Acoustic Carrier Research Articles

The emergence of phonetic structure

To explain the unique efficiency of speech as an acoustic carrier of linguistic information and to resolve the paradox that units corresponding to phonetic segments are not to be found in the signal, consonants and vowels were said to be encoded into syllabic units. This approach stimulated a decade of research into the nature of the speech code and of its presumably specialized perceptual decoding mechanisms, but began to lose force as its implicit circularity became apparent. An alternative resolution of the paradox proposes that the signal carries no message: it carries informat~on concerning its source. The message, that is, the phonetic structure, emerges from the peculiar relation between the source and the listener, as a human and as a speaker of a particular language. This approach, like its predecessor and like much recent work in child phonology and phonetic theory, takes the study of speech to be a promising entry into the biology of language. The earliest claim for the special status of speech as an acoustic signal sprang from the difficulty of devising an effective alternative code to use in reading machines for the blind. Many years of sporadic, occasionally concentrated effort have still yielded no acoustic system by which blind (or sighted) users can follow a text much more quickly than the 35 words a minute of skilled Morse code operators. Given the very high rates at which we handle an optical transform of language, in reading and writing, this failure with acoustic codes is particularly striking. Evidently, the advantage of speech lies not in the modality itself, but in the particular way it exploits the modality. What acoustic properties set speech in this privileged relation to language? The concept of encodedness was an early attempt to answer this question (Liberman, Cooper, Shankweiler, & Studdert-Kennedy, 1967). Liberman and his colleagues embraced the paradox that, although speech carries a linguistic message, units corresponding to those of the message are not to be found in the signal. They proposed that speech should be viewed not as a cipher on linguistic structure, offering the listener a signal isomorphic, unit for uni t, with the message, but as a code 0 The code collapsed the phonemic segments (consonants and vowels) into acoustic syllables, so that cues to the *Also in Cognition, 1981, 10, 301-306. +Also Queens College and Graduate Center,. City University of New York. Acknowledgment. I thank Alvin Liberman, Ignatius Mattingly, and Bruno Repp for much fruitful discussion and advice. Preparation of the paper was supported in part by NICHD Grant HD 01994 to Haskins Laboratories. [HASKINS LABORATORIES: Status Report on Speech Research SR-67/68 (1981)J

Apparatus and method for filtering signals in a logging-while-drilling system

The disclosure is applicable for use in a logging-while-drilling apparatus for obtaining subsurface measurements during drilling in a fluid-filled borehole. Acoustic carrier waves are generated downhole in the borehole fluid and are PSK modulated in accordance with digital data representative of downhole measurements. The PSK modulation is obtained by momentarily unidirectionally either decreasing or increasing the frequency of the acoustic carrier signal until either a desired phase lag (for a decrease in frequency) or phase lead (for an increase in frequency) is imparted to the acoustic carrier signal. An uphole receiving subsystem includes transducers for converting the modulated acoustic carrier waves to electronic signals and circuitry for demodulating the electronic signals to recover the measurement information taken downhole. In accordance with an important feature of the invention, a filter is provided for selectively filtering the electronic signals before the demodulation thereof. The filter is provided with a bandpass characteristic having a center frequency which is displaced from the nominal frequency in the directon of the unidirectional decrease or increase of frequency applied when originally modulating the acoustic carrier waves. The center frequency is preferably offset from the nominal carrier frequency by an amount which is a function of the bit rate of the digital information, preferably an amount equal to one-half the bit rate of the digital information. In the preferred embodiment of the invention, the bandpass characteristic of the filter means is skewed in the same direction as the offset.

Band elimination processor for an experimental parametric acoustic receiving array

In the parametric acoustic receiving array (PARRAY), a low‐frequency acoustic wave modulates a locally generated high‐frequency acoustic carrier wave. The information in the low‐frequency signal wave then appears as low‐level, modulation sidebands of the carrier. The function of the receiver electronics is to suppress the high level carrier while simultaneously amplifying the low level near sideband signals. A band elimination processor has been developed that is capable of detecting signals with carrier to sideband ratios approaching 180 dB. Cascaded crystal filters provide the high order of carrier suppression. Additional circuitry is incorporated to permit independent observation of the upper and lower sidebands. The band elimination processor is described in detail and test data for sideband signals from 40 to 800 Hz centered about the 65‐kHz carrier are presented.

