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Journal of the Acoustical Society of America

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Oct 1990

Volume 88, Issue 4, pp. 1679-2056

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A novel method for the measurement of acoustic speed

I. Y. Kuo, B. Hete, and K. K. Shung

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1679-1682 (1990); (4 pages) | Cited 10 times

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Traditional methods for measuring acoustic speed require knowledge of either the specimen thickness or the distances between the transducers and the specimen. In general, the accuracy in measuring these quantities determines the accuracy of the experimental technique for measuring speed. This problem is particularly acute in measuring sound speed in biological specimens. A new method for measuring acoustic speed of materials, which eliminates the need for determining these quantities, has been developed. The technique, which necessitates the use of only one transducer, requires measurement of four times of flight of a sound pulse and the knowledge of the speed of sound in a reference fluid medium in which the specimen is placed. Ultrasonic speed in stainless steel and Plexiglas was measured using this method to verify its validity. Results on measurements on porcine liver, myocardium, and soft fat are also reported.
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43.80.Ev Acoustical measurement methods in biological systems and media
43.80.Cs Acoustical characteristics of biological media: molecular species, cellular level tissues

Formant frequencies of Dutch vowels in a text, read at normal and fast rate

R. J. J. H. van Son and Louis C. W. Pols

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1683-1693 (1990); (11 pages) | Cited 5 times

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Speaking rate is thought to affect the spectral features of vowels. Target‐undershoot models of vowel production predict more spectral reduction and coarticulation of vowels in fast‐rate speech than in normal‐rate speech. To test this prediction, a meaningful Dutch text of about 850 words was read twice by an experienced newscaster, once at a normal speaking rate and once as fast as possible. All realizations of seven different vowels and some realizations of the schwa (/E/) were isolated. The first and second formant frequency values of all realizations were measured at five different points, each time by making cross sections at different points in the vowel realization. The different selections of these points are based on procedures used in literature, such as maximal F1 or mean formant value. No spectral vowel reduction was found that could be attributed to a faster speaking rate, neither was a change in coarticulation found. The only systematic effect was a higher F1 value in fast‐rate speech irrespective of vowel identity. This possibly suggests a generally more open articulation of vowels, speaking louder, or some other general change in speaking style by our speaker when he speaks fast.
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43.70.-h Speech production
43.70.Fq Acoustical correlates of phonetic segments and suprasegmental properties: stress, timing, and intonation

Some factors affecting the magnitude of comodulation masking release

Brian C. J. Moore, Joseph W. Hall, III, John H. Grose, and Gregory P. Schooneveldt

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1694-1702 (1990); (9 pages) | Cited 7 times

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This paper examines some of the factors that can affect the magnitude of comodulation masking release (CMR). In experiment I, psychometric functions were measured for the detection of a 1‐kHz sinusoidal signal in a ‘‘multiplied’’ narrow‐band noise centered at 1 kHz (reference condition) and the same noise with two comodulated flanking bands added. The functions were slightly steeper for the comodulated than for the reference masker. Thus CMRs measured at a high percent correct point were slightly (0.4 dB) larger than CMRs measured at a low percent correct point. Large individual differences were found for the reference masker but not for the comodulated masker. Experiment II compared CMRs obtained with narrow‐band Gaussian noise and multiplied noise, using a single flanking band. For a flanking band remote from the signal frequency, the CMRs were smaller and more variable for the multiplied noise than for the Gaussian noise. This variability arose mainly from individual differences in the reference condition. Experiment III compared growth‐of‐masking functions for a signal centered in Gaussian noise and multiplied noise. Thresholds were lower for the multiplied than for the Gaussian noise, and the differences were greatest at high noise levels. The results are consistent with the idea that, for multiplied noise, some subjects can detect a change in the distribution of the envelope of the stimulus, when the signal is added to the masker. Such subjects have low thresholds in the reference condition, and give small CMRs. Other subjects are relatively insensitive to this cue. They have higher thresholds in the reference condition, and give larger CMRs. For Gaussian noise, thresholds for the reference condition are relatively stable across subjects and CMRs tend to be substantial, even for flanking‐band frequencies remote from the signal frequency.
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43.66.Dc Masking
43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music

Spectro‐temporal integration in signal detection

Willem A. C. van den Brink and Tammo Houtgast

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1703-1711 (1990); (9 pages) | Cited 18 times

