.

[0022] By directly measuring the transit time of the transmitted ultrasonic reception signal after passing through the measuring section, the measurement errors known from the prior art, such as those that occur when using the phase shift between the transmitted and received signal, are avoided according to the invention.

[0023] Investigations of test materials, such as transformer oils, during exposure to ultrasonic signals and the subsequent determination of the transit time, the amplitude of the transmitted received signal, and the amplitude of the reflected received signal have shown that changes in the test material are significantly reflected in the resulting data sets consisting of transit time, transmission amplitude, and / or reflection amplitude. Conventional acoustic spectroscopy, which previously only measured signal attenuation, is thus expanded according to the invention to determine not only the specific and frequency-dependent attenuation for each frequency, i.e., the ratio between input amplitude and transmission amplitude or the ratio between input amplitude and reflection amplitude, but also the specific and frequency-dependent transit time (time of flight / TOF).By measuring the flow time, the dispersivity of the material under test can be determined. Combining the measurement of attenuation and dispersivity allows for a highly detailed characterization of the test material.

[0024] With the device according to the invention, in contrast to previous testing methods, the transit time of the ultrasonic signal itself is measured, rather than the phase shift. In order to be able to determine the properties of the material under investigation with sufficient resolution, it is particularly advantageous if the transit time can be determined with a resolution of at least 100 picoseconds in the first processing device. Preferably, the transit time should be able to be determined with a resolution of at least 10 picoseconds.

[0025] Which time measuring device is provided in the first processing device to determine the transit time is fundamentally arbitrary. In order to achieve resolutions when measuring the transit time, especially in the range of less than 100 picoseconds, it is particularly advantageous to use so-called time-to-digital converters. These time-to-digital converters are electronic components that can measure time intervals in the range of < 100 picoseconds and convert them into a digital output. The time measurement is based on the known transit time of an electrical signal through the components of the TDC converter. The transit time is then determined by how many of the electrical circuits with a known transit time are passed through between the transmission of the transmitted signal on the one hand and the reception of the input signal on the other.This number of passes of electronic assemblies with known pass time is then counted, whereby the measured pass time then corresponds to the multiplication of the known pass time of a single circuit by the number of passes.

[0026] The second ultrasonic receiver receives the ultrasonic reception signals, which, after the ultrasonic transmission signal has been emitted at its initial amplitude and after passing through the test material or reference material, are first reflected at the interface between the test material and the reference material and then pass through the test material or reference material a second time. The geometric arrangement of the second ultrasonic receiver therefore corresponds exactly to the part of the measuring section where the ultrasonic transmitter is also located. To reduce the technical complexity, it is therefore particularly advantageous if the second ultrasonic receiver is implemented by switching the ultrasonic transmitter. In other words, this means that when the device is in operation, the ultrasonic transmission signal is first generated by the ultrasonic transmitter and then transmitted into the measuring section.The ultrasonic transmitter is then switched to operate as an ultrasonic receiver. The transmitted ultrasonic signal then passes through the reference material or test material for the first time and is subsequently reflected at the interface between the test material and the reference material. The reflected ultrasonic signal then passes through the reference material and test material for a second time and finally returns to the beginning of the measuring section, where it can be received and measured by the ultrasonic transmitter, which is switched to operate as an ultrasonic receiver.

[0027] The material used for the reference material of the measuring section is fundamentally arbitrary. It is particularly advantageous if the reference material is made of a solid, particularly plastic or glass.

[0028] The device according to the invention can be used to test transformer oils. For this purpose, a cavity is formed between the interface of the reference material and the first ultrasonic receiver. The length of this cavity between the interface on the one hand and the ultrasonic receiver on the other hand must be precisely determined. During the actual test, this cavity is then filled with the test material, and the actual measurement is performed. Alternatively, it is also conceivable to allow the test material to flow through the cavity during the test.

[0029] The purpose for which the device according to the invention is used is fundamentally arbitrary. The device offers particularly great advantages for the online measurement of transformer oil, which must be replaced regularly. This is because online measurement of the oil allows the oil to be changed based on wear, thus avoiding the disadvantages of regularly performed maintenance intervals. The device according to the invention can be used particularly well for assessing the condition of transformer oil. For this purpose, the device is connected to a vessel containing the transformer oil to be assessed. During the actual examination, the cavity between the interface of the reference material and the first ultrasound receiving device is filled with the oil from the vessel or is flowed through.The multispectral analysis of the oil is then used to determine the density, sound transmission velocity, acoustic damping coefficient or acoustic impedance of the oil.

