Acoustic Discontinuities Research Articles

Seismic evidence of gas hydrates, multiple BSRs and fluid flow offshore Tumbes Basin, Peru

Identification of a previously undocumented hydrate system in the Tumbes Basin, localized off the north Peruvian margin at latitude of 3°20′—4°10′S, allows us to better understand gas hydrates of convergent margins, and complement the 36 hydrate sites already identified around the Pacific Ocean. Using a combined 2D–3D seismic dataset, we present a detailed analysis of seismic amplitude anomalies related to the presence of gas hydrates and/or free gas in sediments. Our observations identify the occurrence of a widespread bottom simulating reflector (BSR), under which we observed, at several sites, the succession of one or two BSR-type reflections of variable amplitude, and vertical acoustic discontinuities associated with fluid flow and gas chimneys. We conclude that the uppermost BSR marks the current base of the hydrate stability field, for a gas composition comprised between 96% methane and 4% of ethane, propane and pure methane. Three hypotheses are developed to explain the nature of the multiple BSRs. They may refer to the base of hydrates of different gas composition, a remnant of an older BSR in the process of dispersion/dissociation or a diagenetically induced permeability barrier formed when the active BSR existed stably at that level for an extended period. The multiple BSRs have been interpreted as three events of steady state in the pressure and temperature conditions. They might be produced by climatic episodes since the last glaciation associated with tectonic activity, essentially tectonic subsidence, one of the main parameters that control the evolution of the Tumbes Basin.

Acoustic-Phonetic Approaches for Improving Segment-Based Speech Recognition for Large Vocabulary Continuous Speech

Segment-based speech recognition has shown to be a competitive alternative to the state-of-the-art HMM-based techniques. Its accuracies rely heavily on the quality of the segment graph from which the recognizer searches for the most likely recognition hypotheses. In order to increase the inclusion rate of actual segments in the graph, it is important to recover possible missing segments generated by segment-based segmentation algorithm. An aspect of this research focuses on determining the missing segments due to missed detection of segment boundaries. The acoustic discontinuities, together with manner-distinctive features are utilized to recover the missing segments. Another aspect of improvement to our segment-based framework tackles the restriction of having limited amount of training speech data which prevents the usage of more complex covariance matrices for the acoustic models. Feature dimensional reduction in the form of the Principal Component Analysis (PCA) is applied to enable the training of full covariance matrices and it results in improved segment-based phoneme recognition. Furthermore, to benefit from the fact that segment-based approach allows the integration of phonetic knowledge, we incorporate the probability of each segment being one type of sound unit of a certain specific common manner of articulation into the scoring of the segment graphs. Our experiment shows that, with the proposed improvements, our segment-based framework approximately increases the phoneme recognition accuracy by approximately 25% of the one obtained from the baseline segment-based speech recognition.

Optimal Disturbance in a Flow Duct with a Lined Wall

Acoustic lining is widely used to reduce sound. However, for specific liners and under particular flow conditions, some experiments have shown that a convective hydrodynamic instability may grow on the liner and is likely to lead to a sound amplification. In this paper, such a phenomenon is studied through a direct optimal growth analysis of the linearized Euler equations in a two-dimensional flow duct with the bottom wall which is partly lined. A Discontinuous Galerkin scheme is used for spatial discretization, which proved to be quite efficient in handling the acoustic impedance discontinuity at the interface between the lined and hard wall. The optimal perturbation exhibits a spatial amplification similar to the one predicted by a local stability analysis. Moreoever, we can observe a mechanism of conversion of the hydrodynamic surface mode, which appears above the lined wall, into a high amplitude acoustic wave which travels downstream of the liner.

