Alexander Ekimov

Nonlinear acoustic interaction on contact interfaces and its use for nondestructive testing

Recent theoretical and experimental studies demonstrated that a weakly or incompletely bonded interfaces exhibit highly nonlinear behavior. One of acoustic manifestations of such nonlinearity is the modulation of a probing high-frequency ultrasonic wave by low-frequency vibration. The vibration varies the contact area modulating the phase and amplitude of higher frequency probing wave passing through the interface. In the frequency domain, the result of this modulation manifests itself as side-band spectral components with respect to the frequency of the probing wave. This modulation effect has been observed experimentally for various materials (metals, composites, concrete, sandstone, glass) with various types of contact-type defects (interfaces): cracks, debondings, delaminations, and microstructural material damages. Study of this phenomenon revealed correlation between the developed modulation criterion and the quantitative characteristics of the interfaces, such as its size, loading condition, and bonding strength. These findings have been used for the development of an innovative nondestructive evaluation technique, namely Vibro-Acoustic Modulation Technique. Two modifications of this technique have been developed: Vibro-Modulation (VM) and Impact-Modulation (IM), employing CW and impact-induced vibrations, respectively. The examples of applications of these methods include crack detection in steel pipes, aircraft and auto parts, bonded composite plates etc. These methods also proved their effectiveness in the detection of cracks in concrete.

Vibration and sound signatures of human footsteps in buildings

The acoustic signature of a footstep is one of several signatures that can be exploited for human recognition. Early research showed the maximum value for the force of multiple footsteps to be in the frequency band of 1-4 Hz. This paper reports on the broadband frequency-dependent vibrations and sound pressure responses of human footsteps in buildings. Past studies have shown that the low-frequency band (below 500 Hz) is well known in the literature, and generated by the force normal to the ground/floor. The seismic particle velocity response to footsteps was shown to be site specific and the characteristic frequency band was 20-90 Hz. In this paper, the high-frequency band (above 500 Hz) is investigated. The high-frequency band of the vibration and sound of a human footstep is shown to be generated by the tangential force to the floor and the floor reaction, or friction force. The vibration signals, as a function of floor coverings and walking style, were studied in a broadband frequency range. Different walking styles result in different vibration signatures in the low-frequency range. However, for the walking styles tested, the magnitudes in the high-frequency range are comparable and independent of walking style.

Modulation of torsional waves in a rod with a crack

Results of experiments concerning nonlinear effects for elastic waves in metal rods with the crack-type defects are presented. The dependence of the nonlinear effect on the contact type is discussed. The crack has been modeled by cutting a rod and tightly filling the crack with metal plates. Two types of contact have been studied: a dry contact and a contact with lubricant. The modulation of high-frequency torsional waves (20 and 22.8 kHz) in the rod under the effect of low-frequency flexural vibrations has been investigated. Flexural vibrations have been excited by a shock and by a vibrator. The modulation of high-frequency torsional waves under the effect of low-frequency vibrations has been observed only in the rod with a crack. The level of modulation drastically decreases in the presence of liquid lubricant.

A Review of Human Signatures in Urban Environments Using Seismic and Acoustic Methods

Techniques for sensing footstep vibrational frequencies, typically below 100 Hz, by seismic sensors are well- developed human detection methods. Walking styles (standard, soft, or stealthy) and the background noise floor limit the detection range of footsteps. Walking style changes the dynamic footstep force on the ground and the influences the footstep detection range. The seismic background noise floor is much higher in urban areas and in buildings than in rural areas, dramatically influencing detection range. Alternatively, high- frequency passive and active ultrasonic methods for human detection are being developed. High-frequency sound produced by friction forces between a foot and the ground/floor allow passive footstep detection in urban areas and in buildings. The active method utilizes continuous-wave Doppler ultrasound. Simultaneously collecting Doppler motion and footstep ultrasonic signals reveals correlated timing features between the footstep friction and the Doppler shift from the human motion. Discrimination between human and other moving targets is accomplished by analyzing the envelopes of footstep signatures.

