Abstract: Photoacoustic cells for gas sensors are described. In some instances, the photoacoustic cell may be configured to provide an increased internal path length of the light beam in the photoacoustic cell relative to, for example, a conventional cylindrical photoacoustic cell. The photoacoustic cell may be shaped to provide increased internal reflection of the light within the photoacoustic cell, thereby increasing the absorption of the light by a gas to be detected in the photoacoustic cell. One example photoacoustic cell that can provide such increased internal reflection may be a generally conical-shaped.

Abstract: The instant disclosure provides a photoelectric gas sensor device and a manufacturing method thereof. The manufacturing method comprising the steps of: (A) providing at least two half-housing modules from at least one corresponding mold; (B) forming a reflecting layer on the ellipsoidal inner surface of the chamber unit; (C) forming a chamber unit having a reflective ellipsoidal inner surface defining a chamber space from the half-housing modules; (D) forming a reflecting layer on each inner surface of the two half-housings; and (E) disposing an emitter assembly having an energy emitter at the first focal point of the ellipsoidal chamber. A fine-adjustment mechanism may be further provided to enable clearance adjustment between the half-housing modules.

Abstract: A photoacoustic gas detector includes an integrated source, infrared filter and an acoustic sensor. The source, filter and acoustic sensor can be integrated onto one or more semiconductor substrates, such as silicon. Processing circuitry can also be integrated onto the substrate. Further, the source, filter and acoustic sensor can be integrated into a single component package, such as a metal can transistor package.

Abstract: The invention relates to a gas sensor having a mechanical microresonator, which has an excitation apparatus for optically exciting a mechanical oscillation of the microresonator as well as a reading apparatus for detecting the oscillation of the microresonator, wherein the reading apparatus comprises a waveguide which is implemented together with the microresonator on a dielectric or semiconducting substrate and is intended to optically read the oscillation of the microresonator, and wherein the excitation apparatus has an optical waveguide which is implemented on the same substrate and optically connects an excitation light source to the immediate surroundings of the microresonator. The invention also relates to a use of such a sensor to analyse a gas composition.

Abstract: An oscillator element is provided, e.g., for use in a photo acoustic detector for detecting a concentration of a sample in a sample mixture using a light beam for excitation of molecules of the sample in proximity of an excitation area of the oscillator element. The oscillator element includes piezoelectric material for generating a voltage when mechanically distorted. Electrodes at least partially cover a surface of the oscillator element for detecting the generated voltage. The excitation area is arranged in such a way that heating of the electrodes in the excitation area by the light beam is avoided.

Abstract: A method and apparatus for the photo-acoustic identification and quantification of one or more analyte species present in a gaseous or liquid medium in low concentration utilizing a laser and a resonant optical cavity containing the medium and having within the cavity at least two partially transparent mirrors, one of which is a cavity coupling mirror and one of which is moveably mounted on an assembly responsive to an input signal.

Abstract: A method and apparatus for the photo-acoustic identification and quantification of one or more analyte species present in a gaseous or liquid medium in low concentration utilizing a laser and a resonant optical cavity containing the medium and having within the cavity at least two partially transparent mirrors, one of which is a cavity coupling mirror and one of which is moveably mounted on an assembly responsive to an input signal.

Abstract: A method and apparatus for the photo-acoustic identification and quantification of one or more analyte species present in a gaseous or liquid medium in low concentration utilizing a laser and a resonant optical cavity containing the medium and having within the cavity at least two partially transparent mirrors, one of which is a cavity coupling mirror and one of which is moveably mounted on an assembly responsive to an input signal.

Abstract: The invention relates to a pipe system comprising a pipe, a gas sensing station and a remote output system. The pipe comprises a pipe gas cavity, such as an annulus, extending lengthwise in part or all of the length of the pipe; the gas sensing station comprises a sensing gas cavity which is in gas communication with the pipe gas cavity: The sensing gas cavity comprises a photoacoustic spectroscope. The pipe system comprises at least one optical feeding fiber for feeding light to the photoacoustic spectroscope and a transmission path for transferring a signal from the photoacoustic spectroscope to the remote output system, the transmission path from the gas sensing station to the remote output system is an optical transmission path. The pipe may for example be a flexible pipe e.g. an umbilical or a pipe for the transportation of crude oil from a well to an off shore or on shore station, for example a ship or a platform. The gas sensing station may be integrated in the pipe, e.g.