[0030] When testing transformer oil with the device according to the invention, it is particularly advantageous if the water content or acid content in the transformer oil is subsequently derived from the acoustic material properties of the transformer oil. These two parameters provide information about the aging of the transformer oil and can be used to ensure that the transformer oil is only replaced after a certain degree of aging, measured by the water content or acid content, has been reached.

[0031] With regard to the particularly advantageous assessment of transformer oil, a transformer for converting an input alternating voltage into an output alternating voltage is also proposed, wherein the transformer is filled with transformer oil and comprises a device according to the invention for online examination of the transformer oil.

[0032] The method according to the invention is characterized by determining the transit time of the transmitted ultrasonic received signal, the amplitude of the transmitted ultrasonic received signal, and the amplitude of the ultrasonic received signal reflected at the interface between the test material and the reference material. These two measured values are then used to calculate the desired acoustic material properties of the test material for different frequencies, i.e., in particular, the density, the sound transmission velocity, the acoustic attenuation coefficient, and / or the acoustic impedance.

[0033] The type of ultrasonic waves used for examination using the method or device according to the invention is fundamentally arbitrary. Longitudinal ultrasonic waves in the lower megahertz range are particularly suitable. Furthermore, it is conceivable for the ultrasonic wave signals of different frequencies to be emitted by the ultrasonic transmitting device individually, sequentially, or in successive groups.

[0034] Alternatively, the ultrasonic transmission signal for the different frequencies can also be modeled onto a common carrier signal using the ultrasonic transmission device and transmitted. It is also possible for the ultrasonic transmission signal of different frequencies and the ultrasonic transmission device to be superposed onto a common carrier signal and transmitted together.

[0035] The invention will now be explained by way of example with reference to an embodiment shown schematically in the drawings.

[0036] They show: Fig. 1 a device according to the invention with its various functional modules as a schematic diagram; Fig. 2 the measuring section of the device according to Fig. 1 in an enlarged view; Fig. 3 the collection of formulas for calculating acoustic material properties; Fig. 4 the evaluation device of the device according to Fig. 1 with the formulas stored therein for calculating the acoustic material properties of the test material to be examined.

[0037] Fig. 1 shows in a schematic diagram the basic structure of a device 01 according to the invention, as it can be used for the online examination of a test material 02. The test material 02 is transformer oil 30 (see Fig. 2 ) from a transformer 31. Through connecting lines 32 and 33 and through operation of a pump 34, a cavity 35 is connected to the transformer 31, so that the transformer oil 30 can be pumped from the interior of the housing 36 in circulation through the cavity 35.

[0038] The structure of the measuring section 37 during the actual examination of the test material 02 is briefly described below using the sketch in Fig. 2 be explained.

[0039] The measuring section 37 consists, on the one hand, of a reference material 38 and the test material 02 to be examined, which in the illustrated embodiment is the transformer oil 30. The reference material 38 can, for example, be a solid made of glass or plastic, wherein the acoustic material properties of the reference material 38, i.e. the acoustic damping coefficient α R , the acoustic impedance ZR , the density ρ R , the sound transmission speed c R must be at least partially known. In addition, the length x R of the reference material 38 and the length x M of the test material, i.e. the clear width of the cavity 35 in the signal direction, are known.

[0040] At the beginning of the measuring section 07 is an ultrasonic transmitter 39, which generates ultrasonic transmission signals with an initial amplitude A 0 and couples them into the measuring section. The ultrasonic transmitter 39 is a combined device that, when switched, can also function as an ultrasonic receiver 40. At the end of the measuring section 37 is also an ultrasonic receiver 41, which can receive ultrasonic signals.

[0041] During the actual examination of the test material 02, the cavity 35 is first flowed through by the pump 34. The measurements of the transformer oil can be performed online without opening the housing 36. As soon as the cavity 35 is completely filled with transformer oil 30, the ultrasonic transmission device 39 generates an ultrasonic transmission signal with an initial amplitude A 0 and couples it into the measuring section 37.

