bluuebunny/crossref_metadata_2025 huggingface dataset:
The Density and Flow Resistivity of Some Commercial Fibrous Acoustical Materials
Authors: Lemmerman R. D. , Hemond C. , Sapega A.
Published: September 2005

Studies have been of the flow resistance and density of a series of commercially produced fibrous acoustical materials. Random samples of Fiberglas PF Board, TWF Wool, and Spuntex felt were taken from approximately 35 carloads of these various materials. The range of flow resistivities and densities were measured and are reported here. These data are correlated and compared to that previously reported.
Economic Factors Involved in the Design of Acoustical Components of Test Cells
Authors: Hemond C. J. , Lemmerman R. D.
Published: September 2005

Too often the design of acoustical treatments for various test cell configurations has not taken into account the balancing of economic factors involved. Frequently, the balancing of economic factors against acoustical performance may result in the necessity for sacrificing acoustical performance for the sake of economy. This paper is a first attempt, in the light of considerable practical experience, to evaluate standard design components on a decibel-per-dollar basis. Cost figures presented include not only the unit cost of the acoustical treatment, but the unit cost of the structure necessary to house the treatment properly. Treatment costs are based on actual figures taken from completed projects by ISC, while building costs are a combination of actual figures from completed projects by ISC, or from unit building cost figures as published in the April issue of Engineering News Record. Comparative analysis is presented of the following types of treatment: (a) lined bends and chambers, (b) high frequency baffles, (c) low frequency baffles and ducts, (d) soundstream absorbers, (e) circular mufflers.
A Practical Noise Reduction Treatment for Diesel Engines
Authors: Ancell James E.
Published: September 2005

A noise reduction treatment for diesel engines will be described, which, though not the ideal approach to the problem, is practical, economical, and is applicable to both new engines and those already in service in the field. This treatment was designed making use of the results of a survey of the relative importance of the various noise sources on the diesel engine. The treatment consisted of a set of sheet metal covers, lined internally with acoustical absorbing material, which were mounted directly over the noisy areas of the engine by means of built-in vibration isolation. Noise reductions for the direct sound were at least 8 to 12 db, which were observed at a distance of one foot from the engine. The total loudness of the engine noise at six feet was reduced from 320 to 220 sones, which alleviated an almost impossible situation with regard to voice communication in the immediate vicinity of the engine. The Speech Interference Level of the engine noise at six feet has been reduced from 87 to 82 db, which allows satisfactory speech intelligibility by shouting at a distance of one foot.
Automatic Recognition of Phonetic Elements in Speech
Authors: Davis K. H.
Published: September 2005

This paper describes a method of identifying phonetic elements in speech. The device continuously correlates the measured frequency spectrum of incoming speech with a set of reference spectra, corresponding to the phonetic elements to be recognized. That spectrum producing the best correlation at any time is indicated as the phonetic element present. In the circuit which has been built, the number of frequency bands used in the spectrum analysis was arbitrarily limited to ten, and the number of phonetic elements identified was also limited to ten. This phonetic element recognizer has been incorporated in an improved form of a digit recognizer; its operation will be shown in a short sound motion picture. Other uses of the phonetic element recognizer would be in a voice typewriter or in the narrow band phonetic element transmission system to be described in a companion paper.
Selective Compression of Speech Sounds
Authors: Smith Caldwell P.
Published: September 2005

A “raw” speech signal is poorly constituted for transmission over a noisy channel. Articulation tests have shown that the weak consonant sounds tend to be obliterated at about unity signal-to-noise ratio, while the vowel sounds, due to their higher intensity and the greater sensitivity of the ear to their periodicity, continue to be identified accurately at much poorer signal levels. A study has been made of a technique for “reinforcing” voice signals against interference by processing the voice signal at the source, in a selective compressor amplifier. A circuit of special design acts as a linear amplifier for the majority of the consonant sounds, while automatically reducing the amplitude of the vowel and vowel-like sounds. The distinctive temporal patterns of the consonants are not altered, and the mushy, distorted quality of speech that has been highly compressed or clipped is avoided. When a voice is being amplified in a reverberant hall, under ordinary conditions the “overhang” of the vowel sounds reduces intelligibility by masking the succeeding consonants; the use of selective compression should improve the clarity of a speech signal under these conditions. Some results of articulation testing and details of the compression system are presented.
Improvements in the Vocoder
Authors: Milner R. L.
Published: September 2005

A number of improvements in reliability and quality have been made in the vocoder over the past several years which have not been reported in the literature. These include several basic changes in the pitch control channel which give greater reliability for different talkers as well as allow operation over greater volume ranges. Changes have also been made to optimize the quality as a function of the required band width for transmission. In particular, the band pass filter system has been proportioned to follow the Koenig aural scale (similar to mel). Demonstrations are included to illustrate the operation of the vocoder for several different talkers as well as its action when applied to individual musical instruments.
On the Specification of Zero Hearing Loss
Authors: Harris J. D. , Myers C. K. , White C. E.
Published: September 2005

