METHOD FOR DETERMINING A PROPERTY OF A SAMPLE FLUID OR A RESONANCE FREQUENCY OF A RESONATOR CELL

Description

[0001] The present invention relates to a method for determining the amplitude or phase of an acoustic response signal from an acoustic resonator. The present invention also relates to a photoacoustic method based on the aforementioned method for determining a property of a sample fluid. Furthermore, the present invention relates to a device with which the aforementioned methods can be carried out, as well as the use of this device for exhaust gas measurement in a motor vehicle or for measuring pollutants in ambient air.

[0002] Protecting the environment from pollutants has always been a crucial issue, particularly given air pollution in major cities and the challenges of climate change. Against this backdrop, the demand for analytical methods and devices for determining the concentration of pollutants in fluids is growing. This applies especially to gaseous pollutants such as nitrogen oxides and carbon dioxide, as the former has a direct adverse effect on human health and the latter significantly contributes to the greenhouse effect in the atmosphere and thus to climate change. Photoacoustic methods are among those used to determine the concentration of these and other gases in a gas mixture.

[0003] In a photoacoustic method, a fluid mixture, particularly a gas mixture, for which the concentration of a sample gas (such as nitrogen dioxide) is to be determined, is irradiated with incident electromagnetic radiation. The electromagnetic radiation has a frequency of its alternating electromagnetic field that is equal to an absorption frequency of the sample gas. As a result, the sample gas molecules contained in the gas mixture absorb the incident electromagnetic radiation, leading to heating and thus thermal expansion of the entire gas mixture.

[0004] To determine the concentration of the sample gas from the irradiation of the gas, the incident electromagnetic radiation is modulated such that the thermal expansion of the gas mixture occurs periodically, thus generating a sound wave within the gas mixture. By measuring the intensity of this sound wave, for example using a microphone, the concentration of the sample gas within the gas mixture can be deduced. When performing such a photoacoustic method, the gas mixture is advantageously contained within a resonator cell, which acts as an acoustic resonator with a resonant frequency. The intensity of the sound wave can then be amplified by setting the modulation frequency of the incident electromagnetic radiation to the value of the resonant frequency. This results in an improved signal-to-noise ratio when measuring the intensity of the sound wave.

[0005] As described earlier, the electromagnetic radiation to be absorbed is generally modulated such that the frequency of the sound wave equals the resonant frequency of the acoustic resonator, in order to achieve the best possible amplification of the sound wave intensity. However, the resonant frequency is not a fixed value. It varies, in particular, depending on changing environmental parameters such as temperature, gas composition, and pressure. Therefore, it is necessary to know the actual resonant frequency of the acoustic resonator continuously during the photoacoustic process in order to vary the incident electromagnetic radiation accordingly. In other words, the resonant frequency of the acoustic resonator must be tracked during the process to ensure the desired amplification is maintained.

[0006] Currently, such tracking can be achieved by testing different modulation frequencies of the incident electromagnetic radiation absorbed by the sample gas molecules to determine the resonance frequency of the acoustic resonator. However, this is very time-consuming, and the actual measurement to determine the sample gas concentration must be repeatedly interrupted. Furthermore, in the very common cases of very low sample gas concentrations, the signal-to-noise ratio when determining the resonance frequency is very low due to the measurement principle of the photoacoustic method.

[0007] Tracking can also be achieved by using a loudspeaker within the acoustic resonator to generate a sound wave, and the resonance frequency is determined by modulating this sound wave. However, this use of a loudspeaker involves considerable design complexity.

[0008] As an alternative to tracking the resonant frequency, it has been suggested to construct the acoustic cell with temperature stabilization, since temperature variation has a significant influence on the detuning of the resonant frequency. However, this can only be achieved with an additional, costly construction.

[0009] From WO 2018 / 096091 A1, a measuring device for measuring a property of a component in a sample gas and a method for operating this measuring device are known. The sample gas is photoacoustically excited in a measuring chamber, the sound generated is recorded by a sound transducer, and the recorded signal is evaluated in an evaluation unit to determine the measured property. The evaluation unit corrects the determined measured property using a correction function. To determine at least one parameter of the correction function, a test pressure is set in the measuring chamber at least once, and the absolute pressure in the measuring chamber is determined. At least one test measurement is carried out at this absolute pressure during a test excitation, and the at least one parameter is determined based on the absolute pressure and the test measurement result of the test measurement.

[0010] CN 109 490 216 A discloses a calibration-free laser photoacoustic spectroscopy trace gas detection device and method. The laser photoacoustic spectroscopy trace gas detection device comprises a laser light source, an optical collimator, a photoacoustic cell, a photothermal conversion component, a microphone, an air inlet valve, an air outlet valve, an illumination driver, a signal acquisition and processing circuit, and a PC. The photothermal conversion component is amplified in the conventional photoacoustic gas absorption cell; the generated photoacoustic signal serves as a reference signal; the gas concentration is measured using a wavelength modulation technique and a 2f / 1f detection method; and calibration-free gas concentration measurement is achieved during changes in the power of the laser light source.

[0011] Against this background, the object of the present invention is to provide a method and a device with which the resonance frequency of an acoustic resonator can be determined as simply as possible. By carrying out a photoacoustic method, the resonance frequency of the acoustic resonator can thus be easily tracked.

