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Journal of the Acoustical Society of America

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Jun 1978

Volume 63, Issue 6, pp. 1677-1945

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Contribution of ultrasonic measurements to the study of liquid crystals. II. Smectics, cholesterics, and mixtures

G. G. Natale

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1677-1693 (1978); (17 pages)

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The study of ultrasonic propagation in liquid crystals, greatly intensified in recent years, has provided a useful contribution to the knowledge we currently have about these systems. A previous paper (Part I of a two‐part series) contains a review and analysis of the ultrasonic work done on the nematic mesophase. In the present paper, most ultrasonic experimental investigations performed to date in smectic and cholesteric compounds, and some liquid‐crystalline mixtures, are presented in a systematic manner; the results are discussed and compared, whenever possible, to the predictions of several theories (outlined in the paper) for the various mesophases and the many possible phase transitions. In the course of this analysis, several areas which need further ultrasonic studies are identified; these areas are summarized in the concluding section of the paper.
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43.10.Ln Surveys and tutorial papers relating to acoustics research; tutorial papers on applied acoustics
43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions
61.30.-v Liquid crystals

Acoustic scattering from silicone rubber cylinders and spheres

C. M. Davis, L. R. Dragonette, and L. Flax

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1694-1698 (1978); (5 pages) | Cited 1 time

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Scattering of acoustic waves from silicone rubber cylinders and spheres is considered. The effect of shear is found to be negligible. Special consideration is given to cases where the specific impedance of the rubber equals that of water.
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43.20.Fn Scattering of acoustic waves

Theory of resonant scattering from spherical cavities in elastic and viscoelastic media

G. C. Gaunaurd and H. Überall

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1699-1712 (1978); (14 pages) | Cited 8 times

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Plane incident p‐waves which propagate through continuous (possibly absorptive) media are scattered by a fluid‐filled spherical cavity contained in it. Using an approach familiar in nuclear scattering theory but novel to acoustics and elastodynamics, it is possible to express the scattering amplitudes (or the partial waves contained in them) for the scattered p‐and s‐waves in a form which clearly exhibits their dependence on two interacting contributions, one being the broad background of an ideally soft cavity, and the other the superimposed narrow spikes due to resonances excited in the cavity fluid. The cavity appears as a perfectly soft obstacle to the incident waves at all frequencies except in the near vicinity of the cavity eigenfrequencies where there is wave penetration into the filler fluid. When this happens, the interference of this wave with the ’’potential scattering’’ of the background is seen to cause the fluctuating character of the amplitudes (or cross section). We have numerically computed these isolated contributions for a variety of material combinations. The isolated resonances are traced (as Regge pole trajectories) as they reappear at the higher frequencies in subsequent partial waves. Due to its application in the analysis of acoustic‐coating performance we have further studied the case of air‐filled cavities in lossy rubber. The following findings have emerged: (1) The resonances are comparatively narrow, (2) their locations are apparently independent of the amount of absorption present, (3) absorption only affects the background, (4) shear absorption F1 only affects the mode‐converted amplitude fps, and (5) the dilatational absorption, controlled by the parameter F, only influences the nonmode converted amplitude fpp.
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43.20.Fn Scattering of acoustic waves
43.20.Ks Standing waves, resonance, normal modes
43.35.Mr Acoustics of viscoelastic materials

Merged seawater sound‐speed equations

Jack R. Lovett

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1713-1718 (1978); (6 pages) | Cited 2 times

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Recent checks of published seawater sound‐speed measurements indicate that the Del Grosso–Mader data provide the best relationships with temperature and salinity, while the Wilson data have the preferred pressure dependence. Possible error sources in the original measurements are discussed. Anderson’s pressure‐dependence, based on the Wilson data, is applied to Del Grosso’s equation and to the Del Grosso‐Mader data, resulting in three new equations. The third and simplest equation seems preferable for most applications. The new equations agree well with Anderson in comparison with acoustically determined convergence‐zone ranges at sea.
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43.30.Cq Ray propagation of sound in water
92.10.Vz Underwater sound

Transient scattering by arbitrary axisymmetric surfaces

H. C. Neilson, Y. P. Lu, and Y. F. Wang

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1719-1726 (1978); (8 pages)

