bluuebunny/crossref_metadata_2025 huggingface dataset:
Sound Diffracted by a Turbulent Boundary Layer Flow off a Sharp Edge
Authors: Chandiramani K. L.
Published: October 2005

The problem of diffraction of aerodynamic noise off the sharp edge of a rigid semiinfinite plate has received close attention recently. The approaches of Ffowcs-Williams and Crighton, dealing with scattering from a volume distribution of quadrupoles near the plate edge, have recently been recast by Chase with the aim of relating explicitly the magnitude of the scattered sound in the farfield with the wall pressure statistics characterizing a turbulent boundary layer (TBL) flow. Chase's treatment started with the known Green's function for the problem. In this paper, the same problem is treated in terms of diffraction off the plate edge, when the incident wave is pseudosound; it travels parallel to the plate surface with a velocity less than sonic and decays exponentially in a direction normal to the plate. The resulting solution of the diffracted sound wave in the far field, when combined with Corcos' model of the TBL wall pressure statistics, yields good agreement with Chase's results. Comparison of these estimates with Hayden's data is made.
Nearfield Pressure in Perturbed and Unperturbed Turbulent Jets
Authors: Barefoot G. L. , Arndt R. E. A.
Published: October 2005

Consideration is given to the necessary nearfield pressure data necessary for predicting farfield acoustic radiation. A method for measuring two-point pressure correlations within a turbulent flow is developed and applied to three variations of a turbulent, axisymmetric jet: unperturbed, screen perturbed, and one containing a 10:1 ellipsoid in the potential core. Typical length scales for turbulent fluctuations are found to be smaller than equivalent velocity scales. The effects of a screen on the pressure field are significant. On the other hand, a body perturbed jet suffers a large increase in turbulence intensity but little change in the Reynolds stress and pressure field. Consideration is given to low-frequency roll-off problems and a limited frequency response. It is shown that meaningful data can be obtained in the range 1 < x/D < 7 provided the phase lag of each probe is matched. It is also noted that there is an optimum jet velocity for a given facility in which acoustic contamination is minimized.
Wall Pressure Fluctuations at Smooth and Rough Boundaries under Turbulent Boundary Layers with Positive and Negative Freestream Pressure Gradients
Authors: Burton Thomas E.
Published: October 2005

Turbulent boundary layers were fully developed over either smooth or rough walls. Reynolds numbers based upon boundary layer thickness and freestream velocity are on the order 104. Fluctuating pressures at the boundary were investigated within a local region, where a freestream pressure gradient was applied. The flow reequilibrated to the negative pressure gradient and was self-preserving, in the region of measurement. With a positive pressure gradient, the flow tended toward separation with increasing downstream distance and was not self-preserving. Two flush-mounted microphones provided pressure spectral densities and time-autocorrelations at single points on the boundary, as well as cross spectra and cross correlations from pairs of points. The influence on wall pressure of roughness is explained in terms of its effects on the mean velocity profile. Fluctuating pressure models, which incorporate a unique dispersion relation c(f) (the phase velocity c is obtained from the cross-spectral density) are found to be inapplicable to flows with a positive freestream pressure gradient. [Research supported by NSRDC.]
Laboratory Simulation of Spacecraft Acoustic Environment within a Cylindrical Enclosure
Authors: McGregor H. N.
Published: October 2005

Spacecraft integrity is verified by subjecting it in the laboratory to acoustic excitation, which simulates the vibroacoustic environment that occurs during launch, maximum aerodynamic pressure during exit from the earth's atmosphere, and the subsequent aerodynamic pressure fluctuation, which occurs during entry into the Martian atmosphere. Scale model tests were performed to determine if an enclosure approximating the shape of the spacecraft fairing could be used as an inexpensive test chamber. Tests were conducted with chambers having the configuration of the spacecraft fairing and a cylindrical shape. Results of the tests show that the acoustic environment can be replicated in a fairing shaped enclosure over 85% of the top surface and 95% of the bottom surface of the spacecraft. With a cylindrically shaped enclosure, they can be replicated over 95% of the upper and lower surfaces of the spacecraft. A low-pressure cold jet located in the lower half of the enclosure is used to simulate entry acoustics.
Recent Studies of Wall and Air Resonances of a Violin
Authors: Jansson E. V.
Published: October 2005

Normal modes of the walls are studied by means of halogram and laser-speckle interferometry and their vibration patterns, frequencies, and Q:s are registered. Resonance frequencies and corresponding Q:s of the air volume enclosed by the walls of the sounding box are measured by especially designed electroacoustical equipment and a first image of the normal modes has been registered. The experimental findings will be presented and discussed.
A Method for Visualizing the Eigenmodes of Free-Free Violin Plates
Authors: Hutchins Carleen M.
Published: October 2005