[0012] This problem is solved by a method according to claim 1. According to the invention, this method comprises the following steps: a) According to a first alternative, providing an acoustic resonator, wherein the acoustic resonator comprises an absorption element with a surface section, providing a sample fluid or a fluid mixture containing the sample fluid, such that the acoustic resonator is filled with the sample fluid or the fluid mixture containing the sample fluid, wherein the surface section of the absorption element is in contact with the sample fluid or the fluid mixture containing the sample fluid, or according to a second alternative, providing an acoustic resonator, providing a fluid mixture containing a sample fluid, such that the acoustic resonator is filled with the fluid mixture, wherein the fluid mixture comprises, in addition to the sample fluid, at least one absorption fluid or one or more absorption particles, b) According to the first alternative,Irradiating the surface section of the absorption element with first electromagnetic radiation, such that the absorption element at least partially absorbs the first electromagnetic radiation in the region of the surface section, or according to the second alternative, irradiating the fluid mixture with first electromagnetic radiation, such that the absorption fluid or the absorption particles at least partially absorb the first electromagnetic radiation, so that in both alternatives pressure energy is generated in the sample fluid or the fluid mixture containing the sample fluid by absorption, wherein the first electromagnetic radiation is amplitude- or frequency-modulated with a modulation frequency, c) measuring an acoustic response signal of the acoustic resonator, which is generated by the absorption of the first electromagnetic radiation carried out in step b).d) Determining at least one amplitude (A) or phase (ϕ) of the acoustic response signal exhibiting the modulation frequency of the first electromagnetic radiation; determining a property of the acoustic resonator, wherein one or both of the following steps are provided: e) determining a resonance frequency of the acoustic resonator (5) from at least the amplitude (A) or phase (ϕ) determined in step d); f) determining a damping property of the acoustic resonator (5).

[0013] In one embodiment of the method according to the invention, the sample fluid is a sample gas or the fluid mixture containing the sample fluid is a gas mixture containing the sample gas.

[0014] It should be noted that the method according to the invention can generally be used for the analysis of fluids, in particular all types of trace gases in gas mixtures, and is not limited to the use of pollutants. In a particular embodiment, however, the method is specifically used for the detection of pollutants.

[0015] An acoustic response signal of an acoustic resonator is understood to be a pressure variation within the acoustic resonator caused by the absorption of the initial electromagnetic radiation. This pressure variation is physically described as a sound wave or sound pulse. From a metrological perspective, the acoustic response signal is, for example, a time series of pressure recorded by a single local acoustic sensor.

[0016] An absorption element of the acoustic resonator, as defined in the first alternative, is an element of the acoustic resonator capable of at least partially absorbing the first electromagnetic radiation. An absorption element can be either an additional structural element or an element inherent in the acoustic resonator, such as a side wall or the resonator base. In one embodiment of the method according to the invention, a first side wall of the acoustic resonator, or a portion thereof, constitutes the absorption element.

[0017] The method according to the invention makes it possible to determine a resonance frequency of the acoustic resonator and thus provides, among other things, the basis for a photoacoustic method in which the resonance frequency can be efficiently tracked.

[0018] Furthermore, the method according to the invention makes it possible to determine a property of the fluid or fluid mixture located in the acoustic resonator, such as the speed of sound of this fluid or fluid mixture.

[0019] In a particular embodiment of the present invention, ambient air or a comparable gas mixture is contained in the acoustic resonator. In this embodiment, the speed of sound is determined using the inventive method described above. Additionally, the temperature and humidity of the ambient air or the comparable gas mixture are determined. From the parameters thus determined—speed of sound, temperature, and humidity—the concentration of carbon dioxide in the ambient air or the comparable gas mixture is then determined in this embodiment.

[0020] In one embodiment of the method according to the invention, step b) is performed at least twice, wherein the first electromagnetic radiation is modulated with a first modulation frequency during a first performance of step b) and is modulated with a second modulation frequency during a second performance of step b), wherein the second modulation frequency differs from the first modulation frequency, such that in step d) at least a first amplitude or phase (ϕ1) for the first modulation frequency and a second amplitude or phase (ϕ2) for the second modulation frequency are determined.

[0021] According to this embodiment of the invention, the modulation frequency of the first electromagnetic radiation is tuned, preferably within a previously defined frequency range. This can advantageously be used to locate and determine the resonance frequency of the acoustic resonator. In one embodiment, the frequency range in which such tuning of the modulation frequency preferably takes place is determined by an expected value for the resonance frequency. This expected value, in turn, depends on the geometric shape of the acoustic resonator, which largely determines the resonance frequency, and optionally on simultaneously measured environmental parameters such as the ambient temperature or the humidity of a gas or gas mixture located in the acoustic resonator.

[0022] In one embodiment of the method according to the invention, the following further step is provided: g) Determining an amplitude response or phase response dependent on the modulation frequency from the at least two amplitudes or phases of the acoustic response signal determined in step d).

[0023] In mathematical terms, an amplitude response or phase response dependent on the modulation frequency is a two-dimensional matrix in which amplitude or phase values ​​are assigned to modulation frequencies. Based on such a matrix, a graph can be used to graphically represent the assignment of amplitude or phase values ​​to the modulation frequencies of the first electromagnetic radiation. The shape of this graph is referred to as the amplitude response or phase response. However, for the purposes of the present invention, the matrix underlying such a graph is also to be understood as the amplitude response or phase response.

[0024] From an amplitude response or phase response determined in accordance with this embodiment of the invention, the resonance frequency of the acoustic resonator can be determined particularly easily by determining the maximum of the amplitude or phase response, wherein the modulation frequency assigned to this maximum then represents the desired resonance frequency.