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A numerical method using Kirchhoff’s retarded potential integral equation to determine the time‐dependent pressure distribution on the surface of an arbitrary smooth, convex, and axisymmetric rigid scattering body due to the impact of a plane acoustic wave having discontinuous wave front incident from any direction is developed and presented. This numerical technique is developed for general purposes, and it is applicable not only to single structures of same shape but also to composite structures of different shapes such as circular cylindrical, spherical, conical, and toroidal segments as well as flat plate. Typical numerical solutions for a hemisphere‐capped cylinder impinged upon by a unit‐step‐pressure pulse from three different directions, i.e., axial incidence, side‐on incidence, or oblique incidence of 30° off the longitudinal axis of the cylindrical section are presented. Numerical solutions of transient scattering of acoustic waves of normal incidence by a sphere are also given, and results agree very well with the analytical solutions obtained by other investigators.
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43.20.Fn Scattering of acoustic waves
43.20.Bi Mathematical theory of wave propagation

Numerical calculation of the intensity distribution in sound channels using coherence theory

Alan M. Whitman and Mark J. Beran

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1727-1732 (1978); (6 pages)

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We have demonstrated that coherence theory can be applied to the practical computation problem of the propagation of a beam in an infinite half‐space with stratified but arbitrarily varying speed of sound variations. The intensity distributions found in this manner are the same as those of an equivalent point source in regions removed from the caustics of the geometrical ray family, but the intensity formula is continuous everywhere in the field and may be used in the vicinity of caustics. The computer time and storage requirements are equivalent to those of a ray program.
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43.20.Mv Waveguides, wave propagation in tubes and ducts
43.20.Bi Mathematical theory of wave propagation

Acoustic radiation pressure on an absorbing sphere

Takahi Hasegawa and Yumiko Watanabe

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1733-1737 (1978); (5 pages) | Cited 5 times

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The theory of the acoustic radiation pressure on an elastic sphere, placed freely in an inviscid fluid in a field of plane progressive sound waves, is modified to include the effect of a hysteresis type of absorption of shear and compressional waves in the solid material of the sphere. This type of absorption is typical of some types of polymer materials. Calculated results are presented for polymethylmethacrylate lucite and polyethylene spheres in water for ka values between 0 and 24. The results are of interest in view of the use of radiation pressure measurements to determine sound intensity.
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43.25.Qp Radiation pressure
43.20.Fn Scattering of acoustic waves

Theory of nonlinear electroacoustics of dielectric, piezoelectric, and pyroelectric crystals

D. F. Nelson

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1738-1748 (1978); (11 pages) | Cited 19 times

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Based on a fully electrodynamic Lagrangian theory of elastic dielectrics, a completely deductive derivation of the dynamical equations and constitutive relations for elastic, electric, and electroelastic phenomena is presented. The equations apply to crystals of arbitrary symmetry, structural complexity, and nonlinearity. These include pyroelectrics as well as dielectrics and piezoelectrics. Emphasis is placed on the lowest order nonlinearities, that is, ones depending either bilinearly or quadratically on the elastic and electric variables. A relation between the electrostriction tensor and the low‐frequency limit of the elasto‐optic tensor, different from any previous relation in the literature, is derived and discussed.
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43.25.Cb Macrosonic propagation, finite amplitude sound; shock waves
43.20.Bi Mathematical theory of wave propagation
43.40.At Experimental and theoretical studies of vibrating systems

Distortion of the sonic‐boom pressure signature by high‐speed jets

Sydney‐Lynne V. Hall

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1749-1752 (1978); (4 pages)

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Mach‐2.5 (0.30 caliber) projectiles were used to simulate supersonic aircraft; the acoustic impedance mismatch was provided by either of two rectangular nozzles having exit Mach numbers of 1.00 and 1.82 when delivering correctly expanded helium jets. These nozzles were operated from 40%–150% expansion at a fixed position relative to the presure transducer when the projectile miss distance was 36 or 108 body diameters from the transducer. Schlieren optical photographs demonstrated that the bow shock was always folded or ’’wrinkled’’ after passing through either of the jets. As exit Mach number decreased, the size of the shock‐front folds, as well as the divergent jet spread angle, increase and the bow shock becomes increasingly bent forward by the jet. Pressure signatures measured with a jet operating started 40–80 μs earlier than those measured without a jet. All pressure signatures obtained with a jet operating exhibited similar varieties of waveforms as F‐104, B‐58, XB‐70, SR‐71, and Concorde 002 flight test results. The wide variations in overpressure, rise time, duration, and wave shape are attributed to the velocity and density fluctuations of the turbulent jets.
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43.28.Mw Shock and blast waves, sonic boom
43.50.Nm Aerodynamic and jet noise

