bluuebunny/crossref_metadata_2025 huggingface dataset:
Acoustic Propagation in Granular Media
Authors: Morse R. W.
Published: September 2005

A theory is developed for the sound velocity and attenuation in a medium composed of closely packed solid particles immersed in a fluid. The absorption mechanism considered is that of viscous motion of fluid between the particles, and the macroscopic point of view is taken. For high enough frequencies the attenuation is proportional to the square root of the product of frequency and static flow resistance of the medium. Comparisons are made with data reported previously by others, and the agreement with the theoretical results is good for cases involving particles of nearly uniform size. Still another frequency effect, dependent on the size distribution, is evident for unsorted granular substance.
The Effectiveness of Plastic Focusing Lenses with High Intensity Ultrasonic Radiation
Authors: Bronzo Joseph A. , Anderson John Maxwell
Published: September 2005

An investigation of the behavior of Plexiglas and polystyrene lenses as focusing devices for ultrasonic radiation is described. A fixed frequency of 1 megacycle/second and intensities up to 6.6 W/cm2 were employed. Using a thermocouple probe to obtain records of intensity of radiation in a water tank, it was found that above about 2 W/cm2 the Plexiglas showed signs of physical breakdown. Up to the maximum intensities employed, polystyrene showed no indication of damage, while at 6.6 W/cm2 the Plexiglas developed internal cavities near the outer edge of the lens, where the thickness was 4 mm.
The Information of Elementary Auditory Displays
Authors: Pollack Irwin
Published: September 2005

In contrast to the extremely acute sensitivity of a human listener to discriminate small differences in the frequency or intensity between two sounds is his relative inability to identify (and name) sounds presented individually. When the frequency of a single tone is varied in equal-logarithmic steps in the range between 100 cps and 8000 cps (and when the level of the tone is randomly adjusted to reduce loudness cues), the amount of information transferred is about 2.3 bits per stimulus presentation. This is equivalent to perfect identification among only 5 tones. The information transferred, under the conditions of measurement employed, is reasonably invariant under wide variations in stimulus conditions.
Tentative Estimate of a Hearing Damage Risk Criterion for Steady-State Noise
Authors: Hardy Howard C.
Published: September 2005

The probable shape of a limiting spectrum for noise in a working environment where there is long daily exposure is determined by reasoning from previously determined data of the hearing mechanism. It is assumed that hearing loss is due to a progressive fatigue phenomenon, the fatigue being a direct function of the energy stimulus in the inner ear. From the characteristics of this stimulus as a function of frequency, it is shown that the most sensitive region for loss is around 3000 cps, the low frequencies being as much as 20 db less sensitive than this region. The 4000 cps “notch” in audiograms of industrial workers is thereby explained. The limiting spectra are essentially equal-loudness contours for broad-band noise when the noise spectrum is plotted in octave bands.

Two limiting loudness curves and their corresponding spectra are tentatively suggested as damage risk criteria. If the noise exceeds 100 sones in any octave band, it appears very probable that long time daily exposure will result in hearing damage and in a frequency region slightly above the frequencies of maximum stimulus. If the sound does not exceed 50 sones in any octave band, it appears to be unlikely that damage will occur even with long exposure. These criteria are confirmed by the few literature references which have enough data to be used. The need for more research to test these conclusions is emphasized.
Unsolved Problems Relating to Hearing Loss in Industry
Authors: Hoople Gordon D.
Published: September 2005

Two generally accepted facts are: 1. Noise can cause hearing loss. 2. The noise which will cause a hearing loss in one individual may not be the cause of a hearing loss in another.

These immediately suggest two unsolved problems. 1. Why does a noise cause hearing loss in one person and none in another? 2. How much noise will cause a hearing loss?

The only logical answer to the first question must be that there is a variation in the sensitivity of ears to noise. Discovery of reliable tests which will predict which ear will be sensitive to noise and which will not, is an important unsolved problem, but if solved would do much to clear many disputed questions.

The answer to the second question, “How much noise?,” involves the following: the intensity of the noise, the kind of noise, the duration of the exposure to noise, the amount and character of the hearing loss which noise creates.

Noise in intensities of 120 to 150 decibels are conceded to be injurious to hearing. Noise less than 80 decibels is not a problem. Where between these two limits does noise become injurious to some ears? Noise with predominance of higher frequencies apparently is more injurious to hearing than one with predominance of low frequencies.

The length of exposure to noise depends upon so many factors that it is certainly an unsolved problem. The character of the loss is an injury to the nerve of hearing and presents no problem except the differentiation between loss due to fatigue and permanent hearing loss.

