bluuebunny/crossref_metadata_2025 huggingface dataset:
Preliminary Investigation of the Dynamic Magnetostrictive Properties of Ni-Fe Alloys
Authors: Davis C. M. , Helms H. H. , Ferebee S. F.
Published: September 2005

The dynamic magnetostrictive properties of Ni-Fe alloys containing from 35.0 to 70% nickel were investigated to determine their suitability for use in electromechanical transducers. The material, in the form of toroids made from ring laminations, was evaluated by the motional impedance method. The annealing temperature was varied from 900°C to 1220°C. Also the effect of magnetic annealing was considered. Measured values of the electromechanical coupling coefficient, the mechanical Q in air, the reversible permeability, the dynamic magnetostrictive constant, and other parameters are given. A maximum electromechanical coupling coefficient of approximately 0.37 was obtained for 40 Permalloy annealed at 1000°C. The electrical resistivity and saturation flux density of 40 Permalloy exceeded that of nickel by factors of 6 and 2.5, respectively.
Design of Low-Temperature Transducers
Authors: Fitzgerald James W. , Greenwell Robert F. , Leek Stanley C.
Published: September 2005

Although a substantial amount of information is available relating to the propagation of sound in liquefied gases, little or nothing has been done on the design and calibration of transducers at these low temperatures. This paper presents design considerations and measured characteristics of quartz, lithium sulfate, and barium titanate transducers for use at temperatures in the vicinity of −200°C. Measurements were made in liquid nitrogen of dielectric constant, piezoelectric coupling coefficient, and frequency constant in the neighborhood of 400 kc/sec. Completed transducers were calibrated for both receiving and transmitting responses by reciprocity techniques. Complex impedance and beam patterns were also obtained. With proper design, units withstand the thermal shock of sudden immersion LN2 without failure. Noise voltages caused by boiling of the LN2 represent a problem in impedance measurements at resonance. [Work carried out under contract from Bogue Electric Manufacturing Company, Paterson, New Jersey.]
Underwater Sound Velocity Meter
Authors: Wennerberg Sigfrid B.
Published: September 2005

An automatic velocity meter for underwater sound propagation studies will be described. The meter is principally a fixed-path fixed-frequency delay line utilizing 500 kc continuous sound transmission through the water. The acoustical length is about 50 wavelengths. The velocity-dependent phase of the receiver signal is followed by a phase-shift condenser by means of a phase-sensitive servo-system. The dial of the phase-shift condenser is direct-calibrated in sound velocity. A relative velocity change of 10−4 can be measured. When only small velocity changes are to be measured, e.g., horizontal gradients or microstructure, the servo-system is disconnected and the output of a phase detector is recorded. In this way velocity differences of the order of 10−5 can be recorded.
Torsional Wave Ultrasonic Viscometer for Industrial Use
Authors: Bradfield G.
Published: September 2005

The instrument is based on Mason's research device. A composite oscillator of the Quimby-Rose type is made by combining the author's split tube torsional barium titanate transducer with a hollow stainless steel immersible head. Strip element ultrasonic viscometers of the magnetostrictive type have been used industrially but have been found to be somewhat fragile. The new device is far stronger at any given sensitivity; moreover the construction is readily adapted to use new stable piezoelectric materials. In a modified form hot liquids can be measured and in another form, high surface displacements can be generated; this feature may be required for testing two-phase systems incorporating large particles. Electronics require little modification for use with either strip or torsional type transducers. Remote control and indication together with computation from multiple test heads can be effected with either.
Variational Computation of Plate Contour Modes and Applications
Authors: Baerwald Hans G.
Published: September 2005

