Frequency spectrum, acoustic

The fracture of a brittle material is often accompanied by acoustic emission, which results from the release of energy.This acoustic emission has both an energy and a frequency spectrum, Acoustic emission may be used as one indication of the onset of failure. For example, in the testing of adhesion by the scratch test, the coated surface is scratched by a rounded diamond point and the load on the point is increased while monitoring the acoustic emission using a piezoelectric accelerometer to detect the onset of fracture. 

In order to obtain a high signal-to-noise ratio sufficient acoustical power is necessary. For special applications a programmable pulser (transmitter) is required in order to optimize the frequency spectrum. 

The disturbance caused by a noise depends on its intensity (equivalent pressure level L in dB(A)), its frequency spectrum (that is its energy distribution), and the acoustic characteristics of the medium in which the listener is kicated. 

Fig. 1.1 The regions for transient cavitation bubbles and stable cavitation bubbles when they are defined by the shape stability of bubbles in the parameter space of ambient bubble radius (R0) and the acoustic amplitude (p ). The ultrasonic frequency is 515 kHz. The thickest line is the border between the region for stable cavitation bubbles and that for transient ones. The type of bubble pulsation has been indicated by the frequency spectrum of acoustic cavitation noise such as nf0 (periodic pulsation with the acoustic period), nfo/2 (doubled acoustic period), nf0/4 (quadrupled acoustic period), and chaotic (non-periodic pulsation). Any transient cavitation bubbles result in the broad-band noise due to the temporal fluctuation in the number of bubbles. Reprinted from Ultrasonics Sonochemistry, vol. 17, K.Yasui, T.Tuziuti, J. Lee, T.Kozuka, A.Towata, and Y. Iida, Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles, pp. 460-472, Copyright (2010), with permission from Elsevier...

Fig. 13.21 shows another example of oscillatory burning of an RDX-AP composite propellant containing 0.40% A1 particles. The combustion pressure chosen for the burning was 4.5 MPa. The DC component trace indicates that the onset of the instability is 0.31 s after ignition, and that the instability lasts for 0.67 s. The pressure instability then suddenly ceases and the pressure returns to the designed pressure of 4.5 MPa. Close examination of the anomalous bandpass-filtered pressure traces reveals that the excited frequencies in the circular port are between 10 kHz and 30 kHz. The AC components below 10 kHz and above 30 kHz are not excited, as shown in Fig. 13.21. The frequency spectrum of the observed combustion instability is shown in Fig. 13.22. Here, the calculated frequency of the standing waves in the rocket motor is shown as a function of the inner diameter of the port and frequency. The sonic speed is assumed to be 1000 m s and I = 0.25 m. The most excited frequency is 25 kHz, followed by 18 kHz and 32 kHz. When the observed frequencies are compared with the calculated acoustic frequencies shown in Fig. 13.23, the dominant frequency is seen to be that of the first radial mode, with possible inclusion of the second and third tangential modes. The increased DC pressure between 0.31 s and 0.67 s is considered to be caused by a velocity-coupled oscillatory combustion. Such a velocity-coupled oscillation tends to induce erosive burning along the port surface. The maximum amplitude of the AC component pressure is 3.67 MPa between 20 kHz and 30 kHz. - ...

The cantilever is excited into resonance by electrically exciting the piezoelectric cantilever mount. The frequency of the excitation wave is scanned in a given frequency range, and the frequency of maximum cantilever amplitude is taken as the resonance frequency. The frequency spectrum of the cantilever response shows the fundamental frequency as well as the harmonics of cantilever vibration. The cantilevers, however, also resonate in response to ambient conditions such as room temperature or acoustic noise without requiring any external power. 

In much of the interference theory of the acoustic microscope, and especially the theory of how the contrast varies with defocus, it is assumed that the microscope is perfectly monochromatic. Of course, the monochromatic theory could be summed over a frequency spectrum actually used in a given... 

Srulovicz and Goldstein, 1983] Srulovicz, P. and Goldstein, J. L. (1983). A central spectrum model a synthesis of auditory-nerve timing and place cues in monaural communication of frequency spectrum. J. Acoust. Soc. Am., 73 1266-1276. 

As in the study of water dynamics, the power spectrum density is useful to detect the long-time correlation or to detect decay slow energy transfers between optical and acoustic modes in the model given in Eq. (11). The relaxation inside optical modes is much faster than that in acoustic modes, since the frequency spectrum in optical modes is sharply localized and almost resonant while the spectrum is broadly spread in acoustic modes. 