The paper ends with a plea for all who are interested to join in solving these questions so that noise can be less injurious to hearing.
Unsolved Basic Physical Research Problems in the Field of Noise
Authors: Hardy Howard C.
Published: September 2005

Many of the basic unsolved problems in noise research could be attacked by the application of statistical mathematics and techniques instead of use of the conventional “sine wave” approaches. Such techniques could be applied to many sound radiation and diffraction problems. The fields of aerodynamics and acoustics should be brought closer together, which would result in better understanding of the noise of a turbulent medium. Among the various unsolved noise measurement problems are the design of meters for measuring transients, intensity meters, and better sound level meters. Several other basic projects are mentioned briefly.
Unsolved Problems in the Field of Aircraft Noise
Authors: Fehr R. O.
Published: September 2005

Noise-control problems in connection with turbo-jet engine testing as well as operation are experienced by the engine manufacturer, the airframe manufacturer, and the operators of aircraft and of airports. Some of these noise-control problems can be solved by isolation of the noise source, others require reduction of the noise at the source. The important still unsolved problems are stated, together with background information, which may be useful to know when attempting their solution.
An Exact Solution of the Acoustical Field Near a Circular Transducer
Authors: Guptill E. W.
Published: September 2005

An exact solution for the acoustical field of an extended source is obtained by the use of boundary conditions differing from those of the simple piston.
Some Design Considerations for High Frequency Anechoic Tanks
Authors: Weinstein Marvin S.
Published: September 2005

By an extension of the techniques of Lindsay, the reflection from a semi-infinite attenuating medium is derived for the case of oblique angles of incidence. The results are applied to the design of a water tank each wall of which is assumed to be covered by a sheet of absorbing material. The optimum physical properties of the absorber required to make the tank anechoic in the mesacycle region are then determined theoretically. Experimental measurements at 3.35 megacycles/sec indicate that the properties of tan rho-c rubber are very close to optimum. With a 14-inch thick sheet backed by aluminum the reflection coefficient is about −30 db at normal incidence and rises to −20 db at an incident angle of 60°. Beyond 60° the rubber is of little value in reducing the reflectivity. The results obtained are in good agreement with those expected from the theoretical treatment when modified for reflections from the back surface and the phase difficulties previously described by the author.
The Influence of Consonant Environment upon the Secondary Acoustical Characteristics of Vowels
Authors: House Arthur S. , Fairbanks Grant
Published: September 2005

The consonant environments of vowels were varied by forming nonmeaningful stimulus syllables consisting of 72 combinations of six vowels and 12 consonants. The syllables were spoken by subjects, and the duration, fundamental frequency, and relative power of the vowels were measured. All three factors varied significantly in response to changes of the consonant environment. The variations were systematically related to the attributes of the consonants, the most powerful attribute being the presence or absence of vocal fold vibration, followed by manner of articulation and place of articulation, in that order.
Electromechanical Pick-Up Calibration by the Interferometer Method
Authors: Ziegler Charles A.
Published: September 2005

Electromechanical pick-ups have been calibrated in the frequency range from 250 to 7000 cps using an interferometer to measure the vibration amplitude. A description of the theory and the experimental method is given.
Characteristics of Jet-Edge-Resonator Whistles
Authors: Nyborg W. L. , Woodbridge C. L. , Schilling H. K.
Published: September 2005

A jet-edge-resonator (JER) whistle consists essentially of a jet-edge (JE) system, such as may itself produce edge tones, and a resonator. Sound fields generated by such sources are described, as well as dependence of various aspects of these fields on airflow and whistle dimensions. Empirical findings are interpreted in terms of independently determined properties of JE systems and of resonators. A method is described for adjustment of a given whistle to maximum or near-maximum output for a given frequency. Typical output data so obtained are presented for each of several models. Approximate values are also given of the modulation coefficient M for various circumstances. This quantity is an index of effectiveness with which steady flow into a whistle is converted into a sinusoidally varying flow out of its mouth.
A Temperature Invariant Impedance Point in the Frequency Response of Circular Barium Titanate Transducers
Authors: Hueter T. F.
Published: September 2005

In studying radiation from a circular barium titanate transducer, it is found that at one particular frequency the mechanically derived part of the electrical input impedance is independent of temperature.
Early Work on the Instrumental Ear
Authors: Taylor H. O.
Published: September 2005

Editor's Note: Recently, I received from Professor H. O. Taylor an interesting letter along with a paper for publication. More than anyone else, Professor Taylor has been responsible for the development of the tube method of measuring acoustic impedance, even though there were no suitable microphones or loudspeakers available when he performed his original experiments in 1912. Believing that our members would like to hear from Professor Taylor, I am reproducing parts of his letter here. Professor Taylor is now living at 3077 Olive Avenue, Altadena, California and is Dean of Ambassador College in Pasadena.—L. L. BERANEK.
Periodic Fluid Flow through Circular Orifices
Authors: Thurston George B. , Martin Charles E.
Published: September 2005

The results of an experimental investigation of both the linear and nonlinear acoustic impedance of circular orifices with water and 10-centistoke Dow Corning silicone fluid are presented. Experimental determinations of the end corrections for acoustic resistance and acoustic inductance are in good agreement with the classical end correction as given by Rayleigh for acoustic inductance. A series of determinations of the criteria for the onset of nonlinearity acoustic resistance and acoustic inductance together with photographs of the fluid flow patterns developed are presented. A characteristic of one of the fluid flow patterns is correlated with the onset of nonlinearity.
Small-Signal Impedance of Short Tubes
Authors: Nolle A. W.
Published: September 2005