[0025] In one embodiment of the method according to the invention, in step b) the first electromagnetic radiation is amplitude- or frequency-modulated such that a broadband acoustic response signal of the acoustic resonator is generated, wherein the following further step is provided: h) Determining an amplitude response or phase response dependent on the modulation frequency from the broadband acoustic response signal measured in step c), preferably by applying a frequency analysis and particularly preferably by applying a Fourier transform to the broadband acoustic response signal measured in step c).

[0026] This method step can be used additionally or alternatively to tuning the modulation frequency of the first electronic radiation to determine the amplitude or phase response. An advantage of this embodiment is that exciting a broadband acoustic response signal enables a particularly fast determination of the amplitude or phase response, and thus also a particularly fast determination of the resonance frequency of the acoustic resonator.

[0027] In one embodiment of the method according to the invention, the first electromagnetic radiation is amplitude-modulated with a square wave signal. Due to the steep edge of the square wave signal and the multitude of modulation frequencies contained therein, such modulation advantageously leads to the excitation of a broadband acoustic response signal, so that, in conjunction with a frequency analysis, an efficient determination of the amplitude response or phase response can be carried out.

[0028] Frequency analysis is particularly often performed by applying a fast Fourier transform (FFT).

[0029] In one embodiment of the method according to the invention, the first electromagnetic radiation has a frequency of the alternating electromagnetic field that is not equal to an absorption frequency of the sample fluid, wherein the first electromagnetic radiation preferably has a wavelength or wavelength range that lies between 380 nm and 1200 nm.

[0030] According to the invention, in a further step a property of the acoustic resonator is determined, wherein in particular one or both of the following steps may be provided: e) Determining a resonance frequency of the acoustic resonator from at least one amplitude or phase determined in step d), f) Determining a damping property of the acoustic resonator.

[0031] In this embodiment, the method according to the invention is advantageously used to determine a property of the acoustic resonator in order to optimize another method to be carried out with the acoustic resonator. A damping property of the acoustic resonator can be the damping coefficient of the acoustic resonator or the damping coefficient of a partial region of the acoustic resonator, for example, a wall region of the acoustic resonator.

[0032] The method according to the invention can be a method for determining a property of the acoustic resonator.

[0033] In one embodiment of the method according to the invention, a property of the fluid or fluid mixture located in the acoustic resonator is determined in a further step, wherein in particular one or both of the following steps may be provided: i) Determining the speed of sound of the fluid or fluid mixture located in the acoustic resonator or j) Determining the density of the fluid or fluid mixture located in the acoustic resonator or the concentration of the sample fluid if the sample fluid is a component of a fluid mixture located in the acoustic resonator.

[0034] Steps i) and j) can also be intertwined in such a way that first a speed of sound is determined according to step i) and a density or concentration is determined from the speed of sound according to step j).

[0035] In one embodiment, the method according to the invention is a method for determining a property of the fluid or fluid mixture located in the acoustic resonator.

[0036] In one embodiment of the method according to the invention, this represents a photoacoustic method for determining a property of a sample gas with the following steps: aa) Determining a resonance frequency of the acoustic resonator using an embodiment of the method described above, bb) Irradiating the sample gas or the gas mixture containing the sample gas with a second electromagnetic radiation, wherein the second electromagnetic radiation has at least one frequency of its alternating electromagnetic field that is equal to an absorption frequency of the sample gas, and wherein the second electromagnetic radiation is amplitude- or frequency-modulated with a modulation frequency, the modulation frequency being substantially equal to the resonance frequency of the acoustic resonator determined in step aa), cc) Measuring an acoustic response signal of the acoustic resonator generated by absorption of the second electromagnetic radiation by the sample gas as carried out in step bb), dd) Determining the property of the sample gas from the acoustic response signal of the acoustic resonator.

[0037] This embodiment of the method according to the invention enables the user to advantageously determine the resonance frequency of the acoustic resonator when performing a photoacoustic method, namely without having to use the second electromagnetic radiation, which is used to heat the sample gas and thus to determine a property of the sample gas, to determine the resonance frequency itself. This facilitates the tracking of the resonance frequency. The photoacoustic method thus becomes more efficient and at the same time more robust to varying circumvention parameters such as temperature, gas composition, or pressure.

[0038] In step aa) of the photoacoustic method, preferably the first alternative of the method according to claim 1 or the second alternative of the method according to claim 1 with an additional absorption gas is used, but not the second alternative of the method according to claim 1 with absorption particles, since absorption particles present in the gas mixture also absorb the second electromagnetic radiation at least partially, and this leads to a worse signal-to-offset ratio in photoacoustic methods.

[0039] In one embodiment, the acoustic response signal to be measured in step cc) and the acoustic response signal to be measured in step aa) are two acoustic response signals that can be measured successively. In this embodiment, the irradiation with the first electromagnetic radiation and the irradiation with the second electromagnetic radiation occur at different times.

[0040] In an alternative embodiment, the acoustic response signal to be measured in step cc) and the acoustic response signal to be measured in step aa) are two temporally superimposed acoustic response signals that are measured together in a common step. In this embodiment, the irradiation with the first electromagnetic radiation and the irradiation with the second electromagnetic radiation occur at least temporarily simultaneously, wherein the modulation frequency of the first electromagnetic radiation differs from the modulation frequency of the second electromagnetic radiation.In this case, the acoustic signal measured in a common step is subsequently separated in order to separate the acoustic response signal generated by the absorption of the second electromagnetic radiation by the sample gas in step bb) from the acoustic response signal generated by the absorption of the first electromagnetic radiation in step b).

[0041] In one embodiment of the method according to the invention, the property of the sample gas is its density or concentration. This property is often of particular relevance, especially in applications in the field of environmental protection.

