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Journal of the Acoustical Society of America

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Oct 1989

Volume 86, Issue 4, pp. 1223-1638

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An experimental study of acoustoelectric transducers with nonuniform distribution of the piezoelectric coefficient

François Chapeau‐Blondeau, James F. Greenleaf, William B. Harrison, and M. R. B. Hanson

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1223-1229 (1989); (7 pages)

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In this paper, an experimental test of a theoretical model published previously is presented that describes the behavior of an acoustoelectric transducer with a nonuniform distribution of the piezoelectric coefficient within its bulk. Results of this theoretical model are first reviewed. Uniform and nonuniform piezoelectric transducers were fabricated, following a procedure described herein. The receive transfer functions of the transducers were recorded experimentally, and a comparison is made with the theoretical transfer functions predicted by the model, which shows good agreement. The transmit transfer functions of the uniform and nonuniform transducers were also measured and are reported. Numerical calculations of the different transfer functions given by the theoretical model for a uniform transducer associated with different backing materials are also presented, and the results are shown to be equivalent to the results following from the Mason equivalent circuit. Comparisons with experimental results and with Mason’s equivalent circuit verified the new theoretical model.
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43.38.Ar Transducing principles, materials, and structures: general
43.38.Fx Piezoelectric and ferroelectric transducers
43.35.Yb Ultrasonic instrumentation and measurement techniques

Edge effects in short‐pulse piezoelectric transducers

Dov Hazony

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1230-1233 (1989); (4 pages)

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The electroelastic properties of thin‐thickness‐driven piezoelectric transducers are analyzed when the passive edges are free. The principal signals are relatively short stress pulses. It will be seen that these pulses travel close to or at the speed of sound in an unbounded medium and that, like in the one‐dimensional space, circuit models, may be associated with these transducers. Special cases will be pointed out by examples.
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43.38.Ar Transducing principles, materials, and structures: general
43.38.Fx Piezoelectric and ferroelectric transducers

Finite element modeling of radiating structures using dipolar damping elements

Regis Bossut and Jean‐Noël Decarpigny

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1234-1244 (1989); (11 pages) | Cited 10 times

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The finite element modeling of a radiating structure can be performed by surrounding this structure with a limited fluid domain, upon the external boundary of which a nonreflexion condition is prescribed for the acoustic field. This condition can be implemented with the help of damping finite elements which are attached to the external boundary and are designed to absorb the successive components of the pressure field multipolar expansion [Bayliss et al., ICASE Rep. No. 80/1, NASA, Langley (1980)]. This paper describes an axisymmetrical finite element which damps the monopolar and dipolar components of the radiated field. Then, it provides an original extrapolation algorithm to compute farfield quantities (sound pressure level or transmitting voltage response, directivity patterns,...) from the previously obtained nearfield. Finally, it demonstrates the accuracy of the method in two test cases and describes its application to the analysis of a free‐flooded ring transducer used in sonar devices.
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43.38.Fx Piezoelectric and ferroelectric transducers
43.20.Tb Interaction of vibrating structures with surrounding medium
43.20.Px Transient radiation and scattering
43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration

Analysis of a radiating thin‐shell sonar transducer using the finite‐element method

Bernard Hamonic, Jean Claude Debus, Jean‐Noël Decarpigny, Didier Boucher, and Bernard Tocquet

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1245-1253 (1989); (9 pages) | Cited 3 times

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The design of efficient low‐frequency flextensional sonar transducers is a present challenge which is made difficult by a partial lack of general accurate mathematical models. Thus the application of the finite‐element method to this problem is a promising approach which has been worked recently. To test the ability of the finite‐element code atila [Decarpigny et al., J. Acoust. Soc. Am. 78, 1499–1507 (1985)] to predict the in‐air and in‐water dynamic behavior of such structures, an axisymmetrical thin‐shell transducer was built, and its acoustical behavior was experimentally and numerically analyzed. This paper first presents the modal analysis of this projector, using different finite‐element meshes as well as a mixed finite‐element–plane‐wave model and the comparison of numerical displacement field values to holographic measurements. Second, it describes an in‐water harmonic analysis in which the model of the infinite fluid domain is reduced to a portion of the acoustic nearfield, limited by a spherical boundary upon which special damping finite elements are attached. All farfield quantities are then calculated using an efficient extrapolation algorithm [R. Bossut and J. N. Decarpigny, J. Acoust. Soc. Am. 86, 1234–1244 (1989)]. Finally, measured transmitting voltage response and directivity patterns are compared to the finite‐element predicted values. The comparison proves the ability of the finite‐element approach of provide detailed and accurate insights in the behavior of transducers working with shell vibrations.
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43.38.Fx Piezoelectric and ferroelectric transducers
43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration

