Index of content:
Volume 58, Issue 3, September 1975
58(1975); http://dx.doi.org/10.1121/1.380704View Description Hide Description
The evolution of acoustical consulting over the past 40 years is traced from the early days of simple measurement and little technical information through to the emergence of the consultant as a central participant in the quieting of large machines and airfleets that affect the lives at work and at home of large segments of the population. The part the acoustical consultant plays in national noise regulation and control is discussed. Acoustical consultants are urged to take a broader view of their intraprofessional relationships and raise their level of professionalism, not only to their clients but also to each other.
Subject Classification: 10.60, 50.10, 55.10.
58(1975); http://dx.doi.org/10.1121/1.380705View Description Hide Description
Digital spectrum analysis of harmonic signals can result in amplitude estimates in error as much as 3.92 dB. The corresponding frequency estimates are not exact. The paper presents a general method for obtaining significantly improved estimates of amplitude and frequency. Criteria are given which allow specification of an error limit. Specific equations are given for sample signals that are unmodified (open window) and for signals modified by a Hanning data window. The phenomenon called ’’leakage’’ is shown to result from discontinuities imposed by the computation process at the periodically extended ’’ends’’ of a sample signal and not, as is often supposed, by discontinuities presumed to (but do not) exist at the ’’ends’’ of the open window. Criteria for window selection to reduce leakage are discussed. Calibration is specifically treated. When any data window other than the open window is used, a different calibration must be applied to periodic and random components in a signal. Although discussion is limited to a single harmonic signal, the method can be applied in a straightforward way to signals with multiple harmonics.
Subject Classification: 10.60; 60.10; 30.80; 40.60; 50.85, 60.50; 85.32, 85.84.
58(1975); http://dx.doi.org/10.1121/1.380706View Description Hide Description
The scattering of a plane acoustic wave by a rigid prolate spheroid of large fineness ratio is investigated analytically. The solution for the diffracted field is presented in terms of the classic modal series of spheroidal harmonics. The series are numerically evaluated for wavelengths much smaller than the length of the spheroid, but comparable to the minor radius. For this range of wavenumbers, use was made of available asymptotic formulas of the spheroidal wave functions, where the convergence of the double infinite series is discussed in terms of the value of the product of wavenumber and various characteristic lengths of the spheroid. The directivity functions are presented for several values of plane‐wave incidence, reduced frequency, and fineness ratio. The total scattering cross section is evaluated and compared to the scattering cross section of rigid spheres. An estimate is given to the size sphere required to scatter approximately the same power as the spheroid of given size and orientation with respect to the incoming plane wave.
Subject Classification: 20.15, 20.30.
58(1975); http://dx.doi.org/10.1121/1.380707View Description Hide Description
The backscattering of a plane acoustic wave from a rigid prolate spheroid of large fineness ratio (length‐to‐diameter ratio of 22) is investigated analytically. The backscattering cross section is computed with the use of large argument asymptotic expansions of spheroidal wave functions. It is shown that in the short‐wavelength limit, a spheroid backscatters in the axial direction nearly the same mean energy as a sphere whose radius equals the radius of curvature of the spheroid at the pole end. When the spheroid is positioned broadside to the incident wave, the backscattered mean energy is found to correspond to that backscattered from a sphere of diameter equal to the length of the spheroid.
Subject Classification; 20.15, 20.30.
58(1975); http://dx.doi.org/10.1121/1.380708View Description Hide Description
The advantages of using fused silica spheres as the target in the radiation force method of acoustic intensity determination are considered. Experimental determination of the radiation force function, for fused silica spheres in water in the range of k a 2–24, was carried out. The experimentally obtained values of the radiation force function agree within 2% of those calculated from the theory of the radiation force on solid elastic spheres. This makes possible determination of ultrasonic intensity and calibration of acoustic probes with uncertainties <2% to values of k a=24, where the accuracy of the radiation force method with stainless steel spheres becomes reduced. The experimental conditions necessary for the realization of this accuracy are discussed.
Subject Classification; 25.45; 30.80; 35.80; 85.22.
