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Dec 1984

Volume 76, Issue 6, pp. 1609-1883

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A new approach to transducer design applied to a foil electret acoustic antenna

Ilene J. Busch‐Vishniac, James E. West, and R. L. Wallace, Jr.

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1609-1616 (1984); (8 pages)

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A new approach to transducer design is introduced in which a transducer is viewed as a continuum of infinitesimal, locally reacting elements. The transducer response is the coherent sum of the responses of the infinitesimal elements. Hence variations in transducer response characteristics will be produced by variations in the relative contribution of each element. It is thus possible to produce designed transducer characteristics through selection of a suitable local sensitivity shading function. This new approach is used to design a foil electret acoustic antenna which achieves with a single transducer direction characteristics similar to these of a linear microphone array. Five methods for accomplishing sensitivity variation in the electret microphone have been identified: variation of active width, variation of charge density, variation of actual airgap, variation of foil thickness, and variation of effective airgap. It is shown that compared to a linear microphone array, the single transducer acoustic antenna offers advantages of lower sensitivity to small construction errors, directional behavior over a wider frequency range, easier construction, and potentially higher signal to noise ratio.
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43.38.Ar Transducing principles, materials, and structures: general
43.38.Fx Piezoelectric and ferroelectric transducers

Regional differences in the cyclic variation of myocardial backscatter that parallel regional differences in contractile performance

Jack G. Mottley, Robert M. Glueck, Julio E. Perez, Burton E. Sobel, and J. G. Miller

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1617-1623 (1984); (7 pages) | Cited 2 times

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Previous reports from our laboratory indicate that ultrasonic backscatter from myocardium exhibits a cyclic variation during the cardiac cycle that is reduced sharply by ischemia, a process which impairs both systolic contraction and diastolic relaxation. These results suggest that the cyclic variation of backscatter may be related to the cyclic variation of the contractile performance of the myocardium. Because contractile performance of the left ventricle is known to exhibit regional variability, the present study was undertaken to determine whether such regional differences in contractile performance are paralleled by differences in the magnitude of the cyclic variation of ultrasonic backscatter. Measurements obtained from representative zones of three regions of the hearts of ten open‐chest dogs indicate that the magnitude of the cyclic pattern of variation of backscatter parallels the regional differences in contractile performance throughout the left ventricle with the maximum variation (5.5±0.9 dB peak‐to‐peak amplitude) occurring at the apex, intermediate values (4.3±0.8 dB) at the midwall, and minimum (0.5±1.0 dB) at the base. These results suggest that the ultrasonic backscatter may be sensitive to the regional myocardial contractile performance.
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43.80.Cs Acoustical characteristics of biological media: molecular species, cellular level tissues
43.35.Yb Ultrasonic instrumentation and measurement techniques

Vowel‐to‐vowel coarticulation in Catalan VCV sequences

Daniel Recasens

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1624-1635 (1984); (12 pages) | Cited 1 time

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Electropalatographic and acoustical data on vowel‐to‐vowel (V‐to‐V) coarticulatory effects were obtained for Catalan VCV sequences, with the consonants representing different degrees of tongue‐dorsum contact (dorsopalatal approximant [j], alveolo‐palatal nasal [ν], alveolo‐palatal lateral [Y], and alveolar nasal [n]). Results show that the degree of V‐to‐V coarticulation in linguopalatal fronting and F2 frequency varies monotonically and inversely with the degree of tongue‐dorsum contact, carryover effects being larger than anticipatory effects. The temporal extent of coarticulation also varies with the degree of tongue‐dorsum contact, much more so for anticipatory effects than for carryover effects. Overall, results indicate that V‐to‐V coarticulation in VCV sequences is dependent on the mechanical constraints imposed on the tongue dorsum to achieve dorsopalatal closure during the production of the intervening consonant. Moreover, anticipatory effects, but not carryover effects, involve articulatory preprogramming.
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43.70.Aj Anatomy and physiology of the vocal tract, speech aerodynamics, auditory kinetics
43.70.Fq Acoustical correlates of phonetic segments and suprasegmental properties: stress, timing, and intonation

