Excess Attenuation Research Articles

Grazing incidence propagation over a soft rough surface

Attenborough has recently proposed [J. Acoust. Soc. Am. 98, 1717–1722 (1995)] that the effects of surface roughness on an otherwise flat surface of finite acoustic admittance can be modeled as a smooth flat surface with an effective acoustic admittance β*. This effective admittance is determined by the normalized admittance βs and wave number ks of the original smooth surface, as well as a surface roughness parameter σν. Attenborough tested his theory by conducting excess attenuation measurements over plywood and Styrofoam surfaces with two-dimensional roughness elements. Specifically, he investigated the interference pattern between the direct and reflected signals over both smooth and rough surfaces. Experiments are conducted here with three-dimensional roughness elements at grazing incidence. Such a configuration is typical for outdoor sound propagation problems. The results presented here generally confirm Attenborough’s model and open up the analogy to a broader class of problems. It was also found that placing the effective admittance plane at the top of the roughness elements rather than at the bottom yielded slightly better results for the higher acoustic frequencies investigated.

Hands-free circuitry providing audio signal adjustment based on speaker volume level

A hands-free circuitry in which, in order to prevent howling caused by acoustic coupling between a speaker and a microphone, a loss is inserted in a transmit audio path in a receive mode while a loss is inserted in a receive voice path in a transmit mode and values of the losses are controlled according to the volume set value of the speaker. Since the quantities of the losses are minimized when a howling margin is sufficient, simplex communication can be prevented that a user cannot listen to party's receive voice signal due to excessive attenuation.

Sound propagation from a dipole source near an impedance plane

A closed-form analytic solution has been derived for an arbitrarily oriented dipole placed above an impedance plane. Asymptotic approximations for the total sound field can be written in a form similar to the classical formula for a monopole. Analytic approximations derived for horizontal and vertical dipoles give predictions that agree well with those obtained from a program of the fast field type. The asymptotic results have been confirmed also by laboratory measurements. Compared with numerical methods, the analytic approximations offer the combined advantages of easier physical interpretation and a much reduced computational time. It is found that the variation of the sound-pressure level from a vertical dipole above an impedance plane differs significantly from that due to a monopole above this plane, particularly as a result of differences in the ground wave component. However, the excess attenuation due to a horizontal dipole is found to be rather similar to that for a monopole except for source and receiver heights that are comparable with range.

Outdoor sound propagation measurements using an MLS technique

The maximum length sequence (MLS) technique has been applied to the investigation of outdoor sound propagation. MLS leads to a significant increase in the signal-to-noise ratio compared to conventional, stationary excitation. In addition, it offers the advantage that the travel time of the signal is known for each of the pulse sequences. This aids in correlating the measured excess attenuation to the momentary wind speed. First results have been obtained at propagation distances of 50, 100 and 200 m for flat, grassy terrain. For a neutral atmosphere, the experimental results are in excellent agreement with theoretical expectations. In the presence of a positive or negative sound gradient, comparison is difficult because the appropriate value of the sound gradient is unknown.

Excess attenuation of leaky Lamb waves due to viscous fluid loading

In two recent papers [J. Acoust. Soc. Am. 97, 3191–3193 (1995) and 98, 1057–1064 (1995)], Zhu and Wu presented an analytical technique to assess the effect of viscous fluid loading on the propagation properties of Rayleigh and Lamb waves in fluid-loaded solids. They modeled the viscous fluid as a hypothetical isotropic solid having rigidity c55=−iωη, where η denotes the viscosity of the fluid and ω is the angular frequency. In this way, the vorticity mode associated with the viscosity of the fluid is formally described as the shear-mode in the fictitious solid. In this paper this technique is further developed by removing certain inconsistencies that unnecessarily reduce the accuracy and the range of validity of Zhu and Wu’s results. By properly accounting for viscous effects on the bulk compressional wave in the fluid and applying a rigorous treatment of the field equations and boundary conditions, the exact dispersion equations that are not limited to low frequencies and viscosities are derived. Examples of these results are presented to illustrate the effect of fluid viscosity on the lowest-order symmetric and antisymmetric Lamb modes. One interesting feature revealed by these calculations is the presence of a sharp minimum in the viscosity induced attenuation of the lowest-order symmetric mode of thin plates either immersed in or coated with a viscous fluid. This minimum occurs at a particular frequency where the otherwise elliptical polarization of the surface vibration becomes linearly polarized in the normal direction.

