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Nov 1976

Volume 60, Issue S1, pp. S1-S125

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back to top Session H. Underwater Acoustics II: Signal Processing and Noise. Precis Poster Session
Contributed Papers
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Efficient computation of array patterns (A)

Victor C. Anderson

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S17-S17 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The impact of a symmetrical array geometry, the use of a quantized stored cosine function, the exploitation of Digital Fourier transform algorithms and the application of trigonometric interpolation in the computation of array patterns is discussed. Careful selection of parameters permits sampling the array pattern only 6% above the theoretical Nyquist limit. Reconstruction of array patterns showing −20, −30, and −40‐dB relative interpolation errors are presented. A saving of 8000: 1 in computation time over direct “brute force” array pattern computation is illustrated for a hypothetical array. [Research supported by the Office of Naval Research and the Advanced Research Projects Agency.]
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Design of transducer arrays with tapered side‐lobe heights (A)

Geoffrey L. Wilson

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S17-S17 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Transducer arrays for many underwater sound applications have customarily been designed for equal sidelobe levels. This is, perhaps, as much due to the convenience of the analytical Dolph—Chebyshev technique as to any specific requirement. Using a simple adaptation of a previously reported numerical technique for symmetrical arrays [G. L. Wilson, J. Acoust. Soc. Am. 59, 195 (1976)], designs can be obtained whose directional response has a taper on the envelope enclosing the side lobes. The method is equally applicable to sum and to difference patterns. Examples are given. [This work was sponsored by the Naval Sea Systems Command, Code SEA‐034.]
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Method for increasing the number of beams from a time‐domain beamformer (A)

J. H. Stockhausen and L. D. Walkty

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S17-S17 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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In a digital delay‐and‐add beamformer for a line array the number of beams that can be steered between broadside and endfire is equal to the travel time between elements divided by the sampling interval. This number is independent of the number of elements and if the array is long there may be insufficient beams to cover the directional space completely. Without increasing the sampling rate, additional beams may be steered by interpolating between successive samples from every second element. In typical cases the degradation of performance is insignificant.
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Cross‐sensor beam forming with sparse arrays (A)

H. P. Bucker

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S17-S17 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The quadrature field of a narrow‐band signal measured at a sensor, or sensor group, in an array is a complex number that rotates with time. Thus time averages of the quadrature field tend toward zero. However, elements of the cross‐sensor field, i.e., the quadrature field at one sensor multiplied by the complex conjugate of the quadrature field at a second sensor does not rotate and can build up with time. By using the cross‐sensor field, a beam pattern (for a one‐wave, no‐noise acoustic field) can be generated for a spare array which is the same as for a filled array with the same aperture. When real world effects, such as noise and multiwave acoustic fields, are considered the performance of the sparse array degrades more than the performance of the filled array. However, by time averaging the cross‐sensor field the performance of sparse array is greatly improved. Calculations for a 23‐element sparse line array in a realistic simulated acoustic field show significantly improved performance when compared to a 23‐element uniform array.
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Nearfield Fourier transform imaging (A)

Donald A. Murphy

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S17-S18 (1976); (2 pages)

Online Publication Date: 11 Aug 2005

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The two‐dimensional Fourier transform provides an efficient method of forming beams for a line array. The field as sampled by the evenly spaced array is the Fourier transform of the farfield source. Its resolution is limited by the receiving aperture and the signal‐to‐noise ratio at the array elements. When the field is stationary, the resolution can be increased by integrating the Fourier transforms magnitude squared. If the field is also homogeneous the resolution can be increased by extending the aperture without adding additional elements by the method described by Nuttall et al. This method constructs the two‐dimensional spectrum of objects in the farfield without grating lobes from the cross spectra of elements of the array. The array spacings must all be multiples of a basic spacing in order to make the calculation efficient. The method has been extended to apply to objects in the nearfield as well, and allows estimation of noise measured in the nearfield with sparce arrays with an efficient computer program.
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Parametric receiving array beam patterns using a phase demodulator (A)

