Parallel Phase-shifting Interferometry Research Articles

Experimental studies with parallel phase-shifting interferometry

One of the most popular of devices currently used for acousto-optic sensing is the laser Doppler vibrometer (LDV), an interferometer originally designed to measure the vibration velocity of objects. LDVs are a popular way to measure and visualize acoustic fields, however it requires scanning the laser, which limits the measurement sound field. Recently, as the camera captures 2D images of the sound field on thousands of pixels simultaneously, the sound fields that can be measured are no longer limited to those that can be repeated. A high-speed camera captures 2D sound fields with tens or hundreds of thousands of frames per second, making it possible to film a slow-motion video of propagating sound in real time. The parallel phase-shifting interferometry (PPSI) with polarized high-speed cameras has been developed and introduced in the acoustics field as well. It has demonstrated impressive visualizations of airborne acoustic phenomena due to its high sensitivity and spatiotemporal resolution. It has also found many applications in recent years. This paper presents our experimental studies, especially measurement of aerodynamic sound, sound radiated from fast-moving sources or musical instruments, determination of acoustic center, etc.

Experimental and quantitative evaluation of frequency modulation caused by Doppler effect around high-speed moving sound source.

High-speed train noise remains a wayside environmental issue. For accurate noise prediction, the characteristics of a moving sound source must be revealed. In this work, the frequency modulation of sound waves emitted from a high-speed moving sound source was experimentally investigated. In the experiment, the sound field around a running train model emitting a 40 kHz pure tone was measured by an optical measurement technique, parallel phase-shifting interferometry, which can visualize instantaneous sound fields. For quantitative evaluation, a lens distortion correction was also developed and adopted for the visualization results. From the measured result of a sound source moving at a running speed of 280 km/h, the frequency modulation, known as the Doppler effect, was observed, and the measured frequency agreed well with the theoretical values.

Analysis of the sound field in circular spaces using small scale models and parallel phase-shift interferometry

In acoustic design, the use of circular spaces is generally avoided. This is because these spaces present several acoustic problems, for example, “whispering gallery,” and low diffusivity of sound. These room acoustic problems have generally been dealt with by installing diffusers on walls. In this study, we designed the acoustics of the circular space by applying diffusers based on primitive roots and analyze the sound field. These diffusers were proposed by Schroeder and take advantage of the randomness of the numerical sequence generated from primitive roots. We designed several small circular models attached the diffusers and printed them with a 3D printer. The sound field in the models were measured and visualized using parallel phase-shifting interferometry, one of the optical sound measurement methods. As a result, “whispering gallery” phenomenon was improved, and focal points were less likely to occur by the application of diffusers based on primitive roots.

Physical-model-based reconstruction of three-dimensional sound field from multi-directional measurement by parallel phase-shift interferometry

The high-spatial-resolution nature of optical non-invasive measurement and the reconstruction of three-dimensional sound fields are important for evaluating acoustic transducers. The computed tomography (CT) method has been used to reconstruct sound fields from optical measurement results because of its contactless feature. However, since the CT method does not assume any physical property of sound, it may not be suitable for the reconstruction of sound fields. In contrast, a physical-model-based method is able to reconstruct sound fields more accurately by considering the physics of sound. In this study, we propose a scan-free and physical-model-based reconstruction of a three-dimensional sound field. A sound field is multi-directionally measured using parallel phase-shifting interferometry and a high-speed polarization camera by rotating an acoustic transducer. A physical-model-based method is applied to reconstruct the sound field from the recorded optical interferograms. We also discussed the effectiveness of the proposed method to evaluate acoustic transducers.

Sound visualization of impact noise using parallel phase-shifting interferometry and development of noise reduction attachment

Impact wrenches are one of the most commonly used tools in factories, and the noise generated by their use is loud. In order to improve the environment for workers, it is desirable to reduce the generation of such noise. In this study, we characterize the sound generated during the use of impact wrenches, one of the noise sources in factories, and propose a method to reduce the generated sound. Specifically, we first made visualization measurements of sound generated by the impact wrench using a high-speed polarization interferometer. As a result, a loud noise was generated from the part where the socket at the tip of the impact wrench contacts the fastening part at the same time as it rotates, and the vibration was transmitted to the fastening part, causing the noise. Next, we developed a sound deadening attachment to be attached to the impact wrench. We designed a sound-absorbing mechanism that matches the frequency band to which humans are most sensitive and the characteristic frequency generated when tightening the impact wrench, and also designed a vibration-absorbing mechanism using anti-vibration rubber and vibration-damping sheets. As a result, the characteristic metallic sound was reduced and the noise level was also reduced.