In vitro ultrasonic heating of fetal bone

J. L. Drewniak, K. I. Carnes, and F. Dunn

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1254-1258 (1989); (5 pages) | Cited 2 times

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The temperature increase measured in vitro in human fetal femurs exposed to 1 MHz, continuous wave ultrasound at 37 °C is reported. The temperature is measured with a thermocouple probe and is given for several gestational ages. The initial rate of the temperature increase in the specimens is evaluated and compared to known values of absorption in soft tissue. For example, the initial rate of temperature increase in the 108‐day gestational age specimen resulting from exposure to ultrasound is 30 times greater in the fetal bone than that of soft tissue with an absorption coefficient of 0.05 cm−1.
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43.80.Gx Mechanisms of action of acoustic energy on biological systems: physical processes, sites of action
43.35.Wa Biological effects of ultrasound, ultrasonic tomography

Reverberant overlap‐ and self‐masking in consonant identification

Anna K. Nábělek, Tomasz R. Letowski, and Frances M. Tucker

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1259-1265 (1989); (7 pages) | Cited 12 times

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Two effects of reverberation on the identification of consonants were evaluated for ten normal‐hearing subjects: (1) the overlap of energy of a preceding consonant on the following consonant, called ‘‘overlap‐masking’’; and (2) the internal temporal smearing of energy within each consonant, called ‘‘self‐masking.’’ The stimuli were eight consonants /p,t,k,f,m,n,l,w/. The consonants were spoken in /s–at/ context (experiment 1) and generated by a speech synthesizer in /s–at/ and /–at/ contexts (experiment 2). In both experiments, identification of consonants was tested in four conditions: (1) quiet, without degradations; (2) with a babble of voices; (3) with noise that was shaped like either natural or synthetic /s/ for the two experiments, respectively; and (4) with room reverberation. The results for the natural and synthetic syllables indicated that the effect of reverberation on identification of consonants following /s/ was not comparable to masking by either the /s/‐spectrum‐shaped noise or the babble. In addition, the results for the synthetic syllables indicated that most of the errors in reverberation for the /s–at/ context were similar to a sum of errors in two conditions: (1) with /s/‐shaped noise causing overlap masking; and (2) with reverberation causing self‐masking within each consonant.
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43.71.Es Vowel and consonant perception; perception of words, sentences, and fluent speech
43.66.Dc Masking
43.55.Hy Subjective effects in room acoustics, speech in rooms

The perceptual effects of child–adult differences in fricative‐vowel coarticulation

Susan Nittrouer and D. H. Whalen

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1266-1276 (1989); (11 pages) | Cited 2 times

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Earlier work [Nittrouer et al., J. Speech Hear. Res. 32, 120–132 (1989)] demonstrated greater evidence of coarticulation in the fricative‐vowel syllables of children than in those of adults when measured by anticipatory vowel effects on the resonant frequency of the fricative back cavity. In the present study, three experiments showed that this increased coarticulation led to improved vowel recognition from the fricative noise alone: Vowel identification by adult listeners was better overall for children’s productions and was successful earlier in the fricative noise. This enhanced vowel recognition for children’s samples was obtained in spite of the fact that children’s and adults’ samples were randomized together, therefore indicating that listeners were able to normalize the vowel information within a fricative noise where there often was acoustic evidence of only one formant associated primarily with the vowel. Correct vowel judgments were found to be largely independent of fricative identification. However, when another coarticulatory effect, the lowering of the main spectral prominence of the fricative noise for /u/ versus /i/, was taken into account, vowel judgments were found to interact with fricative identification. The results show that listeners are sensitive to the greater coarticulation in children’s fricative‐vowel syllables, and that, in some circumstances, they do not need to make a correct identification of the most prominently specified phone in order to make a correct identification of a coarticulated one.
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43.71.Es Vowel and consonant perception; perception of words, sentences, and fluent speech
43.71.Gv Measures of speech perception (intelligibility and quality)
43.70.Fq Acoustical correlates of phonetic segments and suprasegmental properties: stress, timing, and intonation

