Extra Attenuation Research Articles

Anomalies in Giant Quantum Attenuation of Sound Waves in Bismuth at High Magnetic Fields. II. Existence of Extra Attenuation Peaks

In the course of studying and angular dependence of the ultrasonic attenuation in bismuth at high magnetic fields and low temperatures, some extra attenuation peaks were observed in a limited range of magnetic field directions. These are not assigned to any known Landau levels, and interpreted as due to the existence of a slight warping of the electron Fermi surface. It is suggested that this warping is one cause of the anomaly in the temperature dependence described in the preceding paper.

Excess Ultrasonic Attenuation inAs2S3Glass after Electric Field Removal

Increased ultrasonic attenuation is found in ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ glass at 4.2 K after removal of a high electric field. The extra attenuation persists for a few minutes before disappearing, but can be reduced, or practically eliminated, by infrared radiation of moderate intensities. The effect is attributed to atomic relaxation accompanying electronic transitions in gap states where injected carriers have been trapped. The gap states may be valence-alternation pairs.

Regional dependence of very low‐frequency sound attenuation in the deep sound channel: Correlation with internal wave measurements

The most promising mechanism for the very‐low‐frequency sound attenuation observed in the deep sound channel is diffusive scattering by internal waves. Mellen et al. [J. Acoust. Soc. Am. 60, 1053–1035 (1976)] have obtained estimates for the extra attenuation using the Garrett‐Munk internal wave model and find consistency with the lower experimental values reported. Kibblewhite et al. [J. Acoust. Soc. Am. 63, 1169–77 (1978)] have shown a definite regional dependence in the Pacific. In this paper we compare regional oceanographic measurements with the Garrett‐Munk internal wave model and also correlate local acoustic measurements with estimates of the extra attenuation.

Experimental Study of Outdoor Propagation of Spherically Spreading Periodic Acoustic Waves of Finite Amplitude

The outdoor propagation of spherically spreading sound waves of finite amplitude was investigated. The main purpose of the experiments was to determine the extent to which the outdoor environment, mainly random inhomogeneity of the medium, affects finite amplitude propagation. Periodic sources with fundamental frequencies in the range 6 to 8 kHz and source levels SPLlm from 140 to 149 dB were used. The sources were an array of 7 to 10 horn drivers and a siren. The propagation path was vertical and parallel to an 85 m tower, whose elevator carried the traveling microphone. The general conclusions drawn from the experimental results were as follows. The inhomogeneities caused significant fluctuations in the instantaneous acoustic signal, but with sufficient time averaging of the measured harmonic levels, the results were comparable to results expected for propagation in a quiet medium. Propagation data for the fundamental of the siren approached within 1 dB of the weak shock saturation levels. Extra attenuation on the order of 8 dB was observed. The measurements generally confirmed the predictions of several theoretical models. The maximum propagation distance was 36 m. The narrowbeam arrays were much weaker sources. Nonlinear propagation distortion was produced, but the maximum value of extra attenuation measured was 1.5 dB. The maximum propagation distance was 76 m. The behavior of the asymetric waveforms received in one experiment qualitatively suggested that beam type diffraction effects were present. The role of diffraction of high intensity sound waves in radiation from a single horn was briefly investigated.

Outdoor propagation of spherically spreading acoustic waves of finite amplitude

The outdoor propagation of spherically spreading sound of finite amplitude was experimentally investigated. The main purpose of the experiment was to determine the extent to which the outdoor environment, mainly random inhomogeneity of the medium, affects finite amplitude propagation. Periodic sources with fundamental frequencies of 6–8 kHz and source levels SPL1m from 140 to 149 dB were used. The sources were an array of horn drivers and a siren, The propagation path was vertical and parallel to an 85‐m tower, whose elevator carried the traveling microphone. Measurements were made in late summer under a wide range of weather conditions. The inhomogeneities caused significant fluctuations in the instantaneous acoustic signal, but with sufficient time averaging of the measured harmonics levels, the results were comparable to results expected for propagation in a quiet medium. Propagation data for the fundamental of the siren approached within 1 dB of the weak shock saturation levels. Extra attenuation on the order of 8 dB was observed. The measurements generally confirmed the predictions of several theoretical models. [Supported by NASA, AFOSR, NOAA, and ONR.]

