Volume 60, Issue 4, October 1976
Index of content:
60(1976); http://dx.doi.org/10.1121/1.381158View Description Hide Description
The radiation from a spherical source vibrating with an arbitrary, axisymmetric, time‐harmonic velocity distribution while positioned at an arbitrary point wholly outside a fluid sphere while both are embedded in another infinite fluid medium, is computed. The translational addition theorem for spherical wave functions is used to express series of wave modes centered at one sphere in terms of modes centered at the other, thereby facilitating the task of satisfying the boundary conditions. By letting the fluid sphere become either infinitely dense or infinitely compliant, the special cases of a spherical source radiating in the presence of a perfectly reflecting spherical scatterer are obtained. The source output and farfield directional response pattern are computed as a function of source and fluid sphere separation for some representative values of relative fluid density and sound speed. Numerous results are presented showing the modification of the radiation load on monopole and dipole sources of various wavelength sizes due to a nearby rigid or pressure release, spherical scatterer of the same size. The effect of the perfect scatterer is only important when the source and scatterer are very close and the pressure‐release scatterer has considerably more effect than the rigid one.
Subject Classification: 20.30, 20.55.
60(1976); http://dx.doi.org/10.1121/1.381159View Description Hide Description
We present a theory of attenuation of sound by turbulence. The mechanism underlying this theory is the turbulence‐induced broadening of finite beams of sound. It is thus conjectured that attenuation of sound by turbulence is not an intrinsic property of the medium, nor even of its dynamic state, but depends on the particular details of the experiment, such as beamwidth, beam orientation, etc. This point of view is at odds with that of some other theories, in which attenuation by turbulence is regarded primarily as a scattering process. Some conceptual flaws in these other theories are pointed out. It is shown that the present theory yields results which are in qualitative agreement with observations.
Subject Classification: 28.40, 28.60; 20.35.
60(1976); http://dx.doi.org/10.1121/1.381160View Description Hide Description
60(1976); http://dx.doi.org/10.1121/1.381161View Description Hide Description
The phase integral approximation is modified to describe wave propagation in three dimensions without any singularities at classical turning points. For stratified media a rigorous ray tracing scheme is derived. Stratified media are shown in general to induce dispersion. The connections to both the normal mode and filter theory methods are discussed. The effects of oceanic surface thermoclines upon rigorous ray tracing are shown to be frequency dependent.
Subject Classification: 30.20; 20.15.
60(1976); http://dx.doi.org/10.1121/1.381162View Description Hide Description
Normal‐mode calculations are made of wave propagation in a bilinear medium with a free surface (the surface duct). The WKB approximation is used. In the usual way, the integral expression for the field is transformed into an infinite series of the residues (normal modes) of the integrand at its poles (eigenvalues). Approximations for the eigenvalues of the trapped normal modes are obtained from WKB formulas. The eigenvalues of the untrapped modes are obtained by quadratic extrapolation of the eigenvalues of the trapped modes. The accuracy of the real part of the eigenvalues of all modes is improved by taking into account the imaginary part when solving the characteristic equation. On comparing a number of WKB calculations of transmission loss with exact calculations, it is found that when there is little fine structure in the transmission loss curve, the WKB result is a good representation of the exact result. When there is significant fine structure, however, the WKB results are often in error by several decibels, and occasionally by up to 20 dB.
Subject Classification: 30.20; 20.15; 30.50.
General treatment of source motion on the total acoustic field with application to an isospeed channel60(1976); http://dx.doi.org/10.1121/1.381163View Description Hide Description
A treatment of the effects of arbitrary motion of a cw source and depth‐dependent sound speed on the total acoustic field at a fixed receiving point is considered for an ocean with horizontal boundaries. Application of our general method is made to a constant sound‐speed channel in which the range‐to‐depth ratio is large, when the source follows a short straight‐line path with constant velocity. Total‐field phase is investigated as a function of receiver time for various source trajectories and phase rate is examined in terms of an arbitrary, but fixed, acoustic frequency. It is shown that source motion may be accounted for by assuming the sound source to be stationary, and by replacing its frequency by approximate Doppler frequency. For long source trajectories, cumulative phase can be approximated as a hyperbolic function of time. The outputs of two uniform colinear arrays, together with power spectra there, are employed to illustrate one method for determing source speed, location, bearing, and frequency.
Subject Classification: 30.20; 20.20..
60(1976); http://dx.doi.org/10.1121/1.381164View Description Hide Description
The acoustic pressure versus time signature of an underwater explosive source is a sensitive function of the charge weight and the depth of the explosion. If those parameters are known, then the amplitude of the primary shock pulse and the amplitudes and spacings of the following bubble pulses may be accurately predicted. The power spectrum of the signal is characterized by a scalloped form, which also is determined by the charge weight and depth. Even though the signal observed at long range is distorted due to propagation effects, the scalloped spectral structure is preserved. By using power cepstrum techniques, it is possible to determine the characteristic period of the spectrum. From the relation between the spectral period and the bubble pulse period, the exact detonation depth may be determined from a received shot signal if the charge weight is known. An example of such a measurement is presented in this paper. A series of signals from 1.8 lb Mk 61 SUS charges detonated at nominal depths of 60 ft was examined. The bubble pulse period of each source was determined from recordings made onboard the ship from which the shots were deployed. Shots were received at ranges of 250–310 NM at one location and at ranges of 625–700 NM at another location. The depths, as estimated at the remote receiver, were distributed about the onboard measurements with standard deviations of 0.49 ft at the shorter ranges and 0.52 ft at the longer ranges.
