Measurements Of Acoustic Response Research Articles

Measurements of acoustic response of car interion for structural excitations

1. Cao J, Liu J, Wang J, Lai X. Acoustic vector sensor: Reviews and future perspectives., IET Signal Processing. 2017;11(1):1-9. https://doi.org/10.1049/iet-sp.... CrossRef Google Scholar

The assessment of the technical condition of SO-3 engine turbine blades using an impulse test

The paper presents the effects of the application of an experimental impulse test as a method of diagnosis of the technical condition of an SO-3 engine turbine blade fitted in a TS-11 Iskra aircraft. The aim of the test was to investigate the frequency characteristics of the blades and discuss differences between the frequency spectrum of the investigated component before and after its damage. The acoustic response measurements were performed to the signal generated by an impact hammer on the fully functional and intentionally damaged blades. The recorded signals were converted from the domain of time to the domain of frequency using the Fast Fourier Transform (FFT). The results of the FFT were the Frequency Response Functions (FRF) of the sound of the blades, based on which the statistical analysis of the resonance frequencies was carried out. The influence of the mechanical damage of the blade on the shape and characteristics of the frequency spectrum was confirmed, which substantiated the effectiveness of the impulse test in the diagnostic assessment of jet engine components.

A history of cepstrum analysis and its application to mechanical problems

It is not widely realised that the first paper on cepstrum analysis was published two years before the FFT algorithm, despite having Tukey as a common author, and its definition was such that it was not reversible even to the log spectrum. After publication of the FFT in 1965, the cepstrum was redefined so as to be reversible to the log spectrum, and shortly afterwards Oppenheim and Schafer defined the “complex cepstrum”, which was reversible to the time domain. They also derived the analytical form of the complex cepstrum of a transfer function in terms of its poles and zeros. The cepstrum had been used in speech analysis for determining voice pitch (by accurately measuring the harmonic spacing), but also for separating the formants (transfer function of the vocal tract) from voiced and unvoiced sources, and this led quite early to similar applications in mechanics. The first was to gear diagnostics (Randall), where the cepstrum greatly simplified the interpretation of the sideband families associated with local faults in gears, and the second was to extraction of diesel engine cylinder pressure signals from acoustic response measurements (Lyon and Ordubadi). Later Polydoros defined the differential cepstrum, which had an analytical form similar to the impulse response function, and Gao and Randall used this and the complex cepstrum in the application of cepstrum analysis to modal analysis of mechanical structures. Antoni proposed the mean differential cepstrum, which gave a smoothed result. The cepstrum can be applied to MIMO systems if at least one SIMO response can be separated, and a number of blind source separation techniques have been proposed for this. Most recently it has been shown that even though it is not possible to apply the complex cepstrum to stationary signals, it is possible to use the real cepstrum to edit their (log) amplitude spectrum, and combine this with the original phase to obtain edited time signals. This has already been used for a wide range of mechanical applications. A very powerful processing tool is an exponential “lifter” (window) applied to the cepstrum, which is shown to extract the modal part of the response (with a small extra damping of each mode corresponding to the window). This can then be used to repress or enhance the modal information in the response according to the application.

Depth estimation of laser glass drilling based on optical differential measurements of acoustic response

We present the first steps of a device suitable for characterization of complex 3D micro-structures. This method is based on an optical approach allowing extraction and separation of high frequency ultrasonic sound waves induced to the analyzed samples. Rapid, non-destructive characterization of 3D micro-structures are limited in terms of geometrical features and optical properties of the sample. We suggest a method which is based on temporal tracking of secondary speckle patterns generated when illuminating a sample with a laser probe while applying known periodic vibration using an ultrasound transmitter. In this paper we investigated lasers drilled through glass vias. The large aspect ratios of the vias possess a challenge for traditional microscopy techniques in analyzing depth and taper profiles of the vias. The correlation of the amplitude vibrations to the vias depths is experimentally demonstrated.