Abstract: A carbon dioxide sensor comprising a first beam that includes a functionalized surface and a second beam that includes a functionalized surface such that reduced-drift differential sensing of carbon dioxide may be performed by monitoring changes in the resonant frequency of the first beam relative to the resonant frequency of second beam.

Abstract: Various embodiments of the application provide a photoacoustic sensor, which includes: a gas cell having an opening; a light source to generate a radiation to radiate sample gas within the gas cell; a detector to detect the sample gas within the gas cell, and to generate electrical signals in response to acoustic signals generated by pressure fluctuations of the radiated sample gas caused by the radiation; and an active valve having a speaker aligned with the opening of the gas cell. The speaker having a voice coil and a diaphragm attached to the voice coil. A control signal is applicable for the speaker to control access of the gas cell. During sampling, the control signal causes the voice coil of the speaker to repeatedly or constantly lift the diaphragm from contact with the opening of the gas cell to allow sample gas enter the gas cell. While during detecting, the spring force of the voice coil causes the diaphragm in tight contact with the opening of gas cell to seal the gas cell.

Abstract: Device for detecting a gas, comprising: excitation means, for exciting said gas by means of an electromagnetic wave having a wavelength corresponding approximately to that of said gas; and detection means, for detecting the excitation of said gas, characterized in that it comprises: a waveguide (1) connected to said excitation means, a part (11) of which forms a movable element designed to be in contact with said gas and capable of being set into vibration by the impact of the excited gas molecules; and a measurement sensor (4), for measuring the vibration of said element, said measurement sensor and said element forming said detection means.

Abstract: An art of hydrogen concentration measurement in which the hydrogen concentration of any given location of an object to be measured can be nondestructively obtained is provided.

Abstract: A photoacoustic detection device including a nanophotonic circuit including a plurality of semiconductor lasers capable of emitting a different frequencies; input couplers connected to optical waveguides; a multiplexer; an output optical waveguide, emerging into a recess; a tuning fork having its free arms arranged at the output of the output optical waveguide; and means for detecting the vibration of the tuning fork, all these elements being assembled in a monolithic component.

Abstract: A photo acoustic detection cell (6) is located within the optical cavity (3) of a cavity enhanced absorption spectroscopy apparatus (3,4,5). When a sample in the cell (6) absorbs radiation from a pulsed radiation beam coupled into the cavity (3) pressure waves are generated that are detected by a microphone (9). A detected signal (10) output by the microphone (9) may be processed to determine a value for the concentration of an absorber in the sample.

Abstract: A photoacoustic gas detector and photoacoustic gas detection method are disclosed. The detector includes a laser source, an acoustic resonator, and at least one tuning fork positioned along a longitudinal length of the resonator. The detector is capable of performing fast measurements of the concentration of one or more target gases over a broad temperature range.

Abstract: A system for analysing plasma. The system comprises at least one sensor which is co-operable with a plasma source for providing an analog signal representative of a characteristic of plasma from the plasma source, and a sound card installed on a computing means in communication with the sensor for converting the analog signal into a digital signal for facilitating digital processing thereof.

Abstract: Embodiments of the apparatus, systems, and methods relate to a photoacoustic cell including an excitation source, a chamber, and a quantum dot substrate between the excitation source and the chamber. The excitation source generates a light spectrum. The quantum dot substrate, when subjected to the light spectrum, emits a specific wavelength of light to be received by the chamber. The photoacoustic cell apparatus may be part of a portable gas sensing system, and may be used to detect and measure the concentration of one or more gases. Additional apparatus, systems, and methods are disclosed.

Abstract: A photoacoustic sensor includes a sensor system for photoacoustic detection, at least one noise canceling pressure sensor and a control system in operative connection with the noise canceling pressure sensor to actively cancel the effects of noise in the environment on the sensor system. Another photoacoustic sensor includes a measurement volume, a source of light energy, a photoacoustic pressure sensor, and at least one vibration canceling sensor (for example, a microphone or an accelerometer). A further photoacoustic sensor includes a measurement volume, a source of light energy and a photoacoustic pressure sensor. The measurement volume has an inner surface that is continuously curved over substantially the entire inner surface other than where a window in optical connection with the source of light intersects the measurement volume.