Determination of the complex acoustic scattering matrix of a right-angled duct

A method for determining the complete higher-order scattering matrix of an acoustic discontinuity is developed. The method is demonstrated for a right-angled waveguide bend, and the magnitude and phase of the reflection and transmission coefficients are extracted precisely. The procedure is straightforward and based on the solutions to the Helmholtz equation by the finite element method (FEM). The consistency of the scattering coefficients found by this method is verified by their properties of symmetry, and their accuracy is established by the conservation of energy. The reliability of the new technique is further proved by means of an arbitrary sound source and by comparing the direct FEM response to the reflection matrix calculation. Some features of the scattering matrix as a function of frequency are surprising, such as the steps and reversion of the phase evolution or the complete loss of transmission of the incoming wave. The methodology detailed in this paper can be extended to other multiport junctions, such as T-junctions or size discontinuities in ducts.

Enhanced low-temperature thermal conductance by quantum dot made of material with high sound velocity in a three-dimensional semiconductor nanowire

We investigate ballistic phonon transport and thermal conductance in a three-dimensional semiconductor nanowire modulated with different materials in a single quantum dot using elastic wave continuum model. In general, the acoustic discontinuities at such system cause the decrease of the thermal conductance. However, at very low temperatures, the material with higher sound velocity in quantum dot do enhance the thermal transport, despite that phonons are scattered due to the acoustic mismatch at the interfaces. The enhancement originates from the longer wavelength of the mode (0,0) in the material with higher sound velocity.

Definitions of Trollskär formation and Sandön formation in the Archipelago Sea, northern Baltic Sea

The formal stratigraphy of Late-Quaternary late- and post-glacial sediments in the Archipelago Sea, northern Baltic Sea, is revised. The Trollskar Allomember and Sandon Allomember were previously incorrectly defined as allostratigraphic units based on an acoustic discontinuity, even though their contact is gradational in the studied sediment cores. These allostratigraphic units are formally redefined herein as the lithostratigraphic units Trollskar Formation and Sandon Formation, which belong to the Korppoo Alloformation. The purpose is to forestall misconceptions concerning temporal relationships and depositional processes at the transition of these two units.

Consonant landmarks: Automatic detection and interpretation.

Consonant landmarks are acoustic discontinuities in the speech signal that correspond to the closures and releases in speech production, and have been proposed as critical elements in speech processing [Stevens (2002)]. The three types of consonant landmarks represent the onset and offset of salient acoustic events: glottal vibration, turbulence noise, and sonorancy (e.g., nonvocalic voicing). While earlier work [Liu, (1996)] evaluated the success of identifying single candidate landmarks of all three types, this work focuses on two tools for evaluating strings of landmark candidates. First, a bigram model representing the physiologically feasible sequences of consonant landmarks is used to evaluate candidate strings. Second, a graphical method is used to identify the regions where the landmarks are reliably detected versus where they are ambiguous. Together these tools substantially improve the performance for landmark detection, identify regions in need of further acoustic analysis, and model the ‘grammatical’ structure of landmark sequences. Furthermore, the reliable regions in the proposed representation often correspond to structural elements such as lexical stress and word boundaries. Thus, the proposed representation is potentially useful in analyzing speech not only at the phoneme level but also at the word and phrase levels. [This work was supported by NIH/NIDCD DC02978 and T32DC00038.]

The development of relative amplitude changes across acoustic discontinuities in nasal+vowel syllables

In adult productions of nasal+vowel syllables, relative amplitude changes occur in various frequency regions across acoustic discontinuities and provide important cues as to the place of articulation of nasal consonants. The purpose of this study was to investigate the development of relative amplitude changes in children as potential acoustic correlates to place of articulation. Four age groups of eight participants each (3, 5, 7, adult) served as speakers. Participants produced five productions each of CV syllables comprised of [m] and [n] in the context of four vowels ([i æ u ■]). These syllables were segmented into approximately 25 ms segments of the murmur and 25 ms segments of the vowel bordering the period of discontinuity. The relative amplitude changes in low- and high-frequency ranges from the murmur to the vowel were determined using summed amplitude information from fast Fourier transform (FFT) analyses. Previous research showed systematic differences in relative amplitude between [m] and [n] in adult syllable productions, but only marginal differences were observed in very young children’s speech. The relative amplitude property, a landmark cue representing place of articulation of nasal consonants, is predicted to develop with age based on these preliminary analyses. [Work supported by NIH and an ASHA SPARC Award.]