Several dozen young healthy men, aged 17–25 with negative auditory history, were used to determine SPL under a PDR-8 earphone at threshold for each of 8 frequencies. Careful psychophysical techniques were used, and a calibrated sound probe was used to measure SPL. In addition, an audiogram was taken by a common clinical technique. Average SPL of our population differs only a very few db from previous sound probe studies, being 12–20 db fainter than audiometer zero. However, at all but the lower frenquecies, SPL as inferred from the clinical audiogram agreed very well with SPL as measured by sound probe. It seems clear that the audiometer zero does not represent what the normal young ear can do when handicapped by the relative coarseness of clinical practice, as often alleged, but that the original data, on which the audiometer zero is based, contains an appreciable admixture of partial deafness.
Phenomenological Theory of Ferroelectric Ceramics
Authors: Baerwald Hans G.
Published: September 2005

A ferroelectric ceramic represents in its virgin state, mildly idealized, a three-dimensionally isotropic continuum; in the polarized state, however, a two-dimensionally isotropic one. A general conservative medium of the latter type has ten independent adiabatic elastodielectric parameters. It is shown by phenomenological reasoning that in case of a ferroelectric continuum this generality is somewhat reduced. The two interrelations found to exist between the elastic and piezoelectric parameters, in addition to a rather esoteric dielectric inequality, are predicated on the assumption of conservative behavior above and below, but not through, the secular relaxation region and similar mild idealization, as well as on the fact that the only “large-scale” process possible without rupture of the material is the dielectric polarization. Assumption of conservative behavior through the relaxation region, in particular, embracing the process of permanent polarization itself, as made in earlier theories, is untenable as in contradiction with the observed facts.
Effect of Losses on Transmittivity of Plates
Authors: Fay R. D.
Published: September 2005

The hypothesis that the observed reduction in transmittivity at transmission maxima is due to losses associated with shear waves in the plate material has been investigated. Assuming a small loss factor in the propagation function of shear waves, it is found that the major effect on transmittivity can be expressed in terms of the impedance mis-match which is introduced at the incident interface. The magnitude of the loss factor was determined so as to give agreement with an observation made at a frequency of 112 megacycles a second in a special case. The impedance mis-match was then calculated over a wide range of transmission maxima. The loss predicted by this procedure agrees with observations within the experimental error (±2 db). The loss varies from practically zero to more than 20 db. No data are available at present concerning the dependency of loss factor on frequency.
Acoustical Vibrations of Rings
Authors: Kaiser Elmer R.
Published: September 2005

The frequencies of tones when metal rings are struck in radial and axial directions have been investigated in a variety of steel and bell-metal rings. The classical formulas relating frequency with ring dimensions and physical properties of the materials compare closely with actual measurements except for thick rings. The fundamental tone is predominant in the smaller rings, but the fundamental fades and the overtones predominate as the rings increase in diameter and decrease in width and thickness. Comments are given on musical properties of rings which may be promising but which have not been fully explored.
Sound Radiation from a Plane Radiator Having a Prescribed Pressure Distribution
Authors: Pritchard R. L.
Published: September 2005

A theoretical investigation has been made of the sound radiation from a (flexible) plane radiator having a prescribed pressure distribution. For such a radiator vibrating in an infinite rigid plane, the problem is of the mixed-boundary-value type and is not solved easily in general. However, the sound field resulting from a freely vibrating, flexible plane radiator may be calculated from an integral quite similar to that given by Rayleigh for a plane radiator having a prescribed velocity distribution. For the particular case of the circular disk with a uniform pressure distribution, solutions have been obtained for the sound-pressure and velocity distributions and for the radiation impedance. These results are compared with the corresponding results for the flexible disk vibrating in an infinite plane, which are obtained directly from an existing solution for sound diffraction by a circular aperture. An interesting four-way comparison also is made between this set of results and the corresponding set for the rigid circular disk. Application of the results to the analysis of the electrostatic loudspeaker employing a thin plastic diaphragm is discussed.
The Attenuation of Repeated Shock Waves in Carbon Dioxide
Authors: Werth Glenn C.
Published: September 2005

The propagation in tubes of high-amplitude sound waves which have assumed the sawtooth-repeated shock form has been studied. The results in carbon dioxide are of interest because of the molecular vibration phenomenon. When reciprocals of the sound pressure amplitudes are plotted against the number of wavelengths, straight lines result in accordance with the theory of I. Rudnick. The slope is a measure of the attenuation, and high values as compared to air are obtained with a fundamental frequency around 1000 cycles per second. The 600-cycle data give low values indicating a frequency dispersion not found in air or argon. A comparison of data obtained in several gases will be made as well as some interpretive comments.
Measurements on Sound Passing through a Shock Wave
Authors: Bohn J. L. , Nadig F. H. , O'Day Marcus
Published: September 2005