Multitube turbojet noise‐suppression studies using cross‐correlation techniques

Dennis R. Regan and William C. Meecham

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1753-1767 (1978); (15 pages)

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The noise‐producing region of a suppressed turbojet exhaust is studied by cross‐correlating static pressure fluctuations within the exhaust with farfield sound for Mach numbers up to Mj=0.99, using a 31‐tube nozzle having an area ratio Ar=3.1. Measurements made with an unsuppressed turbojet exhaust having an equivalent exit area and operating under effectively equal thrust levels serve as the experimental control for this work. Static pressure‐level measurements, made with a calibrated high‐temperature acoustically damped probe tube, show that noise suppression by multitube nozzles results from reduced turbulence levels. The maximum fluctuating static‐pressure level in the unsuppressed turbojet exhaust is typically 5–6 dB higher than static‐pressure levels in the suppressed exhaust under conditions of effectively equal static thrust. This suggests that the turbulence intensity in the multitube suppressor flow is reduced in excess of 20% compared with the unsuppressed jet exhaust. Static pressure fluctuation spectrums in the suppressor flow are peaked at the eddy passage frequency and a double spectral peak is found in the region where the suppressor jets coalesce. Estimates of the eddy‐scale‐length to jet‐diameter ratio made from measurements of local jet velocity and eddy passage frequency indicate that a reduction in eddy size occurs when neighboring suppressor tube flows are brought into close proximity to one another. Maximum normalized cross‐correlation coefficients in the unsuppressed turbojet exhaust of approximately cPF?0.2 are found for Mj=0.99 at an angle of 30° from the jet axis. Normalized cross‐correlation coefficients are significantly lower in the interior of the multitube suppressor flow. Measurements of the partially normalized cross‐correlation function show that sound radiated from within the hot, high‐speed suppressor flow is refracted by the flow from neighboring jets. This work demonstrates that the high‐frequency sound radiated directly to the farfield by a multitube suppressor nozzle is primarily from the exterior tubes and that the overall suppression of noise can be attributed to a reduction in turbulence intensity.
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43.28.Ra Generation of sound by fluid flow, aerodynamic sound and turbulence
43.50.Nm Aerodynamic and jet noise

Diffraction by a screen above an impedance boundary

Sven‐Ingvar Thomasson

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1768-1781 (1978); (14 pages) | Cited 3 times

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This paper is aimed at solving the problem of diffraction by a screen above a locally reacting, infinitely large plane. A general solution is given in terms of an integral equation on the diffracting object. An explicit solution is obtained in the sense of physical optics. Comparisons are made with other solutions by Lindblad and Jonasson. The main disadvantage of these solutions is that their validity is unknown; for sufficiently low screens they are not applicable. The present solution avoids these problems and also extends the knowledge to screens of finite length. To improve the solution numerically, a new method of evaluating integrals of two‐dimensional Fourier transforms is discussed and applied. This technique may also be used to evaluate the radiated field from a vibrating surface in a plane infinite baffle. In an experimental section, a close agreement is found between theory and experiments in model scale, indoors as well as outdoors, which also indicates that the method of measuring the ground impedance is useful. The influence of various parameters on the insertion loss is discussed in connection with the model experiments.
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43.28.Fp Outdoor sound propagation through a stationary atmosphere, meteorological factors
43.20.Fn Scattering of acoustic waves
43.50.Vt Topographical and meteorological factors in noise propagation

Use of the complex exponential expansion as a signal representation for underwater acoustic calibration

Louis G. Beatty, James D. George, and A. Zed Robinson

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1782-1794 (1978); (13 pages)

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A modified Prony method is developed to provide a complex exponential signal representation for use with underwater acoustic calibration waveforms. The approach is tested using simulated waveforms, and the simulation results are validated with limited experiments. Results show the complex exponential is an ideal signal representation for underwater acoustic calibration because of its remarkable ability to extrapolate calibration waveforms beyond the actural observation period. Results are shown of real reciprocity calibration experiments that are valid down to 25 Hz where the calibration waveforms are generated from a 5‐ms current ramp and where a 5‐ms observation period is used. The period of the low‐frequency limit is eight times the observation period used in the calibration.
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43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration

