Sound Packets Research Articles

Acoustics and Hydrodynamics of the Drop Impact: Two Modes of Sound Packet Emission

Acoustics and Hydrodynamics of the Drop Impact: Two Modes of Sound Packet Emission

Visualization of the Flow Pattern of the Impact of a Freely Falling Drop during the Generation of Sound Packets

Visualization of the Flow Pattern of the Impact of a Freely Falling Drop during the Generation of Sound Packets

Акустика и гидродинамика импакта капли: два режима излучения звуковых пакетов

The paper studies acoustic signals in two modes of merging with water in a freely falling water droplet by the matched optical and acoustic methods. Only the primary sound packet was observed in the intrusive mode at the low drop speed, when the cavity was formed with a delay in the colored wake of the inflowing liquid. Experiments demonstrated the drop speed influence on the primary signal parameters. At the high droplet speed, the cavity started to form from the moment of the initial contact. The flow pattern became more complicated, the droplet substance was distributed over the emerged cavity surface and the grown crown in the form of separate fibers forming the line and mesh structures. The falling section of the primary acoustic signal was formed by a group of pulses with the complex spectral composition. Further, one or more low-frequency sound packets were registered after the pause. Spectral portrait of the secondary packets was related to the shape of the detached gas cavity. Smooth axisymmetric cavity was radiating the monochromatic packet. The complex-shaped cavity was transforming into a gas bubble and emitting the multi-frequency signal. Duration and amplitude of the secondary sound signals were depending on the transformation rate of the separated gas cavity into a smooth spheroidal bubble. The acoustic signal parameters were changing nonmonotonically with increasing velocity of the merging drop

Visualization of the Flow Pattern of the Impact of a Freely Falling Drop during the Generation of Sound Packets

Visualization of the Flow Pattern of the Impact of a Freely Falling Drop during the Generation of Sound Packets

Sound emission of solid surfaces through bubble layer

The analysis of the influence of a thin homogeneous bubble layer on sound emission from a solid surface is carried out. Sound pulses and monochromatic wave packets with a carrier frequency equal to the resonant frequency of the bubbles forming the bubble layer are considered. It is shown that the bubble layer transforms short sound pulses into wave sound packets and significantly reduces the amplitude of the emitted sound. The structure of a sinusoidal wave packet is transformed similarly. A long sound pulse is stored in the form of a pulse, its shape changes significantly. A homogeneous bubble layer near a solid radiating surface is an open resonator. The layer generates far-field radiation with spectral lines depending on the method of layer excitation and the internal properties of the bubble layer. The resonant frequency of the bubble is the limiting frequency in the spectrum, but it is not distinguished by a separate line.

Пакеты капиллярных и акустических волн импакта капли

Flows, capillary waves, and acoustic signals generated by a drop of water falling into a pool of degassed liquid were recorded by a high-speed video camera, hydro-phone, and microphone. A large-scale analysis of the system of equations was performed. The fast conversion of available surface potential energy is traced. The converted energy is stored in a thin layer in the vicinity of the merged free surfaces and creates large perturbations of temperature, pressure and flow velocity. Capillary waves start to radiate simultaneously with the formation of a cavity and the rise of the crown. New groups of capillary waves arise with all changes in the flow structure --- the formation and immersion of a splash, come back of secondary drops, the formation of cavities, the immersion of a streamer and droplets. Simultaneously with the waves, ligaments --- thin near-surface flows are formed that affect the transport and rupture of gas cavities. Thin flows quickly decay and form again when a new group of capillary waves is generated. A comparison of flow patterns and acoustic signals indicates that the generation of resonant sound packets is synchronized with the pinch-off gas fragments from the cavity or their breaking. The duration of the sound depends on the initial heterogeneity of the geometry of the sounding cavity, gradually transforming into a smooth spheroidal form

Evolution of the Structure of Acoustic Signals Caused by the Impact of a Falling Drop on a Liquid