Experimental results have been obtained to show that sound will pass through a shock wave with small attenuation. The shock waves were produced by high-speed bullets and the sound frequencies used were 480 cps and 5000 cps. Various baffles and filters were used. One microphone was used to trigger the oscilloscope, and another one was used to receive the sound signal. Repetition of sweeps was prevented by a mechanical switch operated by the bullet. The measuring techniques will be described and oscillograms will be shown.
Short-Range Propagation in the Atmosphere
Authors: Ingard Uno , Kuckes A. F. , Dyer Ira
Published: September 2005

Average sound attenuation and fluctuations caused by wind have been studied by transmission measurements over distances varying from 20 feet to 320 feet and with frequencies from 200 to 7000 cps. Curves showing the fluctuation as a function of distance and frequency as well as attenuation due to wind are presented. It is found that the fluctuation in the sound is practically independent of wind direction, but the attenuation depends on the wind direction.
Wave Theory of Sound Transmission in a Randomly Inhomogeneous Medium II
Authors: Mintzer David
Published: September 2005

The analysis previously reported has been extended to include the correlation of pressure amplitude pulses spaced in time. This gives information concerning the time dependence of the refractive index inhomogeneities. Comparison will be made with data.
The Information of Elementary Multidimensional Auditory Displays
Authors: Pollack Irwin , Ficks Lawrence
Published: September 2005

A given amount of information may be incorporated within elementary auditory displays by utilizing a small number of finely subdivided stimulus dimensions or by utilizing a relatively larger number of crudely subdivided stimulus dimensions. Experimental interest to date has been confined largely to the former type of display; the present study considers the latter type. Specifically, the informational transmission with elementary auditory displays of a large number (6–8) stimulus dimensions was investigated. In general, nearly perfect identification was obtained with skilled listeners when each dimension was crudely subdivided into two alternative states. Finer subdivision of each dimension does not produce a proportional gain in information transmission with the display.
Comparison of the Masked Thresholds of a Simulated Moving and Stationary Auditory Signal
Authors: Wilcott R. C. , Gales R. S.
Published: September 2005

The masked thresholds of a simulated moving signal produced by a linear stepwise change in interaural time delay and with movement at two different locations in the phenomenal field were compared with the thresholds obtained with a stationary signal. Using a noise band extending from 75 to 850 cps as the signal and presented against a heterophasic background noise, masked thresholds were made under the following conditions: (1) signal moving at a perceptually slow and fast rates, (2) a perceptually fast moving signal with the signal making four excursions across the phenomenal field during each presentation period, (3) with the signal movement controlled by the observer, and (4) during an extended listening period in which the observer was given no cue as to when the signal was to be presented. The results of the threshold comparisons with these different conditions indicate that the masked threshold of a moving signal is a function only of its localization at any given instant in time and that a moving signal does not elicit a lower masked threshold than a stationary signal.
Acoustic Diffraction Effects
Authors: Pridmore-Brown David C. , Ingard Uno
Published: September 2005

The phenomenon of acoustic diffraction into a zone of silence (shadow zone) is discussed. Preliminary measurements of this effect made in a model “weather chamber” are shown. The temperature distribution to be expected within the chamber taking into account the variable thermal conductivity of air is indicated. Methods of solution of the wave equation in a stratified medium are reviewed.
Propagation of Elastic Waves in Plates and Cylinders
Authors: Harrison Mark
Published: September 2005

Some additional solutions have recently been given for the problem of elastic waves in plates as originally formulated by Lamb. Using recent solutions for the reflection of elastic waves at a free surface, it is possible to construct a wave system that satisfies the boundary conditions for the plate. The construction of these waves surfaces make it possible to understand Holdens results in geometrical terms. This is analogous to the elementary explanations of electromagnetic wave guides in terms of the reflections of plane waves. The utility of the construction is that it makes it possible to gain an intuitive understanding of mathematically intractable problems. It is also possible to better understand the recent solutions for elastic wave propagation in solid circular cylinders by this method.
Wedge Horns for Use with Line Hydrophones
Authors: McKinney C. M. , Anderson C. D.
Published: September 2005

A study has been made of wedge-shaped horns used in conjunction with line sources. The horns are constructed from aluminum plates covered with cellular rubber. Measurements were made over the frequency range of 60 kc to 150 kc using horns of lengths from 3 inches to 15 inches and horn angles up to 90°. Results are given graphically in the form of directivity patterns as well as relative gain in sensitivity and beam width as a function of the several parameters. At 80 kc beam widths as low as 10° are obtained with very small secondary lobes. The horns are easily fabricated and the beam width and sensitivity of the transducers are easily controlled by varying the horn angle.
A Third-Order Pressure Gradient Microphone
Authors: Witchey A. L. , Hawley Mones
Published: September 2005