J. K. Beard and J. J. Truchard

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S18-S18 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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In this paper, closed form solutions are derived for several configurations of the parametric receiving array with a phase demodulator receiver. The expression for parametric nonlinear acoustic phase modulation of a high‐frequency spherical wave from a point source by a low‐frequency plane wave is calculated. The result is in turn used to generate the solution for parametric receiving arrays with various pump transducers, including a truncated line source, a rectangular piston, and a circular piston. The solution of the parametric receiving array and a truncated line receiver is also found. A series of experiments was conducted with a 48‐ft parametric receiving array at a pump frequency of 90 kHz. These experiments duplicated several of the geometries used to obtain the theoretical results. Good agreement between theory and experiment was achieved. [Work supported by ONR]
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Signal‐to‐noise ratio optimization in hydrophone—pre‐amplifier systems (A)

J. W. Young

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S18-S18 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The decrease in signal‐to‐noise ratio caused by electronic noise is an important factor in the design of an acoustic receiving system. Normally, this loss is dominated by the hydrophone, cable, and proamplifier which make up the “front‐end” of the system. In this paper, the signal‐to‐noise degradation factor (SND) for these components is defined. It is shown that the SND can be minimized by proper impedance matching of components. This can be accomplished either by use of a transformer or by the choice of a particular interconnection scheme for a multielement hydrophone. The minimum value of SND which can be achieved is determined by the product of figures of merit (FOM) of the hydrophone and preamplifier. For piezoelectric hydrophones, the FOM is a function of fundamental material parameters and the volume of the ceramic element. The amplifier FOM depends on the equivalent voltage and current noise sources of the device. Both charge amplifier and voltage amplifier configurations have been considered. The charge amplifier is found to have inherently poorer noise performance.
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Programmable system for real‐time data analysis and display (A)

J. L. Bardin, V. M. Moore, and D. K. Raley

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S18-S18 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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A computer‐based system for real time analysis and display of acoustic data is described. A mini computer and special purpose Fourier transform hardware are combined to permit continuous FFT and auto spectral analysis, and/or coherency analysis. In addition, software is available to perform various post‐FFT analyses, including spectral whitening, automatic narrow‐band energy detection and harmonic analysis. A second computer has been dedicated to the formatting and displaying of analysis results. Being programmable allows operation of the computer‐controlled display in various modes, such as A scan, time‐frequency‐amplitude alphanumeric, and combinations thereof. High‐speed rotational engine radiated noise and line component data have been analyzed and results are presented.
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Spectral estimation techniques with special reference to sonar signal processing (A)

A. Arcese and L. E. Bergeron

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S18-S18 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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In this talk we present a comparative evaluation of several spectral estimation techniques. The estimation procedures investigated are the FFT and autoregressive modeling of the time domain waveform. For the autoregressive modeling the estimation procedures considered are the Durbin recursion, Burg recursion, Kalman filtering, and a gradient technique. We also present a new maximum entropy gradient spectral estimation procedure. The results of applying these spectral estimators to real submarine target data are presented.
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Signal excess and detection probability of fluctuating sonar signals in noise (A)

R. J. Urick

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S18-S18 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Signal excess is defined as the excess or deficiency of signal level relative to that required for a detection probability of 50%; curves of detection probability versus signal excess are sometimes called “transition curves.” In this paper, transition curves for a nonfluctuating signal in Gaussian noise and for Gaussian, Rayleigh, and log—normal signal fluctuations are obtained. A comparison with one set of detection data obtained in a sea test with a towed sound source indicates that log—normal signal excess fluctuations with a standard deviation of 6–8 dB give a reasonable fit to the data. The method suggested here—of using transition curves along with a known or assumed relation between transmission loss versus range—permits quicker computation of detection probability against range than does the conventional way of employing ROC curves, and can accommodate a signal excess that fluctuates with time about a mean value. [Work supported by Naval Surface Weapons Center and ASW Systems Project Office, Code ASW‐13.]
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Generalized performance (A)