Deep sound-field denoiser: optically-measured sound-field denoising using deep neural network.

This paper proposes a deep sound-field denoiser, a deep neural network (DNN) based denoising of optically measured sound-field images. Sound-field imaging using optical methods has gained considerable attention due to its ability to achieve high-spatial-resolution imaging of acoustic phenomena that conventional acoustic sensors cannot accomplish. However, the optically measured sound-field images are often heavily contaminated by noise because of the low sensitivity of optical interferometric measurements to airborne sound. Here, we propose a DNN-based sound-field denoising method. Time-varying sound-field image sequences are decomposed into harmonic complex-amplitude images by using a time-directional Fourier transform. The complex images are converted into two-channel images consisting of real and imaginary parts and denoised by a nonlinear-activation-free network. The network is trained on a sound-field dataset obtained from numerical acoustic simulations with randomized parameters. We compared the method with conventional ones, such as image filters, a spatiotemporal filter, and other DNN architectures, on numerical and experimental data. The experimental data were measured by parallel phase-shifting interferometry and holographic speckle interferometry. The proposed deep sound-field denoiser significantly outperformed the conventional methods on both the numerical and experimental data. Code is available on GitHub (https://github.com/nttcslab/deep-sound-field-denoiser).

Experimental observation of the sound field around a moving source using parallel phase-shifting interferometry

Railway noise is still one of the issues in the wayside environment despite of various countermeasures. For effective countermeasures, it is important to reveal characteristics of sound sources and sound propagation. Using parallel phase-shifting interferometry (PPSI) which measure the air density by interfering the reference light with object light, we tried to observe the sound field around a moving source. This system utilize laser and high-speed camera makes it possible to observe unstedy phenomena and visualize sound waves accurately. As a moving source, a speaker emitting 40kHz sinusoidal sound was mounted on a model, and the model was launched at 100 km/h. As A result, we succeed in observing the sound waves generated from the moving source clearly and visualizing the frequency modulation by Doppler effect. Furthermore, the result was averaged in sub-pixel to understand easily. These results clearly show the difference in frequency depending on the relative position to the sound source as it is in theory.

Determination of microphone acoustic center from sound field projection measured by optical interferometry.

This article presents a method for determining the acoustic center of a microphone from a sound field measured by optical interferometry. The acoustic center defines the equivalent point source position of a microphone serving as a sound source where the spherical waveform starts to diverge. The value is used to determine the effective distance between microphones for free-field reciprocity calibration. Conventionally, it is determined from the inverse distance law properties of a point source using the transfer function method. In this study, the acoustic center was determined from the projection of the sound field of the microphone. Parallel phase-shifting interferometry was used to measure the line integration of the sound pressure from a microphone. The acoustic center is determined as the position where the squared error between the measured data and the projection model of a point source is minimized. Experiments with the B&K 4180 (Brüel & Kjær, Nærum, Denmark) microphone were performed for frequencies from 10 to 50 kHz. The best acoustic center estimation was obtained at a microphone distance of 0 mm, with a difference of 0.17 mm to the IEC 61094-3 value and 0.36 mm to the Barrera-Figueroa et al. [J. Acoust. Soc. Am. 120(5), 2668-2675 (2006)] result at a measurement frequency of 20 kHz.

High-speed optical imaging and spatio-temporal analysis of sound sources of edge tone phenomena

Aerodynamic sounds are one of the noises of high-speed trains, automobiles, and wind turbines. To understand the characteristics of those noises, measuring sound sources is important. In general, microphones are used for measuring aerodynamic sounds. However, measuring the sound fields inside flow fields is difficult for microphones because they disturb flows. Thus, optical measurement methods have been applied to visualize aerodynamic sounds. The optical method can measure the sound fields without installing devices inside measurement fields. Therefore, it can capture the sound around sources. In this paper, we performed visualization and spatio-temporal analysis of sound sources of edge tones using parallel phase-shifting interferometry (PPSI). We experimentally confirmed the difference in pressure fluctuations near the sound source depending on the frequency of the edge tones.