Perceiving vowels in the presence of another sound: Constraints on formant perception

C. J. Darwin

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1636-1647 (1984); (12 pages) | Cited 21 times

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Speech is normally heard against a background of other sounds, yet our ability to isolate perceptually the speech of a particular talker is poorly understood. The experiments reported here illustrate two different ways in which a listener may decide whether a tone at a harmonic of a vowel’s fundamental forms part of the vowel. First, a tone that starts or stops at a different time from a vowel is less likely to be heard as part of that vowel than if it is simultaneous with it; moreover, this effect occurs regardless of whether the tone has been added to a normal vowel, or to a vowel that has already been reduced in energy at the tone’s frequency. Second, energy added simultaneously with a vowel, at a harmonic frequency near to the vowel’s first formant, may or may not be fully incorporated into the vowel percept, depending on its relation to the first formant: When the additional tone is just below the vowel’s first formant frequency, it is less likely to be incorporated than energy that is added at a frequency just above the first formant. Both experiments show that formants may only be estimated after properties of the sound wave have been grouped into different apparent sound sources. The first result illustrates a general auditory mechanism for performing perceptual grouping, while the second result illustrates a mechanism that may use a more specific constraint on vocal‐tract transfer functions.
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43.71.Es Vowel and consonant perception; perception of words, sentences, and fluent speech
43.66.Jh Timbre, timbre in musical acoustics

Harmonic‐intensity analysis of normal and hoarse voices

Nobuaki Hiraoka, Yasuhiro Kitazoe, Hisashi Ueta, Shinzo Tanaka, and Masahiro Tanabe

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1648-1651 (1984); (4 pages) | Cited 2 times

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Objective evaluation of normal and hoarse voices is performed considering the characteristic that hoarse voices show a prominent fundamental frequency intensity compared with harmonics in the voice spectrum. The relative harmonic intensity Hr, obtained from a stable portion of the sustained vowel /a/, is defined as the intensity of the second and higher harmonics expressed as a percentage of the total voice intensity. Ninety‐five percent of the normal voices examined have Hr larger than the critical value of 67.2%, whereas 90% of the hoarse voices have Hr smaller than the critical value. The harmonic‐intensity analysis thus provides good discrimination between normal and hoarse voices.
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43.70.Dn Disordered speech
43.72.Ar Speech analysis and analysis techniques; parametric representation of speech
43.70.Aj Anatomy and physiology of the vocal tract, speech aerodynamics, auditory kinetics

Which syllable does an intervocalic stop belong to? A selective adaptation study

Arthur G. Samuel, Donna Kat, and Vivien C. Tartter

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1652-1663 (1984); (12 pages) | Cited 1 time

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Three selective adaptation experiments were conducted to investigate whether intervocalic stops are perceived as the end of the preceding syllable or as the beginning of the following one. The pattern of adaptation effects (and just as importantly, noneffects) indicated that intervocalic stop consonants are perceptually more like syllable‐initial than syllable‐final ones. From this it might be concluded that the perceptual system breaks down a vowel–consonant–vowel (VCV) utterance into a V–CV sequence. However, the similarity of an intervocalic stop to a syllable‐initial one is quite limited; the consonant in a VCV is apparently treated as essentially different from consonants in either VC or CV utterances. These results clarify, and perhaps complicate, the role of the syllable in models of the speech perception process.
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43.71.Es Vowel and consonant perception; perception of words, sentences, and fluent speech
43.70.Fq Acoustical correlates of phonetic segments and suprasegmental properties: stress, timing, and intonation

A multispeaker analysis of durations in read French paragraphs

Douglas O’Shaughnessy

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1664-1672 (1984); (9 pages)