Toward a more general integral formulation of the pressure field of an echosonde aperture antenna

The basis for improvements in acoustic echo-sounding (echosonde) antenna characteristics is analyzed. The results are based on a more general radiation integral formulated from the time-independent wave equation for an acoustic wave, using a cylindrical antenna as a model. Employing common forms of aperture field distributions across the upper transmitting aperture of the antenna, a Fresnel-region field quantity, which is expressed in a closed form using Lommel’s series, is obtained. An echosonde antenna design meeting the requirements of previous specifications is made from the numerical results of the farfield patterns in order to show the validity of the results. The design permits the operation of the antenna in urban environments since irritating noise levels are kept low. The height of the antenna shielding cuffs reduces the 90° side-lobe level (ground level) by 3.85 dB at the height of 1 ft through 40 dB at 13 ft. A 90° side-lobe reduction of 38.6 dB, which agrees with predictions, is obtained for the more appropriate height of 12 ft; diameters of 4 and 12 ft for the lower illuminating—and upper transmitting—apertures, respectively. The results, reported for acoustic carrier frequencies of 2 and 5 kHz, indicate that small values of the antenna height are untenable in echo-sounding operation. Subject Classification: [43]85.40; [43]20.15, [43]20.30, [43]20.55.

Frequency smearing of sound forward-scattered from the ocean surface

The results of an open sea experiment are presented in which CW acoustic signals generated by an omnidirectional source and forward-scattered by the ocean surface were measured. The statistics of the surface waves were simultaneously measured by a surface sensing buoy. It is found that for acoustic frequencies in the range of 100–500 Hz and for conditions of small surface roughness, the acoustic spectrum contains the discrete carrier frequency component with sidebands symmetrically positioned about the carrier, as predicted theoretically. The sidebands are directly related to the surface wave spectrum through a response function, which in general depends on both the acoustic carrier and surface wave frequencies. The experimental response function is compared with response functions predicted theoretically by Roderick and Cron, Parkins, and Williams. In addition, for conditions of moderate roughness, marked asymmetry in the acoustic spectra and strong spectral components that are not prominent in the surface spectra are found.

Vitreous As2Se3; Investigation of Acousto-Optical Properties and Application to Infrared Modulator

Acoustic and acousto-optical measurements have been carried out on vitreous As2Se3. Values of acousto-optical figures of merit M (=n6p2/ρ v3) and M′ (=n7p2/ρv) for longitudinal acoustic waves are 1090 × 10−18 sec3/g and 1600 × 10−7 cm2 sec/g, respectively, for He–Ne laser radiation of 1.153 μ polarized along the propagation direction of the acoustic wave. These values are the largest among those for known materials which are transparent in the near-infrared spectral region. Acoustic attenuation has been evaluated as 11 dB/cm at 200 MHz, and the material is considered useful for a modulator element operable up to several hundred MHz. Experimental pulse modulation and transmission of the 1.153-μ laser radiation have been demonstrated using the As2Se3 modulator at an acoustic carrier frequency of 140 MHz. The transmitted pulse pattern has been displayed by the use of TeO2 light deflector.

A general treatment of modulation scanning as applied to acoustic linear arrays — Part 2

Limitations imposed by the acoustic carrier frequency are examined in detail and their implications discussed. An extension of the matrix approach developed in Part 1 leads to multistage modulation systems of particular relevance to rapid scanning of a narrow beam over a wide sector. Optimum arrangements are derived and phasing requirements relating to multi-stage systems examined. Finally the basic and practical advantages of modulation scanning are surveyed.

CW Phase Measurements across Block Island Sound

A shallow-water acoustic range 40 m deep has been investigated at a range of 33 km to determine the magnitude and temporal character of phase variations in the received signals. Variations sinusoidal in nature with peak-to-peak amplitude on the order of 20 cycles of the acoustic carrier were observed at the tidal periods. The phase of the received signal retarded during flood tide and advanced during ebb tide. Carrier frequencies of 1700 and 1704 Hz were employed in separate experiments conducted under mean sound velocities of 1470 and 1450 m/sec, respectively. The results of both tests demonstrate that there is a temporal scale of phase variability closely related to the tidal phenomena.

Transmission Rates for a Binary Data Acoustic Telemetry System

A discussion of the error rates in AM, PM, and FM telegraphy as a function of the signal-to-noise ratio is followed by the derivation of an equation relating the optimum acoustic carrier frequency to the distance separating the transmitter from the receiver. It is then shown how the maximum transmission rate over a given distance may be calculated when the maximum permissible error rate is specified.