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This paper is concerned with aspects of temporal integration and across‐frequency integration in signal detection. Previous experiments on the detection of brief broadband signals (clicks) in continuous broadband noise revealed efficient spectral integration. The extent to which this effect is restricted to a critical time window was investigated by manipulating the temporal relations among the signal components in different frequency regions. In a typical experiment, the signal consists of nine brief Gaussian‐shaped tone pulses, equally distributed at 1/3‐oct intervals, each with a spectral width of about 1/3 oct, and each equally detectable in white noise. In the synchronized condition (i.e., coinciding peaks of the nine Gaussian envelopes), the detection threshold is reached when the levels of the nine individual tone pulses are about 8 dB below their individual threshold levels (efficient spectral integration). When the signal is progressively desynchronized (i.e., noncoinciding peaks of the Gaussian envelopes), detection threshold is found to increase. This suggests that efficient spectral integration in signal detection is confined to a narrow time window, with a typical value of 30 ms. Similar experiments were performed with respect to the efficiency of temporal integration. For constant‐duration signals (100 ms), the detection threshold is found to increase when progressively widening signal bandwidth. The data indicate that the efficient temporal integration in signal detection is confined to a narrow frequency window, which, not surprisingly, corresponds to the critical bandwidth.
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43.66.Dc Masking
43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music

Hearing a mistuned harmonic in an otherwise periodic complex tone

William Morris Hartmann, Stephen McAdams, and Bennett K. Smith

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1712-1724 (1990); (13 pages) | Cited 29 times

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The ability of a listener to detect a mistuned harmonic in an otherwise periodic tone is representative of the capacity to segregate auditory entities on the basis of steady‐state signal cues. By use of a task in which listeners matched the pitch of a mistuned harmonic, this ability has been studied, in order to find dependences on mistuned harmonic number, fundamental frequency, signal level, and signal duration. The results considerably augment the data previously obtained from discrimination experiments and from experiments in which listeners counted apparent sources. Although previous work has emphasized the role of spectral resolution in the segregation process, the present work suggests that neural synchrony is an important consideration; our data show that listeners lose the ability to segregate mistuned harmonics at high frequencies where synchronous neural firing vanishes. The functional form of this loss is insensitive to the spacing of the harmonics. The matching experiment also permits the measurement of the pitches of mistuned harmonics. The data exhibit shifts of a form that argues against models of pitch shifts that are based entirely upon partial masking.
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43.66.Fe Discrimination: intensity and frequency
43.66.Hg Pitch
43.66.Jh Timbre, timbre in musical acoustics
43.66.Ba Models and theories of auditory processes

Effects of fluctuating noise and interfering speech on the speech‐reception threshold for impaired and normal hearing

Joost M. Festen and Reinier Plomp

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1725-1736 (1990); (12 pages) | Cited 123 times

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The speech‐reception threshold (SRT) for sentences presented in a fluctuating interfering background sound of 80 dBA SPL is measured for 20 normal‐hearing listeners and 20 listeners with sensorineural hearing impairment. The interfering sounds range from steady‐state noise, via modulated noise, to a single competing voice. Two voices are used, one male and one female, and the spectrum of the masker is shaped according to these voices. For both voices, the SRT is measured as well in noise spectrally shaped according to the target voice as shaped according to the other voice. The results show that, for normal‐hearing listeners, the SRT for sentences in modulated noise is 4–6 dB lower than for steady‐state noise; for sentences masked by a competing voice, this difference is 6–8 dB. For listeners with moderate sensorineural hearing loss, elevated thresholds are obtained without an appreciable effect of masker fluctuations. The implications of these results for estimating a hearing handicap in everyday conditions are discussed. By using the articulation index (AI), it is shown that hearing‐impaired individuals perform poorer than suggested by the loss of audibility for some parts of the speech signal. Finally, three mechanisms are discussed that contribute to the absence of unmasking by masker fluctuations in hearing‐impaired listeners. The low sensation level at which the impaired listeners receive the masker seems a major determinant. The second and third factors are: reduced temporal resolution and a reduction in comodulation masking release, respectively.
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43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music
43.66.Sr Deafness, audiometry, aging effects
43.71.Gv Measures of speech perception (intelligibility and quality)

Binaural masking experiments using noise maskers with frequency‐dependent interaural phase differences. I: Influence of signal and masker duration