Range limitation for seismic footstep detection

Seismic methods for footstep detection exploit low frequency vibration waves, typically below 100 Hz. There are two limiting factors for detection of human footsteps at these frequencies: walking styles and the background noise floor. The walking style changes the dynamic footstep force on the ground and, therefore, limits the maximum distance at which walkers may be detected. For seismic frequencies, the background vibration noise floor is higher in urban areas than in quiet areas. This article presents and discusses test results of human footstep measurements as a function of distance using the seismic method in quiet and urban areas.

Structural vibrations of buried land mines

Buried landmines exhibit complex structural vibrations, which are dependent on interaction between soil and mines as well as on their respective properties. This paper presents experimental and theoretical studies of multimodal vibrations of buried mines and discusses the effects of burial depth and soil properties on dynamics of the soil-mine system. The two-dimensional model of the soil-mine system that accounts for soil-coupled mine’s multiple vibration modes and spatial distribution of vibrations over the soil surface is introduced. The model was tested using experiments with the plastic mine simulant. The study reveals that the soil shear stiffness is one of the key governing parameters determining the resonance vibration frequency and the amplitude of the soil-mine system. Burial depth, soil moisture, and consolidation are among factors leading to the increase of the soil shear stiffness, therefore effectively influencing modal vibrations of buried mines.

Nonlinear vibrations of buried landmines.

The seismo-acoustic method is one of the most promising emerging techniques for the detection of landmines. Numerous field tests have demonstrated that buried landmines manifest themselves at the surface through linear and nonlinear responses to acoustic/seismic excitation. The present paper describes modeling of the nonlinear response in the framework of the mass-spring model of the soil-mine system. The perturbation method used in the model allows for the derivation of an analytical solution describing both quadratic and cubic acoustic interactions at the soil-mine interface. This solution has been compared with actual field measurements to obtain nonlinear parameters of the buried mines. These parameters have been analyzed with respect to mine types and burial depths. It was found that the cubic nonlinearity could be a significant contributor to the nonlinear response. This effect has led to the development of a new intermodulation detection algorithm based on dual-frequency excitation. Both quadratic and intermodulation nonlinear algorithms were evaluated at the U.S. Army outdoor testing facilities. The algorithms appear to complement each other in improving the overall detection performance.

Extraction of the velocity of walking human’s body segments using ultrasonic Doppler

The focus of this paper is to experimentally extract the Doppler signatures of a walking humans individual body segments using an ultrasonic Doppler system (UDS) operating at 40 kHz. In a humans walk, the major contribution to Doppler velocities and acoustic scattering is from the foot, lower leg, thigh (upper leg) and torso. The Doppler signature of these human body segments are extracted experimentally. The measurements were made by illuminating one of these body segments at a time and blocking the remaining body segments using acoustic screens. The results obtained in our experiment were verified with the results published by Bradley using a physics-based model for Doppler sonar spectrograms.

Human motion analyses using footstep ultrasound and Doppler ultrasound.

Human footsteps generate periodic broadband frequency envelopes of sound due to dynamic friction forces. Also, human body motion when walking is a cyclic temporal process. The individual body parts have different acoustic cross sections and velocities that form unique human Doppler signatures. The paper introduces an approach to analyze this motion using passive and active ultrasound. The passive method employs a narrowband microphone that is sensitive to the sound from footsteps. The active method utilizes continuous-wave ultrasound to measure the Doppler shifted signal from the body appendages. These two methods show time synchronization between Doppler and ultrasonic human footstep signatures.

Ultrasonic wave generation due to human footsteps on the ground.

Human footsteps generate broadband frequency vibrations in the ground/floor and sound in the air from a few Hertz up to ultrasonic frequencies due to striking and sliding contacts between a foot and the ground/floor. The high-frequency (above 1 kHz) vibrations from footsteps were detected on a building floor, but were not detected on the outdoor ground, even at 1 m from a walker. This paper presents results of ultrasound registration from footsteps on the ground at greater distances. Results are based on sound measurements in air, since the sound absorption in air is less than vibration absorption in the ground.