[0042] In one embodiment of the method according to the invention, the property of the sample gas is determined several times over a process duration at least by steps bb), cc) and dd), wherein during the process duration the resonance frequency of the acoustic resonator is determined several times, preferably periodically and particularly preferably before each execution of step bb) by means of step aa), so that when step bb) is carried out the modulation frequency of the second electromagnetic radiation is always equal to the currently determined resonance frequency.

[0043] This advantageously ensures that the modulation frequency is regularly adjusted to the varying resonance frequency of the acoustic resonator, so that the signal-to-noise ratio of the acoustic response signal of the acoustic resonator is optimized, i.e., essentially maximized, throughout the entire process.

[0044] In one embodiment of the method according to the invention, the second electromagnetic radiation has a wavelength or wavelength range that lies between 780 nm and 1 mm.

[0045] In one embodiment of the method according to the invention, the second electromagnetic radiation is guided in such a way that it is not absorbed by any surface of the acoustic resonator. This ensures that no further absorption occurs by parts of the acoustic resonator other than the intended absorption of the first electromagnetic radiation for determining the resonance frequency. Thus, the determination of the resonance frequency using the first electromagnetic radiation is not disturbed by the second electromagnetic radiation. Furthermore, the measurement of the acoustic response signal resulting from irradiation with the second electromagnetic radiation is not disrupted by unwanted wall absorption.

[0046] The problem underlying the invention is also solved by a device for carrying out the method according to the invention in one of the embodiments described above. This device comprises at least one radiation source for generating first electromagnetic radiation, an acoustic resonator having an absorption element with a surface section, wherein the surface section of the absorption element is arranged such that the surface section is in contact with a sample fluid or a fluid mixture containing the sample fluid when the acoustic resonator is filled with the sample fluid or the fluid mixture containing the sample fluid, and wherein the absorption element is designed such that it at least partially absorbs the first electromagnetic radiation in the region of the surface section.such that pressure energy is generated in the sample fluid or the fluid mixture containing the sample fluid when the acoustic resonator is filled with the sample fluid or the fluid mixture containing the sample fluid, a sound sensor for measuring an acoustic response signal of the resonator, wherein the at least one radiation source and the acoustic resonator are designed and arranged such that the first electromagnetic radiation strikes the surface section of the absorption element, wherein the device is configured such that at least one amplitude or phase of the acoustic response signal is determined and a property of the acoustic resonator is determined, wherein one or both of the following steps are provided: determining a resonance frequency of the acoustic resonator from at least the amplitude or phase, determining a damping property of the acoustic resonator.

[0047] Insofar as aspects of the invention are described here with regard to the apparatus, these also apply to the corresponding method and vice versa. Insofar as the method is carried out with an apparatus according to this invention, the apparatus shall have the necessary features for this purpose. In particular, embodiments of the apparatus are suitable for carrying out the embodiments of the method described above.

[0048] In one embodiment of the device according to the invention, a radiation source for generating a second electromagnetic radiation is provided, wherein the acoustic resonator has a first opening or a first window which is at least partially transparent to the second electromagnetic radiation, wherein the acoustic resonator and the radiation source of the second electromagnetic radiation are arranged in such a way that the second electromagnetic radiation enters the acoustic resonator through the first opening or the first window.

[0049] This embodiment of the device according to the invention particularly enables the advantageous implementation of a photoacoustic method according to the invention with a first and a second electromagnetic radiation.

[0050] In this embodiment, the radiation source for generating a first electromagnetic radiation and the source for generating a second electromagnetic radiation can be separate radiation sources or a single radiation source.

[0051] In the case of a single radiation source, preferably at least one additional optical element is provided, which is configured such that the electromagnetic radiation emitted by the single radiation source is split into a first and a second electromagnetic radiation, and the wavelength of the first electromagnetic radiation and the wavelength of the second electromagnetic radiation differ. Alternatively, a single radiation source for generating electromagnetic radiation is provided, which is configured and arranged such that the beam path of the radiation is variable and, in a first setting of the beam path, the generated electromagnetic radiation is the first electromagnetic radiation, and in a second setting of the beam path, the generated electromagnetic radiation is the second electromagnetic radiation.In this alternative, the irradiation with the first and the irradiation with the second electromagnetic radiation do not occur simultaneously, but one after the other.

[0052] In one embodiment of the device according to the invention, the acoustic resonator has a second opening or a second window which is at least partially transparent to the second electromagnetic radiation, wherein the acoustic resonator and the radiation source of the second electromagnetic radiation are designed and arranged such that the second electromagnetic radiation passes out of the acoustic resonator through the second opening or the second window.

[0053] In one embodiment of the device according to the invention, the acoustic resonator is designed such that, in the case of an acoustic resonator filled with the sample fluid, a standing sound wave with at least one pressure maximum is formed within the resonator by irradiating the surface section with the first electromagnetic radiation, wherein the acoustic resonator and the radiation source of the first electromagnetic radiation are designed and arranged such that the surface section irradiated by the first electromagnetic radiation is located at a position that is essentially the same as the position of the pressure maximum of the standing wave.

[0054] Thermal excitation of the surface section by means of the first electromagnetic radiation in the region of the pressure maximum is particularly advantageous, since the signal-to-offset ratio of the acoustic response signal resulting from this excitation is maximal.

[0055] In one embodiment of the device according to the invention, a first radiation source for generating the first electromagnetic radiation and a separate second radiation source for generating the second electromagnetic radiation are provided, wherein preferably the first radiation source is a light-emitting diode (LED) in the visible spectral range and the second radiation source is an LED in the infrared spectral range.