Speed of sound in NaCl, MgCl2, Na2SO4, and MgSO4 aqueous solutions as functions of concentration, temperature, and pressure

Chen‐Tung Chen, Lee‐Sea Chen, and Frank J. Millero

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1795-1800 (1978); (6 pages)

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The speeds of sound in aqueous solutions of NaCl, MgCl2, Na2SO4, and MgSO4 have been measured relative to pure water from 0 to 1 molal ionic strength, 0° to 55 ° C, and 0 to 1000 bar applied pressure. The 1‐atm results have been fitted to polynomial equations of concentration and temperature with standard deviations of about 0.1 m/s, U0−UW0=am+bm3/2+cm2 where U0 and UW0 are the 1‐atm sound speeds in solution and in pure water, respectively; m is molality; and a,b, and c are temperature‐dependent parameters. The effect of pressure on the relative speeds of sound has been fitted to equations of the form (UP−UWP)−(U0−UW0) =dm+em3/2+fm2, where UP and UWP are the sound speeds of the solution and water at applied pressure P and d,e, and f are temperature‐ and pressure‐dependent parameters. The standard deviations of the least‐squares fits are within 0.12 m/s for all the salts.
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43.30.Cq Ray propagation of sound in water
43.58.Dj Sound velocity
43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions

Observations of the phase and amplitude of individual Fermat paths in a multipath environment

Terry E. Ewart, John E. Ehrenberg, and Stephen A. Reynolds

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1801-1808 (1978); (8 pages) | Cited 1 time

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FM slide and pulsed‐tone signals at 2, 4, 8, and 16 kHz were transmitted from a source to a receiver at the same depth 1100 m distant. Three wholly refracted paths were observed in the return signals, and a discussion of the six‐hour time series of the arrival times and amplitudes for each path is presented. Evidence is given of acoustic frequency‐dependent scattering, and scattering from oceanic fine structure is suggested as the most likely physical mechanism to explain the observations.
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43.30.Bp Normal mode propagation of sound in water
43.30.Cq Ray propagation of sound in water
43.20.Dk Ray acoustics

Joint volume reverberation and biological measurements in the tropical Western Atlantic

Barry A. Gold and Wayne E. Renshaw

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1809-1819 (1978); (11 pages)

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Volume reverberation and biological data were gathered during January in the western tropical Atlantic Ocean. Day and night scattering strengths in 1/3‐octave bands from 1.0 to 20.0 kHz are presented which are compared to other data collected in the same area by a different agency. Identification of fishes captured, their lengths and computed swimbladder volumes, as well as resonance frequencies are given.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries

Coherent ray tracing—measured and predicted shallow‐water frequency spectrum

J. Wakeley, Jr.

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1820-1823 (1978); (4 pages)

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Experiments have been conducted by the Applied Research Laboratory, The Pennsylvania State University, to study the acoustic signals received from underwater explosive charges under various environmental conditions. This paper describes a computer simulation study incorporating a model of a pressure signature for an underwater explosive charge and ray‐tracing techniques to predict the received energy spectrum in an environment where more than one acoustic path may exist between the source and the receiver. Measured and calculated one‐third‐octave received energy levels are compared for a standard Mk 83 SUS detonated at a depth of 18.3 m at a range of 18–128 km in water 35–79‐m deep. The site of the experiment was approximately 64 km east–northeast of Daytona Beach, Florida. The model shows good agreement with the experimental data in the prediction of a low frequency rolloff and frequency window in the received signal energy spectrum that varies with range separation between source and receiver.
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43.30.Bp Normal mode propagation of sound in water
43.20.Dk Ray acoustics

Surface‐wave rays in elastodynamic diffraction by cracks

A. K. Gautesen, J. D. Achenbach, and H. McMaken

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1824-1831 (1978); (8 pages) | Cited 1 time