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Understanding how the durations of acoustic segments vary in natural language can lead to more intelligible synthetic speech, and to improved automatic recognition. Toward this goal, a 111‐word French paragraph was read by 29 native speakers from France. Measured durations of acoustic segments were significantly shorter than those in earlier studies of stressed words in French sentences read from a list. Previously recognized trends (short schwa vowels and function words; long unvoiced fricatives, nasalized vowels, and prepausal syllables) are confirmed and quantitative results are given. Vowels were longer preceding voiced fricatives (but not prior to /r/), and were also longer at sentence‐internal pauses than at the end of a sentence. Standard deviations of acoustic segment durations (at fixed positions in the paragraph) across speakers averaged less than 25% in most cases. The exceptional, larger deviations occurred primarily in segments adjacent to pauses. Speaking rate variations could account for only one‐sixth of the deviations, the rest being attributable to relatively free variation across speakers. A generative model of French durations, suitable for synthesis‐by‐rule, is presented, and applications to automatic recognition are discussed.
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43.70.Fq Acoustical correlates of phonetic segments and suprasegmental properties: stress, timing, and intonation
43.70.Kv Cross-linguistic speech production and acoustics

The effect of correcting fundamental frequency on the intelligibility of deaf speech and its interaction with temporal aspects

Ben Maassen and Dirk‐Jan Povel

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1673-1681 (1984); (9 pages) | Cited 1 time

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This study investigates the role of intonation for the intelligibility of deaf speech. The intonation contours of Dutch sentences spoken by deaf children were manipulated using digital signal processing techniques, including LPC analysis. Sentence intonation was corrected by replacing the original F0 contour of the deaf utterance with an artificial contour derived from a formalized intonation grammar. Three types of intonation corrections were produced, differing with respect to the underlying accent structure and the type of F0 movements used. The overall results show that intonation correction yields a small but significant improvement in intelligibility of 7% (from 20% to 27% words correctly identified). The largest gain is obtained after removal of over‐accentuations. To evaluate the interaction with temporal aspects, intonation corrections were also implemented on temporally corrected sentences. Total growth in intelligibility due to these combined corrections amounts to 13%. Thus it is concluded that no dramatic gain in intelligibility may be expected if speech pathologists succeed in teaching their deaf pupils to have better control over the suprasegmental aspects of their speech.
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43.70.Dn Disordered speech
43.71.Ky Speech perception by the hearing impaired
43.71.Gv Measures of speech perception (intelligibility and quality)
43.66.Sr Deafness, audiometry, aging effects

The detection of notes incompatible with scalar structure

Peter Howell, Robert West, and Ian Cross

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1682-1689 (1984); (8 pages)

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A fundamental question in the perception of melody is how listeners use pitch information to arrive at a sense of musical scale. We investigated possible ways in which this might be achieved. Listeners were required to detect notes incompatible with scalar structure in the course of melodic sequences. Within the parameters of our experimental sequences, the results showed that detection of nonscalar notes could be based on identifying nonscalar interval combinations (e.g., F♯, A, Bb), as well as by matching successive notes to a developing scalar schema. The strength of the scalar schema was influenced by relations between notes in the circle of fifths, with notes closer together in the circle of fifths (e.g., C, G, D, A) leading to a stronger schema than notes further apart (e.g., C, G, E, B).
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43.66.Lj Perceptual effects of sound

Performance on frequency‐discrimination tasks by musicians and nonmusicians

Murray F. Spiegel and Charles S. Watson

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1690-1695 (1984); (6 pages) | Cited 9 times

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Auditory discrimination abilities of professional musicians were compared with those of nonmusicians. The stimuli for the frequency‐discrimination tasks were 300‐msec sinusoidal tones, 300‐msec square waves, and tone patterns consisting of ten 40‐msec tones played sequentially. The musicians’ difference thresholds for single tones were between Δf /f=0.001 and 0.0045. One‐half of the nonmusicians attained thresholds almost as low; the rest attained larger thresholds, up to Δf /f=0.017. The results for the pattern stimuli show a clearer separation between the nonmusicians and musicians, whose median difference thresholds were about three times smaller. However, nonmusician listeners who had previously trained with patterns not in the test set had different thresholds, substantially smaller than those obtained by the musicians. The appropriateness of preferential recruitment of musicians for psychoacoustic research is discussed. The responses to a musical background survey and correlations between the survey items and discrimination performance are contained in a supplement to this paper [PAPS JASMA 76, xxx‐xx].
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43.66.Fe Discrimination: intensity and frequency
43.66.Ba Models and theories of auditory processes