Combined high-resolution optic and acoustic methods are used for the first time to synchronously detect the fine structure of processes initiated by submersion of a freely falling water drop in a tank with degassed liquid. The emphasis is on analyzing the evolution of the fine structure of flow pattern, the accompanying capillary waves, and the amplitude–frequency characteristics of sound packets. The sequence of acoustic signals includes a shock pulse, which is caused by the initial contact of the drop with the surface of the liquid at rest, and a series of consecutive sound packets. The shock pulse being reliably reproduced in experiments has a complex structure, which includes short segments of high-frequency oscillations and longer segments of low-frequency oscillations. The number of emitted sound packets and their parameters vary in an arbitrary way from one experiment to another while the size of drop and the distance of fall remain invariable. A series of experiments reveals cases of single and secondary packet emission with different spectral compositions of packets, as well as cases of their total absence. Analysis of data is performed with allowance for the effect of the “double layer”, which replaces the coalescence surface of liquids and retains the disturbances caused by transformation of available potential surface energy to other forms, such as thermal energy, pressure disturbance, and energy of fine currents actively affecting the gas cavities.

Fast Superfine Components and Sound Packets in Phenomena Induced by the Impact of a Drop on a Target Fluid in Quiescent Conditions

Fast Superfine Components and Sound Packets in Phenomena Induced by the Impact of a Drop on a Target Fluid in Quiescent Conditions

СОУДАРЕНИЕ КАПЛИ СО СВОБОДНОЙ ПОВЕРХНОСТЬЮ: АКУСТИЧЕСКОЕ ИЗЛУЧЕНИЕ В ТОЛЩЕ ЖИДКОСТИ

An experimental study of the emission of sound by droplets falling into an unperturbed liquid was performed using wideband hydrophones and a high-speed video camera. Collision of a drop with a water surface is accompanied by air entrainment and the formation of underwater gas cavities, which leads to the appearance of surface forces and fast flows, generating extreme accelerations, which are sufficient for resonant acoustic excitation of cavities and emission of sound packets. The sequence of emitted signals contains a shock pulse, as well as one or more sound packets generated by resonant bubbles detached from underwater cavities. The number of resonating bubbles varies from experience to experience, depending on the collision scenario and further behavior, including, in particular, the process of fragmentation of the primary cavities. The dimensions of the bubbles, measured on video frames, correspond to the values calculated in accordance with the Minnaert resonance frequency.

New modulation method in acousto-optic spectrometers

A new method is proposed for modulating radiation in collinear acousto-optic (AO) spectrometers, which is based on the sequential diffraction of incident polarised optical radiation using two sound packets with the same frequencies and initial phases. A theory of diffraction of optical radiation on two successive packets of sound waves propagating in a crystal is constructed, and the instrumental function of the AO spectrometer is found. It is established that the instrumental function of such a spectrometer becomes time-dependent, and the modulation frequency of this dependence, in turn, depends on the intensity of the sound wave and on the detuning from the phase-matching conditions. The frequency dependence of the measured photocurrent for some finite number of sound packets is obtained explicitly, and it is shown that its measurement allows the spectral composition of the incident optical radiation to be determined with greater accuracy.

Fine structure of acoustic signals caused by a drop falling onto the surface of water

The temporal structure of sound radiation upon a drop falling onto a free liquid surface is investigated experimentally by high-resolution high-speed videorecording synchronized with a broad-band measurement of the acoustic pressure. Groups of short and relatively prolonged sound packets with frequency filling from 2 to 50 kHz and the corresponding flow patterns including the simultaneous formation of resonating bubbles and their interaction processes with an originating cavern are isolated. The temporal dependence of the determining parameter, i.e., the Weber number, which is stably reproduced in a series of experiments by a power function with a fractional index, is constructed.

Underwater and air sound signals in the case of a droplet falling onto a liquid surface

