Volume 33, Issue 7, July 1961

Array Steering in a Layered Waveguide
View Description Hide DescriptionThe usual way to determine the direction of a radiating source with an array is to steer the array for maximum output. The steering can be done by time delays or by mechanically turning the array. The output of the array can be expressed as the sum of the time average of the products of the pressure 〈P_{m}P_{n} 〉, observed at detectors m and n. The value of 〈P_{m} (τ_{ m })P_{n} (τ_{ n })〉 can be maximized by a proper choice of time delays τ_{ m }, τ_{ n }. This procedure is straightforward in an infinite homogeneous medium. If the medium is a layered waveguide, there are many more possibilities for submaxima of the P_{m}P_{n} terms.
The normal mode solution of the radiation field of a band limited noise point source is used to calculate the acoustical pressuresP_{m} and P_{n} . The value of 〈P_{m} (τ_{ m })P_{n} (τ_{ n })〉 as a function of time delay is compared with the value of P_{n}P_{m} obtained with mechanical steering. For these calculations, the noise source is assumed to have a bandwidth of one fifteenth of the center frequency, and the depth of the water is assumed to be about 2λ over a thick layer of unconsolidated sediment. The number of maxima of the 〈P_{m} (τ_{ m })P_{n} (τ_{ n })〉 is related to the number of modes propagating in the waveguide if the steering is done with time delays. Mechanical steering yields one maximum that corresponds to the source direction.

Sound Radiation from Prolate Spheroids
View Description Hide DescriptionSpheroidal coordinates and wave functions are used to analyze the sound radiation from a prolate spheroid whose surface vibrates in an arbitrary pattern. Expressions are derived for the pressure distribution, velocity amplitude, radiation impedance, and directivity factor in each spheroidal radiation mode. Numerical results are derived for two special examples: (1) a rigid‐body vibration of a thin spheroid, and (2) an “accordion‐like” vibration of a thin spheroid.

The Computation of Far‐Field Radiation Patterns from Measurements Made near the Source
View Description Hide DescriptionAn analysis is made of two methods of predicting the far‐field radiation pattern from pressure measurements made over a closed surfaceS, containing the source. One of these methods involves the use of Green's functions and yields two subcases according to the particular Green's function that is used. The Helmholtz formula, which results when one uses a Green's function of the form exp(ikR)/R, requires the knowledge of the normal gradient of the pressure ∂p/∂n over S in addition to the pressure p, but it leads to a formula that is relatively easy to evaluate. An approximation to ∂p/∂n is discussed. If, instead, one uses the Green's function which vanishes over the surfaceS, one can evaluate the pressure in the far field, in terms of measurements over S, of p only. The resulting formula is somewhat more complicated than the Helmholtz formula, but it does not require a knowledge of ∂p/∂n.
The second method is based on a proposal recently made by Pachner. It is shown, for the case of prolate spheroidal coordinates, that Pachner's method is completely equivalent to the second approach mentioned above. The applicability of these formulas is demonstrated with experimental data for a line hydrophone. Good agreement is obtained between the experimental radiation pattern and the theoretical patterns predicted by the formulas.

Theoretical and Experimental Study of Underwater Sound Reverberation
View Description Hide DescriptionA theoretical study of the fluctuation of the amplitude and phase associated with the return of a single frequency pulse was made. A vectorial representation was used. From this model the conditions necessary for first‐order Rayleigh distribution of amplitudes are given. Experimental tests were conducted in the Bermuda area. Data on volume reverberation, as well as surface return and surface plus bottom return, were obtained. The data were processed via the USL Datrac‐Datatron system. Analog as well as digital processing was used. An analog electronic circuit was used to generate the Hilbert transform of the returned signal. The return signal and the Hilbert transform were both sampled by the “Datrac” system at a rate of the reciprocal of the bandwidth. The analog values thus obtained were then converted to digital form by the “Datrac” system. From the digital values of the original time function and the Hilbert transform of the time function, the Datatron 205 was programmed to compute the corresponding values of the envelope and the phase of the carrier frequency. Experimental histograms were obtained by the computer for these values. The experimental histograms were compared with the expected theoretical distributions. Statistical tests of goodness of fit were conducted on these data. The results showed the theoretical assumptions to be valid.

Concerning the Fundamental Component of Periodic Pulse Patterns and Modulated Vibrations Observed on the Cochlear Model with Nerve Supply
View Description Hide DescriptionSeebeck demonstrated that there are series of clicks for which Fourier analysis shows a very small fundamental component or none at all, in spite of the fact that a distinct tone complex close to the fundamental component can be heard. Similarly, in a modulated tone, the modulation frequency as such may be heard without being present in the Fourier analysis.
This paper shows that on the model of the cochlea the artificial basilar membrane vibrates at the place corresponding to the absent fundamental or the modulation frequency. The vibration seems to be a consequence of the complicated traveling‐wave patterns and their interferences. This may be of interest for some hearing theories.