Effects of background noise level on detection of tone glides

M. Jane Collins and John K. Cullen, Jr.

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1696-1698 (1984); (3 pages) | Cited 1 time

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Detection thresholds were obtained for short duration rising tone glides and falling tone glides for normal hearing listeners in quiet and with varied levels of background, broadband noise. For signal durations 50 ms, or greater, pure tones were detectable at lower levels than were rising and falling tone glides; no level effects were observed. For signal durations less than 50 ms an interaction between background noise level, signal duration, and signal class was observed. The previously reported tendency for short duration rising glides to be detected at lower sound pressures than pure tones or falling tone glides appears to be unique to a limited range of noise levels (40–75 dB).
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43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music
43.66.Dc Masking
43.66.Cb Loudness, absolute threshold

NoSo and NoSπ thresholds as a function of masker level for narrow‐band and wideband masking noise

Joseph W. Hall and Antony D. G. Harvey

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1699-1703 (1984); (5 pages) | Cited 8 times

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NoSo and NoSπ thresholds for a 500‐Hz signal were determined as a function of masker level for masking noises having 600‐ and 50‐Hz bandwidths centered on 500 Hz. Noise levels of 0, 10, 20, 30, 40, 50, and 60 dB/Hz were used for the 600‐Hz bandwidth, and noise levels of 0, 10, 20, 30, 40, 50, 60, and 70 dB/Hz were used for the 50‐Hz bandwidth. The So thresholds increased as a function of increasing noise level with a slope close to 1.0, for both the 50‐ and 600‐Hz masker bandwidths. For the 600‐Hz‐wide masker, the Sπ thresholds increased with a slope less than 1.0 for masker levels from 0 dB/Hz to approximately 20 dB/Hz, but increased with a slope close to 1.0 for masker levels above approximately 20 dB/Hz. For the 50‐Hz‐wide masker, the Sπ thresholds increased with a slope less than 1.0 for masker levels from 0 dB/Hz to approximately 40 dB/Hz, but increased with a slope close to 1.0 for masker levels above approximately 40 dB/Hz. The results are discussed in terms of different binaural processes involving interaction between monaural critical bands centered on the signal frequency, and interaction involving monaural critical bands outside the monaural critical band centered on the signal. It was suggested that the narrow‐band noise results are probably more appropriate than the wideband noise results when considering binaural interaction involving monaural critical bands centered on the signal frequency.
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43.66.Pn Binaural hearing
43.66.Dc Masking
43.66.Ba Models and theories of auditory processes

Discrimination of the spatial distribution of concurrently active sound sources: Some experiments with stereophonic arrays

David R. Perrott

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1704-1712 (1984); (9 pages) | Cited 1 time

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Several experiments are described in which subjects were required to discriminate differences in the spatial distribution of concurrently active sound sources in stereophonic arrays. For pure tone stimuli and binaural listening, systematic discrimination functions were observed when relatively small intersource frequency differences (approximately 30 Hz) were present. For tonal stimuli, this discrimination task was reliably performed only for frequencies below 1500 Hz. Additional tests were conducted with amplitude‐modulated tonal stimuli, low‐ and high‐frequency uncorrelated noise, correlated low‐frequency noise, and, with the uncorrelated low‐frequency noise, for stimuli presented in both the vertical plane and under monaural listening conditions. The results of all of these manipulations support the notion that the spatial distribution of sources in a stereophonic array can be appreciated if disparate low‐frequency energy is available from the sources in a horizontal configuration. The implications of these results are discussed relative to acoustic processing in the natural environment.
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43.66.Qp Localization of sound sources
43.66.Pn Binaural hearing