Cantilever-based acoustic velocity sensors

This paper discusses progress made on the design of an acoustic velocity sensor. An analytic model was developed for the frequency response of a slender cantilever rod forced by the pressure gradient and particle velocity associated with an acoustic wave propagating in a fluid. The model, validated with acoustic response measurements in air, was used to design cantilever sensors, which respond predominately to acoustic particle velocity. One such design utilizes a short cantilever formed from a 125 μm silica glass fiber immersed in a viscous fill fluid whose lateral tip displacement is detected using a multi-fiber optical probe. This velocity sensor is predicted to be able to detect fairly low acoustic sound levels in water. Progress has been made in instrumenting a large pool at NRL to allow accurate propagating acoustic wave response measurements in water of these new velocity sensors down to frequencies below 5 Hz. Measurements made in this facility on various cantilever sensors will be presented and discussed. [Work supported by ONR.]

Acoustic Response of Reservoir Sandstones during Injection of Supercritical CO2

We report experimental results for acoustic response measurements conducted during injection of supercritical carbon dioxide into a brine saturated sandstone plug. We measured P- and S-wave velocities (Vp, Vs) as a function of effective stress and CO2 saturation in a sandstone plug. We demonstrate that Gassmann's fluid substitution procedure matches the experimental results well for this sample. A 3.5% reduction of P-wave velocity after injection of two pore volumes of CO2 into brine-saturated sample was measured. We conclude that measurement of Vp can be used to estimate CO2 saturations in rock. In addition, x-ray computer tomography (CT) images were acquired at reservoir conditions with a resolution of 33 μm, which provided more detailed information about CO2 saturations and distributions in the rock. It is envisaged that these techniques (seismic and CT) can be combined in the future to enable a more holistic understanding of how fluid-fluid displacement processes are coupled with the acoustic response characteristics of the rock.

Variability in the acoustic response from natural shallow-water marine sediments

The large variation in sediment acoustic response observed between soft clays and sands provides a satisfactory means to estimate sediment properties remotely using acoustic techniques. However, the accuracy of these predictions is limited by variations in sediment properties, both horizontally and vertically on a scale of centimeters to meters, within a given sediment volume, and sets the accuracy limits or ‘‘error bars’’ for prediction of sediment properties. In order to quantify these limits, a series of measurements of acoustic response were made at six sites off Gulfport, MS in sediments ranging from sands to muds. A broadband transducer with an 8° constant beamwidth was mounted on a trolley suspended from an I-beam connected to a pair of legs at each end of the beam. This ‘‘swingset’’ was lowered to the seafloor with the transducer mounted at normal incidence and at an altitude of 3.0 m. Measurements were made at both 30 and 50 kHz at each of eight positions 0.3-m apart along the beam. Results suggest greater variability occurs in the vertical direction (6–24 dB) than in the horizontal direction (6–18 dB).

The Acoustic Impulse Response Method for Measuring the Overall Firmness of Fruit

An acoustic impulse method has been used to measure the maturity of ‘‘Golden Delicious’’ apples throughout the growing season and during eight months cool storage at 1°C and 96% r.h. Over the same period, penetrometer firmness was measured. Comparison of the results has enabled correlations to be made between the elastic coefficient of apple tissue and the mean firmness at a depth from 5 to 9mm below the skin. Under the experimental conditions used, the penetrometer measurements provided better discrimination between immature fruit, while the acoustic impulse response measurements were superior for ripe fruit.The possibility of applying the acoustic impulse response method to fruits with non-uniform internal structures was considered experimentally by studies on tomatoes. In this case, correlations were established between the coefficient of elasticity calculated from acoustic measurements, colour measurements (lightness and hue), the force required to compress the whole fruit by 3% of its equatorial diameter, and the modulus of elasticity calculated from the whole fruit load-deformation curve.Acoustic response measurements gave a reliable indication of the change in mechanical properties of fruit before, during and after harvest. In particular there were indications that the acoustic response may give additional information on fruit water status which is not detectable by conventional firmness measurements. Given the non-destructive nature of acoustic response measurements, the method appears to have considerable promise as a technique for the evaluation of the post-harvest condition of apples.