Abstract: A user-friendly photoacoustic spectroscopy (PAS) system and process (technique) provides an open-field PAS instrument, unit and device to remotely sense explosives, chemicals and biological agents. The PAS system and process can include: a pulsed tunable laser, such as a CO2 laser, a reflector, such as a parabolic reflector, an acoustic reverberant resonator in which a microphone is installed, and a data acquisition and analysis system.

Abstract: A method for detecting a target fluid in a fluid sample comprising a first fluid and the target fluid using photoacoustic spectroscopy (PAS), comprises a) providing a light source configured to introduce an optical signal having at least one wavelength into the fluid sample; b) modulating the optical signal at a desired modulation frequency such that the optical signal generates an acoustic signal in the fluid sample; c) measuring the acoustic signal in a resonant acoustic detector; and d) using the phase of the acoustic signal to detect the presence of the target fluid.

Abstract: Methods and systems to measure simultaneously concentrations or concentration ratio(s) of two or more gas ingredients in a sample area comprising: a wavelength modulated light source; an acoustic detector or a photodetector; and means to analyze the signal from the acoustic detector or the photodetector and calculate the concentrations or concentration ratio(s). The light from the light source is transmitted through the sample area. Part of the light will be absorbed in the sample area by the gas ingredients and generates photoacoustic signal. The acoustic detector is used to sample the photoacoustic signal. Alternatively, a photodetector is used to sample the light intensity after the light is transmitted through the sample area.

Abstract: A method of processing time-discrete measured values, which can be described in their time characteristic by a first exponential function which has a first time constant, the method comprising the steps of: detecting a first measured value and storing the first measured value, detecting a second measured value and storing the second measured value according to a defined time interval with respect to the detection of the first measured value, filtering the first measured value and second measured value by calculating a sum of the first measured value and a weighted difference between the second measured value and the first measured value, thereby generating time-discrete output values which can be described in their time characteristic by a second exponential function having a second time constant different from the first time constant, and outputting the output values is disclosed.

Abstract: The invention relates to a system and method for detecting one or more gases or gas mixtures and/or for measuring the concentration of one or more gases or gas mixtures, said system comprising at least a light source, a sample space, a reference chamber, and a measuring chamber. The measuring chamber is supplied with a gas to be detected or measured. In addition, the measuring chamber is provided with a pressure sensor for detecting a photoacoustic signal generated in the measuring chamber. The pressure sensor comprises either a door, whose movement is measured without contact, or a sensor, whose movement is measured optically.

Abstract: A photo acoustic sample detector (10) is provided for detecting a concentration of sample molecules in a sample mixture (1). The photo acoustic sample detector (10) comprises an input for receiving the sample mixture (1), an acoustic cavity (3) for containing the sample mixture (1), a light source (5) for sending light (50) into the acoustic cavity (3) for exciting the sample molecules and thereby causing sound waves in the acoustic cavity (3) and a pick up element (4) for converting the sound waves into electrical signals (12). The photo acoustic sample detector (10) also comprises a light guide (2) comprising a transparent inner wall (8) at an interface of the light guide (2) and the acoustic cavity (3) and a reflective outer wall (7) at an outside of the light guide (2). The light source (5) is arranged for illuminating the light guide (2). The light guide (2) serves for reflecting the light (50) back and forth through the light guide (2) and the acoustic cavity (3).

Abstract: A system and method for analyzing a target analyte gas concentration using a photoacoustic spectroscopy cell comprising: i) a modulatable light source which provides optical radiation at an absorption wavelength of a target analyte; ii) a resonant acoustic chamber for containing said analyte; iii) a microphone positioned within said chamber whereby the acoustic reactance of the microphone is a substantial factor in determining the acoustic resonant frequency of the acoustic chamber and where the magnitude of the acoustic reactance of the microphone is at least two times the acoustic resistance of the microphone.

Abstract: A user-friendly photoacoustic spectroscopy (PAS) system and process (technique) provides an open-field PAS instrument, unit and device to remotely sense explosives, chemicals and biological agents. The PAS system and process can include: a pulsed tunable laser, such as a CO2 laser, a reflector, such as a parabolic reflector, an acoustic reverberant resonator in which a microphone is installed, and a data acquisition and analysis system.