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A geometrical diffraction theory has been worked out to analyze the fields generated by diffraction of high‐frequency waves by cracks. The theory accounts for curvature of incident wavefronts, curvature of crack edges, and finite dimensions of the crack by providing first‐order corrections to the results for a semi‐infinite crack. The diffracted fields include direct diffractions from the crack edges as well as well as diffractions of signals which travel via the crack faces. On the faces of the crack the main contributions to the diffracted fields come from rays of surface waves. The directions of these surface‐wave rays, and the amplitudes, wavelengths, and phases of the associated surface‐wave motions have been related to the corresponding quantities of the incident body‐wave rays. Reflection and diffraction of surface‐wave rays by the edge of a crack have also been analyzed. As an example, diffraction by a penny‐shaped crack of a plane longitudinal wave under normal incidence has been considered in some detail. Explicit expressions are given for the diffracted fields. In these expressions a correction was introduced to extend the validity of the results to the normal axis through the center of the crack, which is a caustic axis. A simple expression for the scattering cross section is presented.
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43.40.Dx Vibrations of membranes and plates
43.35.Pt Surface waves in solids and liquids
43.20.Dk Ray acoustics
43.20.Fn Scattering of acoustic waves

Vibration of circular double‐plate systems

A. S. J. Swamidas and V. X. Kunukkasseril

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1832-1840 (1978); (9 pages)

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Vibrational characteristics of circular double‐plate systems connected together by concentric, intermediate, elastic ring supports have been considered in this work. The analysis is based on the assumption that both of the plates are thin, elastic, and isotropic. Also, the plates are subjected to initial in‐plane loads. The solutions are shown to be in terms of Bessel functions for the case of complete and annular (with equal in‐plane loads) circular isotropic plate systems. The vibrational characteristics of the systems are illustrated by presenting numerical results for isotropic plate systems with one intermediate connection. When both the plates are identical with identical edge forces and boundary conditions, in‐phase and out‐of‐phase vibration modes are observed.
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43.40.Dx Vibrations of membranes and plates
43.40.At Experimental and theoretical studies of vibrating systems

Studies on the spatial variation of decaying sound fields

Thomas W. Bartel and Edward B. Magrab

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1841-1850 (1978); (10 pages) | Cited 3 times

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The spatial variation of the reverberation time was measured in the NBS reverberation room in the 1/3‐octave bands from 80 to 10 000 Hz to determine the following: (1) the effects on the precision of the spatially averaged reverberation time due to (i) vane speed and vane orientation, (ii) loudspeaker location, and (iii) the area and location of an absorbing panel and its absorption coefficient; (2) the selection of the parameters in (1) above such that the measurement uncertainty of the reverberation time is minimized; and (3) the overall measurement uncertainty for this optimum configuration as a function of the number of microphone locations and the number of decay curves recorded at each microphone location. For an 11‐m2 panel with relatively little low frequency absorption and with the vanes oriented at 22.5° from the vertical and rotating at 7.5 rpm, an analysis of variance indicated that the total uncertainty of the measured average reverberation time (one standard deviation from the mean) was less than 0.5% from 160 to 4000 Hz and less than 1.5% from 80 to 10 000 Hz when 20 decays at each of six microphone locations were used.
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43.55.Br Room acoustics: theory and experiment; reverberation, normal modes, diffusion, transient and steady-state response

Effect of sound‐absorptive facings on partition airborne‐sound transmission loss

Steven M. Brown, Joseph Niedzielski, and G. Robert Spalding

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1851-1856 (1978); (6 pages) | Cited 1 time

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Laboratory measurements of the improvement of partition airborne‐sound transmission loss in the presence of sound‐absorptive partition facings are presented. For a double‐leaf partition of 1/2‐in.‐thick gypsum board on 2×4‐in. studs, the application of such facings has led to improvements in transmission loss in excess of 10 dB in the 1/3‐octave bands above 1 kHz. Corresponding, but smaller, improvements have been measured at lower frequencies.
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43.55.Ti Sound-isolating structures, values of transmission coefficients
43.55.Ev Sound absorption properties of materials: theory and measurement of sound absorption coefficients; acoustic impedance and admittance
43.55.Rg Sound transmission through walls and through ducts: theory and measurement

Normal‐mode theory of enclosures with thermoviscous dissipation at the walls

Stephen H. Burns

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1857-1860 (1978); (4 pages)

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This paper treats a limiting case of normal‐mode theory for which all of the dissipation occurs in the thermoviscous boundary layer in the fluid near the walls. This case differs from that of finite‐impedance walls primarily in its more complicated dependence of the viscous dissipation on the mode numbers. The normal‐mode theory with infinite‐impedance walls is shown on a term‐by‐term basis to be equivalent to the Kirchhoff tube theory. An example of how one might exploit this equivalence is given. Calculated absorption coefficients for infinite‐impedance walls are compared with the experimental coefficients in the literature.
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43.55.Dt Sound absorption in enclosures: theory and measurement; use of absorption in offices, commercial and domestic spaces
43.55.Br Room acoustics: theory and experiment; reverberation, normal modes, diffusion, transient and steady-state response