Acoustic wave propagation in two-phase viscoelastic fluids: The case of polymer emulsions

Wave propagation in two-phase viscoelastic fluid media is studied. A theory based on previous ones for liquid–liquid or liquid–solid mixtures is developed. Acoustical parameters, i.e., velocity of propagation and attenuation, are related to the morphological properties (concentration and size of inclusions) through the thermal and viscoelastic effects. This general theory includes the Newtonian and elastic behaviors as limiting cases. In the case of molten polymer blends, viscoelastic effects may be dominant, unlike most of the Newtonian emulsions studied previously, where thermal effects usually prevail. Moreover, by taking into account the viscoelastic character of the fluids, important changes are obtained not only for the absorption due to the individual components, but also for the excess attenuation in the emulsion.

Ultrasonic wave propagation in reticulated foams saturated by different gases: High frequency limit of the classical models

Transmission experiments are performed on high porosity reticulated polyurethane foams saturated by different gases at ultrasonic frequencies up to 800 kHz. An excess attenuation is observed at high frequencies, when the wavelength is not sufficiently large compared to the lateral dimensions of the fibers. At lower frequencies, these experiments lead by using classical models of equivalent fluids, to a fast and reliable method for determining the characteristic length Λ.

On counting ocean bubbles acoustically

The earliest attempts to determine ocean bubble densities were optical, because everyone knows that ‘‘seeing is believing.’’ Unfortunately, bubbles smaller than 60-μm radius were not always recognized in photographs of the dirty, detritus-contaminated, bubbles. And bubbles greater than 150-μm radius were so infrequently observed that the average values were quite unreliable. Acoustical ‘‘counting’’ by inversion of the linear processes of backscatter, excess attenuation, and dispersion has great advantages over optical measurements. Primarily this is because the acoustic cross sections of a resonating bubble are orders of magnitude greater than optical values for the same radius. The ability to gather data rapidly, the ease of obtaining correlations with other oceanographic variables, and the simple inverse relation between sound frequency and resonant bubble radius have given acoustics great advantages over optical bubble-counting at sea. For 30 years, at several locations, under various ocean conditions, active in-situ pulse echo systems and resonators using broadband transducers have yielded a rich harvest of well-resolved data for bubble radii from 15 to 300 μm. More recently, by passively listening to newly created damped oscillating bubbles, acousticians have provided meteorologists and climatologists with accurate fundamental data of ocean surface gas transfer and hydrosol production due to breaking waves and rainfall. Techniques and results of active and passive linear acoustical monitoring are reviewed. [Research supported by ONR.]

Scale-model investigation of highway noise barriers: Walls, earth berms, and earth berm/wall combinations

Scale-model roadside barriers were studied, using an air-jet to recreate a line source of traffic. Scale-model materials were selected by measuring their excess attenuation, at scale factors of 1:20, 25, 31.5, 40, and 50. For each material, the residuals between a measurement and a theoretical best-fit curves were calculated for each cell of a 2-D array whose axes were scale factor versus effective flow resistivity. An optimal scale factor of 1:31.5 was selected, in conjunction with the selection of three model materials to simulate earth berms and soft ground (150–300 cgs rayl), and vertical walls or roadways (20 000 cgs rayl). Roadways were simulated by varnished particle board, walls by dense polystyrene, and both earth berms and soft ground were simulated using extruded polystyrene. The insertion losses, with and without an A-weighted traffic-noise spectrum, were determined for 38 different noise-barrier configurations. Walls outperformed earth berms by 2 dBA; for earth berms, shallower slopes degraded insertion losses, but when a wall was present the opposite trend was observed. One of the most relevant references is [Hutchins et al., J. Acoust. Soc. Am. 75, 1817–1826 (1984)]. [Work supported by the B. C. Ministry of Transportation and Highways.]

Operational Monitoring of Rainfall over the Arno River Basin Using Dual-Polarized Radar and Rain Gauges

Abstract Reflectivity (ZH) and differential reflectivity (ZDR) measurements collected by Polar 55C over the Arno River basin in Italy are presented. The applicability of dual-polarization (ZDR)-based rainfall algorithms at C band in an operational setting is studied in conjunction with a network of rain gauges. Conventional pointwise comparison of radar and rain gauge estimates of rainfall, as well as statistical comparison of dual-polarization radar and rain gauge data via probability matching procedure, are presented. Error structure of reflectivity rainfall Z-R relation, as well as ZDR-based algorithms, is evaluated as a function of spatial and temporal averaging. Pointwise comparison, as well as statistical evaluation based on cumulative distribution function (CDF) matching, are used to show that in an operational environment with excessive ground-clutter contamination and attenuation problems the dual-polarization-based rainfall algorithm performs better than any arbitrary Z-R relation. In addition, ...