Relations expressing the acoustic inertance and resistance of short open tubes in terms of kva are reviewed. The tube radius is a; kv is the complex wave number for viscosity waves. Analogous relations giving the acoustical impedance of a cylindrical cavity in terms of kTa, where kT is the thermal wave number, are considered. Viscous and thermal losses have sometimes been incorrectly separated in the literature. Past measurements, showing disagreement between theory and experiment for small kva, are cited. New measurements of inertance and associated resistance, in the range of |kva| from 1 to 77, are reported. The method of standing wave analysis used. In some instances the inertance tube is incorporated in an adjustable-cavity resonator. Inertance values after end corrections agree with theory within 2.5 percent. Corrected resistance values generally agree with theory within 3 percent for |kva| < 10. Greater discrepancies for larger values of kva are attributed to excessive losses in very short cavities. Consideration of present results together with published data for larger values of kva indicates close agreement of theory and experiment for a range of kva extending at least from unity to several hundred.
The Physical Interpretation of the Expression for an Outgoing Wave in Cylindrical Coordinates
Authors: Junger Miguel C.
Published: September 2005

The sound field generated by a vibrating cylinder of infinite length, whose dynamic configuration is periodic in φ and z, is expressed in terms of acoustic impedance ratios. It is noted that symmetrical modes of vibration are suppressed at certain frequencies because the corresponding reactive impedance is infinite, and that all z dependent modes become nonradiating below certain “cut-off” frequencies, the corresponding impedance being purely reactive. Graphs are presented for the impedance ratios corresponding to certain modes.

For modes independent of z, the sound field is in the form of concentric cylindrical waves. For z dependent modes, as the plane wave wavelength increases from zero to a certain critical cut-off value, the sound field changes from a set of concentric cylindrical waves to two sets of conical waves of decreasing vertex angle; at and beyond the cut-off point, the conical waves have degenerated into a set of plane standing waves normal to the z axis. Simultaneously, the sound field has ceased being periodic in the radial direction, and the phase velocity in that direction has become infinite.

Practical applications of these phenomena are suggested.
Acoustic Streaming due to Attenuated Plane Waves
Authors: Nyborg Wesley L.
Published: September 2005

Theories for calculating steady streaming associated with sound fields are reviewed, comparing the methods and approximations of various authors. Two illustrative problems are worked out, both for rectilinear flow due to irrotational sound fields. The first deals with a single attenuated plane wave traveling down a tube, as in Cady's quartz wind experiments. In the second, a pair of crossed plane waves is treated, giving rise to a quite different kind of streaming. In obtaining solutions, attention is given to boundary conditions; here, gradients of She excess static pressure, another second-order quantity, come into consideration. Significantly, streaming speeds depend critically upon α, the attenuation constant, where α may be due to any common cause, such as heat conduction, scattering, thermal relaxation, etc. From these results it appears that streaming measurements cannot be used to distinguish between absorption mechanisms. Numerical values are given for a few cases; high flow speeds may be expected in a bubbly medium.
Effect of a Finite Circular Baffle Board on Acoustic Radiation
Authors: Nimura Tadamoto , Watanabe Yoshiyuki
Published: September 2005

The effect of a finite circular baffle board on the acoustic radiation was computed by the use of the oblate spheriodal wave function developed by Kotani and by Boukamp. The directivity, power radiation, and radiation impedance of the vibrating disk with a concentric circular baffle board are shown together with a design diagram for a circular baffle board.
A Fixed Path Ultrasonic Interferometer for Absorption Measurements on Gases
Authors: Bell J. F. W.
Published: September 2005

A fixed path interferometer, using variable temperature for the measurement of the absorption in gases in the low f/p region is described. Its use is illustrated by measurements on argon, neon, air, nitrogen, and nitrogen-hydrogen mixtures at 250 kc. The values found for argon and neon were above classical by 40 and 65 percent, respectively. The discrepancies are ascribed to Rayleigh cross modes in the ultrasonic waves excited by the quartz crystal transducer rather than to any departure from classical theory. Cross modes and their influence on ultrasonic measurements are discussed. Data on the performance of two crystals at 250 and 500 kc are given.
The Theory of Noise in Continuous Media
Authors: Eckart Carl
Published: September 2005

It is shown that the mathematical solution of problems involving the propagation of noise is materially aided by the introduction of the space correlation function ψ(x1, x2, τ), defined as the average, over t, of p(x1t)p(x2t−τ), p being the acoustic pressure in the noise field. The differential equations satisfied by ψ are derived. Its relation to Ψ(α, β, γ) is discussed, α, β, γ being the propagation vector of a sinusoidal wave, and Ψ/2ρc2 being the density of potential energy in the α, β, γ space.

The theory of uniform noise fields, both isotropic and anisotropic, is developed in detail. The anisotropy caused by a reflecting plane is discussed. The radiation of noise by a vibrating plane is discussed, neglecting the reaction of the radiation on the motion of the surface. In particular, it is shown that to this approximation the sea surface cannot radiate subsonic energy into the high atmosphere because the velocity of the surface gravity waves is less than the velocity of sound in air.