Shadowed and simple reaction times in stutterers and nonstutterers

Dan C. Harbison, Jr., Robert J. Porter, Jr., and Emily A. Tobey

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1277-1284 (1989); (8 pages)

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Stutterers may experience difficulties in preparing and executing responses. This study investigated these possibilities as well as the possibility that stutterers may experience difficulty in the selection of responses. Two reaction‐time tasks were investigated: a shadowing response in which speakers exactly repeated vowel sequences they heard, and a simple response in which speakers said [u] regardless of the identity of the vowel stimulus. Two groups of six adult male subjects, stutterers and nonstutterers, participated. Stimuli consisted of vowel–vowel ‘‘syllables’’ whose initial duration (response foreperiod) was randomly varied from 500 to 1500 ms. Electromyographic (EMG) and acoustic measures were obtained for each response condition. The EMG response latencies, acoustic response latencies, and execution times (EMG latency less the acoustic response latency) were examined for the fluent responses. Results indicated stutterers were, on average, 34 ms slower on acoustic responses than nonstutterers regardless of the task or foreperiod. However, stutterers’ and nonstutterers’ EMG latencies were not significantly different. Further analysis indicated that the overall slower acoustic responses of stutterers were accounted for almost entirely by longer execution times. Stutterers’ difficulties thus appear to lie after response initiation suggesting they have problems in coordination of gestures during execution of fluent responses.
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43.70.Dn Disordered speech
43.70.Fq Acoustical correlates of phonetic segments and suprasegmental properties: stress, timing, and intonation

Lateralization of bands of noise: Effects of bandwidth and differences of interaural time and phase

Constantine Trahiotis and Richard M. Stern

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1285-1293 (1989); (9 pages) | Cited 14 times

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The effects of stimulus bandwidth on lateralization of narrow bands of noise were investigated with an acoustic pointing task. Stimuli were narrow bands of noise (centered on 500 Hz with bandwidths ranging from 50–400 Hz) that contained interaural time delays and/or interaural phase shifts. The overall extent of lateralization and sidedness was found to vary greatly as a function of stimulus bandwidth, as insightfully discussed earlier by Jeffress [L. A. Jeffress, Foundations of Modern Auditory Theory, edited by J. V. Tobias (Academic, New York, 1972)]. The data are qualitatively consistent with a weighted‐image model [Stern et al., J. Acoust. Soc. Am. 84, 156–165 (1988)] that specifies and utilizes the shapes and locations of patterns of hypothesized neural activity. These patterns are topographically organized along a two‐dimensional surface, and they describe the cross‐correlation function of the stimuli as a joint function of frequency and the delay parameter of the cross‐correlation operation. In this fashion, lateralization depends upon individual modes of such patterns that are weighed with respect to their straightness (consistency of interaural delay over frequency) and centrality (the extent to which interaural delays are small in magnitude).
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43.66.Ba Models and theories of auditory processes
43.66.Nm Phase effects
43.66.Pn Binaural hearing

Auditive and cognitive factors in speech perception by elderly listeners. I: Development of test battery

J. C. G. M. van Rooij, R. Plomp, and J. F. Orlebeke

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1294-1309 (1989); (16 pages) | Cited 9 times