A microphone has been constructed which exhibits the characteristics of a third-order pressure gradient microphone when tested objectively and subjectively. That is to say, the voltage output of the microphone is proportional to the third derivative of sound pressure incident upon the microphone with respect to the distance from the sound source. When used in high ambient noise levels it clearly exhibits more intelligible speech output than first-order gradient microphones. However, its noise discrimination at low frequencies is limited by noise conducted through the structure of the microphone. The microphone is of the magnetic armature type and employs a single diaphragm fed from an array of pressure pickup points. Considerable improvement in the signal-to-noise ratio may be obtained by talking into a selected position in this array instead of along the axis of the array.
Some Interesting Barium Titanate Transducers
Authors: Miller Harry B.
Published: September 2005

Barium titanate elements of rather simple shapes can be arranged to produce underwater transducers having relatively sophisticated directivity patterns, frequency responses, etc. Some of the possibilities will be described.
The Corona Discharge Transducer
Authors: Blum Burt W. , Dyer Ira , Ingard Uno
Published: September 2005

Measurements have continued and will be reported on the dynamic pressure sensitivity of a point-to-plane corona discharge. It was found previously that the sensitivity of the device using a 6.3-mil diameter wire is proportional to the square of the ratio of current drawn by the discharge to potential across the discharge (conductance). Additional measurements show that the functional relation between sensitivity and conductance depends on wire diameter; for wire diameters ranging from 2.8 to 13.5 mils, the sensitivity-conductance relation ranges from a cube law to a linear law. The point-to-plane corona discharge may also be used as a sound source, provided that the discharge is maintained in the required current-voltage range. Preliminary results on the source properties of the device will be reported.
Mechanical Impedance of the Head and Mastoid
Authors: Corliss Edith L. R. , Koidan Walter
Published: September 2005

Measurements of the mechanical impedance presented to a driver tip by the human forehead and by the mastoid have been made over the frequency range from 60 to 10 000 cps, using a special direct-recording force and acceleration measuring system. The effect of the static force coupling the driving tip to the head has been investigated. At high coupling forces, the mastoid and forehead present similar impedances; the difference between mastoid and forehead impedance is greater at light coupling force. Over a large part of the audio-frequency range, the predominant characteristic of both head and mastoid impedance is a stiffness reactance with some damping. In the high-frequency range, a small mass reactance parameter appears. The mass load due to the whole head influences the impedance only at the low-frequency extreme of the measurement range.
Instrumentation for the Measurement of Head and Mastoid Impedance
Authors: Koidan Walter , Corliss Edith L. R.
Published: September 2005

A method has been developed for determining the resistive and reactive components of the human head and mastoid or other high mechanical impedances at audio-frequencies. A tubular section of polarized barium titanate, rigidly clamped at one end to a massive wall, serves as the driver. A cylindrical container is cemented to the free end of the driver, and the force and velocity sensing elements are mounted on a cap which is clamped over the open end of the container. Force and velocity are measured close to the point of contact with the load to minimize errors due to flexure in the driver or between the sensing elements. The force output is obtained with a high signal-to-noise ratio by means of two identical piezoelectric wafers mounted on opposite sides of the cap and connected in opposition for mutual cancellation at no load; the output produced by mechanically loading the exposed wafer is then due substantially to that wafer. The velocity output is obtained by a conventional mass loaded accelerometer mounted on the under side of the driver cap to avoid dependence upon any supporting structure for a stationary frame of reference. The system is calibrated by loading the driver tip with known masses.
Mechanical Resonances of Biological Cells
Authors: Ackerman Eugene
Published: September 2005

The mathematical theory of mechanical resonances of biological cells has been extended to include the effects of radiation and viscosity. Neither produces any pronounced change in the lowest resonant frequency, but the viscosity strongly damps the vibration as well as eliminating many of the possible modes. Estimates of the viscosity show the Q may be as low as two. The scattering cross sections for these resonances is so low that they could not be observed using plane incident waves. Divergent fields near small cavities, comparable in size to the biological cells, would excite these resonances. Breakdown curves are convenient to observe this phenomenon. The height of the resonant peak is dependent on geometrical factors. Thus, the height of the curve can not be used as an indication of the Q; nor can the width of the peak, which is dependent both on the nonresonant breakdown and also on the distribution of cell sizes.
Thickness Measurement by Ultrasonic Resonance
Authors: Cook Edward G.
Published: September 2005

Calibration of instruments employing ultrasonic resonance for thickness measurements is frequently done by assuming half-wave resonance of the sample. Actual laboratory measurements indicate that this is not valid, particularly for the case of fundamental resonance. A mathematical analysis is given which explains the discrepancy. Theoretical calibration curves, calculated from the analysis, have been investigated experimentally.
Inharmonicity of Piano Bass Strings
Authors: Young Robert W.
Published: September 2005