Interaction of spontaneous oto‐acoustic emissions and external sounds

W. M. Rabinowitz and G. P. Widin

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1713-1720 (1984); (8 pages) | Cited 7 times

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Spontaneous oto‐acoustic emissions (SOAEs) were detected in eight of 19 ears from 12 persons with normal hearing. On a subset of these individuals, additional characteristics of SOAEs were studied including the suppression of SOAE level caused by an external tone. For suppressor tones below and slightly above the frequency of an SOAE, suppression is quite abrupt (about 5 dB of SOAE level reduction per dB increase in suppressor level); however, as suppressor frequency increases above the SOAE, the rate of suppression decreases. A release from suppression was demonstrated by the interaction of an SOAE with two external tones. When a tone above the SOAE frequency causes suppression, a second tone above the suppressor frequency can cause the SOAE to increase nearly to its ambient level. This finding is interpreted as the second tone having suppressed some aspect of the intracochlear influence of the first tone. The growth rate of this secondary suppression appears to be near 1 dB/dB, a value similar to rates derived from existing measures of two‐tone suppression observed in auditory‐nerve‐fiber recordings in laboratory mammals.
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43.64.Kc Cochlear mechanics
43.25.Lj Parametric arrays, interaction of sound with sound, virtual sources
43.64.Yp Instruments and methods

Signaling along elastic plates with wideband acoustic pulses

James E. Barger

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1721-1730 (1984); (10 pages)

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High data‐rate signaling with acoustic pulses along an elastic plate is made difficult by the conversion of a transmitted pulse into many component pulses that arrive separately during a time span that can exceed the travel time of the first component pulse to arrive. In addition, the amplitude of the received signals fluctuates widely with changes in pulse center frequency. But to obtain high data‐rate signaling, many pulses with different center frequencies must be transmitted at closely spaced time intervals. It is important in these circumstances to understand the principal features of the component pulses that originate from a single transmitted pulse. This paper derives analytical expressions for the arrival times, the peak envelope amplitudes, and the center frequencies of the arriving component pulses that are transmitted by wideband line forces acting normal to the plate surface. Each component pulse is the manifestation of a different mode of propagation. By carefully picking the bandwidth and center frequency of the launching pulse, the number of its component pulses (or, alternatively, excited modes) that have significant amplitude can be minimized, if the pulse center frequency is low enough. If the frequency is high enough, so that more than about six propagating modes can exist, then the number of arrivals cannot be reduced significantly. At higher frequencies, several of the modes merge to form the Rayleigh wave, which is centered within both earlier and later modes. A series of photographs show how transmitted Gaussian pulses look when the pulse center frequency is picked to illustrate three regimes; where the Rayleigh wave is not formed; where it is partially formed; and where it is fully formed.
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43.60.Cg Statistical properties of signals and noise
43.40.Dx Vibrations of membranes and plates
43.35.Pt Surface waves in solids and liquids

Reconstructing spatially incoherent random sources in the nearfield: Exact inversion formulas for circular and spherical arrays

Stephen J. Norton and Melvin Linzer

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1731-1737 (1984); (7 pages) | Cited 1 time

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We consider the inverse problem for a two‐dimensional, spatially incoherent random source. Under these assumptions, we show that an exact inversion formula can be derived for recovering the source spectral intensity, as a function of position, from nearfield measurements of the emitted radiation recorded on the circumference of a circle enclosing the source region. Although solutions to the inverse random source problem have been reported in the past, these results have almost always employed farfield approximations. After deriving the inversion formula in two dimensions, we discuss an efficient method for numerically evaluating this formula using the fast Fourier transform algorithm. Finally, a generalization of the inverse problem to a three‐dimensional source enclosed by a spherical recording surface is given.
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43.60.Gk Space-time signal processing, other than matched field processing

A performance comparison of four noise background normalization schemes proposed for signal detection systems

William A. Struzinski and Edward D. Lowe

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1738-1742 (1984); (5 pages) | Cited 5 times