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This study compares performance of 24 young normal‐hearing (aged 18–28 years) and 24 elderly (aged 61–85 years) listeners on auditive (sensitivity, frequency selectivity, and temporal resolution), cognitive (memory performance, processing speed, and divided attention ability), and speech perception tests (at the phoneme, spondee, and sentence level). Its principal aim is to assess whether the tests selected yield meaningful results. The results obtained will be used to reduce the test battery in order to be manageable in a second study on a much larger number of elderly listeners. The relationships between the tests are explored by multivariate statistical methods. The results show that: (a) in young listeners, individual differences in speech perception performance are remarkably small resulting in low correlations between the tests, while in the elderly tests of phoneme, spondee, and sentence perception overlap considerably; (b) speech perception in the elderly seems to be largely determined by hearing loss at the higher frequencies, whereas the effects of other auditive and cognitive factors seem to be relatively small or absent; and (c) performance in the elderly is only partly correlated with age.
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43.66.Ba Models and theories of auditory processes
43.71.Es Vowel and consonant perception; perception of words, sentences, and fluent speech
43.71.Ky Speech perception by the hearing impaired
43.71.Lz Speech perception by the aging

Roving‐level tone‐in‐noise detection

Gerald Kidd, Jr., Christine R. Mason, Merry A. Brantley, and Grace A. Owen

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1310-1317 (1989); (8 pages) | Cited 20 times

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The detectability of tones, or of intensity increments to tones, in bands of random noise was measured for conditions in which the overall level was fixed or was randomly roved from interval to interval of every experimental trial. The purpose of the within‐trial rove was to limit the usefulness of a detection strategy based on overall level or level within a single ‘‘critical band.’’ At ‘‘supracritical’’ bandwidths, the functions relating masked threshold to noise bandwidth for the roved conditions were similar to those obtained when no rove was employed. At ‘‘subcritical’’ bandwidths, thresholds were higher in some roved conditions, but, for the largest rove, were still lower than would be predicted from arguments based purely on level detection—with one exception. A comparison of observer performance relative to the statistical limits imposed by the roving‐level procedure indicated that the traditional critical‐band energy‐detector model could not account for the results, which are attributed to discrimination based on spectral shape or on waveshape.
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43.66.Dc Masking
43.66.Ba Models and theories of auditory processes
43.66.Fe Discrimination: intensity and frequency
43.66.Jh Timbre, timbre in musical acoustics

Clutter interference and the integration time of echoes in the echolocating bat, Eptesicus fuscus

James A. Simmons, Edward G. Freedman, Scott B. Stevenson, Lynda Chen, and Timothy J. Wohlgenant

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1318-1332 (1989); (15 pages) | Cited 6 times

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The ability of the echolocating bat, Eptesicus fuscus, to detect a sonar target is affected by the presence of other targets along the same axis at slightly different ranges. If echoes from one target arrive at about the same delay as echoes from another target, clutter interference occurs and one set of echoes masks the other. Although the bat’s sonar emissions and the echoes themselves are 2 to 5 ms long, echoes (of approximately equal sensation levels—around 15 dB SL) only interfere with each other if they arrive within 200 to 400 μs of the same arrival time. This figure is an estimate of the integration time of the bat’s sonar receiver for echoes. The fine structure of the clutter‐interference data reflects the reinforcement and cancellation of echoes according to their time separation. When clutter interference first occurs, the waveforms of test and cluttering echoes already overlap for much of their duration. The masking effect underlying clutter interference appears specifically due to overlap, not between raw echo waveforms, but between the patterns of mechanical excitation created when echoes pass through bandpass filters equivalent to auditory‐nerve tuning curves. While the time scale of clutter interference is substantially shorter than the duration of echo waveforms, it still is much longer than the eventual width of a target’s range‐axis image expressed in terms of echo delay.
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43.66.Gf Detection and discrimination of sound by animals
43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music
43.80.Lb Sound reception by animals: anatomy, physiology, auditory capacities, processing

The acoustic basis for target discrimination by FM echolocating bats

James A. Simmons and Lynda Chen

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1333-1350 (1989); (18 pages) | Cited 14 times