Detection of Auditory Sinusoids of Uncertain Frequency
View Description Hide DescriptionThe decrease in the detectability of a gated sinusoidal signal in noise caused by deliberately introducing uncertainty about the signal's frequency is no greater than 3 db, even in an extreme condition of uncertainty. In this extreme condition the signal duration is 0.1 sec, and the signal frequency is varied between 500 and 4000 cps. This effect is not critically dependent on signal duration. Moreover, the observers not only detect the signal but display at least gross information about the frequency of the signal in the uncertain frequency conditions.
Several models, suggested in previous studies, are considered. The magnitude of the decrease observed in the data falls far short of the predictions of these models. An interpretation suggested by the data is that the observers in a detection task, even when a signal of fixed frequency is used, are highly uncertain as to the exact physical parameters of the signal. Another way of stating this assumption is to say that the observer never tests for the presence or absence of a signal on the basis of one simple hypothesis. From this assumption we should expect little decrease in detectability from deliberately introducing frequency uncertainty. This interpretation would suggest the same result would be obtained if time were the major experimental variable.

Noise‐Induced Permanent Threshold Shift at 2000 cps and 4000 cps
View Description Hide DescriptionThree samples of industrial workers were drawn from environments in which continuous, steady‐state noise had average sound pressure levels in the octave bands from 150 to 4800 cps that ranged from 77 to 96 db. Each sample was sub‐grouped according to length of time on job, yielding sub‐samples whose mean times on job (mean exposure time for typical work years) ranged from less than one year to over 25 years. Median hearing levels were calculated for 2000 and 4000 cps for each sub‐sample. Age effect (loss of auditory sensitivity due to aging) corrections were applied to the median hearing‐level values appropriate to the mean ages of the sub‐samples. The resulting values were defined as noise‐induced permanent threshold shift (NIPTS). It is shown that: (1) there is a maximum NIPTS produced at 4000 cps which occurs within the initial 10 years of exposure to 8 hr/day exposures of continuous, steady‐state noise of any intensity greater than 70–75 db in the 1200–2400 or 2400–4800 octave bands; (2) the maximum NIPTS values which are produced at 4000 cps are approximately equal to the noise‐induced temporary threshold shift (NITTS) values predicted from the appropriate SPL's of each sample for 8 hr/day exposures; (3) there is no apparent maximum NIPTS occurring at 2000 cps within the exposure ranges studied; and (4) NIPTS for both 2000 and 4000 cps are monotonic functions of exposure time and SPL.

Seismometer Sounds
View Description Hide DescriptionBy time‐compressing the output of a seismometer, it is possible to present seismographic data in an auditory display. Preliminary data suggest that the seismicsounds due to natural earthquakes may be distinguishable from those due to underground explosions. Some functions are presented to indicate how probability of detection varies with amount of initial training and distance of the seismometer from the explosionsource. Other applications of auditory displays in the analysis of seismological data are suggested.

Factors Influencing the Practice Effect for Auditory Thresholds
View Description Hide DescriptionInvestigators have reported appreciable practice effects for low frequency pure‐tone absolute thresholds and no practice effects for thresholds at higher frequencies. It was suggested that practicing subjects are learning to discriminate between the signal and a low‐frequency physiologic noise. No practice effect was observed at any frequency when thresholds were measured against a background of random noise. This finding would be predicted by the hypothesis advanced. However, efforts to induce a practice effect for a high frequency tone by introducing a faint, high‐frequency narrow‐band background noise were unsuccessful. Implications of the findings are discussed.

Studies on the Endolymphatic dc Potential of the Guinea Pig's Cochlea
View Description Hide DescriptionThe effect of loud sound on the endolymphatic dc potential was studied in both normal and hypoxic animals. When the integrity of the cochlear wall was maintained, the endolymphatic dc potential decreased 5 mv with the onset of sound (1000 cps below 140 db) and recovered immediately when the sound was terminated. At intensities of 140 db and above, the dc potential irreversibly decreased. The effect of sound on the dc potential was the same for both normal and hypoxic animals. A negative dc potential was found in the scala media of hypoxic and dead animals. This potential survived the life span of the post‐mortem microphonics. The positive dc potential returned on termination of hypoxia. Oxygen deprivation resulted in the dc potential decreasing before the microphonics. The possible significance of these findings to the physiology of hearing is discussed.

Time vs Intensity in the Localization of Tones
View Description Hide DescriptionSubjects were asked to match the lateral position of one tone, the “signal,” by means of another, the “pointer.” The two tones were presented alternately. The experimenter selected a combination of interaural time and intensity differences for the signal, and the subject adjusted the interaural time difference for the pointer until it seemed to him to be in the same lateral position as the signal. Subjects having normal hearing perceived the signal in two places, one strongly affected by the difference of level at the two ears, the other almost wholly dependent upon the difference of time.

Improved Nickel‐Base Alloys for Magnetostrictive Transducers
View Description Hide DescriptionDynamic magnetostrictive properties have been measured for nickel‐base alloys containing up to 10% cobalt, with from 0 to 6% chromium. The maximum value of electromechanical coupling coefficient 0.51 occurs in a binary alloy containing 4.5% cobalt, balance nickel, for which the magnetocrystalline‐anisotropy constant is zero. The maximum value of potential electroacoustic efficiency for resonance frequencies below 100 kc/sec is found, however, in a ternary alloy containing 1.4% cobalt, 2.3% chromium, balance nickel.
It is concluded that the binary alloy is the most suitable material for the magnetostrictive element of receivers, such as hydrophones, particularly where a reasonably flat response is sought. The ternary alloy, because of its lower eddy‐current losses, is to be preferred for generating high‐frequency acoustic power.