Subject Classification: 30.75.
60(1976); http://dx.doi.org/10.1121/1.381165View Description Hide Description
Theoretical predictions, as well as observations at relatively short ranges and steep grazing angles, show that a sound wave which is forward scattered from the oceansurface has its frequency components smeared by surface‐wave motion [M. V. Brown and G. V. Frisk, J. Acoust. Soc. Am. 55, 744–749 (1974)]. The subject of this paper is the presence of observable sidebands in cw signals which are generated by a moving source received at ranges of up to 200 NM in the deep ocean. The sidebands are approximately 0.1 Hz from the source center frequency and are as strong as 20 dB below the level of the signal received at the source frequency.
Subject Classification: 30.20, 30.30, 30.40.
60(1976); http://dx.doi.org/10.1121/1.381153View Description Hide Description
The title problem is studied using a single‐scatter theory. General expressions are obtained for the distribution of scattered acoustic intensity among the waveguide modes and for a measure of azimuthal beam pattern of the intensity in a specified waveguide mode. Detailed calculations are carried out for a specified waveguide and for specified statistics of a fluctuating sound speed, which, while not being representative of actual ocean conditions allow our drawing conclusions that are applicable to ocean acoustic experiments. The importance, previously noted by Beran and McCoy, of the highly anisotropic nature of the fluctuations caused by internal waves on the scattering process is demonstrated. Several conclusions are drawn of the effects that this anisotropy can be expected to play in the multiple‐scatter region.
Subject Classification: 30.40, 30.50; 60.20.
60(1976); http://dx.doi.org/10.1121/1.381154View Description Hide Description
This paper deals with scattering channels involving signal paths that are reflected several times from the boundary on their way from the transmitter to the receiver. The transmitted signal is a single‐frequency sine wave, and the surfaces from which it is reflected are regarded as being statistically independent. Therefore the autocorrelation function of the received signal has a very simple form consisting mainly of the product of the characteristic functions of the various reflecting surfaces.
Subject Classification: 30.20; 30.40.
60(1976); http://dx.doi.org/10.1121/1.381155View Description Hide Description
An approximate formula is derived for the estimation of the fundamental natural frequency of the simply supported orthotropic rectangular plate with thickness varying linearly in one direction. The accuracy of the formula and the influence of the flexural rigidity D*1 on the natural frequency are discussed.
Subject Classification: 40.24.
60(1976); http://dx.doi.org/10.1121/1.381156View Description Hide Description
The image theory method is used to predict sound pressure levels generated by an acoustic source of known strength in regular rooms, tunnels, and flat rooms. For regular rooms, the results are identical to those of the well‐established diffuse field theory. For tunnels and flat rooms, the predictions are verified by direct measurements taken in underground mines.
Subject Classification: 55.20, 55.55.
60(1976); http://dx.doi.org/10.1121/1.381157View Description Hide Description
The concept of a conditional statistical test is applied to obtain a nonparametric detector based on the dead‐zone limiter nonlinearity. The resulting detector is easily implemented, and its performance is shown to be superior to that of the sign detector for both large and small samples.
Subject Classification: 60.20,60.30.
60(1976); http://dx.doi.org/10.1121/1.381166View Description Hide Description
The spectrum of amplitude‐modulated wide‐band noise is invariant with changes in modulation frequency, and consequently such waveforms cannot convey melodic information by spectral changes. The results of several experiments indicate, however, that sinusoidally amplitude‐modulated wide‐band noise does have pitchlike properties. Simple melodies can be identified when the ’’notes’’ correspond to different modulation frequencies. More importantly, musically trained observers can immediately recognize melodic muscial intervals composed of such waveforms. The pitchlike properties of modulated noise are maintained when the modulated noise is bandpass filtered and when an unmodulated band‐reject masker is added to the bandpassnoise. Presumably the perception of pitch elicited by modulated noise is based upon temporal information in the envelope of the waveform. This ’’temporal pitch’’ is less salient than the pitch elicited by pure tones. Also, temporal pitch can be elicited only over a restricted range of modulation frequencies. The upper limit (approximately 850–1000 Hz) appears to reflect the inability of the auditory system to follow rapid temporal changes.
Subject Classification: 65.54, 65.75.
60(1976); http://dx.doi.org/10.1121/1.381167View Description Hide Description
The least discriminable change in the position of a sound image was measured for pure tone with and without initial interaural delay, as well as for complex tones. The signals were either perfectly regular or jittered. The introduction of jitter allowed the subjects to lateralize equal‐amplitude tones beyond 1500 Hz, which has been assumed to represent the limit for binaural phase discrimination. No frequency limit for jittered tones either in a fixed relationship or as binaural beats could be found. The smallest deviation from regularity employed was 0.2 μsec, and under certain conditions it was effective in producing lateralization. With no jitter in one ear, jitter in the other ear was ineffective. For a given high‐frequency jittered tone,discrimination improves up to a limit with level and jitter magnitude. Complex tones, jittered and unjittered, show smaller just‐noticeable differences for low and medium frequencies. Some other implications of these findings are discussed. A striking similarity between the curves for binaural time discrimination and those for pitchdiscrimination was found.