Hologram interferometry has shown the shape of many of the eigenmodes of free-free violin plates. By hanging a violin plate horizontally from its four corners on rubber bands over a specially designed loudspeaker, and the speaker activated by a sine wave from an audiogenerator, it is possible to visualize readily some plate modes. This is done by using finely cut bits of aluminum that will not roll on the curved surface to make Chladni patterns as the plate is activated at its discrete mode frequencies. This is a particularly useful method of checking the contours and activity of the “ring mode” or plate tap tone, which has been found most effective in indicating where to thin the wood of violin plates before assembly. By combining this with our present plate tap tone method, which gives accurate check on frequency, amplitude, and damping of the “ring mode,” we have a much improved method for controlling sound quality in finished instruments from violin to contrabass.
The 1215-Cent Octave: Convergence of Western and Nonwestern Data on Pitch Scaling
Authors: Dowling W. J.
Published: October 2005

Ward [J. Acoust. Soc. Amer. 26, 369–380 (1954)] showed that Western listeners are extremely precise in making octave judgments with pure tones. Ward's listeners adjusted pairs of pure tones to subjective octaves that were slightly larger than a physical 2/1 frequency ratio (12 semitones or 1200 cents). Ward's data between 250 and 2000 Hz showed mean subjective octave sizes of about 1215 cents. The validity of Ward's procedure for establishing a scale of musical pitch was dramatically reinforced by the agreement between scaled octave judgments and absolute judgments by one listener with absolute pitch. Observations made on the tuning of instruments in several nonwestern cultures show good agreement with Ward's result of 15 cents/oct sharpening. For example, reanalysis of Hood's [UCLA Ethnomusicol. Inst. Sel. Rap. 1, No. 1, 28–48 (1966)] data on the tuning of Indonesian gamelans shows a 15.36 cent/oct sharpening. The results of all available precise measurements for widely divergent cultures are summarized. It appears that humans the world over make octave judgments with precision and that these deviate systematically from the 2/1 ratio in ways that must be explained by the structure of the auditory system. The present results enhance the universal applicability of Ward's pitch-scaling methods.
On High-Frequency Scattering Methods: The Need for Error Bounds and Domains of Validity
Authors: New R. , Eisler T. J.
Published: October 2005

There exists an abundance of high-frequency scattering methods—but relatively little is known of their accuracy. Furthermore, the technical literature is significantly lacking in demonstrating the efficacy of one method of target scattering analysis versus another, and very few comparative analyses have been conducted—why? The Kirchhoff, Keller, and geometrical optics (acoustics) methods are well known, and can be applied to arbitrarily shaped bodies. Comparative results are shown for the problem of scattering from a rigid sphere to demonstrate the differences between methods. Further comparisons to the more exact wave-harmonic and Watson series solutions emphasize the importance of establishing domains of validity for the approximate methods.
k-Space Formulation of the Acoustic Scattering Problem
Authors: Bojarski Norbert N.
Published: October 2005

The acoustic scattering problem is solved by means of a k-space formulation of the field equations, thereby replacing the conventional integral equation formulation by a set of two simultaneous algebraic equations in two unknowns in two spaces (the constitutive boundary condition being an algebraic equation in x space). These equations are solved by an iterative method with the aid of the fast Fourier transform (FFT) algorithm connecting the two spaces, requiring trivial initial approximations. Since algebraic and FFT equations are used, the number of arithmetic multiply-add operations and storage allocations required for a numerical solution are reduced from the order of N2 and N2, respectively (for solving the matrix equations resulting from the conventional integral equations), to the order of N log2N and N, respectively (where N is the number of data points required for the specification of the problem). The advantage gained in speed and storage is thus of the order of N2/log2N and N, respectively. This method is thus considerably more efficient, and permits exact numerical solutions for much larger problems than possible with the conventional integral equation-matrix inversion method. The details and some numerical results of the application of this method to two- and three-dimensional acoustic scattering are presented.
The Impulse Response and Acoustic Radiation Impedance of an Annular Ring in a Planar Baffle
Authors: Stephanishen P. R.
Published: October 2005

The acoustic radiation impedance of an annular ring in a rigid infinite planar baffle has been the subject of several recent papers [A. S. Merriweather, J. Sound Vibration 10, 369–379 (1971); W. Thompson, Jr., J. Sound Vibration 17, 221–233 (1961); and C. J. Boukwamp, J. Sound Vibration 17, 499–508 (1971)]. In contrast to the earlier papers, a time domain approach is presented in this paper to evaluate the radiation impedance. Rather than directly evaluate the well-known Kirchoff double surface integral, the impulse response or inverse Fourier transform of the impedance is first evaluated from the integral. A closed-form expression for the impulse response which is valid for all ring sizes is presented, and several interesting characteristics of the solution are noted and discussed. Since the impulse response and radiation impedance of a ring are a Fourier transform pair, the Fourier transform of the impulse response must be evaluated to obtain the impedance. Although the Fourier transform can not be readily evaluated analytically, an approximate expression for the radiation impedance of the ring can be obtained and is presented. Numerical results for the radiation impedance of different sized rings are also presented as a function of normalized frequency.
Acoustic Radiation from a Point-Excited Infinite Elastic Plate
Authors: Stuart Alan D.
Published: October 2005