Abstract: A photoacoustic gas sensor includes a photoacoustic cell configured to receive a gas mixture having a first gas component and a second gas component. The photoacoustic gas sensor also includes a light source configured to provide light to the photoacoustic cell. The photoacoustic gas sensor further includes a photoacoustic cell controller configured to measure a concentration of the second gas component using a speed of sound through the gas mixture, where the speed of sound is determined based on an absorption associated with the first gas component. In addition, the photoacoustic gas sensor could include a temperature sensor configured to measure a temperature of the gas mixture, where the photoacoustic cell controller is configured to determine the concentration of the second gas component using the speed of sound and the temperature.

Abstract: A device is presented for photoacoustic detection, having a cylindrical acoustic resonator (1), in which means (3a-d) are present for guiding the excitation light such that the sound wave that can be excited by absorption of the excitation light is the second azimuthal resonance of the cylinder oscillation.

Abstract: A spectrophone assembly comprises a single detector chamber, a plurality of lasers, a gas inlet for supplying a gas sample to the single detector chamber, and at least one microphone. The detector chamber has an internal geometry arranged to be simultaneously acoustically resonant at a plurality of different resonant frequencies. Each laser operates at a different wavelength and is positioned to emit radiation into the single detector chamber, and is operable to emit radiation that is amplitude modulated at a frequency rate corresponding to a particular resonant frequency different from the resonant frequency of each other laser, simultaneously with each other laser. The microphone(s) are positioned in the single detector chamber so that each microphone is located at or near a maximum of a corresponding acoustic resonance defined by the internal geometry of the detector chamber.

Abstract: The invention relates to a photoacoustic multipass cell with a light source and means (3) for reflecting light in an acoustic resonator (8), which means are embodied in a concentrating manner, wherein the light source is arranged between the means designed in a concentrating manner. Two spherical mirrors (3) with a common optical axis can be used as a means for reflecting the light. The light source is represented by a fiber (5) guided into the multipass cell or a laser diode.

Abstract: A photoacoustic detector is presented having the following structure: a first and a second light source (1a, b) for providing light of the same intensity while retaining the same spectral distribution; a first beam path allocated to the first light source (1a) and at least one second beam path allocated to the at least one second light source (1b), wherein a different absorption of light can occur in the first and second beam paths in selected wavelength ranges; means (4) for alternately guiding light from the first and from the at least one second beam path into a photoacoustic measuring cell (6), the intensity of the light sources being constant in such a way that the temporal change thereof cannot generate a photoacoustic signal that distorts the measurement.

Abstract: A photo, acoustic detector (200) for detecting a concentration of a sample in a sample mixture. The photo acoustic detector (200) comprises a light source (101 ) for producing a light beam for exciting molecules of the sample and a light modulator (102) for modulating the light beam for generating pressure variations in the sample mixture, an amplitude of the pressure variations being a measure of the concentration. The photo acoustic detector (200) further comprises a detector element (103) for converting the pressure variations into a detector current and a processing section (106) for processing the detector current to generate an output signal representing the concentration.

Abstract: The present invention provides a gas analyzer that can be miniaturized and detect a wide variety of gases with high sensitivity, and a method of gas analysis. A separation column 16 is configured so as to pass a sample gas together with a carrier gas through the inside thereof. A surface acoustic wave device 17 has a base material 21 with an annularly continuous annular surface formed of at least apart of a spherical surface; a surface acoustic wave generating means 22 capable of generating a surface acoustic wave that propagates along the annular surface; and a plurality of reaction parts 23 provided along the annular surface so as to change a predetermined physical quantity of the surface acoustic wave in response to the components of the sample gas. The surface acoustic wave device 17 is arranged so that the sample gas passing through the separation column 16 is reacted with the reaction parts 23.

Abstract: A photo acoustic trace gas detector (100) is provided for detecting a concentration of a trace gas in a gas mixture. The detector (100) comprises a light source (101) for producing a light beam and a light modulator (103) for modulating the light beam into a series of light pulses at a chopping frequency for generating sound waves in the gas mixture. The amplitude of the sound waves is a measure of the concentration of the trace gas. The detector (100) further comprises an optical cavity (104a, 104b) with the gas mixture. The optical cavity (104a, 104b) amplifies the light intensity of the light pulses. A transducer (109) converts the sound waves into electrical signals. A feed back loop (110, 111, 113, 114) regulates a ratio of a length of the optical cavity (104a, 104b) and a wavelength of the light beam for amplifying the light intensity of the light pulses in the optical cavity (104a, 104b).