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This article presents the results of a simulation experiment comparing the behavior of four noise background normalization schemes. The four schemes are the two‐pass mean, the split three‐pass mean, the split average exclude average, and the order truncate average normalizers.
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43.60.Gk Space-time signal processing, other than matched field processing
43.50.Cb Noise spectra, determination of sound power

Free vibration of a cantilever folded plate

Toshihiro Irie, Gen Yamada, and Yukinori Kobayashi

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1743-1748 (1984); (6 pages) | Cited 1 time

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An analysis is presented for the free vibration of a cantilever folded plate. For this purpose, the deflection displacements of the plate are written in a series of the products of the eigenfunctions of a cantilever beam and a cranked free–free beam parallel to the clamped edge of the plate. The kinetic and strain energies of the plate are evaluated analytically, and the frequency equations are derived by the Ritz method. The method is applied to cantilever folded plates with and without structural symmetry, and the natural frequencies and the mode shapes of vibration are calculated numerically giving the results.
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43.40.Dx Vibrations of membranes and plates

The propagation of sound through a gas with an overpopulation of excited states

F. Douglas Shields

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1749-1754 (1984); (6 pages)

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Calculations have been made which predict sound amplification under certain conditions when propagated through a gas with an overpopulation of vibrationally excited states. Three metastable states have been analyzed, where the vibrational temperature is varying slowly on the scale of the sound period. The first is that of a long‐lived vibrational mode in a diatomic gas. Calculations were made for N2, CO, O2, and Cl2. The second is that of a fast v–v exchange followed by a slow v–t. Calculations were made for CD4 as an example of this situation. And the third is that of a slow v–v followed by a fast v–t. Calculations were made for a mixture of N2/CO2/He as an example of this situation. Some of the difficulties in experimentally observing the gain are discussed.
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43.35.Ae Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in gases
51.40.+p Acoustical properties
33.20.Tp Vibrational analysis
43.35.Fj Ultrasonic relaxation processes in gases, liquids, and solids

Nonlinear distortion of ultrasonic waves in small crystalline samples

Bruce D. Blackburn and M. A. Breazeale

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1755-1760 (1984); (6 pages) | Cited 1 time

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Harmonic generation of 30‐MHz ultrasonic waves in small solid samples is analyzed to establish a technique for evaluating combinations of third‐order elastic constants of samples which cannot be grown large enough to allow one to make the infinite plane‐wave approximation. Results are reported for CsCdF3 and KZnF3.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
43.25.Cb Macrosonic propagation, finite amplitude sound; shock waves

Measurements of reflection and transmission coefficients of Rayleigh waves from cracks

Ruiqi Dong and Laszlo Adler

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1761-1763 (1984); (3 pages) | Cited 6 times

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The angle and frequency dependence of reflection and transmission coefficients of Rayleigh waves from surface cracks in titanium has been measured. The measured results are compared to the recently developed theory of Angel and Achenbach [J. Acoust. Soc. Am. 75, 313–319 (1984)]. Very good agreement is obtained between experiment and theory.
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43.35.Pt Surface waves in solids and liquids
43.20.Fn Scattering of acoustic waves
68.35.Gy Mechanical properties; surface strains
68.35.Iv Acoustical properties
62.20.M- Structural failure of materials

The rational approximation to the acoustic wave equation with bottom interaction

Robert R. Greene

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1764-1773 (1984); (10 pages) | Cited 12 times

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The rational‐linear approximation to the wave equation is a full‐wave approach to modeling range‐dependent ocean acoustic propagation with bottom interaction. It is a one‐way wave equation which gives an accurate treatment of high‐angle propagation to angles of about 40° with respect to the horizontal. Reflection from sound speed and density discontinuities is treated using the natural wave equation matching conditions. Bathymetry is allowed to vary in range. A tridiagonal implicit finite‐difference solution of this equation has been implemented. It has several advantages over the tridiagonal Crank–Nicholson solution of the parabolic equation. It more accurately models high angles of propagation, treats attenuation as a function of path length rather than range, and models range‐dependent bathymetry in a way that suits the form of the one‐way wave equation. The numerical methods are fourth‐order accurate in depth. The resulting implicit range step is still in the simple tridiagonal form.
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43.30.Bp Normal mode propagation of sound in water
43.20.Bi Mathematical theory of wave propagation
43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
92.10.Vz Underwater sound