The acoustic pressure radiated by an infinite elastic plate excited by a dynamic point force is investigated. The effects of shear and rotary inertia in the plate have been included to ensure the proper representation of the high-frequency components of the radiated pressure. The pressure radiated by this coupled plate-fluid system is obtained with the use of integral transform methods. The inverse integral representation of the pressure field is evaluated by the asymptotic method of steepest descent, from which the near- and farfield pressure is determined directly. The farfield is associated with the saddle-point contribution, while the nearfield is associated with the poles of the integrand, on either Riemann sheet, that are enclosed by the path of steepest descent. A system of normalized variables is introduced that allows the radiated pressure to be expressed explicitly in terms of the material properties of the plate and the acoustic fluid, while being independent of the plate thickness. The frequency spectrum and directivity pattern of the farfield radiated pressure are obtained for a steel plate in air and in water. At frequencies above coincidence, two distinct peaks in the radiated pressure occur. The first peak to occur commences parallel to the plate; the second peak commences at the normal to the plate; both peaks reach a maximum angle as frequency is increased.
Acoustic Transmission Properties of Submerged Finite Cylindrical Shells
Authors: Pallett David S.
Published: October 2005

Statistical energy analysis procedures have been successfully employed for prediction of acoustic transmission properties of finite cylindrical shells in air in the work of White and of Manning and his associates. Extensive studies of Crocker and Price and their associates have dealt with acoustic transmission properties of plane panels in air. The present study directs attention to essential adaptations to these analytic methods that are necessary to account for fluid loading effects due to the presence of a dense fluid surrounding the exterior of a finite air-filled cylindrical shell. Attention is directed to the relative importance of the resonant and nonresonant acoustic transmission paths. The resonant response and transmission path is based upon resonant modal response properties, whereas nonresonant transmission is estimated upon the basis of transmission properties characterizing an infinite plane interface between two acoustic media. The nonresonant transmission path appears to be dominant for the uniform shells included in this study. Experimental measurements of the acoustic transmission properties of two uniform cylindrical shells have indicated good agreement between analytic and experimental data.
Vibration of a Semiinfinite Fluid-Loaded Membrane
Authors: Davies Huw G.
Published: October 2005

The vibration of and the acoustic radiation from a semiinfinite taut membrane is considered. The plane of the membrane is extended by a semiinfinite rigid baffle. A compressible fluid fills the half-space on one side of the membrane and baffle. Two types of vibration are considered: (i) natural motion of the membrane due to an incoming acoustically slow wave on the membrane incident normally on the edge, and (ii) forced motion of the membrane, the force having sinusoidal space and time dependence. The forced motion thus is somewhat analogous to the single-mode-vibration of a finite membrane. The shape of the membrane and the associated acoustic radiation are discussed and compared with the case when fluid loading is not present.
The Effects of Masker Bandwidth on the Function Relating Detectability to Signal Duration
Authors: Dolan Terrence R. , Robinson Donald E.
Published: October 2005

The function relating detectability (d′) to signal duration for sinusoidal signals in Gaussian noise has been well documented. In general, it has been shown that d′ is at a maximum when the signal duration is between 100 and 300 msec, and decreases for both longer and shorter durations. The present experiment replicated these results for both wide-band (2900 Hz) and narrow-band (70 Hz), continuous maskers. However, for pulsed signal and masker, the decrease in d′ at short signal durations was obtained only when the noise masker employed was wide band. If a narrow-band noise was used as the masker, detectability continued to increase as signal duration was decreased.
Monaural Unmasking
Authors: Wightman Frederic L. , Houtgast Tammo
Published: October 2005

A classical forward-masking paradigm was used to measure the masked threshold of a brief 1000-Hz signal presented shortly after the termination of a long, 1000-Hz tonal masker. To our surprise, when a wide-band noise was added to the tone masker, at a level high enough that the tone masker was barely audible, the threshold for the signal decreased, by as much as 10 dB for some subjects. In other words, detection performance was actually improved by the addition of the rather intense noise masker; the noise somehow reduced the effectiveness of the tone as a masker. We suggest that this result reflects the operation of some kind of lateral-inhibitory process in the auditory system. The neural activity produced by the tone masker is thought to be suppressed, or inhibited by the addition of the noise (in much the same way as the neural activity produced by one tone has been shown to be suppressed by the addition of a second tone), thus reducing the masking effectiveness of the tone. In order for this “inhibitory” effect (and others recently reported in the literature) to be revealed, however, it is essential that the masker and signal not be presented simultaneously. In our experiment, with simultaneous presentation, adding noise never resulted in an improvement in performance. [Research supported in part by the Institute for Perception—TNO, Netherlands, and by a National Science Foundation Postdoctoral Fellowship awarded to the first author.]
Detection of Deletion and Insertion of Tones in Bandlimited Noise
Authors: Bourbon W. T. , Lembo V. L.
Published: October 2005