[0042] As shown schematically in Fig. 2 As shown, the ultrasonic transmission signal 42 first passes through the reference material 38 until it reaches the interface 43 between the reference material 38 and the test material 02. A portion of the ultrasonic transmission signal passes through the interface 43 and passes through the test material 02 until it reaches the ultrasonic receiving device 41 as a transmitted ultrasonic reception signal 44. By evaluating the transmitted ultrasonic reception signal 44, the transit time t G that the transmitted ultrasonic reception signal required to pass through the entire measuring section 37 and the amplitude AT of the transmitted ultrasonic reception signal 44 can then be measured or determined.

[0043] The portion of the ultrasonic reception signal 44 reflected at the interface 43 passes through the reference material 38 in the opposite direction a second time until it reaches the ultrasonic reception device 40 formed by switching the ultrasonic transmission device 39 as a reflected ultrasonic reception signal 45. By evaluating the measured values of the ultrasonic reception device 40, the amplitude AR of the reflected ultrasonic reception signal 45 can be determined. The type of further processing of the measurement signals of the two ultrasonic reception devices 41 is described below using the sketch in Fig. 1 further explained.

[0044] In addition to the combined ultrasonic transmitting and receiving device 39 / 40 and the ultrasonic receiving device 41, the device 01 comprises a signal preparation module 05 for generating the ultrasonic signals to be emitted by the ultrasonic transmitting device 39. A signal generator 06 and signal amplifier 07 are also provided.

[0045] The signal curves recorded by the ultrasonic receiving devices 40 and 41 are first amplified by means of signal amplifiers 08. The transmitted ultrasonic received signals 44 are then split in a signal splitter 09 and distributed to a first processing device 10 and a second processing device 11 for parallel processing. The first processing device 10 is used to determine the transit time required by the transmitted ultrasonic received signal 44 at the respectively set frequency to travel through the measuring section 37. After passing through a signal processing module 12, the transmitted ultrasonic received signal 44 reaches a time-to-digital converter 13, with which the transit time of the ultrasonic signal, iethe time between the radiation at the ultrasonic transmitting device 39 and the reception of the transmitted ultrasonic reception signal 44 at the ultrasonic receiving device 41 can be measured.

[0046] The function of the time-to-digital converter 13 is based on the fact that the transit time of an electrical signal through a plurality of electronic circuits 14 contained in the time-to-digital converter 13 is known.

[0047] To measure the transit time, the trigger signal of the ultrasonic transmission signal is transmitted to the time-to-digital converter 13 at the same time as it is emitted from the ultrasonic transmission device 39 to start the timing process. The trigger signal of the ultrasonic transmission signal then passes through the serially arranged circuits 14 in the TTD converter 13. Each pass of an electronic circuit 14, which corresponds to a predetermined transit time, is added by a counter 15. As soon as the ultrasonic reception signal 44 is then transmitted to the electronic circuits 14, the counter 15 switches off and multiplies the number of added passes by the known transit time of the individual electronic circuits 14. This then results in the total transit time t D for the corresponding measurement.For each transmission frequency, a transit time (t G ) 16 results, which is stored in a memory device 17 in association with the respective transmission frequency.

[0048] In parallel, the transmitted ultrasonic reception signal 44 is evaluated in the second processing device 11, and the amplitude AT of the transmitted ultrasonic reception signal 20 is determined. For this purpose, a signal processing module 18 is provided in the second processing device 11, with which the transmitted ultrasonic reception signal 44 is processed and the amplitude (AT ) 20 of the ultrasonic reception signal 44 is determined. The signal splitter 19 splits and indexes the ultrasonic reception signal 44. In a third processing device 46, the reflected ultrasonic reception signal 45 is processed by means of a signal processing module 18, and the amplitude (AR ) 47 of the reflected ultrasonic reception signal is determined.The determination of the amplitude values 20 and 47 in the processing devices 11 and 46 is carried out in a frequency-dependent manner, and the result data are stored for the respective frequency in the storage device 17 together with the respective processing time 16.

[0049] The result data stored in the storage device 17, namely the transit time (t G ) 16, the amplitude (AT ) 20 of the transmitted ultrasonic reception signal 44 and the amplitude (AR ) 47 of the reflected ultrasonic reception signal 45, are then read out from the storage device 17 and forwarded to an evaluation device 48 for further calculation of the result data.