Total Least Square Technique for Estimating the Vibration Parameters of the Apple from the Time Domain Impulse Response Signal

This paper introduces the Hankel matrix based total least square (HTLS) technique to process the time domain acoustic impulse response signal for vibration parameters of the apple. It has a major advantage of being able to separate the modes in a coupled resonant peak which occurs for some apples. The HTLS technique can be used together with a mode identification technique, which is outside the scope of this paper, to determine the spherical resonant frequency. This may improve the reliability of nondestructive firmness determination of fruit based on acoustic response measurements.

Factors Affecting Acoustic Responses of Apples

Sound pitch levels of fourteen apples were evaluated by human auditory sensing and compared with instrumental measurements of acoustic response of the fruits. Results indicated that the sound pitch level had a direct correlation with the first and the second resonant frequencies of the fruit. A linear model based on amplitudes of the power spectrum at selected frequencies could determine the sound levels of the fruit with a coefficient of multiple determination (R2) of 0.96. The acoustic impulse generating method, test location on the fruit, and fruit holding method did not affect the resonant frequencies of the fruit, but affected the amplitudes of vibrations at the second and higher resonant frequencies.

Acoustic Response Measurements and Standards for Motion-Picture Theaters

A proposal of the Nordisk Film/TV Union (NFTU), document 1069–1969, is discussed in great detail. This document refers to the measurement of acoustic response in cinema theaters for optical 35-mm soundtracks. The instructions, procedures and equipment this proposal offers for consideration are described at length, for the benefit of the service engineer for whom this type of measurement may constitute a new field of activity. Also, the work carried out simultaneously in other countries and by ISO is described. Reference is made to test films and further developments of instrumentation in the recent past. Mention is made of the new possibilities opened up by an ISO Draft Proposal, calling for an “Y”-curve to cover films currently in production or exploitation, and for a “X”-curve for such new systems as the Dolby SVA and the Hue-Modulated Photographic Color Soundtrack. A hope for further research regarding roll-off or no roll-off is also expressed.

Newer concepts of the pitch, the loudness and the timbre of musical tones

It is not widely realised that the first paper on cepstrum analysis was published two years before the FFT algorithm, despite having Tukey as a common author, and its definition was such that it was not reversible even to the log spectrum. After publication of the FFT in 1965, the cepstrum was redefined so as to be reversible to the log spectrum, and shortly afterwards Oppenheim and Schafer defined the “complex cepstrum”, which was reversible to the time domain. They also derived the analytical form of the complex cepstrum of a transfer function in terms of its poles and zeros. The cepstrum had been used in speech analysis for determining voice pitch (by accurately measuring the harmonic spacing), but also for separating the formants (transfer function of the vocal tract) from voiced and unvoiced sources, and this led quite early to similar applications in mechanics. The first was to gear diagnostics (Randall), where the cepstrum greatly simplified the interpretation of the sideband families associated with local faults in gears, and the second was to extraction of diesel engine cylinder pressure signals from acoustic response measurements (Lyon and Ordubadi). Later Polydoros defined the differential cepstrum, which had an analytical form similar to the impulse response function, and Gao and Randall used this and the complex cepstrum in the application of cepstrum analysis to modal analysis of mechanical structures. Antoni proposed the mean differential cepstrum, which gave a smoothed result. The cepstrum can be applied to MIMO systems if at least one SIMO response can be separated, and a number of blind source separation techniques have been proposed for this. Most recently it has been shown that even though it is not possible to apply the complex cepstrum to stationary signals, it is possible to use the real cepstrum to edit their (log) amplitude spectrum, and combine this with the original phase to obtain edited time signals. This has already been used for a wide range of mechanical applications. A very powerful processing tool is an exponential “lifter” (window) applied to the cepstrum, which is shown to extract the modal part of the response (with a small extra damping of each mode corresponding to the window). This can then be used to repress or enhance the modal information in the response according to the application.