Armin Kohlrausch

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1737-1748 (1990); (12 pages) | Cited 2 times

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In this paper previous experiments on auditory filter shapes in binaural masking experiments [A. Kohlrausch, J. Acoust. Soc. Am. 84, 573–583 (1988)] are extended to a wider range of masker and signal durations. The masker was a dichotic broadband noise with frequency‐dependent interaural parameters. The interaural phase difference of the masker was 0 below 500 Hz and π above 500 Hz. Signal frequency varied between 200 and 800 Hz, and the signal was presented either monaurally (Sm) or binaurally in antiphase (Sπ). In the first experiment, the masker duration was fixed at 500 ms and signals of 250 and 20 ms were used. In the second experiment, the signal duration was fixed at 20 ms, and the masker duration was reduced to 25 ms. The results from both experiments are consistent with studies using No or Nπ maskers: The binaural masking level difference (BMLD) increases slightly for shorter test signals and decreases strongly for short maskers. The BMLD patterns of the first experiment are well described by the auditory‐filter model derived for stationary test signals, if the additional influence of ‘‘off‐frequency listening’’ for the short test signal is taken into account. The BMLDs resulting from the second experiment (25‐ms masker), however, are much lower than predicted by this filter model. This outcome supports previous observations that binaural unmasking becomes less effective for very short masker durations and indicates that this effect is even stronger for maskers with a complex structure of interaural parameters.
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43.66.Pn Binaural hearing
43.66.Nm Phase effects
43.66.Dc Masking

Binaural masking experiments using noise maskers with frequency‐dependent interaural phase differences. II: Influence of frequency and interaural‐phase uncertainty

Armin Kohlrausch

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1749-1756 (1990); (8 pages) | Cited 3 times

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This study investigates whether binaural signal detection is improved by the listener’s previous knowledge about the interaural phase relations of masker and test signal. Binaural masked thresholds were measured for a 500‐ms dichotic noise masker that had an interaural phase difference of 0 below 500 Hz and of π above 500 Hz. The thresholds for two different 20‐ms test signals were determined within the same measurement using an interleaved adaptive 3‐interval forced‐choice (3IFC) procedure. In each 3IFC trial, both signals could occur with equal probability (uncertainty). The two signals differed in frequency and interaural phase in such a way that one signal always had a frequency above the masker edge frequency (500 Hz) and no interaural phase difference (So), whereas the other signal frequency was below 500 Hz and the interaural phase difference was π (Sπ). The frequencies of a signal pair remained fixed during the whole 3IFC track. These two signals thus lead to two different binaural conditions, i.e., NoSπ for the low‐frequency signal and NπSo for the high‐frequency signal. For comparison, binaural masked thresholds were measured with the same masker for fixed signal frequency and phase. The binaural masking level differences (BMLDs) resulting from the two experimental conditions show no significant difference. This indicates that the binaural system is able to apply different internal transformations or processing strategies simultaneously in different critical bands and even within the same critical band.
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43.66.Pn Binaural hearing
43.66.Nm Phase effects
43.66.Dc Masking

Active localization of virtual sounds

Jack M. Loomis, Chick Hebert, and Joseph G. Cicinelli

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1757-1764 (1990); (8 pages)

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A simple virtual sound display built around a microcomputer and analog hardware is described. The display implements most of the primary cues for sound localization in the ear‐level plane. Judging both from informal observations by users and from objective data obtained in an experiment on homing to virtual and real sounds, it is concluded that simple displays like the one described are effective in creating the impression of external sounds to which observers can locomote with ease; in particular, this means that simulation of the direction‐dependent spectral shaping effects of the pinnae is not a necessary requirement for extracranial sound localization.
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43.66.Qp Localization of sound sources
43.66.Pn Binaural hearing

Perception of amplitude envelope variations of pulsatile electrotactile stimuli

P. J. Blamey, J. I. Alcantara, R. S. C. Cowan, K. L. Galvin, J. Z. Sarant, and G. M. Clark

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1765-1772 (1990); (8 pages) | Cited 2 times