Dislocation damping and the ultrasonic attenuation in the normal and superconducting states of pure tin

Ultrasonic attenuation studies have been carried out both in the normal and superconducting states in pure (99.9999%) single crystals of tin containing various amounts of dislocations. At low pulse amplitude the normal state attenuation is effectively independent of the dislocation density. The dislocations have been found to introduce significant deviations in the superconducting state attenuations from the BCS values. The extra attenuation Δα s in the superconducting state due to dislocations has been found to show an inverse fourth power dependence on the temperature. Δα s has also been shown to exhibit resonance type of behaviour over the frequency range 3 to 50 MHz. This resonance frequency depends on the dislocation density with different power laws in different ranges of the dislocation density.

Dislocation resonance in superconducting tin

Ultrasonic attenuation in the superconducting state of strained samples of single crystals of tin shows a strong dependence on the dislocation density. The dislocation density is estimated with the help of a suitable etch pit method. The etch pits are square in shape and exhibit symmetric dislocation lines normal to the (100) planes. The number of etch pits reveals a direct dependence on the physical state of the specimen. The dislocation density estimated by this etch pit method can be related to the extra attenuation Δα s shows a resonant behaviour and the resonance frequency is dependent on the dislocation density. It is also observed that the functional relationship between the resonance frequency and the dislocation density is different in different regions of the dislocation density.

Characteristics of a doubly clad optical fiber with a low-index inner cladding

This paper investigates a novel type of optical fiber which is composed of three portions: the core, inner cladding, and outer cladding. It is assumed that the core has the largest refractive index of the three, and the outer cladding index is the next largest. When the index difference between the core and the inner cladding is much larger than that between the outer cladding and the inner, the new fiber has a stronger confinement property than a usual singly clad fiber in the single-mode region. It is also assumed that the inner-cladding thickness is larger than or comparable with the core radius. This confinement is the origin of the following three properties of potential importance in single-mode optical communication. 1) As compared with a singly clad fiber, the largest core area for single-mode operation is roughly twice. 2) The group delay arising from waveguide characteristics has an opposite sign against that of typical glass dispersion. (Singly clad fiber has a dispersion of the same sign.) 3) The field is much more tightly confined within the core as compared with a singly clad fiber. This minimizes extra attenuation due to absorption in the cladding.

Reverberation time measurements of the scattering of sound by turbulence

Measurements of the reverberation time of a small (29 m3) room have been made using the Schroeder impulse response method while the air in the room was agitated by a 0.65-m-diam, 14-bladed, 2.6-kW fan and compared with similar measurements made with the fan off. The decay times were found to be shorter when the fan was on and this decay time difference is ascribed to an effective extra attenuation α1, due to scattering of the sound field by the fan-generated turbulence. Only order-of-magnitude estimates of α, are possible; α1 ∼ 10−3 m−1 and is frequency-independent from 600 Hz to 5 kHz. The results agree to an order of magnitude with recent turbulence scattering theories. Some of the implications of these measurements for present theories and future experiments are discussed.

Theoretical evaluation of sound fields in circular or annular ducts with rigid or nonrigid radial splitters

In aircraft engines, extra attenuation can be obtained by installing lined radial or circumferential splitters or both. A study of the governing wave equation and its solution in the lined radial splitter environment is presented. It is shown that the general solution can be written in the terms of modified Bessel and Neumann functions of complex fractional orders and complex arguments. This is due to the fact that the circumferential wavenumber is in general complex, noninteger, and a function of the radial coordinate. In the analysis, the circumferential wave vector is evaluated first as a function of the radial coordinate and in terms of the angular separation between the splitters and the frequency parameter; this function is then used in the radial integration, yielding the mode shapes and the corresponding complex axial wavenumbers. The model attenuations are higher for two reasons: the increased surface area of absorptive materials and the much higher cutoff frequencies of the modes. This analysis can be extended to the evaluation of sound field between blades.