The detectability of insertion (TI) and deletion (TD) of tones in a noise background was compared. Tones of 500 Hz were gated on or off for 100 msec in noise with a spectrum level of 45 dB SPL. Psychometric functions were determined for two observers in a 21FC task under wideband and bandlimited conditions of the masking noise. Under all conditions, the following results were obtained: (1) TI is more detectable than TD by 3 dB or more; (2) the slope of P(c) vs E/N0 is greater for TD than for TI; and (3) the release from masking obtained with bandlimiting of the noise masker is greater for TD than for TI. The relationships of the present data to those of earlier studies and to critical band data are discussed.
Some Possible Attentional Aspects of Post-Stimulus Signal Processing
Authors: Pastore Richard E.
Published: October 2005

Some possible attentional aspects of post-stimulus signal processing were investigated in a modified masked-signal detection paradigm. The signal was a 1000-Hz sinusoidal waveform 20 msec in duration and sated at positive zero-crossing of the waveform. The signal was masked by a broad-band (360–3360 Hz) Gaussian noise. The effect of a decrease in the amplitude of the masking noise, when confined to a limited frequency range, was investigated in terms of both the frequency range of the change and the temporal relationship between the offset of the signal observation interval and the onset of the decrease. An immediate decrease in masking noise in frequency regions outside the critical band of the signal decreased detectability.
Fricatives—Their Physical Properties and Allophones
Authors: Umeda N. , Monsen R. B. , Molter M.
Published: October 2005

Voiceless fricatives in American English were studied from readings of a continuous text. The wide range of duration (30 msec to more than 200 msec) is explainable in terms of phonological conditions. For example, a fricative in the stressed-syllable initial position is about 20% longer than in the same phoneme sequence in unstressed position; single fricatives are longer than those in clusters with the same stress and positional conditions; a fricative preceded by a nasal is about 30% shorter than one under equivalent conditions with a nonnasal. Overall intensity of fricatives is not simply proportional to duration, but rather depends on stress and boundary conditions. Fricatives in prepausal position are longest but weakest. For a given phonological condition (e.g., word-initial stressed, and single), overall intensity reflects the sentence stress. There are consistent relationships between fricative intensity and spectral distribution. Recently we have argued that the stop consonants vary systematically to mark, among other things, word boundaries and the beginnings of stressed syllables. This study shows that allophonic variations of the fricatives carry the same information.
Voiceless Consonant Durations in VCV Utterances
Authors: Kim Jaemin , MacNeilage Peter F.
Published: October 2005

Durations of the eight English voiceless obstruents, /p/, /t/, /k/, /f/, /θ/, /s/, /s/, /c/ were measured from digitized oscillographic displays. Eight subjects produced each consonant 10 times in each of the four contexts i-i, i-a, a-i, and a-a. They read from randomly ordered lists and were instructed to place equal stress on both the surrounding vowels. The affricate /č/ had the longest total duration (closure plus ‘burst”) and the fricatives /s/ and /š/ the next longest total duration for all subjects in all contexts. There were no consistent differences in total duration of the remaining five consonants. Closure durations for stop consonants were, in decreasing order, /p/, /t/, /k/, and burst durations showed the opposite order, results thus tending towards equal total stop consonant durations. Durational effects of the preceding vowel were not consistent across subjects. Low vowels were preceded by shorter total consonant durations than high vowels. The shorter durations were mainly observed during the burst of stops and affricates, and were least obvious in labials and interdentals.
Temporal Variability of Repeated Utterances by Normal Speakers and Stutterers
Authors: Allen George , Cooper Margaret
Published: October 2005

Ten normal speakers, five stutterers, and five “cured stutterers” each repeated four different speech samples several times. The four samples were (1) an “easy” sentence, (2) a “hard” sentence, (3) an “easy” paragraph, and (4) a “hard” paragraph, where “easy” and “hard” refer to high and low frequency of occurrence in English of the content words used. In a separate fifth task, each subject tapped his finger as steadily as he could for 4 min. From each of these five repeated behaviors will be derived a variability measure (relative variance, or amount of variance per unit of time) that describes each subject's accuracy in repeating the time program of that behavior. The reliability of this measure, i.e., the stability of the measure across tasks for each subject will be examined, and its validity will be related to the normal and stuttering populations tested.
Speaking Rate and the Delivery of Phonetic Commands in Standard American English
Authors: Abler William L.
Published: October 2005

Measurements of the acoustic segments in the sentences, get your feet off the table, get your seat off the table, get your foot off the table, get your soot off the table, in which individual acoustic segments of different intrinsic durations were interchanged, show that these changes do not induce compensation or other temporal or acoustic consequences within the syllable. However addition of new acoustic segments into the two-stress sentences, Jóe díed, Jósie díed, Jóseph díed, Jósephine díed, does induce temporal compensation within the interstress interval. Thus the duration of the stressed /o/ in these sentences contracted 45% from the first to the last sentence when all were spoken at a constant speaking rate. Further measurements of serial repetitions of individual sentences, e.g., Jósie díed, Jósie díed, … show that the acoustic interval most stable in duration is the interstress interval when the sentences are spoken at a constant speaking rate. These findings support the view that the phonetic commands of Standard American English are delivered to the vocal muscles in groups spanning the interstress interval, and not spanning just the syllable.
Acoustic Features of Disyllabic Stress Patterns in Speech and Singing
Authors: Bennett D. N. , Tavener T. T.
Published: October 2005