Piano bass strings are made by winding copper or iron wire over a core of plain steel music wire to increase the linear density so that the length of the string can be kept within the space available. The core itself is stiff and causes inharmonicity (i.e., departure of the natural frequencies of the string from a harmonic series), and the covering winding increases the stiffness still more. A theory has been developed for the limiting stiffness resulting from the covering wire. Experiment indicates that the covering wire does stiffen the string somewhat, but that the stiffening is not nearly as great as it would be if the coils of covering wire just touched each other with the string at rest. Inharmonicity data are presented for the bass strings of a concert grand piano, a medium grand piano, and an upright piano. The inharmonicity is greatest in the extreme bass strings. The inharmonicity is twice as great in the upright as in the medium grand piano, and for the lowest eight strings of the medium grand (which are single) the inharmonicity is, in turn, roughly twice that for the concert grand piano.
A Direct-Reading Harmonic Analyzer for Use in the Study of the Quality of Musical Sounds
Authors: Culver C. A.
Published: September 2005

In the study of the quality of musical sounds it is desirable to be able to observe, simultaneously, the number and relative magnitude of the harmonics in any given tone. An analyzer has been developed which accomplished that end. As now designed, the equipment is arranged to indicate, by means of a series of meters, the first ten harmonics based on the notes A-220 and A-440. Using this analyzer, the author has been engaged in a study of piano tones. The results of this investigation are here presented. The study shows the difference in the sound spectrum of several well-known makes of pianos. It is pointed out how the analyzer may be profitably employed in other similar studies.
Growth and Decay of the Tones Produced by Various Musical Instruments
Authors: Stubbins W. H. , Holland-Moritz E. , Frederick J. R.
Published: September 2005

A sound spectrograph has been used to study the transient response of various musical instruments. Slides are presented which show the buildup and decay of the various harmonics of these instruments. The duration of the note as observed by the ear will be compared with that measured on the spectrograph.
New Definition for Acoustical Impedance and its Practical Use for Calculation of Damping in Ventilation Ducts
Authors: Brüel Per V.
Published: September 2005

The ratio between sound pressure and velocity is determined, not as usual in the absorbing material's surface, but in a plane just behind or in front of the surface where the ratio is a real quantity in distinction to the ordinary complex acoustical impedance. This real impedance is directly connected with the absorption at normal incidence, and in praxis experimentally found by using the standing wave apparatus. When using the real impedance as boundary condition in the wave equation a relatively simple set of formulas for calculation of the damping of different mode of vibrations in rooms is obtained. In this paper is particularly shown a set of formulas—specially made for use in praxis—for determination of sound damping in ventilation ducts. An experimental verification of the theory is made, and a simple measuring method for the damping per unit of length of ventilation ducts with low sound damping is shown. From the results of this investigation some economical considerations are given for the planning and construction of noiseless ventilation systems in praxis. It proves that it is more economical to use a blower with high noise level in connection with a heavy damping duct system instead of trying to make the blower with a lower noise level using only a small amount of damping ducts.
Acoustic Factors in the Design of Noise-Reducing Telephone Booths
Authors: Hewitt F. G. , Bishop D. E. , Callaway D. B.
Published: September 2005

A telephone booth of the partial enclosure type is advantageous because of ease of access for use and cleaning and simplification of ventilation and lighting problems, even though size limitations restrict the amount of noise reduction which may be attained. The noise reduction of a telephone booth can be considered as the decibel difference in octave band-noise levels outside and inside the booth in a random sound field. Much of the experimental work leading to development of new booths was carried out by means of acoustic models of 13 scale. The principal parameters affecting the reduction of an acoustically treated telephone booth were found to be: (1) the sound absorption coefficients of the wall surfaces as a function of frequency; (2) the ratio of open area to the wall area of the phone booth; (3) the transmission loss of the walls of the phone booth; and (4) the arrangement of the booth walls. Model study tests show that for a practical configuration a sound absorption coefficient greater than 0.75 above 300 cps does not increase the sound reduction appreciably. The program resulted in a new design for a noise reducing telephone booth having a 60° triangular shape which lends itself to a variety of mounting positions.
The Effect of Ultrasonic Waves on the Electrodeposition of Copper
Authors: Wolfe W. R. , Chessin Hyman , Yeager Ernest , Hovorka Frank
Published: September 2005

The effects of ultrasonic waves on the electrodeposition of copper have been investigated at 200 and 1000 kc/sec with copper sulfate-sulfuric acid plating baths of various concentrations. The ultrasonic waves produce an appreciable decrease in the potential at which the metal is deposited. X-ray diffraction studies by the glancing-beam technique indicate an increase in the tendency for preferred orientation of the crystals in the deposits from dilute solutions. These effects are explained primarily in terms of the disruption of the concentration gradients at the electrode surface by the ultrasonic waves. Schlieren photographs of these gradients support these conclusions.
Temperature Variation of the Adiabatic Elastic Constants of Aluminum
Authors: Long T. R. , Smith Charles S.
Published: September 2005

The elastic constants of a single crystal of aluminum have been measured by the pulsed ultrasonic technique in the temperature range from 80° to 300°K. It was found that all of the elastic constants decreased linearly with increasing temperature in this range and had the following values for the slopes: ΔC44/ΔT=−1.43 × 108 dyne cm−2 deg−1,ΔC′/ΔT=−1.23 × 108,ΔK−1/ΔT=−1.8 × 108.