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Gross variations of the speech amplitude envelope, such as the duration of different segments and the gaps between them, carry information about prosody and some segmental features of vowels and consonants. The amplitude envelope is one parameter encoded by the Tickle Talker, an electrotactile speech processor for the hearing impaired which stimulates the digital nerve bundles with a pulsatile electric current. Psychophysical experiments measuring the duration discrimination and identification, gap detection, and integration times for pulsatile electrical stimulation are described and compared with similar auditory measures for normal and impaired hearing and electrical stimulation via a cochlear implant. The tactile duration limen of 15% for a 300‐ms standard was similar to auditory measures. Tactile gap detection thresholds of 9 to 20 ms were larger than for normal‐hearing but shorter than for some hearing‐impaired listeners and cochlear implant users. The electrotactile integration time of about 250 ms was shorter than previously measured tactile values but longer than auditory integration times. The results indicate that the gross amplitude envelope variations should be conveyed well by the Tickle Talker. Short bursts of low amplitude are the features most likely to be poorly perceived.
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43.66.Wv Vibration and tactile senses
43.66.Ts Auditory prostheses, hearing aids

Revision of estimates of acoustic energy reflectance at the human eardrum

Michael R. Stinson

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1773-1778 (1990); (6 pages) | Cited 17 times

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An improved analysis procedure has been applied to standing wave patterns measured previously [B. W. Lawton and M. R. Stinson, J. Acoust. Soc. Am. 79, 1003–1009 (1986)] in human ear canals. Revised acoustic energy reflection coefficients, at the eardrum, are obtained for 20 ears for frequencies between 3 and 13 kHz. The new analysis addresses anomalous features of the standing wave patterns, apparent at frequencies above 8 kHz, due primarily to the curvature of the ear canal. Much better agreement is now found, at these higher frequencies, between the theoretical form assumed for the standing wave patterns and the experimental data. The revised values of eardrum reflectance are somewhat smaller, especially for frequencies above 11 kHz. The reflectance rises from about 0.25 at 4 kHz up to 0.7 at 8 kHz, falls to a minimum of 0.5 at 11 kHz, then rises to 0.6 at 13 kHz. Considerable intersubject variability in the results is noted.
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43.64.Ha Acoustical properties of the outer ear; middle-ear mechanics and reflex
43.20.Ks Standing waves, resonance, normal modes

Amplitude and frequency fluctuations of spontaneous otoacoustic emissions

Pim van Dijk and Hero P. Wit

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1779-1793 (1990); (15 pages) | Cited 12 times

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Amplitude and frequency fluctuations of spontaneous otoacoustic emissions have been studied. Spontaneous otoacoustic emissions were recorded from eight human ears and two frog ears (Rana esculenta). Record length typically was 80 s. For a recorded emission signal, the amplitude signal A(t) (average A0) and time intervals T(ti) between successive positive‐going zero crossings (i counts zero crossings) were determined. Emission amplitude and period both showed small fluctuations: δArms/A0 ranged from 0.7×10−2 to 6.3×10−2 for human emissions and was 24×10−2 for both frog emissions; δTrms ranged from 1.4 to 6.9×10−7 s for human emission and was 50.0 and 55.0×10−7 s for the two frog emissions. There was a positive correlation between δArms/A0 and δTrms as determined for different emissions (R=0.9). Spectra of A(t) and T(ti) revealed that amplitude and period were slowly fluctuating functions: cutoff frequency ΔfδA of the amplitude spectrum ranged from 3 to 18 Hz; ΔfδT ranged from 7 to 32 Hz. Results have been compared to amplitude and frequency fluctuations of a second‐order oscillator, that interacts with a noise source. It has been concluded that an oscillator with linear stiffness (for example a Van der Pol oscillator) driven by white Gaussian noise, cannot account for all experimental results. Other possible oscillators (e.g., nonlinear stiffness) and noise sources (e.g., narrow‐band noise), that may account for the observed phenomena, are discussed.
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43.64.Jb Otoacoustic emissions
43.64.Bt Models and theories of the auditory system

What type of force does the cochlear amplifier produce?