A variational method utilizing specially adapted sets of trial functions permits highly accurate computation of the natural frequencies of a plate; errors of 10−5 for secular matrices of orders three to five are typical for fundamentals, and independent treatment of overtones is feasible. The method applies to geometries and types of anisotropy in which the component wave equations are separable. The method was applied to planar-isotropic square plates where, in addition to four known families of contour modes, a hitherto unknown one (5) was discovered; the existence of a sixth is inferred. (5) is of the “breathing” type, but principally distortional, whereas the well-known breathing family is predominantly dilatational. Frequency constants and stress distributions were obtained in terms of Poisson's ratio and applied to precision measurements of the equatorial cross-ratios of polarized ferroelectric ceramics. By including the Lagrangian of an associated external system, the method can be extended to forced vibrations and will yield driving point and mutual impedance functions of electromechanical transducers. As a special application, the modal piezoelectric coupling factors associated with the two breathing fundamentals of ferroelectric square plates were computed and confirmed experimentally. Among the geometries of practical interest amenable to this approach are orthotropic circular cylinders. [Supported in part by the Office of Naval Research under Contract Nonr 1055(00).]
Maximum Vibration Damping by Means of Suitably Composed High Polymers
Authors: Oberst Hermann
Published: September 2005

Materials with extremely high internal energy losses (especially resilient materials) having large loss-factors (or low Q values, resp.) of the order of magnitude I can be systematically obtained with high polymers by suitable adjustment of the transformation temperature at which with growing temperature the material passes over from the glass-like to the rubber-like behavior. This can be done in different ways, for instance by suitable choice of the plasticizer contents. The transformation point must be so situated that mechanical dispersion and maximum absorption which are due to relaxation processes of the segments of the long chain molecules occur in the desired range of frequencies and temperatures. To attain a high damping of sheet metal structures by means of adhesive layers, a sufficiently large stiffness of the layers must be demanded besides high internal losses. Optimum mixtures are obtained with high polymers showing maximum absorption in the desired range (see above), and being suitably filled with light granular materials or combined with fibrous ones. The damping of sheet metal covered with not too thin layers of this kind surpasses that of soft cork plates.
Noise Reduction in Motor Vehicles, Especially Motorcycles
Authors: Bobbert G.
Published: September 2005

It is known that the noise from a motor vehicle depends primarily on the output and rpm of the motor, but this dependence is not the same for different types of vehicles. This is understandable if one considers that several component sources contribute to the total noise. For purposes of noise reduction, it is necessary to determine the individual sources and their contributions. In the case of motorcycles, where the requirements of low weight and small dimensions impose particularly difficult conditions on the sound damping arrangements, the following elements need to be considered as component sources: intake and exhaust of the fuel gas, gears, cooling air, cylinder rattle, and valves. There are different methods for investigating the individual sources separately. In the present state of development, all influences contribute about equally to the total noise. That means that it will now be necessary to reduce the noise from the gear unit, the cylinder, and the valve drive, which have usually been neglected in the past. For purposes of a uniform evaluation of all vehicles, a new standard measuring procedure has been developed in Germany and applied to all newly produced vehicles. Certain maxima of loudness are prescribed by law. Their observance is also being tested by control measurements on traffic.
Noise and Structural Vibrations
Authors: Franklin R. E.
Published: September 2005

If one considers the load on an area A of an infinite baffle placed in a random noise field, it appears that the quantity of significance is the correlation coefficient between the pressure at a given point and that at any other point, viz., R(t; x1, x2). This note discusses, in qualitative terms, some of the implications of this in relation to the problem of structural vibration with the object of outlining an experimental approach. It is shown that large amplitude vibrations are likely to be built up in a plate only when the noise field is being propagated and the transit time matches the periodic time of one of the modes of vibration of the plate. The quantity to be measured is the correlation coefficient with delay, i.e., R(l, t − τ; x1, x2,), for from this can be deduced the transit time and the cross-power spectrum.
Some Experiments Related to the Noise from Boundary Layers
Authors: Hubbard Harvey H.
Published: September 2005

This paper presents the main results of several recent experiments relating to the generation of noise in boundary layers. An attempt is made to correlate data obtained from a wind tunnel, a rotating disk, a helicopter rotor, and an aircraft in flight. These data provide evidence as to the mechanism of noise generation in the boundary layer and show how the noise varies as a function of velocity, density, and surface roughness. Some discussion is also given with regard to the relative importance of the boundary layer noise and the noise from the power plant in high-speed jet aircraft.
Procedures for Estimating the Near Field Noise of Rotating Aircraft Propellers
Authors: Kerwin Edward M.
Published: September 2005