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Past experiments show that echolocating bats of the species Myotis lucifugus and Eptesicus fuscus can discriminate among airborne sonar targets presented in the context of pursuit maneuvers for the interception of prey. These bats distinguish between edible mealworms and inedible spheres of various sizes. Myotis can distinguish between disks and mealworms similar enough in size that the bat’s performance requires the ability to perceive the acoustic equivalent of target shape. Previously observed small differences in the spectrum of echoes from mealworms and disks appear insufficient to distinguish these targets at the performance levels achieved by bats. We measured the acoustic properties of the targets by broadcasting ultrasonic impulses at mealworms, spheres, and disks and recording their echoes, displaying the results in terms of impulse echo waveforms and the frequency response of targets derived from the target transfer function. The echoes from disks and mealworms at various orientations convey the range‐axis profile of the target (number and spacing of reflecting points or glints distributed at different ranges) in terms of the impulse structure of their waveforms and in terms of the locations and spacing of notches or nulls in their spectra. For targets that bats can discriminate and that reflect echoes which do not clearly differ in overall amplitude, the targets appear distinguishable from the acoustic representation of their range profile, which is a feature of targets that bats can perceive with great acuity.
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43.66.Gf Detection and discrimination of sound by animals
43.66.Qp Localization of sound sources
43.80.Lb Sound reception by animals: anatomy, physiology, auditory capacities, processing

Difference limens for phase in normal and hearing‐impaired subjects

Brian C. J. Moore and Brian R. Glasberg

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1351-1365 (1989); (15 pages) | Cited 4 times

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These experiments measure the ability to detect a change in the relative phase of a single component in a harmonic complex tone. Complex tones containing the first 20 harmonics of 50, 100, or 200 Hz, all at equal amplitude, were used. All of the harmonics except one started in cosine phase. The remaining harmonic started in cosine phase, but was shifted in phase half‐way through either the first or the second of the two stimuli comprising a trial. The subject had to identify the stimulus containing the phase‐shifted component. For normally hearing subjects tested at a level of 70 dB SPL per component, thresholds for detecting the phase shift [i.e., phase difference limens (DLs)] were smallest (2°–4°) for harmonics above the eighth and for the lowest fundamental frequency (F0). Changes in phase were not detectable for harmonic numbers below three or four at the lowest F0 and below 5–13 at the highest F0. The DLs increased slightly for the highest harmonics in the complexes. The DLs increased markedly with decreasing level, except for the highest harmonic, where only a small effect of level was found. Subjects reported that the phase‐shifted harmonic appeared to ‘‘pop out’’ and was heard with a pure‐tone quality. A pitch‐matching experiment demonstrated that the pitch of this tone corresponded to the frequency of the phase‐shifted component. For the highest harmonic, the phase shift was associated with a downward shift of the edge pitch heard in the reference (all cosine phase) stimulus. When the phases of the components in the reference stimulus were randomized, phase DLs were much higher (and often impossible to measure), the pop‐out phenomenon was not observed, and no edge pitch was heard. Subjects with unilateral cochlear hearing impairment generally showed poorer phase sensitivity in their impaired than in their normal ears, when the two ears were compared at equal sound‐pressure levels. However, at comparable sensation levels, the impaired ears sometimes showed lower phase DLs. The results are explained by considering the waveforms that would occur at the outputs of the auditory filters in response to these stimuli.
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43.66.Nm Phase effects
43.66.Mk Temporal and sequential aspects of hearing; auditory grouping in relation to music
43.66.Ki Subjective tones
43.66.Hg Pitch

Localization of sound in rooms IV: The Franssen effect

William Morris Hartmann and Brad Rakerd

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1366-1373 (1989); (8 pages) | Cited 14 times

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The Franssen effect is an illusion that causes human listeners to make large errors in localizing a sound source. This paper describes steps taken to convert the illusion into an experiment in order to study the localization precedence effect as it operates in rooms. The results of the experiment suggest that there are two components to the illusion: The first is the inability of listeners to localize a sine tone in a room in the absence of an onset; the second is the obscuring of modulation cues by the irregular transient response of a room. Experiments show that the Franssen effect fails completely in anechoic environment, as expected if the effect depends upon the implausibility of steady‐state cues in a room. The Franssen effects also fails when the spectrum of the sound is dense.
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43.66.Pn Binaural hearing
43.66.Qp Localization of sound sources
43.55.Br Room acoustics: theory and experiment; reverberation, normal modes, diffusion, transient and steady-state response