Trained male singers sang (chest and falsetto registers) and spoke CVCV syllables embedded in a carrier phrase, using as stressed-unstressed/unstressed-stressed paradigm. The consonant /t/ was employed throughout in combination with the vowels /i-a-u/. All syllables were segmented into three acoustic elements: (1) silent interval, (2) noise burst, and (3) phonation. The silent interval plus the noise element were considered to correspond with the consonant, and the phonation segment with the vowel. In appropriate instances, measures were made of duration, amplitude, fundamental frequency, and formant characteristics of the segments from spectrographic records. Relations among segments are reported as a function of modes of utterance and stress. Inferences are drawn concerning possible differential articulatory and phonatory demands on the production system as a consequence of syllabic stress during singing and speech.
Coarticulation and Speech Perception: The Effect of Stressing and Consonant Cluster
Authors: Manning W. H. , Beasley D. S.
Published: October 2005

Degree of coarticulation as defined in previous research [W. Manning, D. Beasley, T. Beachy, J. Acoust. Soc. Amer. 52 (1972)] was investigated as a factor in the perception of human speech. Fifty subjects listened to 360 two-word combinations which provided 180 conditions of greater /sp/, /st/, and /sk/ coarticulation, and 180 conditions of lesser /sp/, /st/, and /sk/ coarticulation. Stimuli were presented in a stressed and nonstressed manner. Subjects were randomly divided into five groups of 10 individuals each and administered the stimuli in one of five S/N ratios. The results reveal that mean percentage correct scores were significantly higher for the more coarticulated consonant clusters under the unstressed condition, while the less coarticulated consonant clusters were perceived more accurately under the stressed condition. Where differences in stressing could not be employed in the decision of s placement, coarticulatory cues appeared to assume greater importance in the perceptual task.
Direct Measurement of Rayleigh Wave Velocity by Schlieren Visualization and Estimates of Shear Velocity Therefrom
Authors: Dragonette Louis R. , Neubauer Werner G. , Bucaro Joseph A.
Published: October 2005

Schlieren visualization experiments are used to obtain direct, accurate measurements of Rayleigh phase velocity on various materials. Illumination of a plane solid surface underwater at the Rayleigh angle produces an easily identifiable null strip in the radiation which uniquely defines the Rayleigh angle. This strip is caused by the mutual cancellation of equal amplitude specular and Rayleigh radiations, which are 180°out of phase at the Rayleigh angle. Rayleigh velocities are calculated from the measured angle by a simple equation. The measured Rayleigh velocities are used to give estimates of shear velocity, and these estimates are compared with direct shear velocity measurements. Attenuation of the Rayleigh wave in the low megahertz region is given for six materials, and shown to agree with theoretical predictions.
Experimental Determination of the Critical Angle for Reflection from a Water-Quartz Interface
Authors: Liu John Moyu , Green Robert E. , Henneke Edmund G. , Jones Gerald L.
Published: October 2005

The critical angle of reflection for a wave incident from water upon the xy plane of quartz has been measured for planes of incidence about the z axis. These experimental measurements are compared with the theory that predicts that the critical angle for reflection from an interface of an anisotropic material occurs when the energy flux vector of the refracted wave becomes parallel to the interface. This differs from the usual definition of the critical angle as that angle for which the refracted wave normal becomes parallel to the boundary. The new definition of critical angle modifies the usual expression sinθc = VI/VR by the factor cosφ/cosβ as discussed in the previous abstract.
Computerized Enhancement of Ultrasonic Echograms
Authors: Meyer C. R. , Carlson D. L.
Published: October 2005

In the area of pulsed ultrasonics, there has been an effort to improve the resolution of compound scan (C-scan) echograms. Most of the effort has been directed toward reduction of the transducer's beamwidth with such developments as composite transducers or focused transducers with variable focal distances. Problems of image saturation and artifact generation have received relatively little attention. This study reports on a method of generating without image saturation, a C-scan echogram whose resolution is independent of the transducer's large beamwidth. Normal C-scan information is recorded on analog magnetic tape and temporally expanded for construction of the target's positional likelihood distribution using a digital computer. Targets possessing vertical symmetry in the region scanned by the ultrasonic beam are used in a water bath to investigate the system resolution. After the likelihood distribution is constructed, it may be plotted, or numerical methods can be used to plot the processed echogram. in constructing the processed echograms, only the positions of significant local maxima are plotted. A processed echogram of the whole region, or of any segment, can be plotted using any desired scale factor. The processed echograms are compared to echograms degraded by simulated image saturation and the improvement in resolution of the processed echograms is clearly shown. Artifacts due to beamwidth and multiple reflections are missing in the processed echograms.
A Survey of Sound Propagation in Soils
Authors: Winter T. G.
Published: October 2005