We will establish the basic principles that govern the behavior of aU acousto-optic devices whether of bulk or waveguide (SAW) construction. An acousto-optic modulator is composed of an acoustic medium (such as water, glass, lithium niobate, rutile, etc.) and a transducer. The transducer converts electrical signals into sound waves propagating in the acoustic medium with an acoustic frequency spectrum that is limited by the bandwidth of the transducer that matches the electrical excitation. The sound wave causes a perturbation in the index of refraction of the material, setting up a refractive index grating of the form... 

GuO, N. Cawley, P. (1992). Measurement and Prediction of the Frequency Spectrum of Piezoelectric Disks by Modal Analysis, The Journal of The Acoustical Society of America, Vol.92, No.6, pp.3379-3388, ISSN 0001-4966... 

Ikegami, S. Ueda, I. Kobayashi, S. (1974). Frequency Spectrum of Resonant Vibration in Disk Plates of PbTiOs Piezoelectric Ceramics, The Journal of The Acoustical Society of America, Vol.55, No.2, pp.339-344, ISSN 0001-4966 Ivina, N. F. (1990). Numerical Analysis of the Normal Modes of Circular Piezoelectric Plates of Finite Dimensions, Soviet Physics - Acoustics, Vol.35, No.4, pp.385-388, ISSN 0038-562X... 

Due to the reversibility of the piezoelectric effect, materials exhibiting such an electromechanical coupling may be used to handle actuation as well as sensing tasks. The different piezoelectric materials are able to provide these properties in a frequency spectrum ranging beyond the level of acoustics. On the one hand, there are several monocrystals and polycrystalline ceramics, which are hard and brittle and therefore are suitable only for relatively small strains. On the other hand, there are semicrystalline polymers, which are soft and elastic but show less pronounced coupling properties. Another kind of... 

Masharov, S.I. (1963). Frequency spectrum of acoustic vibrations of crystals with vacancies. Izvestiya vuzov. No. 5, (May, 1963), pp. 39-42, ISSN 1573-9228. 

Chapter 23 shows how modeling can propose mechanisms to explain experimentally observed vibrations in the cardiovascular system. A control system characterized by a slow and delayed change in resistance due to smooth muscle activity is presented. Experiments on this model show oscillations in the input impedance frequency spectrum, and flow and pressure transient responses to step inputs consistent with experimental observations. This autoregulation model supports the theory that low-frequency oscillations in heart rate and blood pressure variability spectra (Mayer waves) find their origin in the intrinsic delay of flow regulation. A second example presents acoustic classification of vascular... 

Thus far we have discussed the direct mechanism of dissipation, when the reaction coordinate is coupled directly to the continuous spectrum of the bath degrees of freedom. For chemical reactions this situation is rather rare, since low-frequency acoustic phonon modes have much larger wavelengths than the size of the reaction complex, and so they cannot cause a considerable relative displacement of the reactants. The direct mechanism may play an essential role in long-distance electron transfer in dielectric media, when the reorganization energy is created by displacement of equilibrium positions of low-frequency polarization phonons. Another cause of friction may be anharmonicity of solids which leads to multiphonon processes. In particular, the Raman processes may provide small energy losses. 

Abe S, Choi P- K (2008) Effect of frequency on sonoluminescence spectrum from alkali-metal solutions. Nonlinear acoustics- Fundamentals and applications. AIP Confer Proc 22 189-192... 

The PAS phenomenon involves the selective absorption of modulated IR radiation by the sample. The selectively absorbed frequencies of IR radiation correspond to the fundamental vibrational frequencies of the sample of interest. Once absorbed, the IR radiation is converted to heat and subsequently escapes from the solid sample and heats a boundary layer of gas. Typically, this conversion from modulated IR radiation to heat involves a small temperature increase at the sample surface ( 10 6oC). Since the sample is placed into a closed cavity cell that is filled with a coupling gas (usually helium), the increase in temperature produces pressure changes in the surrounding gas (sound waves). Since the IR radiation is modulated, the pressure changes in the coupling gas occur at the frequency of the modulated light, and so does the acoustic wave. This acoustical wave is detected by a very sensitive microphone, and the subsequent electrical signal is Fourier processed and a spectrum produced. 

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