Abstract: A method of identifying and determining the concentrations of multiple species in a gas sample, includes providing a spectrophone assembly having a detector chamber, supplying the gas sample to the detector chamber and simultaneously passing a plurality of radiations of different wavelengths into the detector chamber to produce multiple acoustic resonances of different frequencies. Acoustic resonances in the detector chamber are simultaneously sensed to produce corresponding electrical signals, and the electrical signals are analysed to identify the species present in the gas sample and determine the concentration of each specie.

Abstract: A user-friendly photoacoustic spectroscopy (PAS) system and process (technique) provides an open-field PAS instrument, unit and device to remotely sense explosives, chemicals and biological agents. The PAS system and process can include: a pulsed tunable laser, such as a CO2 laser, a reflector, such as a parabolic reflector, an acoustic reverberant resonator in which a microphone is installed, and a data acquisition and analysis system.

Abstract: Embodiments of the apparatus, systems, and methods relate to a photoacoustic cell including an excitation source, a chamber, and a quantum dot substrate between the excitation source and the chamber. The excitation source generates a light spectrum. The quantum dot substrate, when subjected to the light spectrum, emits a specific wavelength of light to be received by the chamber. The photoacoustic cell apparatus may be part of a portable gas sensing system, and may be used to detect and measure the concentration of one or more gases. Additional apparatus, systems, and methods are disclosed.

Abstract: A photo acoustic trace gas detector (100) is provided for detecting a concentration of a trace gas in a gas mixture. The detector (100) comprises a light source (101) for producing a light beam and a light modulator (103) for modulating the light beam into a series of light pulses for generating sound waves in the gas mixture. The light modulator (103) is arranged for modulating the light beam between a non-zero lower intensity level and a higher intensity level. An amplitude of the sound waves being a measure of the concentration. An optical cavity (104a, 104b) contains the gas mixture and amplifies a light intensity of the light pulses. A transducer (109) for converts the sound waves into electrical signals. A feed back loop (110, 111) with a photo detector (110) for measuring the light intensity of the light pulses regulates the amplification of the light intensity in the optical cavity (104a, 104b).

Abstract: Various embodiments of the application provide a photoacoustic sensor, which includes: a gas cell having an opening; a light source to generate a radiation to radiate sample gas within the gas cell; a detector to detect the sample gas within the gas cell, and to generate electrical signals in response to acoustic signals generated by pressure fluctuations of the radiated sample gas caused by the radiation; and an active valve having a speaker aligned with the opening of the gas cell. The speaker having a voice coil and a diaphragm attached to the voice coil. A control signal is applicable for the speaker to control access of the gas cell. During sampling, the control signal causes the voice coil of the speaker to repeatedly or constantly lift the diaphragm from contact with the opening of the gas cell to allow sample gas enter the gas cell. While during detecting, the spring force of the voice coil causes the diaphragm in tight contact with the opening of gas cell to seal the gas cell.

Abstract: A photo acoustic trace gas detector (100) is provided for detecting a concentration of a trace gas in a gas mixture. The photo acoustic trace gas detector (100) comprises a light source (101), an optical cavity (104a, 104b), ratio modulating means (105, 111) and a transducer (109). The optical cavity (104a, 104b) contains the gas mixture and amplifies light intensity. Maximum amplification is provided when a ratio of a wavelength of the light beam and a length of the optical cavity (104a, 104b) has a resonance value. Ratio modulating means (105, 111) modulate the ratio for transformation of the light beam into a series of light pulses for generating the sound waves, an amplitude of the sound waves being a measure of the concentration of the trace gas. A transducer (109) converts the sound waves into electrical signals.

Abstract: An acoustic detector (10), for detecting acoustic signals generated in a photoacoustic spectroscopy system (1) through absorption of light by a fluid, comprising a sensing unit (11), said sensing unit (11) exhibiting structural resonance at or near a frequency of the acoustic signals. The sensing unit (11) forms at least part of a cavity resonator, which is arranged to enable a formation of standing pressure waves inside said cavity resonator at a cavity resonance frequency substantially coinciding with a structural resonance frequency of the sensing unit (11). The present invention is based on the realisation that an enhanced sensitivity of an acoustic detector in a PAS-system can be obtained by forming the acoustic detector as a cavity resonator with dimensions chosen so that the cavity resonance of the detector cooperates with the structural resonance of the sensing unit comprised in the detector, thereby achieving optimal amplification of the acoustic signals generated in the PAS-system.