Flyover Noise Prediction from Static Data

The use of static ground test data to describe the acoustic characteristics of an aircraft flyover can result in a significant savings both in time and money, providing good correlation with actual flight test measurements is assured. This paper addresses itself to the elements associated with the calculation of EPNL values from static ground test data. Farfield noise level measurements were made over a range of engine thrust settings from idle to takeoff power of a JT3D-3B engine mounted on a test stand. Two key engine operation parameters were used to correlate the SPL values: low-pressure compressor rotor speed, which controls fan noise; and net thrust, which controls jet noise. The SPL spectrum, used to simulate a flyover, is generated by interpolating the measured data at the appropriate in-flight values of rotor speed and net thrust. The extrapolation to inflight condition also considers airplane parameters such as altitude, climb gradient, and number of engines, as well as Doppler shift effects, spherical divergence, atmospheric and extra ground attenuation, and ground reflection. Comparisons are made between EPNL values predicted from static ground test data and values obtained from flight tests. A tape will be played which demonstrates the difference between flyovers of a production B-707 airplane and a B-707 airplane equipped with quiet nacelles.

Stratospheric attenuation in the near ultraviolet

Comparison between measurements of atmospheric attenuation at 3800 Å, made during a rocket flight at 30° S in April 1964, and values computed for a dust-free atmosphere demonstrated the existence of extra attenuation which exceeded that due to Rayleigh air scattering at levels below 30 km. By using a model with a simple exponential decay of extra attenuation coefficient with height, the best fit to the observations was obtained with a scale height of 3.25 km and an attenuation coefficient at 28.9 km of 1.06 x 10 -3 km -1 . The approximate height range over which the model was valid was from 20 to 40 km. Additional measurements from the Ariel II satellite indicated the persistence of this attenuation from April to August 1964 between 30° S and 20° N with significantly less attenuation at 45° S. The attenuation was produced by the eruption at Bali in March 1963, and the scale height of the attenuation was consistent with an annual mean value for the vertical eddy diffusivity in the height range 20 to 35 km at 30° S of between 2000 and 6000 cm 2 /s.

Attenuation of galactic radio noise at 25 Mhz and 21.3 Mhz in the ionosphere over Ahmedabad during 1957-1964

Measurement of the intensity of galactic radio noise at Ahmedabad from 1957 to 1964 has shown that its attenuation has decreased steadily with the decline of solar activity. Calculation of the absorption in the D region and in the F region up to the F2 peak using N(h) profiles and effective collision frequencies of electrons has shown that a large part of the absorption of galactic radio noise takes place in the F region over Ahmedabad, both below and above the F2 peak. It is also found that the nighttime attenuation in low solar activity period (1964–1965) is much larger than can be explained on the basis of electron-ion collision processes alone. It is suggested that scattering of galactic radio noise by inhomogeneities in the F region is a source of extra attenuation during night in a low solar activity period, since it is known that the occurrence of spread F shows a considerable increase during night at Ahmedabad in low sunspot years. The absorption measurements also suggest that electron temperatures are appreciably higher than neutral gas temperatures in the early morning hours and that the excess temperature of electrons is higher in low sunspot years than in high sunspot years.

Second-Sound Attenuation Associated with Hot Ions in Liquid He II

An extra attenuation of a second-sound wave associated with the motion of hot ions in liquid helium II has been investigated in the range of temperatures 0.87-1.40\ifmmode^\circ\else\textdegree\fi{}K. Such an attenuation manifests itself as a threshold phenomenon at a well-defined electric field ${E}_{\ensuremath{\alpha}}$, which is found to coincide with the critical field ${E}_{C}$ at which a "persistence" of the drift velocity $〈{V}_{D}〉$ begins. The existence of a threshold indicates that there is a quite different hydrodynamical regime beyond the threshold. The threshold field has the temperature dependence ${E}_{\ensuremath{\alpha}}=A{e}^{\ensuremath{-}\frac{{\ensuremath{\Delta}}_{S}}{T}}$, with ${\ensuremath{\Delta}}_{S}=7.69\ifmmode\pm\else\textpm\fi{}0.13$\ifmmode^\circ\else\textdegree\fi{}K. ${\ensuremath{\Delta}}_{S}$ varies with pressure analogously to the $\ensuremath{\Delta}$ of rotons measured by neutron diffraction. It is found experimentally that the extra attenuation is an effect strictly associated with the ion. The attenuation is not interpretable in terms of the single-vortex-ring models so far existing. Comparisons are made with analogous experiments where the interaction between second-sound waves and vorticity is investigated.