The values of the elastic constants at room temperature (298°) were found to be: C44=0.2848; C′=0.234; and K−1=0.770, all expressed in units of 1012 dyne cm−2. A thermodynamic argument is used to separate the effects of the volume change through thermal expansion from the other effects of temperature on the elastic constants. This argument shows that the change with temperature cannot be explained solely in terms of the change in volume because there is an explicit dependence of the constants on temperature. This explicit dependence is different in both magnitude and direction for the constants C′ and C44.
Ultrasonic Attenuation in Biological Substances
Authors: Hueter T. F. , Cohen M. S. , Morgan H. M.
Published: September 2005

Measurements of ultrasonic absorption in the frequency range from 200 kilocycles to 40 megacycles have been made on a variety of biological substances. In most cases deviations from a square relationship between attenuation and frequency have been found. The experimental results will be used to check alternative theories on the absorption mechanisms involved.
An Anechoic Tank for Ultrasonics in the Megacycle Region
Authors: Fischell Robert E.
Published: September 2005

The need for an anechoic tank to be used for ultrasonics in the megacycle range of frequency is reviewed in light of recent developments of wide-angle radiation pattern, high-frequency transducers. A tank using rho-c rubber as an absorbent lining is discussed as a possible means of obtaining an anechoic tank for use at these ultrasonic frequencies. Previous theoretical work on the reflection of ultrasonic waves from attenuating solids indicates that rho-c rubber would attenuate most incident ultrasonic radiation. Experimental determination of the reflection coefficient as a function of angle of incidence for rho-c rubber shows that the theory is essentially correct in predicting a high degree of attenuation for angles of incidence which are not too close to the grazing angle. The anechoic tank is constructed by mounting the rho-c rubber onto sheets of plywood and fastening the plywood to the sides, top, and bottom of a rigid, water-tight tank. By analyzing all possible tangential, axial, and oblique radiation which could be emitted from a spherical ultrasonic source located at one end of the tank, the best positioning of baffles which can intercept unwanted reflections can be determined. The anechoic properties of the tank were observed at frequencies of 2.53 and 7.57 megacycles per second using a transducer that projects an intense but narrow radiation pattern. The method by which a fixed receiver in an anechoic tank can measure the radiation pattern of a transducer is discussed, and an experimentally measured radiation pattern is shown and analyzed.
Unexpected Waves Observed in Fluid-Filled Boreholes
Authors: White J. E. , Fost H. H.
Published: September 2005

During a continuing program involving pressure and particle velocity measurements in fluid-filled boreholes, several unexpected results have been observed which can be explained by very simple computations. One is an acoustic square wave, a flat topped pulse which is generated when a slug of water, broken off from the main column by cavitatlon, rejoins the column. In another instance, the geometry is such that a wave along a steel casing is the first signal to arrive at a pressure detector, and the initial pressure is negative, although the explosive pressure generating the transient is initially positive. The manner in which a steel casing causes a reversal of pressure in the “casing-break” is described. A damped low-frequency signal observed in a fluid-filled hole after a small explosion indicates that the gaseous products of the explosive constitute a pressure-release surface and hence that the fluid above the shot is “hanging in space,” acoustically free on both ends. An expression relating the negative pressure observed at the bottom of an uncase hole to the elastic constants of the fluid and solid is also presented.
Application of Correlation Techniques to Geophysical Prospecting
Authors: Horton C. W.
Published: September 2005

A recent development in geophysical research is the application of auto- and cross-correlation techniques to geophysical data in the hopes of improving the signal-to-noise ratio. This work is summarized, and typical correlation functions for land and shallow water areas are presented. The relation between correlation techniques, filtering, and direction patterns is reviewed and illustrated. A theory is developed to explain the shape of the autocorrelation function of a seismogram trace.
Seismic Waves from a Horizontal Force
Authors: White J. E. , Heaps S. N. , Lawrence P. L.
Published: September 2005

As part of a program of fundamental research on seismic waves, a generator was built for applying a transient horizontal force at the surface of the ground, and the resulting seismic waves were observed in some detail. The force is applied when a mass swinging through an arc strikes a target anchored to the earth. Surface geophones along a line in the direction of the force register vertically polarized shear waves refracted back up to the surface, whereas geophones on a line perpendicular to the force register horizontally polarized shear waves. The speeds of the two types of shear waves are often different, indicating anisotropy. Buried geophones below the force show a down-going shear wave. Variation of amplitude with angle and other features are in qualitative agreement with the results given by Rayleigh and others for the waves caused by a force at a point in an infinite solid. Love waves and other surface waves were observed, which of course would not be expected from an interior force.
Automatic Audiometry
Authors: Reger Scott N.
Published: September 2005