W. S. Hodgkiss

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S18-S18 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The performance of a Bayes optimal detector is summarized by its ROC curve. In the general ease where uncertain parameters exist under each hypothesis, knowledge about them is explicitly noted at the outset by an a priori probability density function conditional to H1 and one conditional to H0. The processor's performance then becomes a function of their detailed shape. Often, the functional form of these densities is chosen so that various levels of uncertainty are easily modeled and a family of ROC's is reported. The question then arises: What performance would have been achieved under different prior knowledge assumptions (particularly when outside the class of densities modeled)? Or, more deeply: Does some algorithm exist which will operate on a known ROC for a given pair of priors to yield the ROC for a new set of priors? And, if not: Does a canonical intermediate step exist between observation and likelihood ratio statistics which always may be used as a starting point for the calculation of an ROC based on an arbitrary pair of priors? The intent of this paper is to pursue these questions. The discussion will use as a basis the fundamental concepts of sufficient statistics and reproducing densities. A specific example is presented. [Work supported by ONR.]
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Maximization of reverberation gain (A)

D. Lee

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S18-S19 (1976); (2 pages)

Online Publication Date: 11 Aug 2005

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The problem of maximizing array gain (AG) for situations where ambient noise is the dominant interference has already been extensively investigated, These investigations have resulted in mathematical solutions having significant promise for various practical applications. A closely related and more general problem arises when interference is dominated by reverberation; in this situation the gain against reverberation (the reverberation gain, RG) must be maximized. The same general mathematical approach can be applied to both problems in that the omnidirectional ambient noise limited condition can be considered a specific case of the more general directional ambient noise limited condition and the reverberation limited condition. Much of the existing theory has been directed at the specific cases thus leaving unfinished a comprehensive treatment of the general case. In this presentation, the complete RG‐maximization problem will be addressed; a mathematical procedure to achieve the solution will be outlined; this procedure is developed with sufficient generality so that the maximization of AG is automatically included. An application of this technique is carried out for a three‐dimensional array which demonstrates the validity and the effectiveness of this technique. [Work supported by NUSC.]
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Who needs the octave—or, are standard bands feasible? (A)

L. C. Maples

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S19-S19 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Although with the advent of the fast Fourier transform and digital filtering the constraints of available analog filter specifications are no longer really pertinent, the tendency to use the ANSI standard frequency bands for reference and comparison is widespread, particularly the so‐called ⅓‐octave bands. However, in problems involving complex spectra with shifting narrow‐band components, it becomes obvious that the ⅓‐octave approach is not adequate and may give misleading results, particularly at low frequencies, where the bandwidth is narrow. Similarly, because of their conceptual nature and also the way in which the bands are specified, consistent subdivision into narrow proportional bands is not possible, and such subdivision is now desirable, if not essential. For this reason, a return to the decimal system, in name as well as in fact, is strongly recommended. The problems encountered with standard “⅓‐octave” bands are discussed in detail, in the context of the band specifications, and alternative methods of band specification, which avoid the problems entirely, are described, including a new look at the concept of “spectrum level.” [Work supported by NUSC.]
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Interfrequency correlations of ocean ambient noise (A)

R. H. Nichols and C. E. Sayer

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S19-S19 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The correlation of ambient noise levels in each of several frequency bands with those in other bands was investigated for an 11‐½‐day period in February with a hydrophone laid on the ocean bottom in deep water. The bandwidth for the measurements was 6 Hz; the center frequencies were chosen to cover the low frequency range in which shipping is generally the dominant source of noise, 10–150 Hz [G. M. Wenz, J. Acoust. Soc. Am. 34, 1936–1956 (1962)]. A total of 1562 consecutive 640‐sec samples was taken in each frequency band; the average level for each sample was calculated and used in the computation of the correlation coefficients. Tonal line components were excluded. It was found that the coefficients of correlation between sample levels at a given frequency f0 and those at other frequencies generally decreased with increased frequency spacing, but the patterns differed for different f0′s. Typical patterns will be shown. [Supported by IR&D program.]
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Seasonal dependence of ambient sea noise near the marginal ice zone of the Greenland sea (A)