Cross‐correlation methods for studying near‐ and farfield noise characteristics of flow–surface interactions58(1975); http://dx.doi.org/10.1121/1.380709View Description Hide Description
Systematic methods based on cross‐correlation techniques are presented for experimental studies of near‐ and farfield noise characteristics in airflow–surface interaction problems. Analyses show that, in near‐ and farfields, the noise characteristics due to the surface contribution of fluctuatingpressures and velocities and due to the volume contribution of the turbulence in the flow can be determined separately. Both farfield noise intensities and nearfield acoustic energy fluxes can be expressed in terms of appropriate cross correlations. These correlations can be obtained by making microphone measurements in the farfield, in the nearfield, and on the surface. Examples of the applications to the noise field generated by impinging jets, by surface blowing jets, and by turbulent flow over trailing edges are presented. Advantages of the present method over conventional methods are also discussed.
Subject Classification: 28.65; 50.25, 50.30, 50.55.
Fluctuations of narrow‐band sound amplitudes from long‐range transmissions in the deep Atlantic Ocean58(1975); http://dx.doi.org/10.1121/1.380710View Description Hide Description
For transmission paths between a fixed acoustic source at 527‐m depth near Eleuthera and a fixed receiver at 1723 m near Bermuda, G. E. Stanford [J. Acoust. Soc. Am. 55, 968–977 (1974)] has shown that the spectrum of fluctuations in acoustic intensity is nearly flat up to 3 cycles/h and drops off as 1/f 3 above this frequency. For comparison with internal gravity wave disturbances of the soundvelocity profile, we have analyzed amplitude fluctuations of transmissions from floats deployed as part of the 1973 series of Mid‐Ocean Dynamics Experiments (MODE). These floats carrying 30‐W input, 270‐Hz sound sources pulsed for 1.67 sec at 3‐min intervals were released to drift near 28°N,70°W at a depth of approximately 1500 m. The transmissions were received by fixed hydrophones (MILS system), located on the sound channel axis on the slopes of Eleuthera and Grand Turk. We find a spectrum similar to that of Stanford except that the 1/f 3characteristic begins at 1 cycle/h. For each experiment this rolloff frequency coincides approximately with the Brunt–Väisälä frequency at the turning points of those SOFAR paths with minimum excursion from the axis.
Subject Classification: 30.20.
58(1975); http://dx.doi.org/10.1121/1.380698View Description Hide Description
Fluctuations of CW phase caused by time‐varying multipath interference are simulated with fully coherent ray propagation models. Fixed‐system measurements of fluctuations of CW transmission are presented and compared for three ocean ranges. When the sound‐speed perturbation used for model inputs are adjusted to give the same statistics of transmission‐loss fluctuations (decorrelation time, variance, and mean‐square bandwidth) as observed, then the associated fluctuations of CW phase also have the same statistics as the experimental results. Phase fluctuations are shown to be strongly influenced by the nonlinear multipath process. For low‐frequency internal waves (less than 1 cycle/h), and especially for internal tides, linear or direct fluctuations influence the phase spectrum, but for higher frequencies it seems likely that nonlinear effects dominate. Therefore, the possibility of a direct linear relationship between the internal‐wave spectrum and the spectrum of phase fluctuations is unlikely.
Subject Classification: 30.35, 30.20; 20.20.
58(1975); http://dx.doi.org/10.1121/1.380699View Description Hide Description
Sound propagationmeasurements at various frequencies between 10 and 60 kHz were made in April 1974 under the pack ice near Pt. Barrow, Alaska. With a stationary transmitter at mid‐depth, a series of sound level versus depth profiles at ranges between 40 and 1300 m were obtained and used to calculate the absorption coefficient. Although the sound speed profile contained many small irregularities which caused undesirable fluctuations, it had no large features that would change the average intensity at the ranges and depths at which the sound was received. The average temperature and salinity were −1.6 °C and 32.3 0/00, respectively. The calculated absorption values in decibels per kiloyard were 2.5±0.5 at 10 kHz, 4.5±0.5 at 20 kHz, 8.3±0.5 at 30 kHz, 10.7±0.4 at 40 kHz, and 13.9±0.5 at 60 kHz. These values indicate a relaxation frequency, assumed due to MgSO4, of 27±5 kHz, a value much lower than that predicted by the Schulkin–Marsh equation, but about the same as that determined by Greene in similar low‐temperature water.
Subject Classification: 30.20, 30.80; 35.24.