Signal‐processing techniques for resolving individual pulses in a multipath signal

J. E. Ehrenberg, T. E. Ewart, and R. D. Morris

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1861-1865 (1978); (5 pages) | Cited 11 times

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A maximum‐likelihood procedure for estimating the amplitudes and arrival times of individual pulses in a multipath signal is derived. Computer simulation results are presented that compare this procedure with the more traditional matched‐filter and inverse‐filter techniques. The maximum‐likelihood technique is shown to be significantly more accurate than either the matched or inverse filter. A computer algorithm for implementing the maximum‐likelihood estimation procedure is described.
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43.60.Gk Space-time signal processing, other than matched field processing
43.58.Ta Computers and computer programs in acoustics

Sparse array performance

Charles R. Greene and Roger C. Wood

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1866-1872 (1978); (7 pages)

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In a linear array with equispaced elements, any elements not forming a unique pair spacing with at least one other element are redundant and may be eliminated to form a sparse array. The average power output of a sparse array can be made the same as for the full array by substituting self‐ and cross‐power terms from the remaining elements. Such processing assures identical signal‐to‐noise gain and directivity (beam patterns) for the sparse and full arrays, but the output variability for any finite averaging time will generally be greater for the sparse array. Linear combinations of possible substitution terms for each missing term may be optimized for minimum output variance. This concept has been applied to both sparse and full arrays to develop an optimum processing technique that minimizes output variance. An analysis technique has been developed for evaluating the output variance of sparse arrays operating in sound fields with specified signal‐to‐noise ratio and noise fields with arbitrary portions of independent, isotropic, and directional interference. For example, in isotropic noise with a wave length 2.5 times the basic element spacing and a signal‐to‐noise ratio of 0.001, a five‐element sparse array providing the same average output as a full, ten‐element equispaced line array will have an output standard deviation 2.08 times the standard deviation of the full array. For a signal‐to‐noise ratio of 1.0, the factor is 1.09.
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43.60.Cg Statistical properties of signals and noise
43.60.Qv Signal processing instrumentation, integrated systems, smart transducers, devices and architectures, displays and interfaces for acoustic systems

Amplitude shading of irregular acoustic arrays

Edmund J. Sullivan, Jr.

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1873-1877 (1978); (5 pages)

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A method of generating amplitude shading coefficients for iregular arrays is demonstrated. By applying a scale transformation to the element coordinates and making the assumption that the cross section of the main lobe is elliptical, an existing technique can be applied. The method allows for steering within certain constraints. Although this approach strictly applies only to planar arrays, an example is given of an array which conforms to a conic surface in order to demonstrate the utility of the technique even when the required planar condition is not met. For this example, beam patterns are generated for the broadside case and the 45° steered case.
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43.60.Gk Space-time signal processing, other than matched field processing
43.30.Vh Active sonar systems

Effects of stimulus frequency on two‐tone suppression: A comparison of physiological and psychophysical results

Paul J. Abbas

J. Acoust. Soc. Am. Volume 63, Issue 6, pp. 1878-1886 (1978); (9 pages) | Cited 4 times

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The effects of stimulus frequency on two‐tone suppression were investigated in single auditory‐nerve fibers of anesthetized cats and compared with human psychophysical data. In the physiological experiment, both average discharge rate and phase‐locked activity were measured in response to one‐ and two‐tone stimuli. The first component f1 produced an increase in rate above spontaneous activity when presented alone. The second tone f2 was always well below the fiber’s characteristic frequency and was held at a fixed sound pressure level appropriate to produce two‐tone suppression. Responses were plotted as a function of stimulus level of the first tone both alone and in the presence of f2. For different values of f1 with f2 fixed, suppression was maximum with f1 near fiber CF. In the psychophysical experiment, similar stimulus parameters of f1 and f2 were used as the masker in a forward‐masker paradigm. In this experiment the addition of the second masker tone at frequency f2 could produce less masking of the signal. When f1 was varied with f2 fixed, the relative decrease in masking, analogous to suppression, was greatest when f1was equal to the signal frequency.
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43.64.Pg Electrophysiology of the auditory nerve
43.66.Lj Perceptual effects of sound
43.64.Ri Evoked responses to sounds
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