An empirical correlation of the noise field of aircraft propellers with operating conditions and spatial position is described. The results are useful for estimating the propeller-noise levels over the surface of an aircraft fuselage or other surface in the near-field of a rotating propeller at points separated from the propeller axis by a distance greater than the propeller radius. Data for this study came principally from three sources: (a) NACA Reports, (b) the work of Beranek et al. published circa 1947, and (c) recent measurements by Bolt Beranek and Newman Inc. The parameters considered include the power delivered to the propeller, the propeller diameter and nmnber of blades, the propeller tip Mach numbers (both rotational and helical), and the distance from the propeller tips to the measurement point. The results of the study are presented in a number of charts giving the over-all level, spatial distribution, and frequency spectrum of the propeller noise. The use of the charts is described, and a comparisou of predicted and measured levels is made.
Traffic Noise in West Germany, Evaluation of Noise Levels and Experience in Noise Control
Authors: Meister F. J.
Published: September 2005

In 1932, Bakos and Kagan first attempted to measure traffic noise in Berlin. Later in 1938, Kösters and collaborators formed a “Map of Loudness” for Charlottenburg. At that early date they used a sound-level meter with weighting curves having frequency characteristics recommended by the German Acoustic Committee in 1937 and the logarithmic graphic level recorder of Braünmuhl. Many years later the PTB in Braunschweig and the Acoustics Laboratory in Düsseldorf began new traffic noise studies. In Düsseldorf we published a Map of Loudness for the city and tried to obtain acoustical statistics for all vehicles. The map of loudness so obtained in 1938, 1951, 1952, and 1955 showed successively an increase in the occurrence of levels over 80 db. For the purpose of evaluation, the levels at three points have been found useful in psychoacoustical work and for expert testimony. These are: (1) level of background, (2) average level (by a planimeter), and (3) median peaks per minute. These will be illustrated by an example. Many ways are possible for reducing traffic noise. One of the best is proper construction of the vehicles. Noise control now is known by town planning boards. Two projects are presented to show organized planning in Germany.
Absorption of Shear Waves in Liquids
Authors: Clausnizer G. , Kneser H. O.
Published: September 2005

The damping of the torsional resonance of aluminum cylinders (5 to 150 kc) surrounded by liquid was measured. In the case of most liquids, it was possible to calculate from these measurements a viscosity value that agrees with the quasi-static value. A frequency-dependent value was found only for ethyl cyclohexane, which in this frequency region shows a strong absorption for normal (longitudinal) sound waves. Quantitatively this can be explained by assuming a complex viscosity that depends on the frequency in the same manner as the complex “pressure viscosity” with which simple relaxation processes can be described.
Velocity of Ultrasonics in Compressed Gases near the Critical Point and up to 1200 Atmospheres
Authors: Noury J. , Lacam R. , Martin A. V. J. , Bergeon R. , Galatry L. , Vodar B.
Published: September 2005

The velocity of ultrasonics in compressed gases was measured by the optical method using the diffraction of monochromatic light. Our investigations may be summarized as follows: (1) Measurements on various compressed gases (A, N2, CH4) at pressures up to 1200 atmos, temperatures from 20°C to 200°C, and at frequencies ranging from 0.5 to 3.5 Mc. No definite dispersion was found in that range. The velocity increases as a function of pressure; at high densities its temperature coefficient is negative. The velocities calculated from the experimental PVT data of other authors are in good agreement with our measured velocities. The ratio of specific heats and the adiabatic compressibilities have been calculated. Calculations have been made for A, N2, and CH4 using a series expansion of the velocity of sound with respect to densities; the coefficients of this expansion are related to the first four virial coefficients and their first two derivatives. The usual (6–12) spherically symmetric intermolecular potential has been assumed. It is found that a satisfactory agreement between calculated and observed velocities can be achieved at densities up to 300 amagat units; this agreement is obtained for both PVT and sound velocity data, by taking into account only a constant value of the fourth virial coefficient in our temperature range. It is concluded that a purely thermodynamical interpretation is sufficient to explain the observed velocities in our range and that sound velocity data seem a convenient means to test, jointly with the equation of state, the accuracy of a given intermolecular potential law. (2) Measurements of ultrasonic velocity near the critical point: The decrease of the velocity, previously observed for CO2 and N2O, has been studied in C2H6 and C3H8. Systematic measurements on C3H8 show that no dispersion is present in our frequency range (0.585 to 3.2 Mc). Values of the velocity have been calculated on a purely thermodynamical basis from some semiempirical equations of state. The measured values are a little lower than the calculated values in the immediate vicinity of the critical point. Such a possible disagreement has been tentatively ascribed to the effect on the bulk viscosity of a structural relaxation process. (3) Measurements by a pulse method of the ultrasonic attenuation in nitrogen at pressures from 80 to 1400 atmos are presently in progress. Preliminary results indicate that the attenuation is strongly decreased under pressure.
Ultrasonic Relaxation in Freon Vapors
Authors: Miyahara Y. , Richardson E. G.
Published: September 2005