Paul J. Kolston, Egbert de Boer, Max A. Viergever, and Guido F. Smoorenburg

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1794-1801 (1990); (8 pages) | Cited 4 times

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Recent experimental measurements suggest that the mechanical displacement of the basilar membrane (BM) near threshold in a viable mammalian cochlea is greater than 10−8 cm, for a stimulus sound‐pressure level at the eardrum of 20 μPa. The associated response peak is very sensitive to the physiological condition of the cochlea. In the formulation of all recent cochlear models, it has been explicitly assumed that this peak is produced by the cochlear amplifier injecting a large amount of energy into the cochlea, thereby altering the real component of the BM impedance. In this paper, a new cochlear model is described which produces a realistic response by assuming that the cochlear amplifier force acts at a phase such that the main effect is to reduce the imaginary component of the BM impedance. In this new model, the magnitude of the cochlear amplifier force required to produce a realistic response is much smaller than in the previous models. It is suggested that future experimental investigations should attempt to determine both the magnitude and the phase of the forces associated with the cochlear amplifier.
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43.64.Kc Cochlear mechanics
43.64.Bt Models and theories of the auditory system
43.64.Ld Physiology of hair cells

Optimal time‐domain beamforming with simulated annealing including application of a priori information

W. A. Kuperman, Michael D. Collins, John S. Perkins, and N. R. Davis

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1802-1810 (1990); (9 pages) | Cited 8 times

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A fast simulated annealing algorithm is developed for an optimal time‐domain beamformer. The optimal beamformer has greater resolution than the standard frequency‐domain beamformers, which discard information by averaging the data to form a correlation matrix and remove degrees of freedom by collapsing the number of unknown parameters. The optimal ambiguity function uses the data in raw form and depends on all of the unknown source parameters. This approach is practical with simulated annealing. A specialized simulated annealing algorithm is required to search for the source bearings and time series because the parameters are analogous to a mixture of substances with different freezing points. Examples are presented to demonstrate that the optimal beamformer can be enhanced significantly with a priori information and to illustrate the effects of source level and bandwidth, noise level, array size, and number of sources.
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43.60.Gk Space-time signal processing, other than matched field processing
43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration

Subjective evaluation of acoustic properties of concert halls based on their impulse response

Edward Hojan and Christoph Pösselt

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1811-1816 (1990); (6 pages)

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Impulse responses, binaurally recorded using an artificial head, of six European concert halls have been convolved with selected signals of music and speech. Acoustic signals thus obtained have been evaluated subjectively. Questionnaire techniques with semantic differential scales have been used that reflect four perceptual attributes: ‘‘clarity,’’ ‘‘sharpness of localization,’’ ‘‘spaciousness,’’ and ‘‘overall impression.’’ The greatest correlation among perceptual attributes has been obtained between sharpness of localization and clarity and the smallest for spaciousness and overall impression. In the ranking of concert halls, the highest rank has been awarded to the Concertgebouw in Amsterdam.
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43.55.Gx Studies of existing auditoria and enclosures
43.55.Hy Subjective effects in room acoustics, speech in rooms

Radiation and vibrational properties of submerged stiffened cylindrical shells

A. Harari and B. E. Sandman

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1817-1830 (1990); (14 pages) | Cited 4 times

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The vibratory response of submerged cylindrical shells is investigated. The shell response is presented in terms of the spatial wave‐number spectrum of the normal surface displacement. The power output of the vibrating shell into the fluid and the far‐field radiation from the shell are presented as a function of the wave number of the exciting force. The effects of structural damping and stiffeners are also studied.
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43.40.Ey Vibrations of shells
43.30.Jx Radiation from objects vibrating under water, acoustic and mechanical impedance

Critical behavior of the ultrasonic attenuation and velocity and shear viscosity for the binary mixture of nitrobenzene‐n‐hexane

Issam R. Abdelraziq, S. S. Yun, and F. B. Stumpf

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1831-1836 (1990); (6 pages) | Cited 2 times

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Ultrasonic velocity and absorption as a function of temperature, concentration, and frequency (5–25 MHz) and shear viscosity as a function of concentration and temperature are reported for the binary mixture nitrobenzene‐n‐hexane in the homogeneous phase above Tc. For the observed absorption at critical concentration and critical temperature αc/f2 vs f−1.06 yields a straight line as predicted by the dynamic scaling theory of Ferrell and Bhattacharjee [Phys. Rev. A 24, 1643 (1981)]. Also, the critical amplitudes of the thermal expansion and specific heat have been calculated using the two‐scale factor universality relation. The adiabatic coupling constant g is calculated and compared to the experimental value. In addition, the experimental values of α/αc (where α is the absorption at critical concentration above the critical temperature) for nitrobenzene‐n‐hexane are compared to the scaling function F(ω∗) and show a good agreement with the theory. Finally, the velocity for the system at the critical concentration above the critical temperature appears to decrease linearly with increasing temperature.
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43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions

