Organ-pipe Modes Research Articles

Directional emissivity from two-dimensional infrared waveguide arrays

Fabrication and optical characterization of surfaces covered with open-ended metallic waveguides are presented along with numerical modeling of these structures. Both modeling and measurement of the structures indicate that the 2-D array of 3D metallic waveguides modify both the direction and spectral content of the emissivity, resulting in directionality normal to the surface due to the optical axis of the waveguides and spectrally narrow emissivity due to the lateral dimensions of the waveguides. Furthermore, the optical behavior of these structures is placed in the broader context of other structured emission/absorption surfaces such as organ pipe modes, surface plasmon modes, and coherent thermal emission from gratings.

The roar of Yasur: Handheld audio recorder monitoring of Vanuatu volcanic vent activity

We describe how near-field audio recording using a pocket digital sound recorder can usefully document volcanic activity, demonstrating the approach at Yasur, Vanuatu in May 2014. Prominent emissions peak at 263 Hz, interpreted as an organ-pipe mode. High-pass filtering was found to usefully discriminate volcano vent noise from wind noise, and autocorrelation of the high pass acoustic power reveals a prominent peak in exhalation intervals of ~ 2.5, 4 and 8 s, with a number of larger explosive events at ~ 200 s intervals. We suggest that this compact and inexpensive audio instrumentation can usefully supplement other field monitoring such as seismic or infrasound. A simple estimate of acoustic power interpreted with a dipole jet noise model yielded vent velocities too low to be compatible with pyroclast emission, suggesting difficulties with this approach at audio frequencies (perhaps due to acoustic absorption by volcanic gases).

High frequency organ-pipe modes in amorphous boron carbide observed using surface Brillouin scattering

Amorphous boron carbide films of 2 micron thickness were deposited at room temperature by a thermal deposition process on single-crystal silicon substrates. The elastic constants of an amorphous B4C film have been successfully measured by surface Brillouin scattering as a function of temperature, in the process, revealing a phase transition at about 350°C. Quantized wave-vector components perpendicular to the film surface associated with organ-pipe modes occurring within the film were used in conjunction with elastodynamic Green's function calculations as well as independent measurement of longitudinal frequency from bulk excitations to extract the elastic constants.

Tilt angle dependence of backscattering enhancements from organ pipe modes of open water-filled cylinders: Measurements and models

A simple target for simulating narrow low-frequency resonances of cylinders is an open metal pipe completely filled with water. We have previously described how the high-Q organ-pipe modes having a pressure node near each end are easily observed in backscattering experiments with small cylinders [C. F. Osterhoudt and P. L. Marston, J. Acoust. Soc. Am. 110, 2773 (2001)]. The resonance occurs because of the strong reflection of internal acoustic waves from the open ends of the pipe [H. Levine and J. Schwinger, Phys. Rev. 73, 383–406 (1948)]. In the present research, the dependence of the backscattering amplitude on the orientation of the cylinder is measured and modeled. The tilt angle dependence is affected by the symmetry of the organ pipe mode. An approximation was also developed for the backscattering amplitude at high Q resonances based on energy conservation, reciprocity, and the optical theorem. While this analysis applies to cylinders suspended in water away from boundaries, the organ-pipe modes studied may be useful for investigating scattering processes for buried or partially buried cylinders. [Research supported in part by ONR.]

Shock–induced brittle failure of boron carbide

The mechanism for the failure of brittle materials during uniaxial compressive shockloading has been the subject of much discussion. In particular, the physical interpretation of the yield point, t...

The energy dependence of the conductance and scattering wave functions of a 2D quantum dot

A new wave-function matching formalism is used to calculate wave functions psi and the conductance g12 for a quantum dot as functions of the Fermi wavelength lambda . The dot is formed in a 2D quantum wire with hard walls by pushing in through one wall two identical, hardwall, rectangular fingers with height h, width d and separation s. Calculations of conductance g12 are made for a structure which exhibits two resonant tunnelling peaks below the nominal transmission threshold over the tops of the fingers. These peaks are associated with the open-ended organ pipe modes in the region between the fingers. The two longest organ pipe resonant wavelengths are within 2% and 10% of the calculated peak positions, but wave-function spillage over the tops of the fingers and the space between them has a marked effect on both the resonant wave functions and the resonant wavelengths. Contour plots of mod psi mod 2 for scattering waves are presented and used to elucidate the behaviour of g12.

Intrafilm and interface interaction determination by inelastic helium atom scattering

Abstract Organ pipe modes have recently been measured by the inelastic helium atom scattering (HAS) technique for epitaxial films (2–20 monolayers) of sodium on Cu(001). For the Kr/Ag(111), Ar/Ag(111), and KBr/NaCl(001) multilayer systems measured previously by HAS the phonon frequencies as a function of the number of monolayers do not follow the organ pipe sequence. Within the framework of a force constant model the interface condition for the organ pipe modes is derived, and we find that the vibrational frequency distribution in the case of epitaxial thin films depends not only on the intrafilm interaction but also strongly on the interface interaction. The theory provides a consistent explanation of the above phenomena observed by helium atom scattering.