James R. McGrath

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S19-S19 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Omnidirectional ambient sea‐noise measurements in the 20–60‐Hz band near the marginal ice zone were recorded in the deepwater basin of the Greenland sea. This experiment, conducted between August 1972 and July 1973, provided 1‐min samples of sea noise every 4 h using an omnidirectional hydrophone 306 m below the surface. During this time, the ice edge moved over the recording site, providing evidence that the iceline acts as a noise source, raising local levels above both open‐ocean and ice‐field values. Seasonal conditions favor the lowest ambient noise levels during midwinter (January) and early summer (June), while the highest levels occur during early spring (March) and early fall (November). Evidence indicates that the maximum ambient noise levels near the marginal ice zone are 12–16 dB higher than the maximum Arctic Ocean values under contiguous ice cover.
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Measurements of directional ambient noise (A)

S. O. McConnell

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S19-S19 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Measurements of the directional ambient noise field have been made in a bay using an acoustic lens for which the beam pattern closely approximates a circular piston for the individual transducers (DI=28.7 at 25 kHz). The intensity radiation pattern of the noise at frequencies between 5 and 40 kHz is found and compared to models of the form sin2nθ where θ is the grazing angle and n=1, 2, 3. This form for the radiation pattern is used for comparison since previous measurements made at frequencies below 5 kHz can be fitted by a sin2θ pattern at frequencies above ∼ 500 Hz. Since the ambient noise at these frequencies appears to be generated at or near the sea surface due to such mechanisms as splashing of droplets, the intensity of the noise should be a function of conditions at the sea surface; a supposition borne out by these and previous measurements. Measurements are made of the wind shear and one‐dimensional wave spectrum and related to the intensity of the noise.6 Measurements of the rate of rainfall and observations of intermittent noise sources are also made.
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Noise generated by axisymmetric turbulent boundary layer flow (A)

G. C. Lauchle

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S19-S19 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The noise generated by hydrodynamic flow over an axisymmetric body with a blunt nose is described quantitatively. Flush‐mounted piezoceramic hydrophones were used to measure that part of the turbulent boundary‐layer pressure fluctuations that propagates as true sound. Power spectra of the sound pressure were measured in a 3–50‐kHz frequency range over a wide range of Reynolds numbers (U∞D/ν  ⩽  3.03 × 106, where D is the diameter of the body) for the model operating in the Garfield Thomas Water Tunnel. The use of flush‐mounted hot‐film probes to locate turbulence transition is also described. The power spectra of the noise measured in the laminar flow regions correspond closely to those measured in the transition and fully developed turbulent regions of the flow, The exceptions were those spectra measured on the flat part of the nose, but correction for diffraction loss effects suggests that the noise measured there is due to the noise generated by the turbulent part of the flow. Nondimensionalization of the noise spectra measured at various arc lengths with theoretical expressions for the noise expected, that include a critical boundary layer thickness at the beginning of turbulence, show a general collapse of data to within 6 dB. When applicable, comparisons of the radiated noise measured on buoyant bodies are made and agreement was found to be excellent.
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Infrasonic flow‐noise measurements using an H‐58 hydrophone (A)

O. M. Griffin, J. R. McGrath, and R. A. Finger

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S19-S20 (1976); (2 pages)

Online Publication Date: 11 Aug 2005

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Infrasonic and low‐frequency flow‐noise measurements were made during laboratory tests of an H‐58 omnidirectional hydrophone. The hydrophone was tested in three configurations: bare, framed, and faired. Water speeds were varied from 0.25 to 0.45 knots and corresponded to a Reynolds number range from 8800 to 16 000. For different hydrodynamic reasons, the faired and framed configurations developed flow‐noise levels lower than the bare hydrophone configuration. The bare hydrophone developed the highest noise levels at the lowest frequencies of the 1–50‐Hz band. Hydrodynamic considerations related to the acoustic measurements made during this test are discussed; the importance of hydrophone configuration used in very low‐frequency measurements is demonstrated.
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Noise from cavitating hydrofoils as influenced by boundary‐layer development (A)

William K. Blake and F. Ellsworth Geib, Jr.