Some new observations on pulse sequence dependent diffusion related edge enhancement in MR microscopy.

Self-diffusion of nuclear spins has been suggested to cause edge enhancement in images especially on a microscopic scale. According to previously published work, theory suggests that edge enhancement is caused by motional narrowing due to the boundaries and spin self-diffusion during the data acquisition period. More careful examination reveals that edge enhancement due to motional narrowing develops only under a few specific conditions. This lack of generality of motional narrowing theory, as well as experimental observations, indicate that edge enhancement due to effects other than motional narrowing alone can exist. It is found that edge enhancement depends greatly on the data acquisition mode; therefore, the images obtained are different depending on the pulse sequence employed. For example, excessive attenuation of DC components due to diffusion can result in edge enhancement in the spin echo signal. However, in the case of FID-like signals, DC components are preserved while positive high frequency parts are attenuated, thereby degrading resolution. The new phenomenon observed has been termed selective spectral suppression since the observed edge enhancement results from the selective attenuation of certain frequency components in the nuclear signals due to diffusion-dependent signal attenuation for a given pulse sequence.

A Model for Determining the Excess Attenuation in Upwind Zones and a Sample of its Implementation at Atatürk Airport

In this research, the effects of wind and temperature gradients on the propagation of sound in the presence of a ground, have been investigated and the formation of shadow and light zones has been studied, A model that may be used to calculate the excess attenuation values in upwind zones using the proposals of Wiener and Keast is presented and the application of this model is described. Following the implementation of the model, the up-wind and down-wind zones have been determined at the Atatürk Airport, and the change of the these zones depending upon the time of the day has been displayed.

Absorption of sound by noise in one dimension.

Experimental results are presented for the excess attenuation of mono‐ frequency waves in the presence of high‐intensity noise in one dimension. The theory by Rudenko and Chirkin predicts a Gaussian attenuation of a weak signal in the presence of shockless noise, thus showing that in one dimension translational invariance has been broken. The theory is modified to incorporate wall losses providing, only then, excellent agreement with experiment. The agreement is shown as the frequency, noise level, and distance from the source are varied. In addition, the spectral intensity of the high‐frequency tail of fully developed shockless noise is observed to be an f−3 power law in the frequency f, in accord with theory. This power law is a consequence of the far off equilibrium nature of the system. [Work supported by ONR.]

Sound propagation over impedance discontinuities with the parabolic approximation

Long‐range, low‐frequency sound propagation over varying terrain conditions is an important problem with both civilian and military applications. This work considers the modeling of cw acoustic signals as they propagate over variable‐impedance topography. The NASA implicit finite‐difference (NIFD) implementation of the parabolic approximation sound propagation model that incorporates variable‐impedance ground surfaces was used. The accuracy of this numerical model is demonstrated with a pair of test problems. An application of the results is used to predict excess attenuation of pure‐tone signals whose propagation path includes portions of a flat lake surface.

Atmospheric emission measurements of attenuation by microwave radiometer at 19.4 GHz

Attenuation measurements on an Earth space path are presented using a passive microwave radiometer operating at 19.4 GHz in the emission mode. Attenuation measured under clear weather showed variation between 0.2 to 1.1 dB, whereas for cloud conditions attenuation as high as 1.0 dB have been recorded. Attenuation measurements for rain events have been correlated with rainfall rate using a fast-response 10 seconds opto-electronic rain gauge. The values of attenuation versus rainfall rate varied between A (dB)=0.01+0.18 R(mm/h) at the minimum and A (dB)=0.01+0.25 R(mm/h) at the maximum, showing considerable variability in the values of attenuation from year to year. The attenuation statistics for different seasons have also been computed and they show considerable changes from season to season-the largest attenuation in excess of 10 dB recorded in July-August-September, whereas minimum attenuation in excess of 2 dB recorded in December-March for nearly two years of data. Comparison of measurements made over New Delhi with those reported elsewhere show that for 0.02% of time attenuation values lie between those of Slough, England, and Crawford Hill, NJ. The concept of effective path length has been discussed based on the relationship between effective path length and the rain rate.

