Том 62, № 1 (2016)

Nonlinear gas oscillations excited in an open tube by a flat piston at one of the tube ends are studied. The sinusoidal piston oscillations in the shock-free wave mode are created by a vibration exciter near the first eigenfrequency. Expressions for gas pressure oscillations are obtained for a tube with a nonrounded end without a flange and secondary flow velocity components. The influence of the piston displacement amplitude on the pressure range and secondary flow velocity of gas is studied. The theoretical calculations of the gas pressure are compared with experimental data. An estimate for the velocity of particle motion along the tube axis is presented with calculated values of the secondary flow velocity.

The paper discusses a model for a screen with dissipative and nonlinear elastic properties that can be used in acoustic sound absorption and frequency conversion systems. Calculation and estimation schemes are explained that are necessary for understanding the functional capabilities of the device. Examples of the nonlinear elements in the screen and promising applications are described.

The paper presents the results of experimental and theoretical studies of processes of phase-conjugate ultrasonic wave propagation in a liquid flow containing gas microbubbles. It is shown that a signal from a phase-conjugate wave, which is recorded by a transceiving transducer, contains information on the flow velocity of scatterers and their concentration. In this case, the flow velocity is determined both in the presence and absence of moving scattering objects. A theory developed on the basis of the generalized reciprocity principle for a moving inhomogeneous medium represents the main experimentally observed features of the formation of signals from a phase-conjugate wave scattered by a disperse liquid flow.

Experimental data are analyzed, obtained during the recording of seismoacoustic signals with a two-coordinate laser strainmeter; the signals were generated by a surface vessel moving over the shelf and deep water. Based on analysis of the obtained data, we substantiate the prospect of applying shore-based laser strainmeters for the direction finding of vessels and the creation of monitoring systems for large water areas.

The study analyzes the characteristics of surface Love waves excited by the moment of an oscillating torsional force with a point of action that moves uniformly and rectilinearly along the free flat boundary of a medium having the structure of a “layer on a half-space.” The azimuthal–angular distribution of the amplitude and Doppler shift in frequency of the wave modes is studied as a function of the motion velocity of a vibrating source and the parameters of the medium.

The acoustic characteristics of a large-scale model of a wing with high-lift devices in the landing configuration have been studied in the DNW-NWB wind tunnel with an anechoic test section. For the first time in domestic practice, data on airframe noise at high Reynolds numbers (1.1–1.8 × 10⁶) have been obtained, which can be used for assessment of wing noise levels in aircraft certification tests. The scaling factor for recalculating the measurement results to natural conditions has been determined from the condition of collapsing the dimensionless noise spectra obtained at various flow velocities. The beamforming technique has been used to obtain localization of noise sources and provide their ranking with respect to intensity. For flap side-edge noise, which is an important noise component, a noise reduction method has been proposed. The efficiency of this method has been confirmed in DNW-NWB experiments.

The paper presents acoustic measurement results for two concert halls in which nonexponential sound decay is observed. Quantitative estimates are given for how the obtained decay laws differ from exponential. Problems are discussed that arise when using reverberation time to assess the quality of room acoustics with nonexponential sound decay.

A technique for detecting direct signal pulses based on steep rising edges of acoustic pressure is developed. The technique consists in calculating the mirror derivative of the received signal and normalizing it in a specific manner. This makes it possible to amplify weak direct signals and suppress strong reflected ones. A key feature of this technique is that it ensures a high probability of detection of direct signal pulses while keeping the number of false detections at a minimum.

A theory of propagation of torsional waves excited by an electromagnetic–acoustic transducer in a pipe is proposed. This theory takes into account the excitation parameters, geometry, viscosity, and the elastic characteristics of an object. The main testing parameters (the frequency and geometry of the transducer) that determine the possibilities of guided-wave testing of pipelines of various dimensions using torsional waves are theoretically substantiated.

For damped acoustic metamaterials, a discrete model is proposed in the form of a periodic structure with cells of the simplest type. The effective parameters of the model are determined by criteria based on the equality of dispersion of waves. The general properties of the model are studied. An example of one negative type of metamaterials is presented. The model is useful for analyzing wave properties and creating metamaterials with given acoustic properties.

Experimental measurements are presented for sound wave attenuation in foam without additives (standing wave method) and in foam with added particles (pulse method). A setup is developed that makes it possible to obtain a standing sound wave in stable foam and estimate the attenuation coefficient. A comparison is made of the coefficients of sound attenuation in foam in the sonic and ultrasonic frequency ranges, which have been published in a number of works. It is shown that the introduction of particles into foam leads to an increase in sound wave attenuation and may be the result of the viscous mechanism of sound wave energy loss.