[0050] Fig. 3 represents the collection of formulas for calculating acoustic material properties, as required by the evaluation device 48 when calculating the result data.

[0051] Fig. 4 shows schematically the evaluation device 48, with which with the help of the Fig. 4 The result data for the acoustic impedance Z m of the test material, the reflectance R at the interface 43, the acoustic damping coefficient α M of the test material, the sound transmission velocity c M of the test material, the density ρ M of the test material and the compressibility of the test material can be calculated from the measured values, namely the transit time t G , the amplitude (AT ) of the transmitted ultrasonic reception signal 24 and / or the amplitude (AR ) of the reflected ultrasonic reception signal 45 using the formulas shown.

[0052] After determining the acoustic material properties of the transformer oil 30 by calculation in the evaluation device 48, the water content or acid content in the transformer oil 30 can be determined in a next step. In addition, other insulation parameters such as the breakdown voltage (according to BDV) and the dissipation factor (tangent delta) can also be determined.

Claims

1. A device (01) for examining a test material (02) via acoustic spectroscopy, comprising a measuring distance (37) which is formed from a reference material (38) and the test material (02), the acoustic material parameters of the reference material (38), i.e., the density (ρR), the sonic transmission speed (cR), the acoustic attenuation coefficient (αR) and the acoustic impedance (ZR), being known for different frequencies, and the length (xR) of the reference material (38) being known, and the length (xM) of the test material (02) being known, and comprising an ultrasonic transmission device (39) for transmitting an ultrasonic transmission signal (42) having an initial amplitude (A0) through the measuring distance (37), and comprising a first ultrasonic reception device (41) for receiving the transmitted ultrasonic reception signal (44) after said signal (44) has passed through the measuring distance (37), and comprising a second ultrasonic reception device (40) for receiving the ultrasonic reception signal (45) reflected on the boundary surface (43) between the test material (02) and the reference material (38) after said signal (45) has twice passed through the reference material (38) or test material, said transmission device (39) being configured for giving ultrasonic transmission signals (42) having different frequencies (f) and the two reception devices (40, 41) being configured for receiving corresponding ultrasonic reception signals (44, 45) having different frequencies, wherein the time of flight (tG) of the allocated transmitted ultrasonic reception signals (44) can be measured, after said signals (44) have passed through the measuring distance (37), using a first processing device (10) of the device (01) for the ultrasonic transmission signals (42), and wherein the amplitude (AT) of the allocated transmitted ultrasonic reception signals (44) can be identified, after said signals (44) have passed through the measuring distance (37), using a second processing device (11) of the device (01) for the ultrasonic transmission signals (42), and wherein the amplitude (AR) of the allocated ultrasonic reception signals (45) reflected on the boundary surface (43) between the test material (02) and the reference material (38) can be identified, after said signals (45) have twice passed through the reference material (38) or test material, using a third processing device (46) of the device (01) for the ultrasonic transmission signals (42), and wherein the density (ρM) can be calculated by the formula the sonic transmission speed (cM) can be calculated by the formula the acoustic attenuation coefficient (αM) can be calculated by the formula and / or the acoustic impedance (ZM) can be calculated by the formula for different frequencies (f) using an evaluation device (48) of the device (01).

2. The device according to claim 1, wherein the time of flight (tG) can be identified with a resolution of at least 100 picoseconds, in particular with a resolution of 10 picoseconds, in the first processing device (10).

3. The device according to claim 1 or 2, wherein the first processing device (10) comprises a time-to-digital converter by means of which the time of flight (tG) is identified.

4. The device according to any one of the claims 1 to 3, wherein the second ultrasonic reception device (40) is configured for receiving the ultrasonic reception signal (45) reflected on the boundary surface (43) between the test material (02) and the reference material (38), after said signal (45) has twice passed through the reference material (38) or the test material, via switching the ultrasonic transmission device (39) from the transmission mode to the reception mode.

5. The device according to any one of the claims 1 to 4, wherein the reference material (38) consists of a solid body, in particular of plastic or glass.

6. The device according to any one of the claims 1 to 5, wherein a cavity (35) exists between the boundary surface (43) of the reference material (38) and the first ultrasonic reception device (41), a liquid or gaseous test material (02), in particular transformer oil (30), being able to fill or flow through said cavity (35) during examination.