Binaural speech intelligibility in noise for hearing‐impaired listeners

A. W. Bronkhorst and R. Plomp

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1374-1383 (1989); (10 pages) | Cited 15 times

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The effect of head‐induced interaural time delay (ITD) and interaural level differences (ILD) on binaural speech intelligibility in noise was studied for listeners with symmetrical and asymmetrical sensorineural hearing losses. The material, recorded with a KEMAR manikin in an anechoic room, consisted of speech, presented from the front (0°), and noise, presented at azimuths of 0°, 30°, and 90°. Derived noise signals, containing either only ITD or only ILD, were generated using a computer. For both groups of subjects, speech‐reception thresholds (SRT) for sentences in noise were determined as a function of: (1) noise azimuth, (2) binaural cue, and (3) an interaural difference in overall presentation level, simulating the effect of a monaural hearing aid. Comparison of the mean results with corresponding data obtained previously from normal‐hearing listeners shows that the hearing impaired have a 2.5 dB higher SRT in noise when both speech and noise are presented from the front, and 2.6–5.1 dB less binaural gain when the noise azimuth is changed from 0° to 90°. The gain due to ILD varies among the hearing‐impaired listeners between 0 dB and normal values of 7 dB or more. It depends on the high‐frequency hearing loss at the side presented with the most favorable signal‐to‐noise (S/N) ratio. The gain due to ITD is nearly normal for the symmetrically impaired (4.2 dB, compared with 4.7 dB for the normal hearing), but only 2.5 dB in the case of asymmetrical impairment. When ITD is introduced in noise already containing ILD, the resulting gain is 2–2.5 dB for all groups. The only marked effect of the interaural difference in overall presentation level is a reduction of the gain due to ILD when the level at the ear with the better S/N ratio is decreased. This implies that an optimal monaural hearing aid (with a moderate gain) will hardly interfere with unmasking through ITD, while it may increase the gain due to ILD by preventing or diminishing threshold effects.
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43.66.Sr Deafness, audiometry, aging effects
43.66.Pn Binaural hearing
43.66.Ts Auditory prostheses, hearing aids
43.71.Ky Speech perception by the hearing impaired

Normative thresholds in the 8‐ to 20‐kHz range as a function of age

Patricia G. Stelmachowicz, Kathryn A. Beauchaine, Ann Kalberer, and Walt Jesteadt

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1384-1391 (1989); (8 pages) | Cited 4 times

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Using a prototype high‐frequency audiometer, auditory thresholds in the 8‐ to 20‐kHz range were obtained from 240 subjects ranging in age from 10–60 years. These measurements were obtained in interest of developing a normative database for frequencies above 8 kHz, and to evaluate intersubject variability as a function of age. An analysis of variance (ANOVA) revealed significant effects of frequency, age, and sex, and a significant frequency‐by‐age interaction. The largest changes in sensitivity with age occurred between 40 to 59 years. Below approximately 15 kHz, the intersubject variability of threshold estimates increased as a function of both age and frequency. Further analysis revealed that the age‐related changes in variability were related to absolute thresholds rather than to age per se. When data are converted to dB HL (relative to the youngest group tested), the region of maximum hearing loss shifts to lower frequencies with increasing age, and threshold shifts with age are greatest in the 13‐ to 17‐kHz range.
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43.66.Sr Deafness, audiometry, aging effects
43.66.Yw Instruments and methods related to hearing and its measurement

Comparison of the noise attenuation of three audiometric earphones, with additional data on masking near threshold

E. H. Berger and Mead C. Killion

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1392-1403 (1989); (12 pages) | Cited 1 time