Time-resolved modal decomposition based on the Linear Prediction Method—Applied for crack detection

A nondestructive technique for testing and monitoring structures is presented, this uses guided waves to avoid a long scanning process. In contrast to other techniques that use guided waves this method can be applied to any structure made of a material that can be dispersive, strongly attenuating, and anisotropic, without the need of special adaptation. This is demonstrated on beams made of aluminum and wood. The propagating waves are reflected at any acoustic discontinuity, for example, by cracks and defects. In strongly attenuating materials such as wood, it is necessary to work with low frequencies, but due to the correspondingly long wavelength, the incident and reflected (from defects and boundaries) waves superimpose. The presented new signal processing technique allows the determination of the wave modes and their propagation direction very accurately in the time domain. Incident and reflected waves can easily be distinguished by the propagation direction. Since the wave mode is known from signal processing the right group velocity is applied for an accurate calculation of the defect location.

A role for tracheal resonances in speech perception

Acoustic coupling between the vocal tract and the trachea results in the introduction of pole-zero pairs corresponding to resonances of the uncoupled trachea. If the second formant (F2) passes through the second tracheal resonance (T2) a discontinuity in amplitude occurs. This work explores the hypothesis that the F2−T2 discontinuity affects how listeners perceive the distinctive feature [back] in transitions from a front vowel (high F2) to a labial stop (low F2). We synthesized two versions of an utterance (‘‘apter’’) with an F2−T2 discontinuity at different locations in the initial VC transition. Subjects heard portions of the utterance with and without the discontinuity, and were asked to identify the utterance. Results show that the presence of the F2−T2 discontinuity facilitated the perception of frontness in the vowel. Discontinuities of the F2−T2 sort are proposed to play a role in shaping vowel inventories in the world’s languages [K. N. Stevens, J. Phonetics 17, 3–46 (1989)]. Our results support a model of lexical access in which acoustic discontinuities subserve phonological feature identification.

Temporal variability in speech segments of Spanish: context and speaker related differences

This article reports on segmental duration measurements of eight selected consonants (voiceless obstruents, nasals and liquids) and three vowels in 192 disyllabic (CVCe) nonsense words with stress on the first syllable, spoken in isolation by 12 Spanish speakers. Durations as measured based on acoustic discontinuities are discussed along with speaker variability. The intrinsic and context-dependent duration of consonants /f, θ, x, s, m, n, l, r/ and vowels /a, i, u/, as well as the inter-speaker variability of these phonemes were analysed. Results show sizable differences in the duration of consonants (voiceless fricatives are longer than voiced fricatives) and vowels (/a/ has a longer duration than /i/ and /u/). With regard to contextual effects, there is a remarkable decrease and increase in vowel durations preceding voiceless fricatives and sonorants, respectively. These effects are present in all speakers. Our results on durational effects indicate that (a) the initial consonants /x, s/ and /r/ show larger differences among speakers; (b) effects for the vowel /a/ are greater than for the vowels /i/ and /u/; and (c) voiceless fricative consonants in medial position show greater intra-speaker idiosyncrasy than voiced consonants. The effects of anticipatory consonant-to-vowel coarticulation are discussed, as well as differences in segmental duration among speakers.

A Japanese TTS system based on multiform units and a speech modification algorithm with harmonics reconstruction

This paper proposes a new text-to-speech (TTS) system that utilizes large numbers of speech segments to produce very natural and intelligible synthetic speech. There are two innovations; new multiform synthesis units and a new speech modification algorithm based on a vocoder that offers harmonics reconstruction. The multiform units make it possible to reduce acoustic discontinuities at concatenation points and unnatural sound by preparing synthesis units with various lengths and various F/sub 0/ contours. The new speech modification algorithm, on the other hand, improves the quality of prosody modified speech. This algorithm is extremely effective in synthesizing speech whose prosodic parameters are quite different from those of synthesis units. Listening tests confirm that the new synthesis units yield speech with high intelligibility and naturalness, and that the new speech modification algorithm is superior to all other conventional vocoders and waveform domain algorithms including TD-PSOLA, especially when modifying the F/sub 0/ frequency upward.