Organ-pipe modes of sodium epitaxial multilayers on Cu(001) observed by inelastic helium-atom scattering.

Inelastic helium-atom scattering from epitaxial films (2-20 monolayers) of sodium on Cu(001) reveals a large number of nearly dispersionless phonon modes whose frequencies depend on the thickness and scale as the lower harmonics of an open-ended organ pipe. These data provideevidence for confined acoustic resonances, corresponding to longitudinal standing waves normal to the surface with frequencies about 20% larger than in the bulk

Surface waves in anisotropic media: the secular equation and its numerical solution

The secular equation for waves on an anisotropic half space is examined. It is reduced to a polynomial of degree four when the sagittal plane is a plane of symmetry and to a generalized eigenvalue problem giving twenty-seven roots in the most general case. When the surface, or the plane normal to the direction of propagation, is a plane of symmetry, three families of roots of the secular equation can be identified: modes involving body waves, organ-pipe modes, akin to plate modes, and generalized surface waves of which the Rayleigh wave is an example. Defining equations for these three families are obtained and it is shown that the secular equation, viewed as a polynomial, factorizes when symmetries are present. There are between ten and twelve roots of the factor yielding the Rayleigh wave which are obtained from a generalized eigenvalue problem, a competitive numerical technique. In numerical experiments, a second wave of Rayleigh type, for one direction on the (001) - plane of copper, has been identified.

A real frequency, complex wave-number analysis of leaking modes

Abstract Leaking-mode dispersion and attenuation is computed for two single-layer models. Roots of the dispersion relation are obtained in the complex wave-number plane, using real frequency as the independent variable. At low frequencies the root loci on the (+, +) sheet determine the fundamental propagating mode plus a series of attenuated standing-wave modes in the vicinity of the source. As frequency increases, the complex roots on all four sheets migrate through the wave-number plane, producing (successively) organ-pipe modes, PL modes, and normal modes. For higher frequencies, the modes exhibit a tendency to couple with pure P- (and S-) type motion in the wave guide. These effects are related to similar phenomena which occur for the case of the free elastic plate. Results are also calculated for a six-layer continental model. Here the pseudo-modes on the (+, −) sheet noted by Cochran et al. (using real wave-number analysis) are found to be decoupled P- and S-related modes whose phase-velocity curves freely intersect one another when frequency is assumed real.

Ocean microseism measurements with a neutral bouyancy free-floating midwater seismometer

abstract Seismometers in spherical aluminum pressure housings have been weighted to float stably at midwater depths in the ocean, and thus record water motions in a frequency band of 0.02 to 5 cps. Simultaneous records made with a midwater instrument at 1.2-km depth and a bottom instrument at 4.6-km depth showed coherence at spectral power peaks of leaky organ-pipe frequencies and additional coherence peaks at frequencies down to 0.025 cps. Twenty organ-pipe modes can be tentatively identified. The spectral power can be attributed almost entirely to microseismic motions in wave-guide modes. We conclude that the forcing functions for microseisms are broad enough so that deep ocean-bottom and midwater microseism spectral peak frequencies are characteristic of local bathymmetry.

Diagnostic diagrams and transfer functions for oceanic wave-guides

abstract The variation of frequency F as a function of wave number K and the associated spectral transfer function are computed for different modes in a complex oceanic wave-guide. The model consists of a fluid layer resting upon a three-layer elastic half-space. The layers and the half-space are homogeneous. The comparison of theoretical results with measured power spectra for two records taken in the Pacific Ocean shows qualitative agreement stressing strongly the role of the leaking compressional organ-pipe modes which are not continuations of normal modes beyond cutoff frequency. The mathematical procedure consists in the integration of the second-minor propagator equation of Gilbert and Backus (1966). The determinant representing the secular function is computed directly rather than by summing the products of its elements. This improves both accuracy and computing time. The integration can be reduced to that of a third-order nonlinear differential system which, for K = 0, splits into two Riccati equations. The (F, K)-diagram corresponding to every mode is obtained by a technique based on properties of similar diagrams for simple oceanic and continental structures.

Supersonic Diffuser Instability

Steady operation of supersonic diffusers near critical mass flow is interrupted by a transient process known as buzz. This phenomenon consists of a random sequence of individual relaxation cycles. Mass flow entering the diffuser during steady operation is suddenly cut off by a strong interaction between the subcritical shock and boundary layer on the surface of the external compression generator, which blocks the inlet. Air in the plenum chamber, stored at high pressure, then ?blows down? until the inlet can restart. The subsequent supercritical flow entering the diffuser exceeds the flow rate at the exit and the plenum chamber is re-charged to the original condition. A distinction is drawn between this phenomenon and a high frequency wave-type resonance noticed at low mass flows and during an individual buzz cycle after the diffuser shock system has been expelled. For the large diffuser tested here, this high frequency oscillation compares well to the 8th closed-end organ pipe mode of the diffuser at low mass flows and to the 9th mode during the shock-expelled phase of the buzz cycle. It is shown that burning almost always ceases in the presence of buzz. When burning was maintained during buzz, it was found to have no qualitative effect on the buzz cycle.