From the point of view of their use in supersonic wind tunnels in which it is desired to obtain a high Mach number without great expenditure of power, the substitution of the air by vapors having a low velocity of sound is of interest. The freons (halogen-substituted methanes) suggest themselves for this. It is shown that these vapors have ultrasonic relaxation times in the neighborhood of 10−7 sec coupled with high-absorption coefficients in the ultrasonic range, a fact which may limit their usefulness in wind tunnels. The relation between relaxation times measured by other methods and the present one are discussed.
Precipitation Hardening in an Ultrasonic Field
Authors: Schetky Laurence McD.
Published: September 2005

Ultrasonic vibrations have been used in the study of many of the phenomena of physics and chemistry, but to date little has been reported in the field of metallurgical processes or reactions. One of the deterrents to its use in such investigations is the problem of coupling a high power source of ultrasonics to a solid load. The use of a liquid couplant has inherent disadvantages which obviate its use, particularly where the solid under examination is to be maintained at an elevated temperature. The present study involved the construction of a transducer useful for either liquid or solid metal irradiation. An investigation was carried out on the effect of high power ultrasonics on precipitation in the solid state. Two aluminum alloys, one a high purity copper-aluminum binary, and the other an Al-Mg, Cu, Zn alloy 75S, were used in the investigation. It appears that the rate of hardening of these alloys is increased by irradiation with intense ultrasonic vibrations during aging at 100°C. Suggestions are given as to the nature of the acceleration effect, and mention is made of the probable limitations imposed on its commercial application.
Sonic Analysis of Fracture
Authors: Kolsky H.
Published: September 2005

The passage of stress waves of large amplitude through solid specimens often produces fractures which differ in several respects from those which can occur under any other conditions. Many of the observed phenomena can be accounted for in terms of the acoustical treatment of reflection and refraction of stress waves at the boundaries of the specimen and of diffraction effects in regions where the dimensions of the specimen become comparable with the wavelength of the disturbance. When a fracture occurs it results in a sudden release of stress, and consequently, secondary stress waves travel out from the fracture surfaces. The experimental investigation of these effects by means of high-speed photography, by electrical measurements of stress and strain, and by subsequent examination of the geometry and appearance of the fractured surfaces will be described, and the type of information which such investigations may give about the nature of the phenomenon of brittle fracture will be discussed.
Sound Scattering from Plane Inhomogeneous Surfaces with Acoustic Admittance Changing Periodically along the Surface
Authors: Lysanov Yu. P.
Published: September 2005

A method is given for calculating the scattered sound field from plane inhomogeneous surfaces having normal impedance or admittance independent of the angle of incidence. The expression for the scattered field is represented in the form of power series of a parameter determining the relative inhomogeneity of the surface. A recurrent expression is given for the nth term of the series. Several particular cases are considered in more detail. The proposed method is valid for any ratio λ/L where λ is the wavelength of the incident radiation and L the spatial period of inhomogeneity. A comparison is made with the approximate solution obtained on the assumption that the change of admittance is small at the distance of the order of one wavelength; it means that it is possible to neglect diffraction effects in the immediate vicinity of the surface. A criterion is given for the validity of this approximate method.
A Crystal Pick-Up for Measuring Ultrasonic Wave Velocity and Dispersion in Solid Rods
Authors: Bakanowski A. E. , Lindsay R. B.
Published: September 2005