Diverse Acoustic Cues at Consonantal Landmarks

The consonantal segments that underlie an utterance are manifested in the acoustic signal by abrupt discontinuities or dislocations in the spectral pattern. There are potentially two such discontinuities for each consonant, corresponding to the formation and release of a constriction in the oral cavity by the lips, the tongue blade, or the tongue body. Acoustic cues for the various consonant features of place, voicing and nasality reside in the signal in quite different forms on the two sides of each acoustic discontinuity. Examples of these diverse cues and their origin in acoustic theory are reviewed, with special attention to place features and features related to the laryngeal state and to nasalization. A listener appears to have the ability to integrate these diverse, brief acoustic cues for the features of consonants, although the mechanism for this integration process is unclear.

Labeling a speech database with landmarks and features

A database of utterances has been labeled for acoustically salient landmarks and for phonetic features. The landmarks locate syllabic nuclei, glide midpoints, and acoustic discontinuities at the narrowing or releasing of consonantal constrictions. The consonantal landmarks are labeled with values of the articulator-free features (e.g., [continuant] and [sonorant]), and values of articulator-bound features are recorded at each landmark based on acoustic evidence near the landmark. These features specify, for example, the major articulator and its placement, as well as the activity of secondary articulators such as the glottis and the soft palate. Conventions have been developed for labeling reduced vowels, syllabic consonants, glottal stops, consonant sequences in which closures or releases are not acoustically marked, etc. The labeled database is being used for two purposes: (1) to provide a documentation of instances in which the acoustic/articulatory representation of an utterance deviates from the underlying feature-based lexical representation and (2) to train and to test signal-processing algorithms that are being developed to locate the landmarks and to identify the features attached to these landmarks. An advantage of this labeling method is that it gives a more fine-grained phonologically-relevant representation of an utterance than a conventional labeling in terms of phonetic symbols. [Supported in part by NIH Grant No. DC02978.]

High-definition speech synthesis

This paper describes a method for producing high-quality speech synthesis, without signal processing, using indexing and resequencing of phone-sized segments from a prerecorded speech corpus for the purpose of reproducing the voice characteristics and speaking style of the original speaker to create novel utterances. It describes procedures for indexing and retrieval using pointers into an external speech corpus that enable the synthesizer to be both language- and speaker independent. The prosody-based synthesis unit selection process does not itself produce speech sounds, but yields an index for a ‘‘random-access’’ retrieval sequence into the original speech to produce the closest approximation to a desired utterance from the segments available in a given speech corpus. To find the optimal sequence of segments for concatenation, the synthesizer first creates an inventory of phones and their acoustic and prosodic characteristics, and then selects from among these by a weighted combination of features to give an index of the segment sequences that best match the target prosody while minimizing intersegment acoustic discontinuities. It has, to date, been tested on English, Japanese, Korean, and German using more than 20 speech corpora.

Pressure measurement with resonant PVDF or copolymer

Abstract We present a new method for measuring static force or pressure using a piezoelectric element. Our structure is based on a composite resonator using PVDF, brass electrodes and rubber. This resonator presents a resonance frequency shift under applied stress. The observed phenomenon is mainly due to the variation of the acoustic impedance discontinuity between PVDF and brass electrode. We have developed a one dimensional model to describe the resonator, then the design of the device is realized with controlled roughness of the brass electrode surface. A low cost electronic system is developed to analyze the resonator and to measure the applied stress. Good results are obtained with a composite resonator having the following elements: PVDF with thickness of 110 μm; brass electrode with thickness of 100 μm and roughness of 1 μm; rubber with thickness of 4 mm. With this resonator we obtain a sensibility of 0.92 KHz/N and a measurement range of 350 N.