The theory of noise, as here developed, is analogous to the theory of turbulence, and the relation between noise and turbulence is briefly discussed.
Transient Loading of a Baffled Piston
Authors: Miles John W.
Published: September 2005

The force-time history required to produce a step in the velocity of a circular piston mounted in an infinite baffle is determined with the aid of the Laplace transform. Working from this result, the inverse problem of the velocity response to a step force is calculated approximately. These two results suffice to treat arbitrary distributions of applied force or velocity by Duhamel superposition. The application of the results to a loudspeaker model shows that a system designed for critical damping on the “steady-state” approximation for the air loading will actually be slightly overdamped in its initial motion, but that the characteristic time during which the design assumption is inadequate is of the order of 10−3 second.
Transient Loading of a Baffled Strip
Authors: Miles John W.
Published: September 2005

The force-time history required to produce a unit step in the velocity of a two-dimensional plate mounted in an infinite baffle is determined and applied to more general velocity variations by Duhamel superposition.
Surface Motion Associated with Obliquely Incident Elastic Waves
Authors: Pearson John , Rinehart John S.
Published: September 2005

Well-known laws which govern the reflection of elastic waves that strike free surfaces obliquely are used to deduce particle motion at the free surface of a body. The surface particle motion is presented in the form of diagrams and graphs for the case of an incident longitudinal wave. Considerations indicate that, for oblique incidence, the particle motion at the surface will not, in general, be perpendicular to the surface but will depend on the angle of incidence and the Poisson's ratio of the material. The data are expected to be of value in the solution of problems connected with impulsively loaded bodies such as metal-explosive systems.
Notes on the Transmission of Sound Through Plates
Authors: Fay R. D.
Published: September 2005

A relatively tractable expression for the transmittivity of steel plates immersed in a fluid medium is formulated. Criteria for total transmission and total reflection are inherent in the expression.

An investigation of the discrepancies between calculated and observed transmittivity indicates a probability that the assumption of negligible losses associated with shear waves in steel is not tenable.
A Variation of the Radiation Pressure Method of Measuring Sound Absorption in Liquids
Authors: McNamara Frank L. , Beyer Robert T.
Published: September 2005

An apparatus is described which may be used to measure the ultrasonic absorption coefficient in liquids. The transducer is one-hundred percent modulated at an audiofrequency. This produces the same frequency variation in the radiation pressure. A condenser microphone is used to detect these low frequency pressure changes. The apparatus has been tested in water over the frequency range 9–27 mc and possesses an accuracy which is comparable to that of pulse and optical methods.
Determination of the Acoustic Properties of Blood and its Components
Authors: Carstensen Edwin L. , Li Kam , Schwan Herman P.
Published: September 2005

Measurements of absorption and velocity of sound in blood, plasma, and solutions of albumin and hemoglobin have been carried out in the frequency range 800–3000 kc and temperature range 5–45°C. The absorption departs only slightly from a linear dependence upon frequency. Absorption for the various solutions is in direct proportion to protein content. It is concluded that the acoustic properties of blood are largely determined by the proteins which it contains.
The Measurement of the Lateral Mechanical Impedance of Phonograph Pick-Ups
Authors: Woodward J. G. , Halter J. B.
Published: September 2005

A device is described for the measurement of the lateral complex mechanical impedance of phonograph pick-ups in the frequency range between 30 and 10 000 cps. The device employs four electromagnetically driven, calibrated vibrators resonating different frequencies. The theory of operation is outlined, and results obtained for a typical phonograph pick-up are presented.
An Investigation of the Air Chamber of Horn Type Loudspeakers
Authors: Smith Bob H.
Published: September 2005

The air chamber is treated as a boundary value problem which results in the solution of the wave equation for the general case in which the horn throat enters the air chamber in any circumferentially symmetrical manner. The following specific cases are analyzed: (1) the case in which the horn throat enters the air chamber by means of a single orifice, (2) the horn throat enters the air chamber by means of a single annulus of radius r and width w, and (3) the horn throat enters the air chamber in “m” annuluses of radii r1⋯rm and widths w1⋯wm. The analysis reveals that the radial perturbation caused by the horn throat excites higher order modes. At the resonant frequencies of these modes the horn throat pressure becomes zero and the loudspeaker does not radiate. By suitable choice of annulus radii and widths the first “m” modes may be suppressed and the corresponding nulls in the output pressure eliminated.
Apparatus and Procedures for Predicting Ventilation System Noise
Authors: Beranek L. L. , Reynolds J. L. , Wilson K. E.
Published: September 2005

