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Signal coherence is an important environmental predictor for sonar performance and is often difficult to estimate. This paper examines different methods for estimating signal coherence from data not amenable to coherence estimation. Methods are compared using narrowband, high-frequency, short pulse length continuous wave signals (cw) that typify those used by mine-hunting sonars in complex shallow-water environments in which the spectral frequency resolution is poor using classical overlapping segments and Fourier techniques. Alternate approaches for the spectral and coherence estimation were the autoregressive parametric based approach, harmonic wavelet approach, and concatenation. These methods provide a statistic similar to coherence that is shown to be useful for specialized data. Low model orders in the parametric method yield results that are difficult to interpret. The wavelet approach is more suited for signals whose frequency spans several wavelet levels. Concatenation via narrowband spectral averaging improves spectral resolution but at the expense of pulse train temporal resolution. Generally, these other methods perform no better than classical Fourier-based techniques; however, they can be useful in a limited sense as a relative measure of the 'spectrum sameness' between outputs of a single system. © 1999 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

The performances of the maximum entropy method (MEM) and Wagstaff-Berrou broadband (WB2) high-resolution spatial processing techniques are compared with each other and with the conventional fast Fourier transform (FFT) beamformer. The comparisons are based on the analysis of three sets of measurements acquired at sea during two different environmental acoustics field exercises. Chosen for comparison are the effects of signal-to-noise ratio (SNR) on beamwidth and the effect of the number of array elements on the bearing estimation accuracy and beamwidth of the spatial processors. Results from these data indicate that both of the high-resolution methods yield smaller beamwidths than the conventional FFT processor, and that the high-resolution beamwidths show little dependence on SNR for values greater than 8 dB at the hydrophone input. Also, MEM beamwidths are generally slightly smaller than WB2 beamwidths and show less dependence on aperture. Measured MEM beamwidths show the same general trend as theoretical calculations for sensitivity to SNR, and the average arrival angles of all three beamformers show the same general dependence on the aperture. [ABSTRACT FROM AUTHOR]

An extensive series of sound absorption measurements were taken in air over a range of frequencies from 10-2500 Hz, of temperature from 10°-50 °C, and of relative humidity from 0.3%-92%, all at a pressure of 1 atm. The N2 and O2 vibrational absorption components were extracted from a relatively large background absorption by means of a differential technique using a background gas, 89.5% N2-10.5% Ar, which matches the sound speed of air but has no molecular absorption over the range of experimental frequencies. The measurements reveal: (1) The humidity dependence of the relaxation frequency of N2 in air exceeds that in binary N2-H2O mixtures, possibly because the vibrational modes of CO2 provide a competing relaxation path for V-V exchange between N2 and H2O molecules; (2) the temperature dependence of the relaxation frequency of humid N2 is nearly the same in air as in binary mixtures; and (3) at very low humidities the relaxation frequency of O2 appears to approach a limiting value much lower than that determined from a prior study. The present experimental results, together with those of an earlier study at high frequencies, provide a substantial data base leading to revised formulas for standard values of the relaxation frequencies of N2 and O2. [ABSTRACT FROM AUTHOR]

Sound absorption measurements were conducted by the resonant tube technique to study vibrational relaxation in moist N2 at 301°, 343°, and 387 °K. Analysis of the data yields the following results: (1) The improved measurement accuracy at the higher temperatures establishes vibrational-vibrational (V-V) energy transfer as the operative relaxation path for the de-excitation of N2* by H2O beyond the uncertainty of experimental error. (2) The best-fitted temperature dependence of the V-V rate constant is k30 = 5.41×106 exp(-25.3/T1/3 )(atm·s)-1, an expression which is consistent with seven independent sets of both acoustical and nonacoustical data. (3) The measured humidity and temperature dependence of the relaxation frequency of N2 in air differs substantially from that specified by the recent ANSI Standard S1.26/ASA23-1978. The N2 contribution causes error, up to a few dB/1000 ft, in the total sound absorption coefficient predicted by both the ANSI Standard and ARP866A. [ABSTRACT FROM AUTHOR]

The role of radiation losses in a resonant tube-the transfer of sound energy from the test gas through the confines of the tube to the ambient atmosphere-is investigated both theoretically and experimentally. It is concluded that losses of this sort make no noticeable contribution to the background losses, even at high gas pressures. Nevertheless, if an axial resonant frequency of the test gas closely matches a longitudinal resonant frequency of the tube wall, the internal sound pressure can excite tube displacements of disproportionate magnitude and thus cause excessive losses, not due to radiation but rather to frictional dissipation related to the motion of the tube structure. This mechanism is effective when the mode numbers of the gas and tube have opposite parity, and makes sound absorption measurements at the fundamental gas frequency particularly problematical. An experimental investigation confirms the existence of such additional losses when the parity condition is fulfilled. [ABSTRACT FROM AUTHOR]