Simultaneous Measurement of Vibration and Temperature Using Frequency-Scanned Parallel Phase-Shifting Interferometry

In order to simultaneously detect high-frequency dynamic parameters and low-frequency quasi-static parameters through spectrum acquisition, a novel frequency-scanned parallel phase-shifting interferometry is proposed in this work. A Michelson interferometer (MI) composed of a 3 × 3 coupler is used for high-frequency vibration measurement. Different from conventional 3 × 3 fiber coupler based demodulation methods, a frequency swept laser source with flat intensity output is used. The time-varying vibrations will be encoded on the phase shifted spectra measured at two return ports of the coupler. Using a modified phase shifting algorithm, the optical path difference (OPD) changes along optical frequency can be retrieved, thereby significantly improving the sampling rate of the dynamic MI. Thanks to the full-spectrum scanning, the asymmetric parameters of non-ideal 3 × 3 couplers can be directly measured. Moreover, it provides the possibility of spectral demodulation of multiple quasi-static parameters. Another sensing channel containing multiple fiber Bragg grating (FBG) sensors is used to achieve quasi-distributed temperature measurement. Experimental results demonstrate its effectiveness in dynamic/static multi-parameter measurement.

Physical-model-based reconstruction of axisymmetric three-dimensional sound field from optical interferometric measurement

Optical interferometric measurement methods for sound fields have garnered considerable attention owing to their contactless nature. The capabilities of non-invasive measurement and reconstruction of three-dimensional sound fields are significant for characterizing acoustic transducers. However, three-dimensional reconstructions are typically time consuming because of the two-dimensional scanning and rotation of the measurement system. This paper presents a scan and rotation-free reconstruction of an axisymmetric sound field in the human hearing range. A physical-model-based algorithm is proposed to reconstruct an axisymmetric sound field from optical interferograms recorded using parallel phase-shifting interferometry and a high-speed polarization camera. We demonstrate that audible sound fields can be reconstructed from data measured in 10 ms. The proposed method is effective for the rapid evaluation of axially symmetric acoustic transducers.

Seeing the sound of castanets: Acoustic resonances between shells captured by high-speed optical visualization with 1-mm resolution.

Sound fields radiated from the castanet, a Spanish percussive instrument comprising two shells, were optically visualized. A measurement system, which used parallel phase-shifting interferometry and a high-speed polarization camera, enabled the capture of instantaneous sound fields around the castanets, while the castanets were played, with the spatial resolution of 1.1 mm and frame rate of 100 000 fps. By carefully aligning the tilt of the castanets, the sound fields within the 1-mm gaps between both the shells were captured. From the visualization results, two acoustic resonances between the shells were identified. The first mode appeared between 1000 and 2000 Hz and exhibited a frequency chirp of several hundred hertz for several milliseconds after the impact. This can be explained by the Helmholtz resonance with a time-varying resonator shape, which is caused by the movement of the shells after impact. The second mode showed a resonance pattern with a single nodal diameter at the center of the shells, i.e., the standing wave mode caused by the interior volume. These physical phenomena involved in the sound radiation were identified owing to the unique features of the optical imaging method, such as contactless nature and millimeter-resolution imaging of instantaneous pressure fields.

Determination of frequency response of MEMS microphone from sound field measurements using optical phase-shifting interferometry method

Accurate determination of microphone sensitivity is important to build reliable acoustical instruments. The sensitivity is usually determined by calibration. However, because available microphone calibration methods determine the sensitivity from a mathematical model derived from the geometry of a conventional condenser microphone, they cannot be applied to the calibration of microelectromechanical systems (MEMS) microphone straightforwardly. To compromise this geometry difference with the available calibration methods, some authors have proposed the development of adapters that fits the conventional calibration apparatus and modified the calibration procedure. In this paper, we propose a different approach to calibrate the MEMS microphone. The sensitivity is calculated directly from the measurement of the sound field applied to the MEMS microphone and its output voltage. The projection of the sound field is measured by parallel phase-shifting interferometry (PPSI), and sound pressure on the MEMS microphone is obtained by tomographic reconstruction. Experimental calibration of a MEMS microphone was performed and validated using a microphone substitution method to evaluate the discrepancies of the sensitivity result. It is shown that the proposed method can be used to determine the frequency response of the MEMS microphone in the frequency range of 1000 Hz to 12000 Hz.

High-Speed Imaging of a Laser Ablation Process Using Parallel Phase-Shifting Interferometry

High-Speed Imaging of a Laser Ablation Process Using Parallel Phase-Shifting Interferometry

Optical visualization of a fluid flow via the temperature controlling method.