A method has been developed for measuring the acoustic power output in watts and the frequency spectra of ventilating fans operating into ducts. Both the inlet and the exhaust noise are determined as a function of back-pressure. The fan is ideally connected to a circular section of duct about equal in diameter to the fan diameter and five to ten diameters in length. Three to six diameters from the input end of this duct an air straightener is located. The acoustic measurements are made with a condenser microphone enclosed in a wind screen located four to eight diameters from the inlet. The outlet end of the duct couples through an exponential connector to an anechoic wedge structure two feet square. This is backed by a plenum chamber containing an array of holes that can be progressively closed to produce increased back-pressure. Sound pressure level measurements are made in frequency bands and converted to acoustic watts per band. The inlet noise is measured over a hemispherical surface outdoors to obtain the directivity pattern and the acoustic power for each frequency band. Engineering formulas and charts for applying the data to prediction of the noise produced in rooms are presented, including the effects of room characteristics, duct losses, and the distance from the ventilating grille in the room. Criteria for permissible noise levels in various types of rooms and auditoriums are presented.
Equivalence of Sonic and Thermal Energies
Authors: Parthasarathy S. , Chari S. S. , Srinivasan D.
Published: September 2005

In the absorption of 4 mc/sec sound in liquids the complete conversion of sound energy into thermal energy is substantiated.
Crystal Acoustic Arrays
Authors: Welkowitz W.
Published: September 2005

A point source method for rapid design of a crystal transducer array using three crystal thicknesses in the ratio of 1:2:4 has been developed. Formulas are presented for main lobe width and ratio of secondary lobe amplitude to main lobe amplitude. For a 10×10 array (100 elements) this method of design leads to a relative side lobe level of 22 db as compared to a value for a uniform array of 13.4 db at the expense of having a beam width 1.25 times as wide as the uniform array.
Physical Characteristics of Aircraft Noise Sources
Authors: von Gierke H. E.
Published: September 2005

Available basic characteristics of different aircraft noise sources under the condition of zero forward speed are summarized: Total acoustic power; acoustic mechanical efficiency; directivity and frequency characteristics are given for the rotation and vortex noise of propellers, for the exhaust noise of reciprocating engines, and for different types of jet engines. The physical mechanisms underlying the different noise sources are discussed and the influence of changes in the parameters such as tip speed or pitch of the propeller and diameter or velocity of the gas jet are shown. In cases where measurements on the actual propulsion systems are incomplete, the basic physical conclusions are drawn from experiments on model airscrews and small air jets. The changes in the characteristics of the noise generators during flight are discussed briefly.
Status of Research on Propeller Noise and Its Reduction
Authors: Regier Arthur A. , Hubbard Harvey H.
Published: September 2005

The basic concepts concerning the mechanism of propeller noise generation are reviewed, and equations for calculating the noise field both near and far from the propeller are given. The problem of noise due to non-steady airloads on the blades, and differences in noise from the approaching and retreating blades are discussed. Effect of tip speed, number of blades, blade thickness, and blade width are considered, and it is shown that reducing the tip speed and increasing the number of blades are probably the most effective means of reducing the efficiency of noise generation and alleviating the noise problem. Noise characteristics of such special types as the supersonic, dual-rotating, tandem, and shrouded propeller are also presented. Studies of propeller weight show that substantial noise reduction on transport type propellers would result in a considerable weight penalty. For the propeller-driven aircraft in the 400 to 500 mph speed range, the slower turning quieter propeller is likely to have better propulsive efficiency than the noisier high speed propeller. Hence, in this speed range, the weight penalty of the quieter propeller may be offset by its higher propulsive efficiency.
A Review of the Influence of Meteorological Conditions on Sound Propagation
Authors: Ingård Uno
Published: September 2005

The study of the different atmospheric effects indicates that in short-range sound propagation the attenuation by irregularities in the wind structure (gustiness) often is of major importance in comparison with humidity, fog, and rain, and ordinary temperature and wind refraction. However, the ground attenuation can be of equal importance to the gustiness, in particular, when the sound source and the receiver are sufficiently close to the ground. The effect on the attenuation of the height of the source and the receiver off the ground is presented as a function of frequency for a typical ground impedance. The attenuation curve exhibits a maximum which in most cases lies at a frequency between 200 and 500 cps.
Noise Control for Ground Operation of the F-89 Airplane
Authors: Veneklasen Paul S.
Published: September 2005

A noise control project is described which includes a noise muffler for the ground testing of the F-89 air plane which is powered by two turbojet engines. Included are the prediction of performance, acoustical design considerations, correlation of acoustical and thermodynamic requirements, general construction, acoustical tests during construction, noise survey around the exposed airplane, and neighborhood survey around the completed installation. This noise suppressor is comparatively simple and inexpensive, requires no water cooling even for afterburner operation, and has proved adequate in terms of neighborhood relief and protection of operating personnel.
A Comparison Between Model Study Tests and Field Measurements on an Aircraft Test-Cell Silencer
Authors: Callaway D. B. , Lemmerman R. D.
Published: September 2005