Thermoviscous Attenuation of Plane, Periodic, Finite-Amplitude Sound Waves

The propagation of a plane progressive wave of finite amplitude in a thermoviscous fluid is considered. The wave is taken to be purely sinusoidal in shape at its source. The approach is through Burgers equation, which is a very good approximation of the exact equations of fluid motion when effects of nonlinearity and dissipation are relatively small but definitely not negligible. A complicated but exact solution of Burgers' equation is analyzed. The nature of the solution is found to depend strongly on a parameter Γ, which represents the importance of nonlinearity relative to dissipation. Nonlinear effects prove to be significant when Γ>1, a finding in agreement with the criterion proposed by Gol'dberg (Akust. Zh. 2, 325 (1956) [English transl.: Soviet Phys.—Acoust. 2, 346(1956)]) concerning the appearance of shock waves. When Γ>>1, simple asymptotic representations of the exact solution in terms of Fourier series may be obtained. One of these corresponds to Fay's solution [J. Acoust. Soc. Am. 3, 222 (1931)]. An equivalent “time-domain” representation shows clearly the sawtooth behavior of the wave. The sawtooth region is found to extend approximately to the point x=O.6/α, where α is the dimensional small-signal attenuation coefficient. Curves of the extra attenuation suffered by the fundamental as a result of nonlinear effects are given for values of Γ in the range 1–100 000. If Γ is large, the extra attenuation in decibels approaches the asymptotic value −12+20 log10Γ as the distance from the source becomes large. A value 1 dB below the asymptote is attained at a distance x=1/α. Application to waves in argon and air is discussed. It is found that nonlinear effects may be employed to increase the efficiency of long-range sound transmission. A possible application to low-frequency sound in the ocean is also discussed.

Sound Propagation in Layered Media with a Rough Interface

When sound propagates in a fluid layer of suitable thickness, overlying a much thicker layer of greater density and sound velocity, the normal-mode propagation expected ideally [C. L. Pekeris, “Theory of propagation of explosive sound in the sea,” in Memoir 27 of the Geological Society of America (1948)] is a result of total internal reflection. Anything preventing such reflection, such as a rough interface, should cause attenuation of the sound. A qualitative explanation is that some diffusely reflected sound strikes the interface at less than critical angles and escapes. Such attenuations have been studied with the apparatus of the accompanying paper by Ryan, Eby, and Williams. Small solid spheres were spread on the rubber-water interface; the extra attenuation caused by them was measured in the water layer as a function of frequency, water depth, sphere size, and area density of spheres. This attenuation is roughly proportional to area density of spheres, and to their acoustic scattering cross sections, and varies additionally with sphere size, water depth, and frequency, Frequencies were 100–250 kc; sphere radii 0.07–0.23 cm; water depths 0.5–2.0 cm. Glass, lead, and steel spheres behaved alike. (Supported in part by the Office of Naval Research.)

Second Sound Attenuation in Rotating Helium II

An experiment is described wherein liquid helium II is rotated in an annulus between a metal cylinder which rotates at constant speed and a concentric cylindrical cavity at rest. Second sound pulses, propagated parallel to the cylindrical axis and through a substantially uniform velocity field, are found to suffer an extra attenuation due to the motion of the fluid. If we define $\ensuremath{\sigma}$ as the ratio of the received thermal pulse heights (fluid in motion versus fluid at rest), then for angular velocities of the cylinder $\ensuremath{\omega}$ and second sound path lengths $l$ it appears that $\ensuremath{\sigma}={e}^{\ensuremath{-}\ensuremath{\beta}\ensuremath{\omega}l}$. The coefficient $\ensuremath{\beta}$ decreases somewhat with rising temperature in the range from about 1.2 to 2.0\ifmmode^\circ\else\textdegree\fi{}K, having a value of (8\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ ${\mathrm{rev}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ sec at 1.39\ifmmode^\circ\else\textdegree\fi{}K. The effect is discussed in the light of present knowledge concerning the nature of helium II.