[0056] In one embodiment of the device according to the invention, the radiation source of the first electromagnetic radiation is designed and arranged such that the first electromagnetic radiation enters the acoustic resonator through an opening or a window, wherein in the case of a window the window is at least partially transparent to both the first and the second electromagnetic radiation, or alternatively the radiation source of the first electromagnetic radiation is designed and arranged such that the first electromagnetic radiation is generated within the acoustic resonator.

[0057] Furthermore, the present invention relates to a use of the device according to the invention for exhaust gas measurement in a motor vehicle, for pollutant measurement in the ambient air or for measuring the hydrogen concentration in a gas mixture.

[0058] Further advantages, features and embodiments of the present invention can be seen in the following figures and the accompanying description. They show: Figure 1: a schematic side view of a first embodiment of the device according to the invention, with which a first embodiment of the method according to the invention can be carried out; Figure 2: a schematic side view of a second embodiment of the device according to the invention, with which a second embodiment of the method according to the invention can be carried out; Figure 3: a schematic side view of a third embodiment of the device according to the invention, with which a third embodiment of the method according to the invention can be carried out; Figure 4: a schematic side view of a fourth embodiment of a device according to the invention, with which a fourth embodiment of the method according to the invention can be carried out.

[0059] At the in Figure 1 The first embodiment of the device 100 according to the invention is a special device comprising only as many elements as are necessary to realize the underlying inventive concept. A single radiation source 3 is provided, which generates a first electromagnetic radiation 1. The beam path of the first electromagnetic radiation is divergent. The radiation source 3 is arranged such that the first electromagnetic radiation 1 is generated within an upwardly open resonator cell 5, which is also referred to as an acoustic resonator, and strikes a surface section 9 of the surface of a resonator cell wall 7. The upwardly open resonator cell 5 represents a resonator for longitudinal waves.

[0060] In the course of carrying out a first embodiment of the method according to the invention with the in Figure 1In the first embodiment of the device according to the invention, the electromagnetic radiation is wholly or partially absorbed by the resonator cell wall 7, which, according to the present invention, constitutes an absorption element. This results in local heating of the resonator cell wall 7 in the region of the surface section 9. If the cavity enclosed by the acoustic resonator is filled with a fluid mixture containing a sample fluid, for example, with a sample gas mixture in the form of water vapor (sample gas) and ambient air, heat transfer also leads to local heating of the sample gas mixture in the region of the surface section 9, since the resonator cell wall 7 is in contact with the sample gas mixture. The heating of the sample gas mixture is then accompanied by an increase in the pressure energy within the sample gas mixture.

[0061] Overall, at least some of the radiation energy of the first electromagnetic radiation is therefore first converted into heat energy of the resonator cell wall 7 and then into pressure energy of the sample gas mixture.

[0062] In the course of carrying out the first embodiment of the method according to the invention, the Figure 1The first electromagnetic radiation 1 shown is amplitude- or frequency-modulated. Preferably, amplitude modulation takes place. By modulating the first electromagnetic radiation with a modulation frequency, a periodic variation of the energy transfer with the corresponding modulation frequency is caused. Consequently, pressure energy is periodically generated within the sample gas at the modulation frequency. According to the hydrodynamic laws, this leads to the formation of a pressure wave 10 (sound wave) in the sample gas within the acoustic resonator 5. The pressure wave 10 forms in the acoustic resonator as a standing wave, which is amplified to a lesser or greater degree depending on the proximity of the excitation frequency to the actual resonance frequency.

[0063] The pressure variation generated by such a sound wave 10 in the form of a standing wave within the acoustic resonator 5 is measured by means of a microphone 6 arranged on the underside of the acoustic resonator 5. For the purposes of this application, the measured signal is described as an acoustic response signal 10 of the acoustic resonator, which is generated by absorption of the first electromagnetic radiation. In this sense, a resonant peak in the standing sound wave measured by the microphone is also to be understood as an acoustic response signal. The amplitude or phase of the response signal is then determined in a further step.

[0064] The terms "sound wave" and "acoustic response signal" are used synonymously within the meaning of the present invention. In a narrower sense, the term "sound wave" refers to the physical description of the pressure variation occurring in the acoustic resonator 5, and in a narrower sense, the term "acoustic response signal" refers to the metrological description of the pressure change or resonant peak occurring in the acoustic resonator 5.

[0065] The modulation frequency of the initial electromagnetic radiation is subsequently varied, so that for each modulation frequency an amplitude or phase of the response signal is determined. The amplitude response or phase response is then derived from the majority of these amplitudes or phases.

[0066] From the amplitude or phase response thus determined, conclusions can be drawn, either directly or after first determining the resonance frequency of the acoustic resonator, about the properties of the fluid or fluid mixture located in the acoustic resonator 5, such as the speed of sound, density, or concentration. In particular, if the fluid in the acoustic resonator 5 is gaseous water / water vapor (H₂O), conclusions can be drawn about the speed of sound within the water vapor and thus also about the density of the water vapor.

[0067] The in Figure 1 The arrangement shown can be used in parallel or separately to determine the resonance frequency or other properties of the acoustic resonator 5.

[0068] To determine the resonance frequency, the modulation frequency of the first electromagnetic radiation 1 is also varied, for example, within a frequency interval in which the resonance frequency is expected based on the geometric dimensions of the resonator cell 5. In other words, the modulation frequency of the first electromagnetic radiation 1 is tuned, and the amplitude or phase response of the acoustic resonator is determined by measuring the acoustic response signal. From the amplitude response, it is then determined for which modulation frequency the amplitude of the acoustic response signal has a maximum. The modulation frequency determined in this way corresponds to the resonance frequency of the acoustic resonator.