An acoustic model study was made of the silencer for a propeller engine test cell in which the acoustical treatment consisted of conventional splitters for high frequency silencing and thick splitters designed to give low frequency reduction. The scale chosen for the model study was 13, and absorption coefficients of silencing treatment were simulated at frequencies eight times those of interest in the full-scale structure. After construction of the test cell, field measurements were made of the noise reduction provided by the silencing treatment, using as a sound source both a propeller engine and loudspeakers driven by a thermal noise source. Measurement of the noise reduction provided in the scale model permitted prediction of reductions in the full structure which showed reasonably good agreement with field measurements. On the basis of the model study and field measurements on high frequency splitters, an empirical relationship between noise reduction and absorption coefficient was determined.
Sound Radiation Patterns of Gas Turbine Exhaust Stacks
Authors: Wells R. J. , Crocker B. E.
Published: September 2005

Radiation patterns for gas turbine exhaust stacks have been calculated for the case of wide-band noise, assuming negligibly low gas velocities and temperatures. Both shape of stack and band width of analyzing equipment are considered. Theoretical results are compared with data measured on models and on a prototype exhaust system.
Materials and Structures Problems in Aircraft Noise Control
Authors: Lemmerman Richard D.
Published: September 2005

The materials and structures problems associated with the construction of aircraft noise silencing systems often make classical solutions to the acoustic problems extremely difficult. The associated problems of fluid mechanics and physical construction must be carefully considered along with acoustical requirements at the time of initial design. A review of some of these problems specifically associated with various types of aircraft engines will be useful. Similarly, the various types of acoustical material suitable for broad band attenuation will be considered from their structural standpoint.
Ultrasonic Waves and Electrochemistry. III. An Electrokinetic Effect Produced by Ultrasonic Waves
Authors: Dietrick Harry , Yeager Ernest , Bugosh John , Hovorka Frank
Published: September 2005

If a wire covered with a fiber or a porous coating is submerged in a dilute electrolytic solution and irradiated with ultrasonic waves, an alternating potential of the same frequency is produced on the wire relative to the bulk of the surrounding solution. The effect has been studied with pulse-modulated ultrasonic waves at a carrier frequency of 200 kc/sec. For a double cotton-covered copper wire in 0.0001-molar sodium chloride solution, the open circuit response has been found to be approximately 10−7 volt per dyne/cm2. In general, the acoustical response is dependent on the nature and concentration of the electrolyte, the pH, and the type of covering but independent of the metal. The effect is explained on the basis of the interaction of the ultrasonic waves with the diffuse double layer of ions surrounding the fibers of the covering. Hydrophones involving this effect offer particular advantage at frequencies above 100 kc/sec because of the small dimensions of the acoustically sensitive elements.
Ultrasonic Waves and Electrochemistry. IV. The Production of Alternating Components in the Potential of a Polarized Hydrogen Electrode with Ultrasonic Waves
Authors: Dietrick Harry , Yeager Ernest , Bugosh John , Hovorka Frank
Published: September 2005

If sound waves interact with an electrode upon which a gas is being liberated by electrolysis, an alternating potential of the same frequency is developed on the electrode relative to the bulk of the surrounding solution. The effect has been studied in the case of a polarized hydrogen electrode with pulse-modulated ultrasonic waves at a frequency of 200 kc/sec. Under favorable conditions the amplitude of the alternating potentials is of the order of 10−3 volt per dyne/cm2. The effect has been found to depend on the formation of gas bubbles at the electrode surface. Although the amplitude of the effect is a function of the conductance of the solution and the current density, it is essentially independent of the hydrogen ion concentration of the solution in the range 10−4 to 10−8 molar and is also independent of the specific metal upon which the hydrogen gas is being liberated. On the basis of these results, the effect is attributed primarily to the modulation of the i-r drop in the solution immediately adjacent to the electrode surface by periodically expanding and contracting gas bubbles.
The Effect of Intermolecular Bond Strength on the Onset of Cavitation
Authors: Horton John P.
Published: September 2005

Since bacterial destruction is a cavitation-dependent effect of ultrasound, it has been possible to draw conclusions regarding the factors affecting cavitation by studying bacteriological results. Micro gas bubbles are often described as nuclei for cavitation bubbles. In order to destroy bacteria, however, cavities must form and collapse within several microns of the bacterial cells so that the resulting collapse shock wave will have a high enough energy to rupture the cell wall. Cavitation near the cell occurs most readily when the interfacial linkages between the cell and the surrounding liquid are weakest. When surface-active agents, which strengthen the interfacial linkages, are added to the bacterial suspension, cavitation is inhibited and the ultrasonic death rate decreases markedly. If the bacterial suspension is heated, increasing the energy of the intramolecular bonds, the threshold sound pressure just necessary to kill bacteria is reduced, or, if the sound pressure is constant, the bacteria are destroyed at an increasing rate as the temperature increases. Likewise, if alternate cavitation nuclei which form weaker linkages with the surrounding liquid than the initial bacteria are added to the bacterial suspension, the killing rate of the original bacteria is decreased.
Recent Work on Violins
Authors: Saunders F. A.
Published: September 2005

(a) Loudness-frequency curves of violins disclose peaks caused by natural vibrations of the body and of the inner air. “Overtone peaks” occur when a natural vibration coincides with one of the partials of the tone. Shifting the frequencies of the natural vibrations is possible and can produce a more even distribution of strength.