The Arctic ocean is a unique region acoustically, dominated by the under ice canopy. The ice varies seasonally in coverage, thickness, and roughness, causing varying degrees of acoustic spatial and temporal variability. In this paper, horizontal coherence, from explosive acoustic sources under the arctic ice pack, is examined in the frequency range 90-1400 Hz with a nominal sensor separation of 370 m. Magnitude-squared coherence is computed separately for the direct and ice-reflected propagation paths. Frequency averaging is used to compute the coherence estimate that fluctuates with horizontal range, sensor depth, and frequency. Frequency and depth dependence is associated with background coherence inside and below the acoustic half-channel determined by sound speed. The lowest frequency bands do not always give the highest coherence. Coherence degradation due to ice interaction may be as high as 60% for the higher frequency bands inside the half-channel. [ABSTRACT FROM AUTHOR]

NRL/SSC conducted Arctic acoustics experiments in the Beaufort Sea as part of ICEX-92 in March and April of 1992. One objective was to collect a long-term time series of low-frequency (less than 50-Hz) acoustic ambient noise with a volumetric array to investigate environmental forcing functions and 3-D noise characteristics. A total of 191 nearly continuous hours of acoustic ambient noise was collected with a vertical array and three horizontal line arrays that were separated approximately 60° in azimuth. A geophone array was also deployed coincident with one of the horizontal line arrays. Meterological data collected included wind speed, wind direction, temperature, and barometric pressure. Oceanographic data included CTD's and current velocities. Local environmental conditions varied widely during the data collection period. In addition, both ridging and an opening of the ice occurred within 500 ft of the vertical array during data collection. The focus of this presentation is the first step in the analysis of this data set and examines the relationship between ambient noise level fluctuations observed on an individual hydrophone and changes in local air temperature for different time scales. Gradual surface temperature increases and decreases over 10-h time segments occur simultaneous with an increase in the noise fluctuation. Temperature changes during time periods less than 5 h show minimal effect on noise fluctuations. Geophone data agree well with acoustic data in both level and fluctuation. Comparisons of ambient noise with other local forcing functions will be discussed. [This research was supported by ASW Environmental Acoustics Support Program (AEAS).] [ABSTRACT FROM AUTHOR]

Two high resolution beamformers, maximum entropy method (MEM) and Wagstaff-Berrou broadband (WB2) [R. A. Wagstaff and J. Berrou, J. Acoust. Soc. Am. 75, 1133-1141 (1984)], were compared with each other and the standard FFT beamformer to determine performance degradation when the number of line array elements and the signal-to-noise ratio (SNR) were decreased. Experimental data from a multi-tonal CW source recorded on an equally spaced horizontal line array were used to investigate the main beam broadening resulting from decreased SNR for SNR values ranging from 30 dB to near zero dB. Additional CW data from an equally spaced vertical line array were used to investigate the main beam broadening which resulted when the number of array elements was decreased successively from 31 to 16, 8, 4, and finally 2. Preliminary results indicate that the main beam broadening due to the decrease in array elements for both MEM and WB2 are similar and that both of these methods offer an order of magnitude decrease in the -3 dB beamwidth with respect to the standard FTT beamformer. [Work supported by Naval Electronic Systems Command, Code 612.] [ABSTRACT FROM AUTHOR]

An extensive set of sound absorption measurements was taken in air over a range of frequency from 20-2500 Hz, of temperature from 20°-50 °C, and of relative humidity from 0.3%-100%. Over the lower portion of this frequency range, where relaxation in N2 is prominent (except in very dry air), prior measurements are scanty. This study yielded the following conclusions: (1) The humidity dependence of the relaxation frequency of N2 differs in air from that in binary N2-H2O gas mixtures (246-Hz/atm. mole % in air vs 184-Hz/atm. mole % in binary mixtures at 20 °C). (2) The temperature dependence of the relaxation frequency of N2 is the same in air as in binary mixtures. (3) At low humidities (∼0.01% mole ratio), where relaxation in O2 dominates sound absorption in air, the measured relaxation frequencies of O2 agree with those reported by Harris and Tempest [J. Acoust. Soc. Am. 36, 2390-2394 (1964)] and lie substantially lower than specified by ANSI Standard S1.26-1978. [ABSTRACT FROM AUTHOR]

A low-cost digital system based on an 8-bit Apple II microcomputer has been designed to provide on-line control, data acquisition, and evaluation of sound absorption measurements in gases. The measurements are conducted in a resonant tube, in which an acoustical standing wave is excited, the excitation removed, and the sound absorption evaluated from the free decay envelope. The free decay is initiated from the computer keyboard, after the standing wave is established and the microphone response signal is the source of the analog signal for the A/D converter. The acquisition software is written in assembly language and the evaluation software in BASIC. This presentation will discribe the acoustical measurement, hardware, software, and system performance. Also, measurements of sound absorption in air will be presented as an example. [Work supported by NASA under Grant NAG 1-234.] [ABSTRACT FROM AUTHOR]