Subject Classification: 65.68, 65.62, 65.60.
60(1976); http://dx.doi.org/10.1121/1.381168View Description Hide Description
Thresholds for detection of a 3‐kHz tone embedded in a 2121–4242‐Hz octave band of masking noise or a 6‐kHz tone in 4243–8486‐Hz masking noise were determined before and after fatiguing noise exposure. Masking‐noise levels were varied from 0 to 90 dB (re 20 μN/m2). The fatiguing noise was a 1414− to 2828‐Hz or a 2829–5658‐Hz octave band of noise set at intensities between 90 and 115 dB. Each observation included the following parts: (1) prefatigue threshold determination, (2) 3‐min exposure to intense noise, (3) 90‐sec delay, and (4) postfatigue threshold determination. The results of a series of observations employing the above procedure indicate that tone thresholds were elevated following the fatiguing‐noise exposure by up to 22 dB when the tones were embedded in 0−dB masking noise (TTS2=22 dB). However, tone thresholds were elevated only slightly following fatiguing‐noise exposure when the tone was embedded in 45–90‐dB masking noise. Possible mechanisms that might account for these observations are considered.
Subject Classification: 65.58, 65.66, 65.50, 65.75.
60(1976); http://dx.doi.org/10.1121/1.381169View Description Hide Description
In order to test the relative susceptibility of young and adult ears to loss from intense stimulation, losses in round window cochlear microphonic (CM) sensitivity and hair cell losses were compared in series of experiments using 52 eight‐week old kittens and 59 adult cats. In acute experiments, ears of deeply anesthetized animals were exposed to 50 min of 5.0‐kHz pure tone stimulation at that intensity necessary to produce a maximum CM output (MAX+0), or 10 dB(MAX+10) or 30 dB(MAX+30) higher than that. In chronic experiments, similar measures were made and the animals were allowed to recover for six weeks before their sensitivity was remeasured and their cochleas examined by surface preparation histology. Losses were the same at MAX+0, but at MAX+10 the kittens lost significantly more sensitivity than the cats. At MAX+30 the losses were large (45–50 dB) but not significantly different. The chronic experiments showed that some CM recovery did occur and that hair cell losses were orderly and generally confined to the areas of the basilar membrane involved in the displacement pattern for a 5.0‐kHz tone.
Subject Classification: 65.64, 65.40.
60(1976); http://dx.doi.org/10.1121/1.381170View Description Hide Description
Noise‐intensity discrimination was studied as a function of both signal and masker bandwidth. Five bandwidths of noise—ranging from 100 to 10 000 Hz—were employed. Maskers were presented at each of three spectrum levels (5, 25, and 45 dB r e 0.0002 μbar). Discrimination thresholds were relatively unaffected by changing bandwidth over a two‐decade range when the signal and masker were filtered together, with either continuous or gated presentation of the masker. When the masker bandwidth was greater than that of the signal, the reciprocity between signal power density and signal bandwidth was found to be 5 dB per log unit of bandwidth with continuous maskers and 5–10 dB (depending upon level) with gated maskers. The results were compared with predictions of energy‐detector models of noise‐intensity discrimination. Modifications of simple energy‐detection schemes were discussed.
Subject Classification: 65.58, 65.75, 65.50; 50.70.
60(1976); http://dx.doi.org/10.1121/1.381171View Description Hide Description
Two experiments were conducted investigating the identifiability of vowels excerpted from context. Five phonetic contexts were investigated: neutral (h‐d), initial and final /l/, and initial and final /r/. Vowels were excerpted from these contexts by computer and presented to subjects for identification. The consonantal contexts of initial and final /r/ and /l/ affected the identification of the excerpted vowels in various systematic ways, as compared with the identification patterns of vowels excerpted from the neutral context.
Subject Classification: 70.30, 18.104.22.168.
60(1976); http://dx.doi.org/10.1121/1.381172View Description Hide Description
A common type of interference in speech transmission is that caused by the speech of a competing talker. Although the brain is adept at clarifying such speech, it relies heavily on binaural data. When voices interfere over a single channel, separation is much more difficult and intelligibility suffers. Clarifying such speech is a complex and varied problem whose nature changes with the moment‐to‐moment variation in the types of sound which interfere. This paper describes an attack on the principal subproblem, the separation of vocalic speech. Separation is done by selecting the harmonics of the desired voice in the Fourier transform of the input. In implementing this process, techniques have been developed for resolving overlapping spectrum components, for determining pitches of both talkers, and for assuring consistent separation. These techniques are described, their performance on test utterances is summarized, and the possibility of using this process as a basis for the solution of the general two‐talker problem is briefly considered.
Subject Classification: 70.62, 70.60, 70.65.