[0069] Furthermore, the damping properties of the acoustic resonator can also be inferred from the determined amplitude or phase response of the acoustic response signal.

[0070] For example, the damping coefficient or parts of the damping coefficient tensor of the acoustic resonator can be determined from the amplitude or phase response. Knowledge of the damping coefficient or parts of the damping coefficient tensor of the acoustic resonator, in turn, allows conclusions to be drawn about the concentration of a sample gas within the acoustic resonator.

[0071] Alternatively or additionally to tuning the modulation frequency of the first electromagnetic radiation 1, a broadband excitation of a sound wave 10 can also be performed within the acoustic resonator 5. For this purpose, the first electromagnetic radiation 1 is modulated, for example, with a square wave function. A square wave function contains a multitude of modulation frequencies. Accordingly, a spectrally broadband sound wave 10 is also generated by the absorption of the first electromagnetic radiation 1. The acoustic response signal measured by the acoustic microphone 6 also contains a multitude of frequencies. Using a frequency analysis, which is usually a fast Fourier transform (FFT), an amplitude or phase response can thus be directly calculated from the measured acoustic response signal.The resonance frequency of the acoustic resonator can then be determined by finding a maximum in the amplitude response.

[0072] Figure 2 Figure 2 shows a second embodiment of the device 200 according to the invention. In this embodiment, two separate radiation sources 3 and 4 are provided. The first radiation source 3 generates the first electromagnetic radiation 1, and the second radiation source 4 generates the second electromagnetic radiation 2. In this embodiment, the second radiation source 4 generates a parallel beam path.

[0073] For example, the second radiation source 4 can be an infrared laser.

[0074] The in Figure 2The device 200 shown according to the invention is particularly designed to carry out a photoacoustic method according to the invention, in which the properties of a sample fluid contained in the acoustic resonator 5 are determined by means of the second electromagnetic radiation 2. The second electromagnetic radiation 2 is advantageously amplitude- or frequency-modulated with the resonance frequency of the acoustic resonator 5. The first electromagnetic radiation 1, which strikes a resonator cell wall 7 and is absorbed there in the surface section 9, is used to track this resonance frequency. As with the Figure 1As already described, a method for determining the currently valid resonance frequency of the acoustic resonator can be carried out using the first electromagnetic radiation 1. The result of this method can then be used to adjust the modulation frequency of the second electromagnetic radiation 2 to the currently valid resonance frequency of the acoustic resonator.

[0075] The second electromagnetic radiation enters the Figure 2In the illustrated embodiment, the second electromagnetic radiation passes through a first window 8 into the acoustic resonator. The second electromagnetic radiation has a wavelength corresponding to an absorption wavelength of the sample fluid located in the acoustic resonator. Consequently, along the path of the second electromagnetic radiation 2, the sample fluid is locally heated, and thus—also due to the modulation of the second electromagnetic radiation—a sound wave is generated. This sound wave is perceived by the microphone 6 as an acoustic response signal, which is produced by the absorption of the second electromagnetic radiation by the sample fluid. The second electromagnetic radiation then exits the acoustic resonator through the second window 8' without being absorbed by any element of the acoustic resonator.

[0076] The acoustic response signal, or the sound wave generated by the second electromagnetic radiation, is in the Figures 2 to 4 not depicted. Rather, it is in the Figures 2 to 4 and also in the Figure 1 The sound wave 10 produced by the absorption of the first electromagnetic radiation is shown.

[0077] In Figure 3 is a third embodiment of the device 300 according to the invention. This differs from the one described in Figure 2The device 200 shown differs in that the second radiation source 4 generates a divergent, rather than a parallel, beam path. Furthermore, the first electromagnetic radiation 1 and the second electromagnetic radiation 2 are coupled together into the acoustic resonator 5 via a lens 11, both through a first window 8 of the resonator cell 7. The window 8 is designed to be transparent to both the first and the second electromagnetic radiation. The lens 11 is arranged and designed such that the first electromagnetic radiation 1 is directed by the lens 11 onto the absorption element, here the resonator cell wall 7, and the second electromagnetic radiation 2 is guided by the same lens 11 in such a way that it passes through the resonator cell 5 without touching the wall.

[0078] In Figure 4A fourth embodiment 400 of the device and method according to the invention is shown. This differs from the one shown in Figure 2 In the illustrated embodiment 200, the first radiation source 3 for generating the first electromagnetic radiation 1 is arranged within the acoustic resonator 5 or is connected to the acoustic resonator 5 in such a way that the first electromagnetic radiation 1 is generated within the acoustic resonator 5. Furthermore, an absorber plate 12 is provided as an absorption element, wherein the absorber plate 12 is rigidly connected to the resonator cell 5. In the Figure 4In the illustrated embodiment, the first radiation source 3 is integrated into the resonator cell wall, the resonator cell wall 7 being opposite the absorbing surface section 9 of the absorber plate 12, so that the first electromagnetic radiation 1 is directed at a substantially right angle to the second electromagnetic radiation 2 and strikes the absorber plate 12 opposite the first radiation source 3. To measure an acoustic response signal, unlike in the Figure 2 In the illustrated embodiment 200, an acoustic flow sensor 6 is provided which is attached to a resonator cell wall 7 and which is arranged in the cavity enclosed by the acoustic resonator in such a way that it does not protrude into the beam paths of the first and second electromagnetic radiation.