7. A transformer (31) for converting an AC input voltage to an AC output voltage, wherein said transformer comprises a housing (36) filled with transformer oil (30), and wherein a device (01) according to any one of the preceding claims is provided in or on the transformer (31) for the online examination of the transformer oil (30).

8. A method for examining a biological or non-biological test material (02) via acoustic spectroscopy, comprising a measuring distance (37) which is formed from a reference material (38) and the test material (02), the acoustic material parameters of the reference material (38), i.e., the density (ρR), the sonic transmission speed (cR), the acoustic attenuation coefficient (αR) and the acoustic impedance (ZR), being known for different frequencies, and the length (xR) of the reference material (38) being known, and the length (xM) of the test material (02) being known, and comprising an ultrasonic transmission device (39) for transmitting an ultrasonic transmission signal (42) having an initial amplitude (A0) through the measuring distance (37), and comprising a first ultrasonic reception device (41) for receiving the transmitted ultrasonic reception signal (44) after said signal (44) has passed through the measuring distance (37), and comprising a second ultrasonic reception device (40) for receiving the ultrasonic reception signal (45) reflected on the boundary surface (43) between the test material (02) and the reference material (38) after said signal (45) has twice passed through the reference material (38) or test material, said transmission device (39) being configured for giving ultrasonic transmission signals (42) having different frequencies (f) and the two reception devices (40, 41) being configured for receiving corresponding ultrasonic reception signals (44, 45) having different frequencies (f), wherein several ultrasonic transmission signals (42) having different frequencies (f) are emitted into the measuring distance (37), wherein a) the time of flight (tG) of the allocated transmitted ultrasonic reception signals (44) is measured for the different ultrasonic transmission signals (42) after said signals (44) have passed through the measuring distance (37), b) the amplitude (AT) of the allocated transmitted ultrasonic reception signals (44) is identified for the different ultrasonic transmission signals (42) after said signals (44) have passed through the measuring distance (37), c) the amplitude (AR) of the allocated ultrasonic reception signals (45) reflected on the boundary surface (43) between the test material (02) and the reference material (38) is identified for the different ultrasonic transmission signals (42) after said signals (45) have twice passed through the reference material (38) or test material, d) the density (ρM), is calculated by the formula Z R t G ⋅ c R − x M − x R ⋅ A 0 − A R ⋅ e 2 α R ⋅ x R x M ⋅ c R ⋅ A 0 + A R ⋅ e 2 α R ⋅ x R , the sonic transmission speed (cM) is calculated by the formula x M ⋅ c R t C ⋅ c R − x M − x R , the acoustic attenuation coefficient (αM) is calculated by the formula − 1 x M ⋅ ln A T ⋅ e 2 α R ⋅ x R A 0 − A R ⋅ e 2 α R ⋅ x R , and / or the acoustic impedance (ZM) is calculated by the formula Z R ⋅ A 0 − A R ⋅ e 2 α R ⋅ x R A 0 + A R ⋅ e 2 α R ⋅ x R for different frequencies.

9. The method according to claim 8, wherein the time of flight (tG) is identified with a resolution of at least 100 picoseconds, in particular with a resolution of 10 picoseconds.

10. The method according to claim 8 or 9, wherein the acoustic material parameters of an oil, in particular a transformer oil (30), in particular the density (ρM) of the transformer oil (30), the sonic transmission speed (cM) in the transformer oil (30), the acoustic attenuation coefficient (αM) of the transformer oil (30) and / or the acoustic impedance (ZM) of the transformer oil (30), are identified.

11. The method according to claim 10, wherein the water content and / or the acidity level in the transformer oil (30) is derived from the acoustic material parameters of the transformer oil (30), in particular from the density (ρM) of the transformer oil (30), the sonic transmission speed (cM) in the transformer oil (30), the acoustic attenuation coefficient (αM) of the transformer oil (30) and / or the acoustic impedance (ZM) of the transformer oil (30).

12. The method according to any one of the claims 8 to 11, wherein the ultrasonic transmission signals (42) having different frequencies (f) are modulated onto or added to a shared carrier signal and are subsequently transmitted together in the ultrasonic transmission device (39).