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The noise‐excluding properties of a standard supra‐aural audiometric earphone, a widely used circumaural‐supra‐aural combination, and an insert earphone sealed to the ear with a vinyl foam eartip were measured in a diffuse‐field room complying with ANSI S12.6‐1984. Data on attenuation were obtained monaurally with the nontest ear plugged and muffed. Results for the supra‐aural earphones generally agreed well with previously reported measurements. A broadband masking noise was used to directly test the ANSI S3.1‐1977 permissible background noise levels for measuring to audiometric zero using standard audiometric earphones. This ‘‘ANSI noise’’ raised the average thresholds of 15 normal‐hearing test subjects by 3 to 5 dB at the octave frequencies from 500 to 4000 Hz. With a noise conforming to the less stringent OSHA‐1983 regulation, average thresholds were elevated 9 to 17 dB. An ‘‘ENT office noise’’ with an overall sound level of 54 dBA raised average thresholds even further, by as much as 29 dB at 500 Hz. Use of the circumaural system in the office noise limited the threshold elevation to 11, 5, 2, and 0 dB at the four octave frequencies tested. With the fully (‘‘deeply’’) inserted foam eartips, the threshold elevation in the simulated office noise was 2 dB or less at all test frequencies. Actual threshold elevations agreed closely with predictions based on a critical ratio calculation utilizing measured sound field noise levels and measured earphone attenuation values.
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43.66.Yw Instruments and methods related to hearing and its measurement
43.50.Hg Noise control at the ear
43.38.Si Telephones, earphones, sound power telephones, and intercommunication systems
43.66.Dc Masking

Parametric generalized Gaussian density estimation

Mahesh K. Varanasi and Behnaam Aazhang

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1404-1415 (1989); (12 pages) | Cited 2 times

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The primary objective of this paper is to compare the large‐sample as well as the small‐sample properties of different methods for estimating the parameters of a three‐parameter generalized Gaussian distribution. Three estimators, namely, the moment method (MM), the maximum‐likelihood (ML), and the moment/Newton‐step (MNS) estimators, are considered. The applicability of general asymptotic optimality results of the efficient ML and MNS estimation techniques is studied in the generalized Gaussian context. The asymptotic normal distributions of the estimators are obtained. The asymptotic relative superiority of the ML estimator or its variant, the MNS estimator, over the moment method is studied in terms of asymptotic relative efficiency. Based on this study, it is concluded that deviations from normality in the underlying distribution of the data necessitate the use of the efficient ML or MNS methods. In the small‐sample case, a detailed comparative study of the estimators is made possible by extensive Monte Carlo simulations. From this study, it is concluded that the maximum‐likelihood method is found to be significantly superior for heavy‐tailed distributions. In a region of the parameter space corresponding to the vicinity of the Gaussian distribution, the moment method compares well with the other methods. Further, the MNS estimator is shown to perform best for light‐tailed distributions. The simulation results are shown to lend support to analytically derived asymptotic results for each of the methods.
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43.60.Cg Statistical properties of signals and noise

The use of nonuniform element spacing in array processing algorithms

S. S. Shanan and C. A. Pomalaza‐Raez

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1416-1418 (1989); (3 pages)

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Most array signal processing algorithms use uniform element spacing to estimate source bearing. This paper demonstrates the benefit of using nonuniform element spacing in Bartlett, linear prediction (LP), and minimum variance (MV) array processing algorithms. By using optimum element spacing results obtained by previous investigators for sidelobe reduction of the Bartlett method, better MV and LP performances, in terms of array output power, are obtained.
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43.60.Gk Space-time signal processing, other than matched field processing

Line mobilities of infinite plates

Sten Ljunggren

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1419-1431 (1989); (13 pages)

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Analytical solutions are presented for the input mobilities of an isotropic infinite plate with no fluid load. An infinitely long, massless beam is assumed to excite one of the faces of the plate at a fairly low frequency. The plate is described by means of the general equations of motion for three‐dimensional elastic bodies, which leads to solutions for the plate that beside bending, longitudinal, and shear waves also incorporate low‐frequency Lamb and Love modes. It is shown that the expression for the mobility due to a shear wave excited by an in‐line force is similar to that of the mobility due to the longitudinal wave excited by a horizontal force; the only difference is that the shear wave speed is inserted instead of the longitudinal wave speed. It is also shown that, in the case of excitation with a vertical or a horizontal force, the mobilities with respect to the longitudinal waves are much smaller than those connected with the bending waves. An interesting conclusion is that the influence of the Lamb and Love modes is, in general, less important than in the case of point excitation.
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43.40.Dx Vibrations of membranes and plates