0ne of the most attractive applications of acoustical holography is the location of objects and formations buried near the surface of the earth. But the first step is to learn the sound propagation characteristics of the earth. Dilitational wave velocities in unconsolidated soils range from 500 to 5000 ft/sec within the first 50 ft. Shear velocities are 30%–60% of dilitational velocities. The absorption coefficient is directly proportional to frequency and is typically 1–5 dB/ft at 1000 Hz. The effect of amplitude, frequency, void ratio, pressure, and moisture content on absorption and velocity will be reported. But velocity and absorption coefficients are the least of the problem. The layered nature of the medium leads to multiple transmission paths and the variation of velocity with position in the medium leads to complex refraction effects.
Performance of an Acoustic Parametric Source
Authors: Eller Anthony I.
Published: October 2005

The performance of an acoustic parametric source was investigated at the NRL Acoustic Research Tank Facility. The source utilized a USRD type E8 transducer, driven simultaneously at two frequencies close to the resonance frequency of 1.5 MHz. Narrow-beam difference-frequency waves, at frequencies ranging from 30–200 kHz, were generated by the nonlinear nature of acoustic propagation. Measured values of the difference-frequency SPL and a sequence of radiation patterns illustrate beam formation in the nearfield of the parametric source. The results show that the E8 transducer is suitable for use as a parametric source and also demonstrate the flexibility of the NRL facility as a research tool in nonlinear acoustics.
Absorption of Sound by Sound
Authors: Westervelt Peter J.
Published: October 2005

The exact solution for first order nonlinear scattering of two plane waves of sound [J. Acoust. Soc. Amer. 29, 934 0957)] showed the scattered density to be ρs = c0−2E12+12ω1−1ω2−1(cosθ+12Λ)[sin−2(12θ)]∇2W12. The singularity at θ = 0, which occurs when the primary waves are collinear, may be removed by applying to this solution the operator {1 − exp[(|∂∂t||c0∇|−1−1)]r⋅∇}. The resulting solution is now valid for θ = 0 in addition to all other intersection angles. This new solution has been applied with the conservation of energy [P. J. Westervelt, in L. Cremer, Ed., Proc. Int. Cong. Acoust., 3rd (Elsevier, Amsterdam, 1960), p. 316], to determine the attenuation of one plane wave with wave vector k1, interacting with an isotropic distribution of waves having the energy density spectrum u(k). The general result for the pressure attenuation coefficient is α = (2ρ0c02)−1(1 + 12Λ)2π[ ∫ 0k1 ku(k)dk+2k1 ∫ k1∞u(k)dk+k12 ∫ 0k1k−1u(k)dk]. In the event k1 is much less than any component k of the interacting background, a great simplification results and α = (2ρ0c02)−1(1 + 12Λ)2πk1E, in which E is the energy density of the background radiation. This result is supported by experiments on the attenuation of sound in superfluid helium [H. J. Maris, Phys. Rev. Lett. 28, 277 (1972)]. [This work was supported by the U. S. Office of Naval Research.]
A Unified Analysis of Fan Stator Noise
Authors: Hanson Donald B.
Published: October 2005

A theory is developed for axial flow fans which predicts the harmonic and broad-band acoustic radiation from the stator due to interaction with rotor viscous wakes. Both the harmonic and broad-band spectrum components are calculated from a unified model using methodology from the theory of random pulse modulation. The stator is modeled as a circular array of pulsed dipoles. The amplitude and arrival time of each pulse are random variables whose means correspond to the values calculated for harmonic rotor-stator interaction theory. The standard deviations of these random variables are measures of the turbulence level in the blade wakes. For the model proposed, the solution is exact and the results are instructive: when pulse amplitude modulation (PAM) is imposed on a periodic stator source, new broad-band energy is generated whose spectrum shape is similar to the envelope of harmonics at high frequencies. The harmonic radiation is unchanged. When pulse position modulation (PPM) occurs, new broad-band energy is radiated but at the expense of harmonic energy. The theoretical results are discussed with reference to directivity patterns and radiation efficiency, and the harmonic component is compared with the previous Lowson and Embleton/Theissen analyses. The transfer of harmonic to random energy through PPM is discussed. At frequencies significantly above duct cutoff, it is shown for a fixed stator solidity that the spectrum is essentially independent of the number of stator vanes. In order to show the capabilities of this new theory, calculations are compared in detail with experimental data and modulation levels are discussed.
Effects of Blade-Vane Spacing on Fan Noise
Authors: Semrau W. R.
Published: October 2005

Definitive experimental knowledge of discrete-frequency noise reduction, which can be obtained in an aircraft turbofan engine through increased axial spacing between the fan rotor and outlet guide vanes, has been lacking. An experimental scale-model research program has been undertaken to fill this void. Using a unique heavy-gas scale-model anechoic test facility, rearward radiated discrete frequency and broad-band noise was investigated for a scale model fan. Data has been obtained for fan tip Mach numbers of 0.7–1.4, and rotor to OGV spacings of 0.13, 0.5, 1.0, and 2.0 rotor chords. Discrete frequency noise reductions up to 17 dB were obtained. At close spacing, discrete frequency reduction was at a rate greater than 6 dB per double spacing. Relative harmonic content was not influenced by spacing and reduction was independent of rotor speed. No significant change in broad-band noise was observed.
Test and Evaluation of a Quiet Helicopter Configuration HH-43B
Authors: Bowes M. A.
Published: October 2005