Among fast natural phenomena, exchange pro� cesses involving the atmosphere and the ocean are of particular interest. These processes include the inter� action of rainfall (first of all, rain) with the oceanic surface. The kinetic energy of a rain droplet and the surface energy that becomes free once the droplet has been absorbed result in a series of hydrodynamic phe� nomena. These are formation of a cavern, a cumula� tive jet, streamers, regular capillary waves, and irregu� lar waves. The formation and fillingin of the caverns are accompanied by the detachment of air cavities of both regular and irregular shape, with a part of them emitting sound. The acoustic rain noise generated by falling drop� lets forms the total sonic background. This is an important source of information on the intensity and localization of rain, which is a significant fraction of the total ocean acoustic noise (1). In records of under� water signals obtained within the rain period, the spec� tral peak between 10 and 15 kHz is surely localized (2). Modernization of experimental methods has opened the possibility to measure fluctuations of sound pressure in water, which arise as a result of the fall of both the set of (rain) droplets and an individual droplet with the simultaneous registration of the flow pattern (2). Among competing mechanisms of sound emission, the impact pulse, hydrodynamic impact, and aircavity resonances were analyzed. In experi� ments, along with the sound registration, air bubbles were always observed. Therefore, basic attention was paid to the resonance mechanism for which the soundemission frequency was connected to the bub� ble size by a simple calculation relationship (3). The available experimental data made it possible to obtain a diagram of flow modes in the plane of the dropletfall velocity as a function of the droplet diam� eter. In this plane, a region of stable registration is iso� lated for gas bubbles of the size corresponding to the spectralpeak frequency (4). However, for the reso� nance mechanism, the question remained open on the transformation of the fallingdroplet energy to that of bubble resonance vibrations accompanied by sound emission. In further experiments, it was noted that gas cavities emit underwater sound in the course of recon� structing their irregular shape (with tapers and folds) to the equilibrium (spherical) one. This process is accompanied by the liberation of the potential surface energy (5). The analysis of the processes allows for the fact that a falling droplet leads to accepting the substance liq� uid, the momentum, and the mechanical energy, which results in generating visible and latent flows of different scales. Caverns, coronas, cumulative jets, and streamers correspond to visible flows and manifest themselves in the changing the shape of the free sur� face (6). Largescale latent flows cause chaotic displace� ments of gas bubbles in the bulk of the liquid. The vir� tually unstudied finestructure latent flows manifest themselves in peculiarities of both the freesurface shape and the substancetransport pattern near the interface. They also determine the properties of emit� ted sound packets. In the present paper, for the first time, we thoroughly study the frequencytime param� eters of sound signals initiated in air and water by freely falling droplets.

Emission of the sequence of sound packets during a drop falling onto the surface of water

Emission of sound signals accompanying the fall of a water drop onto a free liquid surface is investigated in a laboratory bath by high-speed video filming and underwater acoustics. Emission of high-frequency sound is detected during the initial contact of the drop with the surface. Low-frequency sound packets are observed after a prolonged silence interval with the formation of nonequilibrium-shaped gas cavities with sharpenings during their reconstruction into spherical bubbles. By virtue of complexity of the flow pattern at the stages of detachment and coalescence of gas cavities, the frequency and intensity of the emitted sound varied in wide ranges even under invariable initial experimental conditions.

Sound generation as a drop falls on a water surface

Impact of a drop on a water surface is accompanied by a series of sound pulses propagating in air and underwater. Depending on the falling mode (drop size and initial velocity), pulses substantially differ in amplitude, duration, and modulation frequency. We study falling modes in which in addition to conventional sound packets—the shock pulse and single resonance sound packets—several packets are observed. Experiments were conducted with simultaneous recording of sound in air and underwater and were accompanied by synchronous video depiction of currents in the drop impact region. Comparison of videograms and phonograms demonstrate that the sources of sound packets are gas cavities of arbitrary shape detached from the underwater cavern under the action of large accelerations (several km/s2) during a sharp change in its surface area, which gradually achieve equilibrial elliptical and spherical shapes.

Codec system and method

A sound codec including a compression portion and a decompression portion. The compression portion separates the incoming sound packet bandwidth into frequency sub-bands using a bank of infinite impulse response (IIR) filters. In accordance with the invention, the lower frequency signals are divided into more sub-bands than the higher frequency signals. Once the signals are divided into sub-bands, each sub-band signal is quantized. The resulting signals of all of the quantized sub-bands are then sent out over a communications link along with the filter state at the end of each sound packet. The decompression portion recombines the individual sub-band signals together (using a bank of infinite impulse response (IIR) filters) to form the audio data using the filter states to configure the reconstruction.