Mechanical Pressure‐Gradient‐to‐Pressure Sound Transducer
View Description Hide DescriptionIn places of high ambient‐noise level, the use of electrical devices as an aid to speech communication is sometimes precluded by safety requirements. A mechanical device may then be used provided that the individuals concerned are close to each other. Such a device can be made to incorporate all the acoustical benefits of a dose‐talking microphone. Sentence intelligibility tests showed that when the articulation index was zero for a nearby speech source the median subject obtained a score of 28% using ear covers alone and 84% with the soundtransducer; the subjects with extreme scores improved from 0 to 14% and from 88 to 100% respectively. No subject using the device suffered a decrease in intelligibility.

Generation of Intense Audio Sound Fields Utilizing Arrays of Multiple‐Driver Horns
View Description Hide DescriptionThere are several major applications for intense audio sound fields: the outdoor propagation of speech and of warning signals, and the sonic fatigue testing of missile and aircraft components. In these applications, several factors combine to make it necessary to limit the portion of the frequency spectrum covered by any one loudspeaker unit. In order to provide adequate power capacity, selection is required between the alternatives of (a) using a large number of identical drivers, (b) using a plurality of drivers each especially designed for a selected portion of the frequency range, and (c) using combinations of these arrangements.
In an array currently being used for sonic‐fatigue testing, a plurality of large open cone loudspeakers is used for low frequencies, and a plurality of horns is used for middle and high frequencies. Each horn is driven by several drivers.
The driver voice coil and diaphragm proportions have been optimized to withstand fatigue stresses under the thermal conditions encountered. The driver is rated at 50‐w input for program material.
Horn assemblies are available having throat arrays for mounting 6, 12, or 24 drivers. Various applications will be illustrated.

Information in Simple Multidimensional Speech Messages
View Description Hide DescriptionSimultaneous pairs of four‐bit messages were presented to listeners at rates of two pairs every 4 sec (4 bits/sec), three pairs every 4 sec (6 bits/sec), four pairs every 4 sec (8 bits/sec), and six pairs every 4 sec (12 bits/sec). Each message of the pair was either [a] or [i], said by a male or a female, as a question (rising inflection) or a statement (falling inflection), and heard in either the right or the left ear.
Sequences were made that were balanced (1) between ears, (2) between male and female voices, (3) between [a]'s and [i]'s and (4) between questions and statements. The subject sometimes listened to both messages but usually listened only for the messages (1) in one ear, (2) of one voice, (3) of one vowel sound, or (4) of one inflection.
The best subject could receive correctly about 6 bits/sec whether this was one of two messages at the 12 bits/sec rate or both of two at the 6 bits/sec rate.

Diffraction of Light by Ultrasonic Waves of Various Standing Wave Ratios
View Description Hide DescriptionThe theory for the diffraction of light by plane ultrasonic waves of various standing wave ratios is derived. The liquid medium disturbed by the ultrasound is considered to act as an optical phase grating. By evaluating the diffraction integral for the light amplitude, expressions for the Doppler shift, the time dependent, and the time average light intensities are found for the diffractionspectrum. Experimental measurements using two adjacent ultrasonic waves progressing in opposite directions to simulate the desired optical phase grating indicate that the theory is valid.

Elastic Wave Mode Conversion at a Solid‐Solid Boundary with Transverse Slip
View Description Hide DescriptionIf two solids are coupled to each other through a liquid layer, transverse slip can occur at the boundary surface when an elastic wave is incident on the boundary. Algebraic expressions for the amplitudes of the various wave modes are derived on the assumption of transverse slip at the boundary, and these are compared with wave mode amplitudes for a solid‐air boundary. Experimental measurements in connection with the design of an indicator for the degree of coupling between an angle probe and a test object are explained on this assumption. Curves are given showing wave‐mode amplitude for: (i) Plexiglas‐air boundary; (ii) Plexiglas steel boundary assuming transverse slip; (iii) Plexiglas steel boundary assuming rigid contact.

Solution of an Integral Equation Occurring the Study of Certain Wave‐Propagation Problems in Layered Media
View Description Hide DescriptionThis paper describes a method of inversion of an integral equation arising the solution of the problem of reflection or refraction of a spherical sound‐wave pulse at a plane interface a layered elastic medium. The solution of this integral equation is equivalent to obtaining the inverse Laplace transform of the operational solution of the problem.

Rayleigh Waves in a Porous, Elastic, Saturated Solid
View Description Hide DescriptionThe equation for the velocity of propagation of Rayleigh‐type surface waves in a porous, elastic, saturated solid is derived and discussed. The inertia coupling between fluid and solid is neglected, and Darcy's coefficient is taken to be independent of frequency.
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