In this Letter, a visualization method of a fluid flow through temperature control is proposed. The proposed method enables us to visualize an invisible fluid flow by controlling the temperature so that its visibility can be easily adjusted. Such ability of adjusting appearance is effective for visualizing the phenomena consisting of multiple physical processes. In order to verify the validity of the proposed method, the measurement experiment of visualization of both flow and sound in air using parallel phase-shifting interferometry, which is a similar condition to the previous research [Opt. Lett.43, 991 (2018)OPLEDP0146-959210.1364/OL.43.000991], was conducted.

Time-directional filtering of wrapped phase for observing transient phenomena with parallel phase-shifting interferometry.

Recent development of parallel phase-shifting interferometry (PPSI) enables accurate measurement of time-varying phase maps. By combining a high-speed camera with PPSI, it became possible to observe not only time-varying but also fast phenomena including fluid flow and sound in air. In such observation, one has to remove static phase (time-invariant or slowly-varying phase unrelated to the phenomena of interest) from the observed phase maps. Ordinarily, a signal processing method for eliminating the static phase is utilized after phase unwrapping to avoid the 2π discontinuity which can be a source of error. In this paper, it is shown that such phase unwrapping is not necessary for the high-speed observation, and a time-directional filtering method is proposed for removing the static phase directly from the wrapped phase without performing phase unwrapping. In addition, experimental results of simultaneously visualizing flow and sound with 42 000 fps are shown to illustrate how the time-directional filtering changes the appearance. A MATLAB code is included within the paper (also in https://goo.gl/N4wzdp) for aiding the understanding of the proposed method.

Simultaneous imaging of flow and sound using high-speed parallel phase-shifting interferometry.

In this Letter, simultaneous imaging of flow and sound by using parallel phase-shifting interferometry and a high-speed polarization camera is proposed. The proposed method enables the visualization of flow and sound simultaneously by using the following two factors: (i) injection of the gas, whose density is different from the surrounding air, makes the flow visible to interferometry, and (ii) time-directional processing is applied for extracting the small-amplitude sound wave from the high-speed flow video. An experiment with a frame rate of 42,000 frames per second for visualizing the flow and sound emitted from a whistle was conducted. By applying time-directional processing to the obtained video, both flow emitted from the slit of the whistle and a spherical sound wave of 8.7 kHz were successively captured.

Optical sensing of sound fields: Non-contact, quantitative, and single-shot imaging of sound using high-speed polarization camera

Imaging of a sound field aids understanding of the actual behavior of the field. That is useful for obtaining acoustical spatial characteristics of transducers, materials and noise sources. For high spatial resolution imaging, optical measurement methods have been used thanks to its contactless nature. This paper presents sound field imaging method based on parallel phase-shifting interferometry, which enables to acquire an instantaneous two-dimensional phase distribution of light. Information of sound field is calculated from the phase of light based on the acousto-optic theory. The system consists of a polarization interferometer and high-speed polarization camera. The number of the measurement points in a single image are 512 × 512 and the interval between adjacent pixels is 0.22 mm. Therefore, the system can image a sound field with much higher spatial resolution compared with conventional imaging methods such as microphone arrays. The maximum frame rate, which is corresponding to the sampling frequen...

High-speed imaging of sound using parallel phase-shifting interferometry.

Sound-field imaging, the visualization of spatial and temporal distribution of acoustical properties such as sound pressure, is useful for understanding acoustical phenomena. This study investigated the use of parallel phase-shifting interferometry (PPSI) with a high-speed polarization camera for imaging a sound field, particularly high-speed imaging of propagating sound waves. The experimental results showed that the instantaneous sound field, which was generated by ultrasonic transducers driven by a pure tone of 40 kHz, was quantitatively imaged. Hence, PPSI can be used in acoustical applications requiring spatial information of sound pressure.

Superresolution of interference fringes in parallel four-step phase-shifting digital holography

A superresolution method for interference fringes obtained by parallel four-step phase-shifting digital holography is proposed. A complex amplitude distribution of an object wave is derived from a recorded hologram by parallel phase-shifting interferometry using two pixels without any interpolation procedures. Multiple distributions are derived by changing one of the two pixels when conducting phase-shifting interferometry. The angular spectrum distribution of the object wave is obtained by both the Fourier transforms and synthesis of the spectrum distribution from the Fourier-transformed images in the spatial frequency domain. Available space bandwidth is extended to half of that of an image sensor.