Serdar H. Yönak

Multiple microphone photoacoustic leak detection and localization system and method

A method and system for multiple microphone photoacoustic leak detection and localization. The leak detection technique uses photoacoustic sounds produced by the interaction of a carbon dioxide (CO2) laser turned to 10.6 microns and a photoactive tracer gas, sulfur hexafluoride (SF6), emitted by calibrated leak sources. As the leaked gas is heated by the laser, it expands and launches a photoacoustic sound pulse. The sound pulses generated by the scanning process are recorded across a broad bandwidth by multiple ultra-sensitive microphones. After the photoacoustic pulses arc recorded, the magnitude is compared with background noise measurement at several frequencies for a determination if a leak is present. If the presence of a leak is found, then the location of the leak is determined. The recorded sound is processed by using Matched Field Processing (MFP) as the signal processing technique. The photoacoustic sounds recorded at the microphones are reversed in time through computer simulation that causes the sounds to converge to the apparent point of origin. No synchronization between the acoustic signal processor and the laser scanner is required.

Photoacoustic detection and localization of small gas leaks

Leak detection and localization are critical manufacturing quality-control processes. Many industrial and domestic machines use or convey pressurized gases or liquids. Unintended leaks from machine components may be detrimental to consumers, manufacturers, and the environment. This paper describes a leak detection technique based on photoacoustic sounds produced by the interaction of a carbon dioxide (CO2) laser tuned to 10.6 micrometers and a photoactive tracer gas, sulfur hexaflouride (SF6), emitted by calibrated leak sources. Acoustic signals generated by a high-speed scan of the laser beam through the cloud of tracer gas formed near the leak are recorded in a bandwidth from 3 to 52 kHz by multiple microphones. From the recorded signals, the presence or absence of a leak may be deduced by comparison with the background noise level at the signal frequencies, which occur at the harmonics of the scan rate. When a leak is present, its location is determined from a simple model of the acoustic environment and matched field processing (MFP). Current results show that a gas leak of 1 cm3 per day can be detected and localized to within +/- 3 mm in a few seconds using four microphones, placed 0.41 m from the leak location, and an incoherent average of the MFP ambiguity surfaces at eight signal frequencies. Comparisons of the Bartlett and minimum-variance-distortionless matched field processors are also presented.

Parametric dependencies for photoacoustic leak localization

Unintended gas or liquid leaks from manufactured components or manufacturing systems may be detrimental to consumers, manufacturers, and the environment. Thus, leak testing is important for quality, safety, and environmental reasons. This paper describes parametric dependencies for photoacoustic leak localization. The technique is based on the interaction of 10.6-micrometer radiation from a carbon dioxide (CO2) laser and a photoactive tracer gas, sulfur hexafluoride (SF6). For the current investigations, acoustic signals are generated by scanning a laser beam at high speed through gas plumes formed above calibrated leaks. These signals are remotely measured with a four-microphone linear array and analyzed using Bartlett and minimum-variance-distortionless (MVD) matched-field processing (MFP) techniques to determine leak location. This paper extends prior work in photoacoustic leak testing through (i) use of more signal frequencies; (ii) parametric study of four different laser scan rates; and (iii) examination of mismatch between the actual acoustic environment and the propagation model used in the MFP; and (iv) presentation of leak localization results on a curved surface. For a 12-watt CO2 laser exciting the small SF6 gas plume produced by a one-cm3-per-day leak with microphones placed 0.41 m from the leak location, root-mean-square localization uncertainties as small as +/-0.5 mm on a line scan of 0.46 m can be achieved when the largest possible number of signal frequencies fall in a measurement bandwidth of approximately 70 kHz.

Gas-phase generation of photoacoustic sound in an open environment

The photoacoustic effect is commonly exploited for molecular spectroscopy, nondestructive evaluation, and trace gas detection. Photoacoustic sound is produced when a photoactive material absorbs electromagnetic radiation and converts it to acoustic waves. This article focuses on the generation of photoacoustic sound from thermal expansion of photoactive gases due to unsteady heating from a laser light source, and extends the work of prior studies on photoacoustic sound generation in an open environment. Starting with the forced free-space wave equation, a simple model is constructed for photoacoustic sounds produced by both acoustically distributed and compact gas clouds. The model accounts for laser absorption through the Lambert-Beer law and includes the effects of photoactive gas cloud characteristics (shape, size, and concentration distribution), but does not include molecular diffusion, thermal conduction, convection, or the effects of acoustic propagation through sound-absorbing inhomogeneous media. This model is compared to experimentally measured photoacoustic sounds generated by scanning a 10.6-micron carbon dioxide (CO2) laser beam through small clouds of a photoactive gas, sulfur hexafluoride (SF6). For the current investigation, the photoactive gas clouds are formed either by low flow-rate calibrated leak sources or by a laminar jet emerging from a 1.6-mm-diam tube. Model-measurement comparisons are presented over a 3- to 160-kHz bandwidth. Signal pulse shapes from simple gas cloud geometries are found to match calculated results when unmeasured gas cloud characteristics within the model are adjusted.

Non-line-of-sight sound source localization using matched-field processing.