(b) The volume of tone is very sensitive to the thickness of the top plate along the line where it is attached to the rest of the body. Thinning a groove part way around the top has produced marked improvements in several instruments.

(c) The air inside the box produces one noticeable peak only in the loudness curve. Tuning forks placed in front of the f-holes produce responses but they are the result of body vibrations.

(d) Experiments on f-holes show that long ones are somewhat better than short, and also that the main function of the bass-bar is to carry vibrations from the active foot of the bridge to the wider areas.

(e) Resonating strings reduce loudness rather than increasing it. Interesting phase differences occur between strings, or the inner air, or parts of the body.

(f) A string gives more tone when the others are relaxed.
A Scale for Specifying Frequency Levels in Octaves and Semitones
Authors: Moon Parry
Published: September 2005

A new system of specifying frequency level is proposed. It utilizes the logarithm (base 2) of the relative frequency, expressed in duodecimal notation. In this way the designation of musical intervals and musical scales is related directly to frequency, yet the numbers repeat in each octave and the semitones of the equitempered chromatic scale are expressed exactly as 0.100, 0.200, 0.300, ⋯.

The new notation is applied to the macrodesignation of semitones and octaves, and to the microdesignation of true intervals and of Pythagorean and Ptolemaic scales. A slide rule is developed to allow easy visualization of the relations among musical intervals.
Diffraction Patterns for Solid Delay Lines
Authors: Mapleton Robert A.
Published: September 2005

Solutions of the vector equation of equilibrium for an isotropic solid are presented for the special case of harmonic time dependence and dissipationless wave propagation. The object is to investigate the angular distribution and speed of wave propagation of the components of the elastic wave field at large distances R from the source, which source region is a plane surface of finite extent. Solutions are obtained for specified discontinuities of stress on an interior plane of an unbounded solid, and prescribed stresses on the bounding plane of a semi-infinite solid. Several examples are considered in which the distribution of a single stress component is selected to simulate the stress produced by a high frequency quartz piezoelectric crystal. For the high frequencies of solid delay line operation and the isotropic solids commonly used, the angular distribution of the dominant components of the elastic wave field are essentially the same for the semi-infinite and unbounded solid, to terms in R−1. The speed of propagation of the dominant field is characteristic of the type of source, shear or compressional, and the angular distribution is characteristic of the configuration of the source region.
The Dispersion of a Pulse Propagated through a Cylindrical Tube
Authors: Anderson D. V. , Barnes C.
Published: September 2005

The dispersion of an ultrasonic pulse propagated through a circular tube filled with water has been observed to agree sufficiently closely with the theory that the envelope of the dispersed pulse may be used for the approximate determination of the frequency spectrum of the original pulse.
Longitudinal Magneto-Hydrodynamic Waves
Authors: Anderson Neal S.
Published: September 2005

The properties of longitudinal magneto-hydrodynamic waves are discussed. For a conducting fluid the presence of a strong magnetic field, on the basis of Alfven's theory, will change the sound propagation velocity. This change results from the additional forces on the conducting fluid due to the motion of the fluid in the magnetic field. The theory is extended to a fluid with finite conductivity. The case discussed deals with plane waves whose propagation direction is perpendicular to the direction of the magnetic field. It is found that the finite conductivity introduces dispersion and absorption into the propagation, which has the form of a reversed “relaxation type” frequency dependence, with the velocity decreasing with increasing frequency. The theory is applied to predict the results that would be obtained for two experiments: the propagation of sound in mercury in a very strong magnetic field; and second, the propagation of sound in sea water and the influence of the earth's magnetic field.
The Absorption of Sound in Suspensions and Emulsions. I. Water Fog in Air
Authors: Epstein Paul S. , Carhart Richard R.
Published: September 2005

The suspended particles are approximated by spheres and the diffraction problem for a fluid sphere in a fluid medium is solved taking into consideration viscosity and thermal conduction. The results are discussed numerically for water droplets in air and a satisfactory agreement with Knudsen's attenuation measurements in water fog is found.
The Scattering of Sound from the Sea Surface
Authors: Eckart Carl
Published: September 2005

It is assumed that the equation of the sea surface is z = ζ(xyt), and that the time average Φ = 〈ζ(x′y′t)ζ(x″y″t)〉 is a function only of ξ = x″ − x′, η = y″ − y′. The scattering coefficient of long-wave sound is calculated and shown to be σ = (c2k2/4π)2∫∫Φ(ξη) exp[−ik(aξ+bη)]dξdη, where 2π/k is the wavelength of the sound, and a, b, c are, respectively, the sum of the x, y, z direction cosines of the incident and scattered rays. For short-wave sound, the formula is more complicated, but independent of the wavelength of the sound, as it should be. However, it is shown that experiments with short waves yield much less information about the sea surface than do those with long waves. This is unfortunate, since the latter experiments are much more difficult than the former.
Anomalous Transient Response of a Crystal Pick-up
Authors: Honnell Pierre M.
Published: September 2005