Wavelet packet based analysis of sound fields in rooms using coincident microphone arrays

This paper presents a passive analysis method for determining the spatio-temporal characteristics of sound fields in small rooms. The analysis finds an approximate directional reflectogram (ADR) which reveals the approximate arrival directions, time delays and amplitudes of the direct sound and early reflections without using a special or known sound source. A coincident microphone array is used to obtain directional recordings. The recordings are analysed by wavelet packet decomposition to determine the direction of the sound source and select wavelet packet coefficients to reconstruct the estimate of the direct sound. ADR is then computed via deconvolution using this estimate. Experiments have been carried out using synthesized recordings that were obtained from actual room impulse responses measured in two rooms for various source locations. The method estimates the source direction with a mean absolute error of about 7°. Calculated ADRs provide a good estimate of the time delays and arrival directions of acoustical reflections, whereas the amplitudes differ slightly.

The acoustic wave equation as a finite difference

The classical wave equation used to describe acoustic phenomena was converted to a finite difference equation. This was done by constraining a sound packet to be in a box of finite length. By using certain mathematical techniques, recursion relations were found which enabled the amplitude of sound to be measured at every point in the box. This form of the equation is useful in modeling certain acoustic phenomena. The finite difference equation is especially useful in calculating wave reflection/transmission phenomena. The classical way of calculating reflection/transmission was by calculating a separate wave for each reflected or transmitted wave. The finite difference equation method cancels the need for many different waves and encompasses all interference from these waves.

Continuously Variable Signal Delays with Ultrasound

A device to introduce continuously variable delays in elec- tric signals with frequencies in the megahertz range is proposed. The basis of the device is the generation of replicas of ultrasonic wave pack- ets by conduction electrons in metals. The construction, advantages, and limitations of the device are discussed. S I. INTRODUCTION URFACE acoustic wave devices are frequently used to delay signals by a fixed amount. We propose an acoustic wave device that produces a continuously varia- ble signal delay. The device uses the generation of repli- cas of ultrasonic wave packets in pure metal single crys- tals at low temperatures in applied magnetic fields. The generation of replica wave packets was first observed in ultrapure gallium (l) and later correctly attributed to the interaction of conduction electrons with ultrasound (2). The proposed device converts an electric pulse into a sound wave packet. An external magnetic field is used to produce a replica of the wave packet that has the same shape as the original or packet, but a smaller amplitude. The replica precedes the primary packet by an amount that depends inversely on the strength of the mag- netic field. Thus the delay produced by the device (the time between the conversion of an electric signal into an ultrasonic wave packet and the subsequent conversion of the replica into an electric signal) is also field-dependent. One varies the delay by varying the strength of the applied field. Although both sound packets are converted back into electric pulses, the pulse that corresponds to the primary wave packet is diverted by a SPDT switch.

Theory of electronic transport of sound packets in metals in parallel magnetic field

A theory is constructed for the propagation of sound pulses in metals in a magnetic field parallel to the wave front. Under the conditions of strong spatial dispersion kl≫ kR≫ 1 (k is the sound wave vector, l is the electron mean free path, and R is the cyclotron radius), because of deformation interaction with electrons in the metal a sound wave in pure samples gives rise to a system of spikes at distances which are multiples of 2R and move synchronously with the sound wave at the speed of sound. It is shown that the spike amplitudes are proportional to the squared local values of the deformation interaction potential Λ(p) and that their attenuation is determined by the averaged value of the potential. Attention is drawn to the possibility of establishing the form of Λ(p) by comparing theoretical and experimental results. The salient features of the formation of spikes are analyzed for an arbitrary electron dispersion relation.

Thermal interference-optical excitation of ultrasonic waves in condensed media

An investigation was made of an effective method for the excitation of ultrasonic waves in thin liquid films based on the pulsed heating of these films by an interference field of coherent radiation. The perturbations induced in the medium were measured from the diffraction of a cw laser beam. It is shown that the optimal conditions for the generation of monochromatic sound packets are achieved for exciting radiation having a pulse duration considerably shorter than the period of the acoustic waves. A method of determining the velocity and attenuation of the sound is proposed and the importance of edge effects in the relaxation processes of the perturbations is studied.