Acoustic diffraction allows sound to travel around opaque objects and therefore may allow beyond-line-of-sight sensing of remote sound sources. This paper reports simulated and experimental results for localizing sound sources based on fully shadowed microphone array measurements. The generic geometry includes a point source, a solid 90° wedge, and a receiving array that lies entirely in the shadow defined by the source location and the wedge. Source localization performance is assessed via matched-field (MF) ambiguity surfaces as a function of receiving array configuration, and received signal-to-noise ratio for the Bartlett and minimum variance distortionless (MVD) MF processors. Here, the sound propagation model is developed from a Greens function integral treatment. A simple 16 element line array of microphones is tested in three mutually orthogonal orientations. The experiments were conducted using an approximate 50-to-1-scaled tabletop model of a blind city-street intersection and produced ambiguity surfaces from source frequencies between 17.5 and 19 kHz that were incoherently summed. The experimental results suggest that a sound source may be localized by the MVD processor when using fully shadowed arrays that have significant aperture parallel to the edge of the wedge. However, this performance is reduced significantly for signal-to-noise ratios below 40 dB.

Planar gradient index photonic metamaterials

Metamaterials operating at optical frequencies, referred to as optical or photonic metamaterials, require features fabricated at a subwavelength scale from 50 nm to 1000nm. In this work a planar gradient index metamaterial is designed and demonstrated at optical frequencies by numerical simulation through a finite-difference time domain method in conjunction with an electromagnetic retrieval technique. We confirm the gradient by simulating the deflection of a light beam passing through a multilayer silver (Ag) and magnesium fluoride (MgF2) slab featured with specially designed nano-rectangular holes. The planar gradient index photonic metamaterials we propose can be fabricated by available nano-fabrication technologies. Optical tests can be performed since the designs are also based on the consideration of the frequency range available for evaluation.

Ultrasonic calibration of MEMS (microelectromechanical systems) microphone arrays.

Ultrasonic microphone arrays have applications in imaging, scale model testing, and sound source localization. For these applications MEMS ultrasonic microphones are emerging as a natural choice because of their small size, low cost, and robustness to manufacturing treatments and environmental conditions. In this talk an array is constructed using commercially available Knowles SiSonic[TM] capacitive MEMS microphones. The absolute sensitivity and phase calibrations of the array elements in the 1–60‐kHz frequency range are obtained through the use of replacement calibration and time‐frequency analysis. For these results, a modulation in sensitivity and phase measurements for frequency ranges over 15 kHz is observed under certain conditions. The origin of this modulation is discussed as well as various techniques for its removal. Relative element calibrations are crucial for array performance and are obtained through a variety of sound source configurations, which vary in size and location relative to the a...

Matched field processing applied to diffraction of sound around 90 deg corners: Modeling and experiment.

Acoustic diffraction allows sound to travel around opaque objects and therefore may allow beyond‐line‐of‐sight sensing of remote sound sources. This presentation reports simulated and experimental results for detecting and localizing sound sources at blind city‐street intersections based on shadowed microphone array measurements. The generic geometry includes a point source, a solid 90 deg wedge, and a receiving array that lies entirely in the shadow defined by the source location and the wedge. Sound source detection and localization performance are assessed via matched‐field (MF) ambiguity surfaces as a function of source frequency, receiving‐array configuration, and received signal‐to‐noise ratio for the Bartlett and minimum variance distortionless MF processors. Here, the sound propagation model is developed from a Green’s function integral treatment and the geometric theory of diffraction. The simulations suggest that sound sources may be localized by fully shadowed arrays for signal‐to‐noise ratios ...

Generation of photoacoustic sound for gases in an open environment

Photoacoustics is the generation of acoustic waves due to unsteady heating from a light source. It was discovered over 119 years ago and is commonly used for trace gas detection and molecular spectroscopy when the photoactive gas or gas mixture is contained in a chamber. However, photoacoustic phenomena are less well characterized in an open environment. In this presentation, scaling laws for radiated photoacoustic sound pressure are developed from dimensional analysis and verified with experiments using a carbon dioxide laser tuned to 10.6 μm and small plumes of sulfur hexafluoride, a photoactive tracer gas. In addition, a forced wave equation that includes laser‐gas heating is developed to predict the generation and propagation of photoacoustic sound. Using this forced wave equation, the shape and amplitude of photoacoustic sound pulses are determined based on the shape of the tracer‐gas plume and the extent of laser‐beam absorption through the Lambert–Beer law. The model results are compared to measure...

Photoacoustic localization of small gas leaks on cylindrical surfaces

Detecting and localizing leaks in or on manufactured components that contain pressurized gases or liquids is a critical quality control process. Previous work has shown that gas leaks on a flat surface with a leak rate on the order of one cubic centimeter per day can be reliably detected and accurately localized using photoacoustic signals, four microphones, and incoherent broadband matched‐field processing (MFP). However, leaks may not lie on flat surfaces. This work considers a case in which the surface where the leak resides is cylindrical. Photoacoustic sound is generated by rapidly scanning a carbon dioxide laser tuned to 10.6 micrometers (μm) over the suspected leak area. Sulfur hexafluoride, which has strong spectroscopic absorption near 10.6 μm, is the tracer gas. Localization results obtained using Bartlett MFP and minimum variance MFP are compared. The physics behind photoacoustic signal generation is discussed. Experimental results are presented. [Sponsored by Ford Motor Company.]