An automatic audiometer is defined in the present paper as a device for presenting auditory stimuli, so constructed that the listener's responses constitute feedback which controls the system by reinserting into it the results of its past performance. A randomized pulse tone system is described in which an incorrect response by the listener results in an increase in the intensity of the following pulse tone pattern, and a correct response results in a decrease in the intensity of the next pulse tone pattern. The intensity levels at which the pulse tone patterns are presented are recorded on a moving audiogram blank. The statistical probability that the listener can guess the number of pulses in a pattern is a known factor. The differences in intensity between thresholds obtained by means of a Békésy-type audiometer (another type of automatic audiometer) and the present instrument will be illustrated and discussed.
Perstimulatory Fatigue as Measured by Heterophonic Loudness Balances
Authors: Egan James P.
Published: September 2005

Perstimulatory fatigue is the decrease in loudness of a steady auditory stimulus during its presentation. In the past it has been measured by requiring a simultaneous dichotic loudness balance between two pure tones, one in each ear. Under these conditions the listener hears a single phantom sound whose localization depends upon the relative intensities of the two tones. The present investigation shows that the process of localizing the sound image in making the loudness balance is not critical to the occurrence of perstimulatory fatigue. Two principal conditions were compared. In the first, the frequency of the comparison stimulus was the same as that of the fatiguing stimulus. In the second, the comparison and fatiguing stimuli differed sufficiently in frequency (cps) so that the listener always could correctly localize the two tones. The amount of perstimulatory fatigue is the same under these two conditions. A procedure is developed which largely prevents the constant errors that result from the formation of a single absolute standard of loudness. This method results in greater measured fatigue than the usual procedure. This research was supported by the Operational Applications Laboratory, Air Force Cambridge Research Center, Air Research and Development Command.
Perstimulatory Auditory Fatigue for Continuous and Interrupted Noise
Authors: Carterette Edward C.
Published: September 2005

Perstimulatory fatigue, or adaptation, is measured by a simultaneous loudness balance or median plane localization of a dichotically presented acoustic stimulus. After one ear has been stimulated for a period of time, it is usually found that the variable or comparison stimulus must be reduced below the prefatiguing intensity in order to maintain the loudness match or localization balance. Five observers made median plane localizations of a continuous 100–5000 cps band-pass noise before, during, and after a seven-minute fatiguing period. The fatiguing stimuli were continuous noises at 30, 60, 87, and 100 db SPL, and noises interrupted at 1, 2, 5, 9, and 12.5 ips with both burst level and noise-time fractions held constant at 90-db SPL and 0.5, respectively. It was found that (a) the time required for fatigue to reach an apparent asymptote is at least seven minutes, about twice that required for pure tones; (b) the maximum fatigue increases with the intensity of the fatiguing stimulus and the function is positively accelerated; (c) for a fixed intensity of fatiguing noise, the maximum fatigue for the highest rate of interruption used (12.5 ips) is less than that obtained with a continuous noise having the same overall level (87 db). During the course of fatigue, the between-observer variations (standard deviations) of the balances tend to be greater than the within-observer variations; the reverse is true for the post-fatigue period.
Underwater Sound from the Splashes of Water Droplets and Sprays of Water Droplets
Authors: Franz G. J.
Published: September 2005

The underwater sound radiated from the splashes of individual water droplets is correlated with the development of the splash. The observed instantaneous sound pressure and the spectrum of the sound energy radiated into the water are compared with theory. The spectra of the sound energy from a large number of individual splashes of droplets of various sizes and velocities are plotted in dimensionless form and compared with a similar dimensionless spectrum obtained from data on the rms sound pressure from a spray of water droplets. An expression is given for the sound pressure spectrum level of the sound to be expected from sprays on the surface in terms of the intensity of the spray, the size and velocity of the droplets in the spray, and the configuration of the spray area on the surface. Estimates are made of the spectrum levels of the underwater sound from the splashing of rain on the surface of a sea in terms of the rate of rainfall.
Backward Scattering of Ultrasonic Energy from Water-Filled Sands with a Wide Distribution of Particle Sizes
Authors: Hoyer W. A. , Mifsud J. F. , Runyan W. R. , Ward M. B.
Published: September 2005