A series of noise control modifications was made to the HH-43B helicopter. The objective of these modifications was attainment of a 50% reduction in forward flight (flyover) octave-band (SPL) signature. Additionally, the effect of each modification was to be evaluated through direct comparison of the acoustic signatures of modified and unmodified configurations. Modifications were made to the aircraft engine, drive, and rotor systems, resulting in substantial reductions in the helicopter noise signature. Significant reductions were achieved in all octave bands of interest, i.e., 63 Hz to 4 kHz, with an average reduction exceeding 8.5 dB. A total of 10 distinct aircraft configurations were tested, and their respective noise control modifications evaluated. Greatest reductions in noise signature resulted from engine and rotor system modifications. Turbine engine noise reductions were obtained through the use of reactively lined inlet and exhaust silencers, which produced substantial reduction of middle- to high-frequency noise components. Rotor system modifications included both blade geometry changes, i.e., increased diameter and chord, modified airfoil section, increased (negative) blade twist rate, etc., and reduced tip speed. These modifications resulted in acoustic signature changes covering the entire frequency spectrum, i.e., 20 Hz through 10 kHz [Work supported by ARPA-DOD, monitored by Eustis Directorate, USAAMRDL.]
Estimates of the Contribution to Jet Engine Exhaust Noise Made by Internal Sources
Authors: Plett E. G. , Summerfield M.
Published: October 2005

The flow unsteadiness generated inside a turbojet engine by unsteady flow interactions with struts as well as the unsteadiness generated by nonsteady combustion processes may be significant sources of noise in high-bypass aircraft engines. By treating the jet exhaust plane as a surface bounding the otherwise free jet, a surface integral of the appropriate turbulence quantities [N. Curle, Proc. Roy. Soc. 231A, (1955)] over the exit plane of the nozzle yields the contribution from the internal sources. Monopoles, dipoles, and quadrupoles have been used to characterize the fluctuations at the exit plane. Source strengths corresponding to turbulence intensities ranging from 1% to 20% are used in calculations. It is shown that if the exit plane and shear region have the same turbulence intensity, then a single exit plane monopole source could produce more noise than 103 shear region quadrupoles over the entire subsonic jet Mach number range. and if the exit plane was filled by a single dipole rather than monopole source, it would be the dominate source to Mach numbers of about 0.6. [Supported by ONR contract and by NASA grant.]
Effect of Studs and Cavity Absorption on the Sound Transmission Loss of Plasterboard Walls
Authors: Rinsk Alan , Lyon Richard H.
Published: October 2005

The experimental transmission loss (TL) of a 9-ft×14-ft single leaf 12-in.-thick plasterboard wall with and without studs compares well with mass law below its critical frequency (3150 Hz) and is dependent on panel resonance level, as measured with an accelerometer, above the critical frequency. The addition of 2×4 wood studs every 16 in. to the single panel increased its resonance radiation efficiency below critical frequency by an amount predicted by the ratio of panel impedance to stud lineal mass impedance. By contrast, the addition of steel channel studs every 24 in. had no significant effect on the panel radiation efficiency. Tests on a double leaf plasterboard wall separated 358 in. by steel channel studs every 24-in. showed that the intermediate TL from source room to the cavity, as determined with a microphone suspended in the cavity, is quite similar to the TL of a single panel between two large rooms. The cavity is shown to behave as an intermediate resonant system between the transmission and receiver rooms. The cavity loss factor, as determined from reverberation times of sound piped into the cavity, could be increased by the addition of felt or foam strips hung inside the cavity. With continued addition of cavity absorption, an upper TL limit of the double panel is reached when transmission becomes dominated by receiver-room-panel resonant modes due to flanking through the steel studs and about equally as much dominated by nonreverberant direct sound transmission through the cavity, virtually unaffected by cavity absorption. This explains why it has been observed that continued increase in cavity absorption beyond a certain point no longer increases the overall TL of a double leaf wall.
Theoretical Considerations in the Determination of Complex Acoustical Impedance of Nonlocally Reacting Noise Control Materials
Authors: Jhaveri Arun G.
Published: October 2005

Various techniques and computer programs have been developed for the determination of complex acoustic impedance of thin (locally reacting) noise-control materials, such as metal felt, polyimide, perforated plate, etc., within last five years. However, there is very little published data available for the calculation of complex impedance and absorption coefficient of thick, homogeneous, and nonlocally reacting noise control materials, such as glass fiber and open-pore polyurethane foam. The present paper establishes theoretical guidelines for the computation of acoustical parameters that determine the complex acoustical impedance of porous, homogeneous, and isotropic materials. These materials have been and are being used for the abatement and control of air-conditioning and air-borne types of environmental noise. A thick or nonlocally reacting type of material is characterized by two complex numbers, namely, the characteristic impedance (z/ρc or w), and the propagation constant (γ), as opposed to only one complex number for thin or locally reacting materials. Then, two separate measurements based on theoretical equations contained herein yielding two complex numbers, would suffice in the determination of required complex acoustic impedance and absorption coefficient as a function of discrete frequency f.
Acoustic Measurements Using Acoustic Pulse Trains of Infinite Duration
Authors: Van Doeren Richard E.
Published: October 2005