The breakdown of the linearized theory and the role of quadrupole sources in transonic rotor acoustics

The retarded Green's function for the linearized version of the equation of the mixed type governing the potential flow around a rotating helicopter blade or a propeller (with no forward motion) is derived and is shown to constitute the unifying feature of the various existing approaches to rotor acoustics. This Green's function is then used to pinpoint the singularity predicted by the linearized theory of rotor acoustics which signals its experimentally confirmed breakdown in the transonic regime: the gradient of the near-field sound amplitude, associated with a linear flow which is steady in the blade-fixed rotating frame, diverges on the sonic cylinder at the dividing boundary between the subsonic and supersonic regions of the flow. Prom the point of view of the equivalent Cauchy problem for the homogeneous wave equation, this singularity is caused by the imposition of entirely non-characteristic initial data on a space—time hypersurface which, at its points of intersection with the sonic cylinder, is locally characteristic. It also emerges from the analysis presented that the acoustic discontinuities detected in the far zone are generated by the quadrupole source term in the Ffowcs Williams-Hawkings equation and that the impulsive noise resulting from these discontinuities would be removed if the flow in the transonic region were to be rendered unsteady (as viewed from the blade-fixed rotating frame).

The Fresnel zone and its interpetive significance

Resolution of the reflection seismic record in the temporal (vertical) dimension has historically dominated the attention of geophysicists. The ability to “resolve” two reflectors with close vertical spacing is of particular importance in stratigraphic analysis. However, resolution is most completely understood when it is considered in a three‐dimensional sense. To what extent can the seismic method resolve a point? If we can understand the resolution limits of seismic sections for expressing a point acoustic discontinuity in subsurface space, then the question of resolution is answered for any reflector configuration (since, under Huygens’ principle, all reflection surfaces may be represented as sufficiently dense arrays of point diffractors).

Reflection study of the Haughton impact crater

The western flank of the Haughton impact crater was imaged with a 10.25‐km‐long multifold reflection profile. The seismic waves mapped 10 acoustic discontinuities in the stratified sedimentary target rocks. The impact disturbed the entire 1900 m of the highly competent Paleozoic strata. The average seismic velocity of over 6000 m/s exhibited by these rock units is a clear indication of the undisturbed impact area's unusually high elastic strength. The dynamic and destructive forces of the cratering process are manifested on the seismic profile by a dense and complex system of faults. These steeply dipping faults penetrated the sedimentary rocks but showed no recognizable extension into the crystalline basement. The recognition of normal faults 3 km west of the faulted annulus suggests a structural diameter of 24 km, some 4 km larger than postulated from earlier geological data. It has been found that the surface morphologically distinct ring zones do not have seismically recognizable subsurface signatures. In the central interior region of the crater, coherent reflection signals are lacking. However, some weak local acoustic discontinuities are recognized. The underlying crystalline basement rocks did not exhibit any seismically mappable impact related zones of alteration.

Phonetic information is integrated across intervening nonlinguistic sounds.

When the fricative noise of a fricative-vowel syllable is replaced by a noise from a different vocalic context, listeners experience delays in identifying both the fricative and the vowel (Whalen, 1984): mismatching the information in the fricative noise for vowel and consonant identity with the information in the vocalic segment appears to hamper processing. This effect was argued to be due to integration of the information relevant to phonetic categorization. The present study was intended to eliminate an alternative explanation based on acoustic discontinuities. Noises and vowels were again cross-spliced, but, in addition, the first 60 msec of the vocalic segment (which comprised the consonant-vowel transitions) either had a nonlinguistic noise added to it or was replaced by that noise. The fricative noise and the majority of the vocalic segment were left intact, and both were quite identifiable. Mismatched consonant information caused delays both for original stimuli and for ones with the noise added to the transitions. Mismatched vowel information caused delays for all stimuli, both originals and ones with the noise. Additionally, syllables with a portion replaced by noise took longer to identify than those that had the noise added to them. When asked explicitly to tell the added versions from the replaced, subjects were unable to do so. The results indicate that listeners integrate all relevant information, even across a nonlinguistic noise. Completely replacing the signal delayed identifications more than did adding the noise to the original signal. This was true despite the fact that the subjects were not aware of any difference.