The true transient response of a piezoelectric vibration pick-up (Brush type BL-301) to square waves and steps of displacement has been obtained with a transient-testing transducer system. The anomalous response of the pick-up to these transient motions is in the nature of superimposed oscillations of high frequency. These spurious oscillations in the step response have their counterpart in the multiple resonances of the crystal element as indicated by the steady-state cissoidal calibration of the pick-up.
A Circular-Orifice Number Describing Dependency of Primary Pfeifenton Frequency on Differential Pressure, Gas Density, and Orifice Geometry
Authors: Anderson A. B. C.
Published: September 2005

A nondimensional number (Δp/ρ)12/ft for primary Pfeifentöne relating the dependence of differential pressure Δp across orifice to density of gas ρ, thickness t of orifice-plate terminating pipe, and frequency f of primary Pfeifentöne is obtained by dimensional analysis. Correlation of experimental data presented on the basis of this number indicates that it is relatively constant in numerical value over range of variation of the parameters, pipe length, orifice diameter, differential pressure across orifice, and Pfeifenton frequency, studied.
On the Diffraction of a Plane Sound Wave by a Paraboloid of Revolution. II
Authors: Horton C. W.
Published: September 2005

Some numerical values have been computed for the functions which arise in connection with the scalar wave equation in rotational paraboloidal coordinates. Tables of these values are given with the aid of which a numerical analysis is carried out of the scattering of a plane sound wave by a rigid convex paraboloid of revolution. The surface of the paraboloid is defined by kξ0=0.25 and the direction of propagation of the plane wave makes an angle of 36.9° with the axis of symmetry of the paraboloid. The asymptotic expansions of the scattered waves are discussed and their amplitudes are tabulated. The magnitude and phase angle of the total pressure are evaluated at points on the surface of the paraboloid.
Sound Propagation in Gross Mixtures
Authors: Ament W. S.
Published: September 2005

For low acoustic frequencies, a mixture (a porous medium or a suspension) is shown to have an effective density which differs slightly from the density given by Archimedes' principle. This effective density is computed from a physically elementary consideration of viscous, incompressible fluid flow. For higher frequencies, pore or particle size in the mixture becomes comparable with the wavelength of shear waves in the fluid, while still small compared with dilatational wavelength. The theory is extended to such frequencies through the known formula for the fluid's resistance to the oscillations of a rigid sphere. In both cases, the effective compressibility of the mixture is taken to be the volume-average of the component compressibilities. From the effective density and compressibility, the acoustic properties of the mixture are predicted. Predictions are compared with previous theories and with experimental results.
Ultrasonic Absorption in Magnesium Sulfate Solutions
Authors: Connolly Philip L. , Fox Francis E.
Published: September 2005

The ultrasonic absorption coefficients in water solutions of magnesium sulfate were measured at 30 Mc as a function of temperature and concentration. Temperatures from 0 to 30°C and concentrations from 0.05 to 2.00 molarity were used. Measured data were used to calculate Q, the absorption cross section per molecule of magnesium sulfate at several temperatures. Q is defined by (2αM/ν2)/N where N is the number of molecules of magnesium sulfate per cc, ν is the frequency, and αM is the amplitude absorption coefficient caused by the magnesium sulfate. These are constant for concentrations up to 1-molarity but decrease by 50 percent for 2-molarity concentration. The constant values of Q×1037, 25 at 25°C, 30 at 20°C and 37 at 15°C indicate an activation energy for the magnesium sulfate absorption process at 30 Mc of 6.8 kcal/mole°K.
Sound Waves Observed in Mud-Filled Well After Surface Dynamite Charges
Authors: Ording J. R. , Redding V. L.
Published: September 2005

An array of three pressure pick-ups 100 ft apart covering a vertical range of 200 ft was used to record sound waves in two wells drilled to depths exceeding 12 000 ft. Dynamite charges were exploded in 80-ft holes at a distance of 500 ft from the well-head. A separate charge was fired with the array at each 500-ft interval in the well. The recorded events were analyzed and found to belong to two categories. The first category includes the pressure pulse arriving at the pick-up by a more or less direct route through the sedimentary rocks. The second consists of events set up in the mud column at points other than adjacent to the pick-up by the compressional wave in the sedimentary rocks. These events then travel through the mud column and are recorded as they pass the pick-up. Anomalies in the hole, such as the ends of casing strings and the bottom of the hole, cause events of larger amplitude than those set up in sections of the hole with no major anomalies.
A Field Meter for Sound Velocity Measurements
Authors: McLoughlin Robert C. , Chiles James R.
Published: September 2005

The travel time of sound across fixed gap between diaphragms of transmitting and receiving transducers is recorded each 1/500 second. From this the interdiaphragm velocity is computed. It is a function of both temperature and wind. Spurious travel time changes introduced by phase shifts in the receiver are minimized by using a 500-cps intelligence signal amplitude modulating a 10-kc carrier. Experimental errors when operating in air with interdiaphragm distances of 3 feet are less than 2 percent in the temperature range from 70° to 200°F, but the application of the instrument is not restricted to this range. A circuit for recording velocity directly on a paper tape is described. Although designed primarily for air, the unit is adaptable to other media. It should also be of special interest to the micrometeorologist.