[0079] For the purposes of the original disclosure, it is pointed out that all features as they can be deduced by a person skilled in the art from the present description, the drawings, and the claims, even if they are specifically described only in connection with certain other features, can be combined individually or in any combination with other features or groups of features disclosed herein, unless this has been expressly excluded or technical circumstances render such combinations impossible or pointless. A comprehensive, explicit description of all conceivable combinations of features is omitted here solely for the sake of brevity and readability.

[0080] While the invention has been illustrated and described in detail in the drawings and the preceding description, this illustration and description are merely exemplary and are not intended to limit the scope of protection as defined by the claims. The invention is not limited to the disclosed embodiments.

[0081] Variations of the disclosed embodiments are obvious to a person skilled in the art from the drawings, the description, and the accompanying claims. In the claims, the word "have" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude multiple features. The mere fact that certain features are claimed in different claims does not preclude their combination. Reference numerals in the claims are not intended to limit the scope of protection. Reference sign

[0082] 1 First electromagnetic radiation 2 Second electromagnetic radiation 3 First radiation source 4 Second radiation source 5 Acoustic resonator (resonator cell) 6 Microphone 6' Acoustic flux sensor 7 Resonator cell wall 8 First window 8` Second window 9 Surface section 10 Sound wave (acoustic response signal) generated by absorption of the first electromagnetic radiation in step b) 11 Lens 12 Absorber plate 100 First embodiment of the device 200 Second embodiment of the device 300 Third embodiment of the device 400 Fourth embodiment of the device AAmplitude of the acoustic response signal ϕPhase of the acoustic response signal FModulation frequency A(F)Amplitude response ϕ(F)Phase response

Claims

1. A method comprising the following steps: a) according to a first alternative, providing an acoustic resonator (5), the acoustic resonator (5) comprising an absorption element (7) having a surface portion (9), providing a sample fluid or a fluid mixture comprising the sample fluid such that the acoustic resonator is filled with the sample fluid or the fluid mixture comprising the sample fluid, wherein the surface portion (9) of the absorption element (7) is in contact with the sample fluid or the fluid mixture comprising the sample fluid, or according to a second alternative, providing an acoustic resonator (5), providing a fluid mixture comprising a sample fluid such that the acoustic resonator is filled with the fluid mixture, wherein the fluid mixture comprises, in addition to the sample fluid, at least one absorption fluid or one or more absorption particles, b) according to the first alternative, irradiating the surface portion (9) of the absorption element with a first electromagnetic radiation (1) such that the absorption element (7) at least partially absorbs the first electromagnetic radiation (1) in the region of the surface portion (9), or according to the second alternative, irradiating the fluid mixture with a first electromagnetic radiation (1) such that the absorption fluid or the absorption particles at least partially absorb the first electromagnetic radiation, such that, in both alternatives, by way of the absorption, pressure energy is produced in the sample fluid or the fluid mixture comprising the sample fluid, wherein the first electromagnetic radiation (1) is amplitude modulated or frequency modulated at a modulation frequency, c) measuring an acoustic response signal (10) of the acoustic resonator (5) generated by the absorption of the first electromagnetic radiation (1) taking place in step b), d) determining at least one amplitude (A) or phase (ϕ) of the acoustic response signal (10) having the modulation frequency of the first electromagnetic radiation (1) characterized in that in a further step a property of the acoustic resonator (5) is determined, wherein one of the following steps or both followings steps is provided: e) determining a resonant frequency of the acoustic resonator (5) from at least the amplitude (A) or the phase (ϕ) determined in step d), f) determining a damping property of the acoustic resonator (5).

2. The method according to claim 1, wherein step b) is performed at least twice, wherein in a first performance of step b), the first electromagnetic radiation (1) is modulated at a first modulation frequency (F1) and in a second performance of step b), is modulated at a second modulation frequency (F2), wherein the second modulation frequency (F2) is different from the first modulation frequency (F1), such that in step d) at least a first amplitude (A1) or phase (ϕ1) of the acoustic response signal (10) for the first modulation frequency (F1), and a second amplitude (A2) or phase (ϕ2) of the acoustic response signal (10) for the second modulation frequency (F2), are determined.

3. The method according to claim 2, wherein the following further step is provided: g) determining an amplitude response (A(F)) or phase response (cp(F)) dependent upon the modulation frequency (F) from the at least two amplitudes (A1 and A2) or phases (ϕ1 and ϕ2) determined in step d) of the acoustic response signal (10).

4. The method according to any one of the preceding claims, wherein in step b) the first electromagnetic radiation (1) is amplitude modulated or frequency modulated such that a wide-band acoustic response signal (10) of the acoustic resonator (5) is generated, wherein the following further step is provided: h) determining an amplitude response (A(F)) or phase response (cp(F)) dependent upon the modulation frequency (F) from the wide-band acoustic response signal (10) measured in step c), preferably by applying a frequency analysis, and particularly preferably by applying a Fourier transform to the wide-band acoustic response signal (10) measured in step c).

5. The method according to any one of the preceding claims, wherein the first electromagnetic radiation (1) comprises a frequency of the alternating electromagnetic field, wherein the frequency of the alternating electromagnetic field is not equal to an absorption frequency of the sample fluid, wherein the first electromagnetic radiation (1) preferably comprises a wavelength or a wavelength range that is between 380 nm and 1200 nm.

6. The method according to any one of the preceding claims, wherein in a further step, a property of the fluid or fluid mixture located in the acoustic resonator is determined, wherein in particular one of the following steps or both of the following steps can be provided: i) determining a velocity of sound of the fluid or fluid mixture located in the acoustic resonator, or j) determining a density of the fluid or fluid mixture located in the acoustic resonator, or a concentration of the sample fluid, if the sample fluid is a constituent of a fluid mixture located in the acoustic resonator.