The backward scattering coefficients of several water-filled sands were measured, as a function of grazing angle, at frequencies of 0.5 Mc and 1.0 Mc. The water-filled sands are various mixtures of particles whose diameters are 0.40 mm and 0.12 ram. The backward scattered energy is assumed to be the sum of the energy scattered by all the particles of sand in a depth of sand α−1, where α is the absorption coefficient of sound in the water-filled sand. Hence, from a knowledge of the backward scattering coefficients, attenuation and porosity for graded sands, whose average particle sizes are 0.40 mm and 0.12 mm, backward scattering coefficients for mixtures of the two sizes of particles are calculated and compared with the experimentally measured values. There is agreement within the accuracy of the experiment. The backward scattering experiments were performed using a highly directional sound beam in a laboratory tank. A pulsed-signal echo-ranging system was used.
A Note on the Effect of Test Distance on Directivity Patterns of Underwater Sound Transducers
Authors: McKinney C. M. , Collins F. A.
Published: September 2005

In underwater sound work that requires large transducers with beams of only a few degrees, a problem often arises in regard to obtaining accurate calibrations with limited testing distances. At many of the available testing sites, directivity patterns must of necessity be made with the transducer either in the Fresnel region or in the critical region for true Fraunhofer diffraction. Several papers appear in the literature relating to corrections that can be made to measurements made on the acoustic axis of the beam, but little has been said regarding the effect on the directivity pattern for other orientations since it is very tedious to compute any such corrections. This paper presents some of the results of an experimental investigation as to the effect on beam width and secondary lobe structure. Measurements were made using both piston radiators and line source hydrophones.
Wedge-Shaped Acoustic Horns for Underwater Sound Applications
Authors: McKinney C. M. , Owens W. R.
Published: September 2005

This paper gives the results of an experimental investigation of wedge-shaped acoustic horns used in conjunction with line hydrophones. An earlier study covered the frequency range from 40 kc to 150 kc. The measurements have been extended to a lower value of 10 kc. Characteristic features of the horns are moderate directivity with minor lobes at least 40 db below axial response. The horns, which are constructed from aluminum planes covered with cellular rubber, are simple devices and very easy to construct. Some related experiments concerning the basic operation of the horns are described.
Control of Noise from Air Force Operations
Authors: Parrack Horace O.
Published: September 2005

The approach to the “man-noise interaction” technical area, used by the Air Research and Development Command in setting up its program for adequate control of noise from Air Force operations, will be discussed. In this program, the aim is to provide a system for noise control which will (a) prevent exposure of men to hazardous noise levels; (b) reduce noise to levels compatible with assigned functions and duties; (c) reduce noise levels to values acceptable to persons living on air bases and in nearby communities. To achieve the aim, a systems approach is necessary.
Correlation of Turbojet Noise with Engine Design
Authors: Mercer Derwent M. A.
Published: September 2005

The dependence of the total acoustic power radiated by a turbojet engine on one dimensionless parameter was put forward by Mawardi and Dyer. This relationship is reviewed in the light of further data, and also the dependence of acoustic power on a number of other parameters, dimensionless and otherwise, is presented. The special limitations of dimensional analysis in this problem are discussed. The results of some comparative measurements using carefully controlled operating conditions varying over a small range are presented, and the precautions necessary to obtain meaningful measurements of engine parameters are discussed. Reasons are given for doubting the agreement of acoustic measurements made at maximum engine conditions and those below this point. The turbojet engine is a complex noise source, and while the jet is the most powerful component, others are not negligible; compressor noise, combustor noise, and noise arising from vibration are discussed.
Noise Characteristics of Turbojet Engines in Test Cells
Authors: Galloway William J. , Pietrasanta Adone C.
Published: September 2005

A number of comprehensive acoustical surveys of several turbojet engine test facilities have recently been completed. Preliminary results of these surveys, including data on the noise characteristics of turbojet engines operating in test cells are discussed. Sound pressure level spectra are presented for several different present-day turbojet engines at various engine operating conditions. Problems introduced in the interpretation of test cell measurements because of the nature and complexity of the turbojet engine noise source are discussed.
General Design Considerations in the Quieting of Jet Aircraft Run-Up Testing
Authors: Hemond C. J. , Lemmerman R. D.
Published: September 2005

The problem of quieting jet run-up testing is becoming of greater importance to the acoustical engineer. This paper will review several of the factors which need to be considered in the design of silencing equipment for jet run-up operations. Details of associated mechanical design will be discussed relative to acoustic criteria and economic operational factors.
Absorption and Velocity of Ultrasonic Waves of Finite Amplitude in Liquids
Authors: Towle D. M. , Lindsay R. B.
Published: September 2005

Measurements have been made which show a dependence of attenuation of ultrasonic waves in several liquid media upon the intensity level at the source of the radiation. Measurements of the velocity showed no observable change at high intensities. The attenuation for distilled water and dilute solutions of acetic acid and sodium acetate was found to increase almost linearly with the acoustic pressure amplitude in the interval from 2 to 7 atmospheres. The attenuation in glycerol showed a similar increase when an unavoidable rise in the temperature of the medium was taken into account. Graphs of data taken at 4, 5, 7 and 9 megacycles/sec will be given and theories of the increase in attenuation will be discussed.