Abstract: A photoacoustic spectrometer apparatus adapted to enable an observation and characterisation of non-radiative sub bandgap defects in narrow and large bandgap materials using photoacoustic spectroscopy techniques, the apparatus providing for an irradiation of a sample material provided within a photoacoustic cell and the subsequent detection and processing of an acoustic signal emitted by the sample, the apparatus comprising a light source having a polychromatic output substantially in the photonic energy range 0.5 eV to 6.2 eV, focusing means adapted to couple the output from the light source onto the sample material, the focusing means providing for an alignment and focusing of the light emitted from the light source so as to provide a substantially parallel incident light onto the sample material, and means for detecting and acquiring the acoustic signal emitted by the sample in response to the irradiation.

Abstract: A gas detector with a selectively adsorbing surface (3) and an acoustic measuring cell (5) is presented. The detector is characterized in that the selectively adsorbing surface (3) and the acoustic measuring cell (5) can be arranged with respect to one another such that gases desorbed by means of thermal desorption from the adsorbing surface (3) reach the acoustic measuring cell (5) and there trigger a pressure wave that can be measured by one or more acoustic pick-ups (13, 14), in particular microphones, which are arranged in the acoustic measuring cell (5). Furthermore, a corresponding method is provided. The detector is particularly suitable for measuring contaminants in interior spaces and ventilation systems.

Abstract: An art of hydrogen concentration measurement in which the hydrogen concentration of any given location of an object to be measured can be nondestructively obtained is provided.

Abstract: An aspirated smoke detector includes a flow path and a generator of acoustic waves in the flow path. Airborne particulate matter in the flow path responds to the acoustic field by particle agglomeration; the resulting larger particles flow into a photoelectric-type smoke sensor. A sensed level of particles can be processed, or compared to one or more predetermined thresholds to establish presence of one or more predetermined conditions.

Abstract: The invention relates to a photoacoustic detector including an acoustically open measuring area which is not completely surrounded by a housing. The detector includes an arrangement for introducing excitation light into the measuring area, such that the excitation light can be absorbed by absorbent materials which are located in the measuring area and which are used to produce acoustic energy. The invention also relates to a detector which includes at least one acoustic sensor and an arrangement is provided in order to concentrate the acoustic energy, in order to reach a local maximum of the acoustic pressure on at least one position. The at least one sensor is arranged in the vicinity of the at least one position, whereon the local maximum of the produced acoustic pressure is present or can be produced. The invention also relates to an associated method.

Abstract: A method of using photoacoustic spectroscopy to determine chemical information about an analyte includes the steps of emitting a light ray for interaction with a sample of an analyte; transmitting the light ray through a fill fluid disposed in a detection cell, the fill fluid having molecules substantially similar to molecules of the analyte to absorb the light ray; producing a thermal wave and oscillation in the fill fluid proportional to an intensity of the light ray; including a pressure oscillation in the fill fluid by the thermal wave; and detecting the pressure oscillation by a microphone to determine information about the analyte sample

Abstract: A photoacoustic sensor includes a sensor system for photoacoustic detection, at least one noise canceling pressure sensor and a control system in operative connection with the noise canceling pressure sensor to actively cancel the effects of noise in the environment on the sensor system. Another photoacoustic sensor includes a measurement volume, a source of light energy, a photoacoustic pressure sensor, and at least one vibration canceling sensor (for example, a microphone or an accelerometer). A further photoacoustic sensor includes a measurement volume, a source of light energy and a photoacoustic pressure sensor. The measurement volume has an inner surface that is continuously curved over substantially the entire inner surface other than where a window in optical connection with the source of light intersects the measurement volume.

Abstract: A photo acoustic detection cell (6) is located within the optical cavity (3) of a cavity enhanced absorption spectroscopy apparatus (3,4,5). When a sample in the cell (6) absorbs radiation from a pulsed radiation beam coupled into the cavity (3) pressure waves are generated that are detected by a microphone (9). A detected signal (10) output by the microphone (9) may be processed to determine a value for the concentration of an absorber in the sample.