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S20-S20 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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This paper describes measurements of noise from cavitating flow over a hydrofoil. Cavitation was generated on a hydrofoil in the presence of a separated laminar boundary layer on the one hand and of a fully turbulent attached boundary layer on the other. The turbulent boundary layer was formed down‐stream of a trip which was positioned near the leading edge. The noise is shown to depend on the type of cavitation produced. Dimensionless spectral densities of the sound are shown for each type of flow. For the cavitation associated with turbulent boundary layer development, the dimensionless spectrum is interpreted in terms of the observed single‐bubble motions in the experiment.
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Confidence limits for measured supertanker‐radiated noise levels (A)

J. Cybulski and E. B. Wright

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S20-S20 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Sound pressure spectral levels were obtained for the supertanker class of ships using a developing technique of aircraft deployed Sonobuoys with a deep hydrophone at 305‐m depth. These measurements were made in the deep oceanic waters over the Seine, Tagus, Iberian and Biscay abyssal plains. The received levels at individual omnidirectional sensors were processed to yield ship radiated noise in 0.2‐Hz bandwidths over the 5–80‐Hz band and transformed to sound pressure levels in dB ‖ μPa at 1 m. Confidence limits that are constructed for these values reflect the effects of navigation, calibration, geometry reconstruction. CPA determination and a transmission loss correction associated with the range between ship and sonobuoy sensor. The variation of these limits is illustrated as a function of CPA, aspect angle and frequency with standard errors as high as 3 dB. The primary sources of error are hydrophone/system calibration, CPA determination and the transmission loss values. The FACT acoustic‐environmental model was employed to obtain the frequency‐range dependence corrections. Directions toward increasing the confidence of these measurements are indicated. [Work sponsored by Naval Electronics Systems Command. Code 320.]
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Ambient sea noise directionality: Measurement and processing (A)

N. Yen

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S20-S20 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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The response of a receiving hydrophone array in a noise field is analyzed for the relationship between the array parameters and the directionality of the noise field. The base functions formulated by array element spacing are considered as the coordinates of a multidimensional space. By orthonormalization of these base functions, the cross‐spectra between hydrophone outputs are transformed directly to noise field directionality. Applying the same method to the array's beamforming output also gives another noise field estimate. When the number of independent measurements from the array's beamforming output is the same as that of the cross spectrum, the noise fields estimated by either methods are equivalent and depend on the array's structure. For stationary noise fields with low variability in the measured data, a further improvement in the angular resolution of the noise field can be achieved by deriving a new set of orthogonal base functions through analytic continuation. Interferences caused by uncorrelated local noise together with limits in the precision of numerical computation are shown to impose a practical limit on the accuracy of the estimated noise field. Thie processing method for noise directionality is illustrated by application to a simulated noise field and sea data acquired from a vertical line array. [Work supported by NAVSEA.]
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Project SPAN 3: Low‐frequency ambient sea noise in the South Fiji Basin (A)

R. W. Bannister, R. N. Denham, K. M. Guthrie, and D. G. Browning

J. Acoust. Soc. Am. Volume 60, Issue S1, pp. S20-S20 (1976); (1 page)

Online Publication Date: 11 Aug 2005

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Measurements of low frequency ambient sea noise near the New Zealand coast show relatively low levels due to a generally sparse shipping density in the Southern Hemisphere. [R. W. Bannister et al., J. Acoust. Soc. Am. 55, 418 (1974)] Project SPAN 3 extends these measurements to a deep ocean basin, specifically the South Fiji Basin located to the north of New Zealand. Ambient noise (10–500 Hz) was recorded continuously for one week on five hydrophones which were suspended between 300 and 2300 m depth in 4000 m of water. Supporting propagation loss measurements and ship density surveys were conducted. The results are in agreement with the previously reported data. [Work supported by NUSC and DSE.]
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