Ultrasonic studies in aqueous solutions of rare earth nitrates

S. P. Tandon and S. Gambhir

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1837-1841 (1990); (5 pages) | Cited 1 time

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Ultrasonic velocity at 3 MHz has been measured at 32 °C using a variable path interferometer in the aqueous solutions of nitrates of lanthanum, cerium, terbium, and dysprosium at various concentrations ranging from 0.01 to 1.0 mol/l. The various acoustic parameters, namely, the molar sound velocity, adiabatic compressibility, specific acoustic impedance, and relative association for these solutions have been computed. The variation of these constants with concentration has been discussed. The dependence of these constants on other parameters like atomic weight and ionic radius of the cation has also been discussed. Ultrasonic velocity and other constants except adiabatic compressibility are found to increase with the concentration.
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43.35.Bf Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in liquids, liquid crystals, suspensions, and emulsions

Observation of optoacoustic amplitude in CS2 at high‐input energies

Stanley A. Cheyne and Henry E. Bass

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1842-1845 (1990); (4 pages) | Cited 2 times

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The optoacoustic amplitude as a function of laser pulse energy has been measured. A nitrogen laser emitting UV (337 nm) pulses 800 ps in duration was used to excite liquid carbon disulfide (CS2), which strongly absorbs the UV radiation. Measurements indicate an optical absorption coefficient of 370 cm−1 implying an optical penetration depth of 27 μm. The spatial profile of the pulse was an ellipse at the focal point with dimensions of 330×90 μm. The energy coupled with the short penetration depth resulted in a high energy density (532 J/cm3). This was varied by placing glass microslides in the beampath. The optoacoustic amplitude as a function of energy exhibited changes in slope as the liquid in the focal volume underwent vaporization. The experimental observations were explained in terms of an energy‐dependent coefficient of thermal expansion and optical absorption.
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43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect

Numerical simulation of the second‐order Kirchhoff approximation from very rough surfaces and a study of backscattering enhancement

Jei S. Chen and Akira Ishimaru

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1846-1850 (1990); (5 pages) | Cited 2 times

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A Monte Carlo simulation is used to study the second‐order Kirchhoff scattering and backscattering enhancement of a scalar wave from one‐dimensional soft Gaussian rough surfaces. The surfaces are very rough with the rms height σ/λ=0.707, 0.5, and 0.3, and the correlation distance l/λ=1. Numerical results of the scattered intensity using the Kirchhoff approximation and second‐order scattering are compared with the exact calculations of the integral equation. With the corrections of incident and scattering shadowing and the propagation shadowing for the second‐order scattering, good agreement between approximate and exact results is obtained. The results also show that the backscattering enhancement may be explained by the second‐order Kirchhoff scattering including shadowing effects. The law of energy conservation and the problem of convergence in the simulation are also investigated. The results of numerical simulation may be useful for the development of an analytical model for very rough scattering.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Hw Rough interface scattering

Environmentally tolerant beamforming for high‐resolution matched field processing: Deterministic mismatch

Henrik Schmidt, A. B. Baggeroer, W. A. Kuperman, and E. K. Scheer

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1851-1862 (1990); (12 pages) | Cited 15 times

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Standard adaptive beamforming or matched field processing requires accurate replica fields finely gridded over the search parameter space for localization with sidelobe control. Multiple constraints to the maximum likelihood method (MLM) technique are introduced in order to construct a beamformer (MCM) more tolerant to environmental mismatch of a deterministic nature. The result is a plane‐wave or matched field beamformer that accommodates some mismatch in the environment while still suppressing sidelobes. This beamformer maintains its localization and sidelobe control over a coarser grid of the search parameter space than the standard MLM beamformer which requires an extremely fine grid for localization and sidelobe control. Examples simulating the performance of the MCM beamformer for plane‐wave and matched field processing for an Arctic environment are given.
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43.30.Bp Normal mode propagation of sound in water
43.60.Gk Space-time signal processing, other than matched field processing

A very‐wide‐angle acoustic model for underwater sound propagation

Robert A. Dalrymple, Levsiri C. Munasinghe, David H. Wood, and James T. Kirby

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1863-1876 (1990); (14 pages)