7. A photoacoustic method for determining a property of the sample fluid, wherein the sample fluid is a sample gas, having the following steps: aa) determining a resonant frequency of the acoustic resonator (5) using a method according to claim 1, the method according to claim 1 comprising step e), bb) irradiating the sample gas or the gaseous mixture comprising the sample gas with a second electromagnetic radiation (2), wherein the second electromagnetic radiation (2) comprises at least one frequency of its alternating electromagnetic field that is equal to an absorption frequency of the sample gas, and wherein the second electromagnetic radiation (2) is amplitude modulated or frequency modulated at a modulation frequency, wherein the modulation frequency is substantially equal to the resonant frequency of the acoustic resonator determined in step aa), cc) measuring an acoustic response signal of the acoustic resonator (5) generated by an absorption by the sample gas of the second electromagnetic radiation which takes place in step bb), dd) determining the property of the sample gas from the acoustic response signal of the acoustic resonator (5).

8. The method according to claim 7, wherein the property of the sample gas is the density or concentration of the sample gas.

9. The method according to any one of claims 7 or 8, wherein the property of the sample gas is determined multiple times over a process duration at least with steps bb), cc) and dd), wherein, during the duration of the method, preferably periodically and particularly preferably before each performance of the step bb), the resonant frequency of the acoustic resonator (5) is determined by means of step aa), such that, when performing step bb), the modulation frequency of the second electromagnetic radiation (2) is always equal to the currently determined resonant frequency.

10. The method according to any one of claims 7 to 9, wherein the second electromagnetic radiation (2) has a wavelength or wavelength range that is between 780 nm and 1 mm.

11. The method according to any one of claims 7 to 10, wherein the second electromagnetic radiation (2) is guided such that it is not absorbed by a surface of the acoustic resonator (5).

12. A device comprising at least one radiation source (3) for generating a first electromagnetic radiation (1), an acoustic resonator (5) comprising an absorption element (7) having a surface portion (9), the surface portion (9) of the absorption element (7) being arranged such that the surface portion (7) is in contact with a sample fluid or a fluid mixture comprising the sample fluid when the acoustic resonator (5) is filled with the sample fluid or the fluid mixture comprising the sample fluid, wherein the absorption element (7) is configured to at least partially absorb the first electromagnetic radiation (1) in the region of the surface portion (9) such that pressure energy is generated in the sample fluid or the fluid mixture comprising the sample fluid when the acoustic resonator (5) is filled with the sample fluid or the fluid mixture comprising the sample fluid, a sonic sensor (6, 6') for measuring an acoustic response signal of the acoustic resonator (5), wherein the at least one radiation source (3) and the acoustic resonator (5) are configured and arranged such that the first electromagnetic radiation (1) is incident upon the surface portion (9) of the absorption element (7), characterized in that the device is configured in such a way that at least an amplitude (A) or a phase (ϕ) of the acoustic response signal (10) is determined, and a property of the acoustic resonator (5) is determined, wherein one of the following steps or both followings steps is provided: determining a resonant frequency of the acoustic resonator (5) from at least one amplitude (A) or phase (ϕ), determining a damping property of the acoustic resonator (5).

13. The device according to claim 12, wherein a radiation source (3, 4) is provided for generating a second electromagnetic radiation (2), wherein the acoustic resonator (5) comprises a first opening or a first window (8) at least partially transparent for the second electromagnetic radiation (2), wherein the acoustic resonator (5) and the radiation source (3, 4) of the second electromagnetic radiation (2) are configured and arranged such that the second electromagnetic radiation (2) passes through the first opening or the first window (8) into the acoustic resonator (5).

14. The device according to claim 13, wherein the acoustic resonator (5) comprises a second opening or a second window (8'), which is at least partially transparent for the second electromagnetic radiation (2), wherein the acoustic resonator (5) and the radiation source (3, 4) of the second electromagnetic radiation (2) are configured and arranged, such that the second electromagnetic radiation (2) passes through the second opening or the second window (8') out of the acoustic resonator (5).

15. The device according to any one of claims 12 to 14, wherein the acoustic resonator is configured so that by irradiating the surface portion (9) with the first electromagnetic radiation (1) in the case of an acoustic resonator (5) filled with the sample fluid within the acoustic resonator (5) a standing sound wave (10) with at least one pressure maximum forms, wherein the acoustic resonator (5) and the radiation source (3) of the first electromagnetic radiation (1) are configured and arranged such that the surface portion (9) irradiated by the first electromagnetic radiation (1) is arranged at a location, which is substantially equal to the location of the pressure maximum of the standing sound wave (10).

16. The device according to any one of claims 12 to 15, wherein a first radiation source (3) for generating the first electromagnetic radiation (1) and a separate second radiation source (4) for generating the second electromagnetic radiation (2) are provided, wherein preferably the first radiation source (3) is an LED in the visible spectral region and the second radiation source (4) is an LED in the infrared spectral region.

17. The device according to any one of claims 13 to 14 and 16, wherein the radiation source (3) of the first electromagnetic radiation (1) is configured and arranged such that the first electromagnetic radiation (1) enters the acoustic resonator (5) through an opening or a window (8), wherein, in the case of a window, the window (8) is at least partially transparent for both the first and second electromagnetic radiation (1, 2), or alternatively, the radiation source (3) of the first electromagnetic radiation (1) is configured and arranged such that the first electromagnetic radiation (1) is generated within the acoustic resonator (5).

18. Use of a device according to any one of claims 12 to 17 for exhaust gas measurement in a motor vehicle or for pollutant measurement in ambient air.