Low‐frequency grazing propagation over periodic steep‐sloped rigid roughness elements

Herman Medwin, Gerald L. D’Spain, Emily Childs, and Stephen J. Hollis

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1774-1790 (1984); (17 pages) | Cited 3 times

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Extensive experimental studies of the coherently forward scattered sound at grazing incidence to low roughness rigid surfaces with periodic steep‐sloped elements have confirmed several of the theoretical predictions of large boundary wave amplitude and subsonic dispersion [Tolstoy, J. Acoust. Soc. Am. 75, 1–22 (1984)]. For example, at short ranges the boundary wave amplitude diverges cylindrically and is proportional to ϵk3/2r−1/2, where ϵ is the scattering parameter, k is the wavenumber, and r is the range; thereby, at sufficient ranges and frequencies it exceeds the amplitude of the spherically diverging direct wave. The dispersion is subsonic and goes as Ak2ϵ2, where A depends on the roughness element. The experiments have also revealed an attenuation factor exp (−δr), where δ=αk6 due to incoherent scatter up to critical range kr=π/(Ak2ϵ2) beyond which an attenuation of form exp (− 1/2 A2ϵ4k6r2) becomes dominant. This leads to frequency‐independent peak amplitudes two to six times the direct wave amplitude for the surfaces studied. Beyond this critical, frequency‐dependent, range the boundary wave catastrophically self‐destructs due to interference. The amplitude, dispersion, and attenuation of the boundary wave is a function of the packing, heights, and slopes or shapes of the scattering elements. Results are presented for spheres, spaced and packed circular cylinders, and several wedge corrugated roughness elements. The boundary wave is also shown to diffract over a ridge in the same manner as the direct wave from a point source.
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43.30.Bp Normal mode propagation of sound in water
43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect
43.20.Bi Mathematical theory of wave propagation

Modified sound refraction near a rough ocean bottom

Herman Medwin and Jorge C. Novarini

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1791-1796 (1984); (6 pages) | Cited 2 times

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It is well established that there is a strong boundary wave during low‐frequency grazing scatter at a rough rigid surface overlaid by a homogeneous medium. Tolstoy [J. Acoust. Soc. Am. 69, 1290–1298 (1981)] has also shown that when source and receiver are on the bottom and the velocity in the medium decreases away from the ocean bottom the boundary wave tunnels into the shadow zone. We consider the case when source and receiver are near the ocean surface. It is predicted that, under certain conditions of velocity gradient, frequency and bottom roughness, the addition of the boundary wave mode to the volume wave mode will change the path of the grazing specularly scattered ray above the rough bottom. As a result, an upward refracted ray will reach the surface at a skip distance significantly greater than for a smooth bottom. This modified refraction phenomenon should be detectable at sea, under suitable indicated experimental conditions.
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43.30.Bp Normal mode propagation of sound in water
43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect
43.20.Bi Mathematical theory of wave propagation
92.10.Vz Underwater sound

A range refraction parabolic equation

Frederick D. Tappert and Ding Lee

J. Acoust. Soc. Am. Volume 76, Issue 6, pp. 1797-1803 (1984); (7 pages)

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Application of the standard parabolic wave equation to solve real problems requires a clever selection of the reference wavenumber k0. An extended parabolic equation, having range refraction capability, is reintroduced in such a manner so as to be totally independent of k0. An already existing Implicit Finite‐Difference (IFD) model was applied to test the range refraction parabolic equation. Results compare favorably with known solutions for weakly range‐dependent environments, but yield significant corrections for propagation through strong oceanic fronts.
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43.30.Bp Normal mode propagation of sound in water
43.20.Bi Mathematical theory of wave propagation
43.30.Es Velocity, attenuation, refraction, and diffraction in water, Doppler effect
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