Sound absorption measurements were conducted in N2-H2O gas mixtures at 297°, 343°, and 387°K to determine the location of the vibrational relaxation peak of N2 on the frequency/pressure (f/P) axis as a function of humidity and temperature. At low humidities, the best fit of the reported data to a linear relationship between (f/P)max and humidity h yields, within experimental error, the same slope (2.00 × 104 Hz/atm mole fraction) at all three temperatures. The slope is lower than the value of 2.6 × 104 reported by Chang, Shields, and Bass at higher humidities [J. Acoust. Soc. Am. 63, 577-581 (1977)], but the two sets of data are shown to be mutually consistent by means of a model in which V-V transfer is assumed to provide the dominant relaxation path. The relationship between (f/P)max and h written into ANSI Standard S1.26/ASA23-1978 contains an excessively large slope, does not account for the observed transition between the low-humidity and high-humidity slopes, and specifies an erroneous temperature dependence. [Work supported by NASA Grant NSG 1324.] [ABSTRACT FROM AUTHOR]

Results investigating the effects of variable bottom composition on modeled high-frequency backscattered levels are presented for a typical shallow water, range-dependent environment. The modeled environment consisted of a single sound-speed profile, a flat sea bottom with range-dependent bottom composition, and a benign sea surface. Coarse and fine grain sandy areas were partitioned in range to create range dependence. Bottom backscattering and reflection loss for each partition were obtained from the recent University of Texas high-frequency ocean bottom backscatter model [N. P. Chotiros and F. A. Boyle, 3264(A) (1994)]. Differences of 5 dB were discernible between the fine-coarse-fine sand bottom and a range-independent fine sand bottom for ranges <500 m. The trend and structure of the backscattered levels were nearly identical for both bottom types. Differences of 42 dB for ranges <500 m were visible in the modeled backscattered level of the fine-coarse-fine sand bottom and the coarse-fine-coarse sand bottom. The trend and structure of the backscattered levels were also different. In all cases, surface reverberation had a strong impact on the backscattered levels, trends, and structures. [Work supported by MCM Tactical Environmental Data Systems (MTEDS) under Program Element 62435N.] [ABSTRACT FROM AUTHOR]

Propagation of high-frequency acoustic signals under a multiyear pack ice results in a scattered acoustic field of fluctuating amplitude and coherence. High-frequency scattering from under-ice surfaces is complicated by out-of-plane and multiple scattering because acoustic wavelengths may be many orders of magnitude smaller than ice relief. In addition, the transition layer between water and ice, ice inhomogeneities, and ice anisotropy become important scattering sources. Measurements are presented of horizontal spatial coherence at 24 and 42 kHz from linear FM slide pulses of 800-Hz bandwidth and 100-ms duration. Measurements were made with a linear array of 16 hydrophones distributed over a 16-m aperture and deployed 61 m below the ice surface. The data presented here were taken at a nominal range of 1000 m and results from array bearings at endfire, broadside, and 45° are compared. Results are presented for a fixed source at depths of 30, 61, and 91 m. Deconvolution of the source waveform is used to separate direct and reflected arrivals. Relative amplitudes of these arrivals are compared with what would be expected for a perfect reflector. The amplitude differences may be as high as 30 dB and the ice reflected coherence may increase slightly with range. In addition to hydrophone spacing, frequency, and source depth, the reflected arrival coherence is found to be sensitive to the physical location of the hydrophone in the aperture. This sensitivity is due to multipath interference from the scattering region. [Work supported by the Office of Naval Technology.] [ABSTRACT FROM AUTHOR]

The Arctic is a unique region acoustically, dominated by the under ice canopy. The ice varies seasonally in coverage, thickness, and roughness causing varying degrees of acoustic spatial and temporal variability. Horizontal coherence from explosive acoustic sources at a depth of 300 m under a relatively smooth Arctic ice pack is examined in the frequency range 100-1500 Hz over source-receiver ranges from 3-50 km. Coherence estimation is a difficult problem and is not defined theoretically for transient broadband signals. Mean-square-coherence is computed using frequency band averaging and is computed separately for the direct and ice-reflected propagation paths. Coherence is found to fluctuate significantly with source-receiver range, and acoustic frequency for both acoustic propagation paths. The highest values of coherence do not always occur at the lowest frequency bands. Inside the acoustic half-channel, above 300 m, coherence is less for the ice-reflected arrival. Changes in coherence due to under ice reflection are smallest for the 100- to 200-Hz octave band, indicating less coherence loss due to under ice scattering, but the change is sometimes as high as 50%. [Work supported by the Office of Naval Technology.] [ABSTRACT FROM AUTHOR]

Estimation of time delays between coherent transient signals is biased by the choice and location of the sampling window. For transients that do not tend to zero within the sampling window (self-windowing), the uniform or boxcar window may not be the best choice due to spectral leakage. This is especially important when processing highly asymmetrical transient signals that are not self-windowing as in the case of acoustic ambient noise. When a sample window is used to suppress spectral leakage, a bias in time delay estimation results from a suboptimal window location. Even though the boxcar window produces the least bias in time delay when the transient is self-windowing, it is shown that sidelobe interference is significant for highly asymmetric signals. This work critically examines these effects and describes an iterative procedure to determine optimal location of the transient within any sample window. [Work supported by the Office of Naval Technology.] [ABSTRACT FROM AUTHOR]