Many workers in the field of acoustics have found that measurements must be performed in nonideal environments in which error contributions from reflections and diffractions can be significant. Furthermore, creation of satisfactory acoustical measurement conditions (e.g., anechoic or reverberant rooms) can be costly both in dollars and space. Use of acoustic pulsetrains of infinite duration has definite advantages for measurement of reflection and transmission coefficient of achitectural panels and for measurement of sound pressure level effects of full-size or scaled noise reduction systems. The experimental pulse system at North Star is described and application of pulsetrain techniques to measurement of reflection, transmission, and sound-pressure-level reduction of scale- and full-size systems is discussed. Experimental results are given for transmission and reflection from architectural-type panels and for a scale model of a highway barrier.
Discrimination of acoustic patterns under various levels of stimulus uncertainty
Authors: Watson Charles S.
Published: October 2005

The discriminability of sequential acoustic events is partially predictable from listeners' well-established abilities to discriminate among the events' component sounds (e.g., single tones, noise bursts, or clicks). Research in our laboratory and in others suggests that failures of such predictions result, in large part, from the greater demands complex sounds place upon the mechanisms of attention and memory. Some acoustic sequences can be resolved as accurately as can their individual components, but only when they are heard in a minimal-uncertainty context. Even a moderate degree of stimulus uncertainty can degrade performance for these sounds by more than an order of magnitude. Although listeners can resolve such sounds with greater accuracy under special conditions, those conditions are not all representative of music or connected discourse. Maximum capabilities to discriminate acoustic sequences under high-uncertainty listening conditions should therefore be considered as important a limitation on auditory processing as other, more primitive, psychophysical measures. [This research was supported in part by a grant from the National Institute of Neurological Diseases and Stroke to the Central Institute for the Deaf.]
Categorical perception and absolute pitch
Authors: Harris G. , Siegel J.
Published: October 2005

Musicians with absolute pitch made magnitude estimates of 20 tone frequencies spaced over five semitones. When averaged over several blocks of trials, the resulting psychophysical functions increased smoothly as a function of tone frequency. However, for absolute pitch subjects the individual block data revealed functions with five discrete steps, such that physically different stimuli were judged by the observers to have the same subjective magnitude. These steps had variable boundaries, so that the discrete character of the functions was obscured when averaging techniques were used. The results suggest that musicians with absolute pitch have categorical perception for tone frequency, based on their knowledge of the notes of the musical scale.
Beam pattern control by the use of real (vice complex) shading coefficients
Authors: Smith David A.
Published: October 2005

The usual method of beam control using complex shading coefficients or complex shading functions involves the algorithm of “phasing or time delaying to a plane,” and then performing a weighted sum. This paper considers the performance of beams formed using real shading coefficients (i.e., no time delay or phase shift being implemented). The algorithm used is a combination of “replica correlations” and the normal “aperture function” for sidelobe control. The array considered in the majority of the work consists of equally spaced finite-sized acoustic elements or staves arranged on the surface of a right circular cylinder. It has been found, however, that the method works for several geometries including volumetric, line, and plane arrays. The functional dependence of beamwidth, bandwidth, sidelobe level, and sensitivity upon array size in wavelengths is presented. Advantages and disadvantages of the use of the method will be enumerated. Experimental proof of the mathematical model used in this study is contained in a companion paper.
Beamforming experiments with a cylindrical array using real (vice complex) shading coefficients
Authors: Moses Bruce O. , Smith David A.
Published: October 2005

A method of beamforming which utilizes real shading (without the use of time delay or phase shift networks) of the element outputs in a cylindrical hydrophone array was tested. Narrow beam patterns formed from the outputs of a 29.62-wavelength-diameter cylinder of elements space 2° apart are compared with the prediction of the mathematical model presented in a companion paper. The accuracy of the prediction is illustrated by close comparison with measurements made to evaluate the effects of shading coefficient errors, focusing, and frequency response.
Gaussian-Laguerre/Hermite formulation for the nearfield of an ultrasonic transducer
Authors: Cook Bill D. , Arnoult W. J.
Published: October 2005

The orthonormal sets of functions known as Gaussian-Laguerre and Gaussian-Hermite functions (G-L/H) have previously been used in quantum mechanisms, electromagnetic waves, and laser optics. These functions satisfy a modified Helmholtz equation which amounts to a Fresnel approximation. Experimental mappings of sound fields using liquid crystals suggest that the problem of the nearfield of an ultrasonic transducer can be rendered in terms of these functions. This field can be expressed as a linear combination of modes, each described by a (G-L/H) function. Each mode has a slightly different phase velocity in the nearfield but has the same spreading nature and phase velocity in the farfield. By using this formulation, the transition from nearfield to farfield is readily explained.