58(1975); http://dx.doi.org/10.1121/1.380700View Description Hide Description
CW acoustic transmissions received by ship‐suspended hydrophones from seamount emplanted sources off San Diego of 750, 1500, and 3000 Hz; and submarine sonars of 1500 and 2500 Hz in the Gulf of Alaska result in attenuation coefficients significantly below values given by Thorp’s equation. The present data, and the Sheehy–Halley very‐low‐frequency Pacific data, suggest a modified equation for CW transmissions.
Subject Classification: 30.20.
58(1975); http://dx.doi.org/10.1121/1.380701View Description Hide Description
Short‐duration, laser‐generated, compressive stress pulses were propagated in a variety of solid and liquidmaterials; and the initially compressive pulses were observed to develop tensile ’’tails’’ for propagation distances of less than 6 cm. This type of pulse distortion cannot be attributed solely to geometric dispersion or materialattenuation. It is shown that the observed pulse distortion can be explained using elementary diffraction theory. Since the transition distance from the Fresnel region (nonattenuative) to the Fraunhofer region (1/zattenuation) is dependent upon the wavelength of the acoustical radiation, stress p u l s e s composed of many wavelengths are distorted and not merely attenuated during propagation. Numerical results are presented to substantiate this observed effect and to provide a convenient means of predicting the shape of a stress pulse at any distance in a nondispersive, nonattenuative medium given the shape initially.
Subject Classification: 35.65, 35.24, 35.26; 25.22.
58(1975); http://dx.doi.org/10.1121/1.380702View Description Hide Description
A technique for the digital simulation of multicorrelated Gaussian random processes is described. This technique is based upon generating discrete frequency functions which correspond to the Fourier transform of the desired random processes, and then using the fast Fourier transform (FFT) algorithm to obtain the actual random processes. The main advantage of this method of simulation over other methods is computation time; it appears to be more than an order of magnitude faster than present methods of simulation. One of the main uses of multicorrelated simulated random processes is in solving nonlinear random vibration problems by numerical methods of simulation. One of the main uses of multicorrelated simulated random integration of the governing differential equations. The response of a nonlinear string to a distributed noise input is presented as an example. nonlinear string to a distributed noise input is presented as an example.
Subject Classification: 45.40; 40.30, 40.22, 40.35.
Measurement of frequency responses and the multiple coherence function of the noise‐generation system of a diesel engine58(1975); http://dx.doi.org/10.1121/1.380703View Description Hide Description
A structural–acoustical system has been established to model theoretically the noise generation of a six‐cylinder diesel engine. In this system, six‐cylinder pressures have been treated as six mutually correlated inputs. The noise measured 3 ft away from the engine has been treated as the single output of the system. Six structural–acoustical frequency responses with respect to the six inputs were evaluated experimentally. The multiple coherence function between all the inputs and the single output has also been evaluated experimentally. Using the multiple coherence function, the engine noise which is coherent to the cylinder pressures was separated from the total engine noise. It has been found that, for the V‐6 diesel engine studied, the coherent noise is almost equal to the total noise. From the structural–acoustical frequency responses, the contributions of the individual cylinder pressures to the coherent engine noise were assessed.
Subject Classification: 50.50; 60.20.
58(1975); http://dx.doi.org/10.1121/1.380711View Description Hide Description
It is shown in geometrical acoustics that ergodic specular enclosures do exist and that in such auditoriums, but not in general, 4V/S′ is the exact mean directed path length (V is volume and S′ is any part of surface area S). Sabine’s expression is then demonstrated to yield the exact reverberation time, provided the enclosure is mixing and provided the inhomogeneous anisotropic surface absorptivity is sufficiently weak. It is further proven that the functional form of Sabine’s expression cannot be modified so as to become correct for large absorption. In an attempt to reassign credit and to reconcile these results with influential findings to the contrary, a short historical account is added. Conditions imposed upon the surface reflectivity—whether the reflectivity be reversible (specular or other) or irreversible (statistical) —by the second law of thermodynamics and by the principle of detailed balance are evaluated. Extensions (e.g., mean length of curved paths in an ergodic auditorium with a thermal gradient) and other applications (electroluminescent‐diode design) are noted.
Subject Classification: 55.20, 55.30, 55.55.