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A parabolic model, valid for wide angles (out to 90° from the assumed propagation direction for a homogeneous environment), is presented. Numerical computations for the model are done almost entirely in the Fourier domain, and the model can be shown to be theoretically exact for a homogeneous ocean. In addition, the model can implicitly handle range‐dependent sound‐speed profiles. An error analysis indicates that the model is more accurate than the standard parabolic equation (SPE) and the modified‐wide‐angle parabolic equation (MWAPE) for constant perturbations of the index of refraction. The accuracy of the model is examined by comparison of computed solutions with exact solutions for range independent cases. Several idealized range‐dependent cases are also examined.
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43.30.Bp Normal mode propagation of sound in water

Scattering from very rough surfaces based on the modified second‐order Kirchhoff approximation with angular and propagation shadowing

Akira Ishimaru and Jei S. Chen

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1877-1883 (1990); (7 pages) | Cited 4 times

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A theory of scattering from very rough surfaces is presented. Both the surface rms height and the correlation distance are close to a wavelength and the rms slope is close to unity. The theory is based on the first‐order and modified second‐order Kirchhoff approximations. The second‐order scattering includes the incident and scattering shadowing and the angular and propagation tapering shadowing. The angular shadowing limits the angular spectrum of the second‐order scattering within those intercepted by the surface, while the propagation shadowing limits the propagation distance within those intercepted by the surface. The calculations are made for the rms height σ/λ=1.5, 1.0, and 0.5 and the correlation distance l/λ=1.8, and show close agreement with the Monte Carlo simulation of the exact integral equation. The results show that the first‐order Kirchhoff scattering is dominant for σ/λ=0.5, but for σ/λ=1.0 and 1.5, the second‐order scattering becomes comparable to the first‐order and produces the enhanced backscattering. Therefore, this theory provides a possible physical explanation for the enhancement.
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43.30.Gv Backscattering, echoes, and reverberation in water due to combinations of boundaries
43.30.Hw Rough interface scattering

The power radiated by a vibrating body in an acoustic fluid and its determination from boundary measurements

Giorgio V. Borgiotti

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1884-1893 (1990); (10 pages) | Cited 18 times

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The power radiated into a specified angular sector by a vibrating object immersed in a fluid is expressed as a quadratic functional of the boundary normal velocity field. The diagonalization of the functional, obtained through the singular value decomposition of its discretized version, identifies a set of orthonormal boundary velocity patterns, each corresponding to a far‐field pattern belonging to a set of functions orthonormal in the angular sector of interest. Any boundary normal velocity field can be represented as a linear superposition of the orthonormal patterns. The velocity patterns having high radiation efficiencies form a subset, whose dimension depends upon the object boundary shape and size in wavelengths. The other velocity patterns do not radiate efficiently and contribute mainly to the evanescent field in the neighborhood of the object. Assuming that some noise is present, only the radiation patterns associated with the efficiently radiating velocity patterns are observable in the far field. Therefore, the dimension of their set defines the number of degrees of freedom of the far field. The efficiently radiating velocity patterns constitute a set of spatial filtering functions, separating the radiating from the essentially nonradiating components of an arbitrary boundary normal velocity field.
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43.30.Jx Radiation from objects vibrating under water, acoustic and mechanical impedance
43.20.Tb Interaction of vibrating structures with surrounding medium

Low‐frequency wind‐generated ambient noise source levels

D. J. Kewley, D. G. Browning, and W. M. Carey

J. Acoust. Soc. Am. Volume 88, Issue 4, pp. 1894-1902 (1990); (9 pages) | Cited 4 times

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Measurements of ambient noise (30–800 Hz) useful in characterizing the wind‐generated component have been obtained in the low ship density Southern Hemisphere. Source levels that characterize the frequency and wind speed dependence of this noise have been obtained by Burgess and Kewley using vertical noise directionality data and had been implemented in a diagnostic computerized noise model. Northern Hemisphere vertical noise directionality data have been examined, processed for source level, and compared to these results. These data and other data summaries, when taking a two‐mechanism viewpoint, are found to be consistent with respect to wind speed and frequency dependence. When a dipole source model, based on the possible physical mechanism of sound production, is used, a set of consistent source levels for wind‐dependent noise are realized from the several data sets examined.
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43.30.Nb Noise in water; generation mechanisms and characteristics of the field
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