Excess first sound attenuation in metastable superfluid3He−4He mixtures

The first sound velocity and attenuation in metastable long-lived superfluid3He−4He mixtures are measured in a temperature range of 30 to 200 mK. The metastable state is reached by increasing continuously the3He concentration during the experiment due to change of the osmotic pressure. The concentration of the mixture is determined by simultaneous measuring the dielectric constant. The first sound is shown to undergo an excess attenuation in the metastable region while the sound velocity does not display any anomalous behaviour in this region. The results for stable mixtures are in good agreement with the kinetic theory but there is no quantitative explanation of the excess attenuation in the metastable solutions.

Critical mechanical behavior in the fluid/solid transition of suspensions

Velocity and attenuation measurements of compressional waves at 3 and 5 MHz, as a function of solid volume concentration, are reported for different suspensions. The suspensions were made of 1 μm size particles of kaolinite or glass beads in water or light oil. At a volume concentration of 40%, a sharp peak in attenuation accompanied by a change in velocity is observed. The volume concentration at which this peak occurs is independent of frequency and particle geometry. The observed critical change in attenuation around 40% is consistent with an excess attenuation induced by the fluid-shearing processes between neighboring particles, and corresponds to the jamming point concentration predicted by particle packing simulations.

Blast sound propagation over hilly land

A blast sound propagates over a long range, because of its large sound power of sources and low-frequency components. Blasting operations are often conducted at a hilly side and as a result, sometimes the sound of the blast influences the life environment beyond the hilly undulations. At both hilly sites with heights of up to several tens of meters and flat lands, excess attenuation of blast sound was measured at points of several hundred meters away. Frequency spectrums of excess attenuation beyond the hilly side show that (1) up to around 100 Hz, excess attenuation increases in proportion with frequency, and for more than 100 Hz, it shows mostly constant values of around 40 dB; (2) shapes of frequency spectrum of the excess attenuation show similar pattern with the ones of flat land; and (3) shielding effects of hills mainly appear at the frequency range around 100–250 Hz. From these results, the authors discuss the possibility of sound propagation modeling by extending usual models for flat land.

Measurements of passive and active microbubbles in the sea

Around 1960 an oceanographer at the Navy Electronics Laboratory dared to question physics laboratory measurements when he wrote an internal memo titled ‘‘Do Invisible Microbubbles Exist at Sea?.’’ Indeed they do! From 1964 to 1974 a series of M.S. students at the Naval Postgraduate School published the first microbubble density distributions at sea. These were measured by several in-situ ocean techniques including: change of transducer impedance; photography; excess attenuation, backscatter and sound-speed dispersion in a small pulse-echo system: Fourier transform of a sawtooth signal propagating between two separated hydrophones. An ‘‘unbelievable’’ immense number of coastal bubbles of radius 15 to 300 μm were found. The 1990s have seen a renaissance in the field. The spatial and temporal variation of bubble numbers have been studied, and acoustical oceanographers now use bubbles as tracers to determine ocean processes near the ocean surface. Sea state noise and rain noise have both been definitively ascribed to the radiation from huge numbers of infant microbubbles. Indeed, the ‘‘noises’’ have now become ‘‘signals.’’ The underwater sound of breaking waves has been inverted to yield the spectrum of ocean wave height and recently the underwater sound of rainfall has been inverted to reveal the real-time rainfall drop size distribution. The distinctive spectrum of underwater sound during rainfall even allows one to describe the clouds from which the rain has fallen. [Work supported by ONR.]

Further investigations on the propagation of sound over flat rough finite impedance surfaces

The propagation of sound over rough surfaces of finite impedance is investigated. Initial experiments have illustrated the excess attenuation caused by roughness, that is, attenuation above and beyond that caused by geometrical spreading alone. These initial experiments were conducted indoors in a well-controlled setting for sound at grazing incidence. These measurements also show good agreement with Attenborough’s proposed model [J. Acoust. Soc. Am. 95(5), 2838(A) (1994)] that incorporates the effects of roughness into an effective admittance. The purpose of this present work is to find out if these trends persist in the propagation of sound over rough surfaces in a real-world setting. Sound signals will be measured above both rough and smooth agricultural surfaces with varying roughness profiles. Additional measurements will be made to determine the impedance and wave number of the surface as well as the roughness profile in order to examine Attenborough’s model in more detail. [Work supported by USDA.]