Probability density approximations of linear system responses to intensity‐modulated Gaussian processes

William D. Mark

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1432-1445 (1989); (14 pages)

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An intensity‐modulated Gaussian process {x(t)} is defined as the product of a non‐negative stationary random process {σx(t)} and a stationary Gaussian process {z(t)} that is statistically independent of {σx(t)}. When such an intensity‐modulated process is the excitation to a linear time‐invariant system, the response process {y(t)}, conditioned on the excitation‐modulating function σx(t), is a generally nonstationary Gaussian process. This property is used to develop an approximation to the probability density function (PDF) of the system response by assuming that the time‐varying conditional response variance σ2y(t), conditioned on the excitation‐modulating function σx(t), is governed by the gamma PDF. The resulting response process PDF is characterized by two parameters: the mean and variance of the conditional response variance σ2y(t). An alternative approximation to the response PDF, characterized by these same two parameters, is developed by using a Taylor’s series representation in the variable σ2y of the response PDF conditioned on σ2y. This latter PDF approximation is shown to be identical to the Gram–Charlier series of type A through terms involving the Hermite polynomial of degree 6. The two response PDF approximations are shown to give essentially identical results for values of the coefficient of variation of σ2y of one‐half or less. Simple, easily evaluated expressions for the mean and variance of σ2y are derived in terms of measurable quantities by utilizing the assumption that fluctuations in σx(t) occur slowly in comparison with those of z(t).
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43.40.Hb Random vibration

Propagation of ground‐borne vibrations from surface sources

A. Trochidis

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1446-1452 (1989); (7 pages)

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The propagation of ground‐borne vibration from surface sources has been investigated theoretically, and a simple method for obtaining the motion on the surface as well as the attenuation of the vertical motion with the distance from the source for various ground models has been presented. Provided that the depth of the layers and the wavenumbers remain small, the analysis used in this paper gives quite acceptable results.
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43.40.Ph Seismology and geophysical prospecting; seismographs

Elastic constants and structure of the vitreous system ZnO–P2O5

A. A. Higazy, B. Bridge, A. Hussein, and M. A. Ewaida

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1453-1458 (1989); (6 pages)

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The elastic moduli of the entire vitreous range of the system ZnO–P2O5 that can be prepared by melting ZnO and P2O5 oxides in open crucibles have been measured by ultrasonic techniques. The bulk, shear, longitudinal, and Young’s moduli, and the Poisson’s ratio are found to be rather sensitive to the glass composition. From these ultrasonic data, it is found that the glass system can be divided into ‘‘three compositional regions.’’ This behavior is qualitatively interpreted in terms of the zinc coordination, cross‐link densities, and interatomic force constants.
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43.35.Cg Ultrasonic velocity, dispersion, scattering, diffraction, and attenuation in solids; elastic constants
62.20.D- Elasticity

A higher‐order parabolic equation for wave propagation in an ocean overlying an elastic bottom

Michael D. Collins

J. Acoust. Soc. Am. Volume 86, Issue 4, pp. 1459-1464 (1989); (6 pages) | Cited 23 times

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A higher‐order elastic parabolic equation (HEPE) is derived for wave propagation in depth‐dependent and weakly range‐dependent fluid/solid media. Galerkin’s method is used to discretize the depth operators in the HEPE within layers in which depth variations in the Lamé constants and density are continuous. Discontinuities in material properties are handled with centered differences for the interface conditions between layers. The numerical solution of the HEPE also involves the method of alternating directions and Crank–Nicolson integration. The HEPE is applied to underwater wave propagation problems involving a water column over an elastic bottom including a weakly range‐dependent problem. The accuracy of the HEPE is demonstrated with benchmark calculations.
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43.30.Bp Normal mode propagation of sound in water
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