58(1975); http://dx.doi.org/10.1121/1.380712View Description Hide Description
This paper presents a Bayes optimum approach to detecting a target of known location by processing the outputs of an array of sensors. Beamforming is not postulated a p r i o r i. The complex exponential Fourier series is used to represent the deterministic signals and random processes. Using Bayes decision theory, important existing results and new results are unified and presented in a single mathematical framework. Optimum receiver structures and performance are derived for detecting a known signal source in a Gaussian noise field. They are also derived for detecting a signal source which generates an uncertain waveform in either a spatially uncorrelated noise field or a noise field with spatial correlation due to a farfield noise source. Important new results concerning factorization of optimum array receivers are presented for detecting signal sources of known location which generate either known or uncertain waveforms. A technique for applying existing scalar results to array problems is presented for several cases. Detection performance results, in terms of the receiver operating characteristics (ROC), which are characterized in some cases by the detectability index, are presented for a number of situations.
Subject Classifications: 60.20; 25.35.
58(1975); http://dx.doi.org/10.1121/1.380713View Description Hide Description
In practical underwater signal detection problems using an array, the statistics of the ambient noise field vary spatially and temporally. This paper considers the problem of detecting a farfield signal source of known location and waveform which generates a known signal field at an array aperture. The power of the ambient noise field is considered to be a random function of space and time and the Bayes approach is used to derive optimum receiver structures and their performance. For all cases considered, the optimum array processor either requires spatial filtering which is not accomplished by a beam former or requires spatial processing in addition to that provided by a beam former. The loss of detection performance due to uncertainty in the parameters of the ambient noise field is determined for several specific cases.
Subject Classification: 60.20; 25.35.
Correlation function of the field between arbitrarily oriented points for a plane wave in a random medium58(1975); http://dx.doi.org/10.1121/1.380714View Description Hide Description
This paper is concerned with the correlation between arbitrarily oriented points for a plane wave propagating in a statistically homogeneous and isotropic medium. The bearing response pattern of a line array in such a medium is obtained and numerical results are presented.
Subject Classification: 60.20; 30.20; 60.30.
58(1975); http://dx.doi.org/10.1121/1.380715View Description Hide Description
This paper studies the ability of a line array to discriminate between two nearfield acoustic point sources from the viewpoint of statistical hypothesis testing. The optimum binary detectors are presented for noise alone and noise plus point source interference environments. Under a small signal‐to‐noise power ratio assumption, general expressions are presented for the performance of these detectors. For a uniformly spaced line array with a large aperture (compared to the source range) in a spatiallywhite noise environment, the detector performance is shown to be linearly proportional to signal‐to‐noise ratio, inversely proportional to hydrophone spacing and proportional to a factor involving the ratio of the separation of the two sources to the acoustic wavelength. When operating in a noise plus interference environment, the performance of this detector is severely degraded. The optimum detector for noise plus interference is shown to perform significantly better.
Subject Classification: 60.20, 60.30.
Measurements of diffraction and interaural delay of a progressive sound wave caused by the human head. II58(1975); http://dx.doi.org/10.1121/1.380716View Description Hide Description
On several occasions the authors have required information on diffraction of sound and interaural time delay caused by the human head, and have measured these parameters in anechoic space. The measurements are of interest since both live subjects and a sophisticated acoustical manikin were used. These measurements are difficult to perform in a reliable fashion at high frequencies on live subjects because of head movement and use of the manikin allowed extension of most measurements to 10 kHz. Different measurement techniques were tried to yield accurate data, and these methods are discussed and results compared.
Subject Classification: 65.60, 65.62, 65.75; 20.30.
58(1975); http://dx.doi.org/10.1121/1.380717View Description Hide Description
Monaural localization of low‐pass noise positioned in the horizontal plane on the side of the unoccluded ear was investigated. Listeners showed no localizing ability until the upper cutoff frequency of the noise was advanced to 5.0 kHz. Performance proficiency increased progressively with the inclusion of still higher audio frequencies. Consonant with the behavioral data were the results of sound spectrum measurements performed on a head–pinna model. The spectral composition of a broad‐band noise positioned at various azimuthal positions became clearly distinguishable, one from another, only at the higher frequencies.
Subject Classification: 65.62, 65.75, 65.54.