A piezoelectric crystal pick-up has been designed, constructed and successfully applied to the detection of internodal distances in a solid metal rod vibrating at resonance and thus to the evaluation of configurational dispersion of ultrasonic compressional radiation in the rod. The use of the pick-up yields results as accurate as the powder method and can be applied in certain cases where the latter fails.
A Method of Calculating Hearing Loss for Speech from an Audiogram
Authors: Fletcher Harvey
Published: September 2005

In the present paper a formula is developed for calculating the hearing loss for speech from an audiogram showing the hearing loss for each of a series of pure tones. The formula is based upon studies of loudness, including the determination of the relative contributions of different frequency regions to the audibility of speech at or near the threshold level. The formula is tested for each of 165 ears involving a wide variety of hearing losses. In every instance an audiogram is available and also an independent observation of the hearing loss for speech. The formula yields a calculated value which generally is in closer agreement with the observation than is the value calculated by the familiar rule of averaging the losses at 500, 1000, and 2000 c.p.s. The agreement is particularly better when the audiogram is not “flat.” A simplified computational rule, indicated by the more complete formula, is found within indicated limits to be almost as reliable as the formula. This simplified rule is to examine the hearing losses measured by means of the audiometer at the three frequencies 500, 1000, and 2000 c.p.s., and to take the average of the two smallest values of loss.
The Acoustical Impedance of a Bubbly Mixture and Its Size Distribution Function
Authors: Davids Norman , Thurston E. G.
Published: September 2005

A statistical method is described which enables an analytical bubble size distribution to be formulated from the rough size grouping usually obtained with practical methods of experimental graphs of bubbly turbulent water. The resulting function is used with certain known formulas for evaluating the acoustical impedance of such a mixture. A brief physical interpretation of the effects predicted by the theory is given.
Transmission of 24-kc Underwater Sound from a Deep Source
Authors: Sheehy Myles J.
Published: September 2005

A few experiments are reported in which thirty-millisecond signals of 24-kc sound were emitted in the ocean every second at depths of 150, 300, 500, and 1000 ft., by a relatively non-directional projector, and were received by similar units at the same depths after having traveled 100 to 3000 yards. A recording technique was used which made it possible to resolve the direct and surface-reflected components. Transmission conditions near the surface were poor during the experiments, and the transmission was found to improve as the projector and receiver depths increased. The average attenuation at the greatest depths was about 6 db/1000 yd. in addition to the loss caused by an assumed inverse-square divergence of the sound energy. Although the emitted signals had rectangular envelopes and were all of the same amplitude, the amplitudes of the received signals fluctuated from signal to signal and within each signal. The fluctuation of the directly transmitted signals showed a general tendency to increase with range in proportion to the square root of the range. The magnitude of this fluctuation changed from about 10 percent of the average amplitude at 100 yards to 45 percent at 3000 yards. The fluctuation of the surface-reflected component was greater than that of the direct component and did not depend on range.
Acoustical Properties of Anisotropic Materials
Authors: Price W. J. , Huntington H. B.
Published: September 2005

Several acoustic properties of anisotropic media have been investigated by the pulsed ultrasonic method and the results analyzed in terms of plane waves traveling through an infinite medium. Single crystals of ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP) were the subjects of this study and in each case velocity measurements were used to compute a complete set of elastic moduli. Coupling between modes which results in the simultaneous generation of two modes by a transducer and in the transfer of energy between modes during a reflection was studied. The phenomenon of acoustical birefringence in ADP was particularly investigated and it was found that all of the three modes with wave normals in the 45° direction in the yz plane are propagated in different directions making non-zero angles with the wave normals. A case in which the anisotropic property of the medium causes anomalous beam spreading is reported and the circumstances leading to this condition are discussed.
Acoustic Filter for Water-Filled Pipes
Authors: Hoover Robert M. , Laird Donald T. , Miller Laymon N.
Published: September 2005

It is anticipated that some of the auxiliary equipment to be used in conjunction with the new water tunnel under construction at The Pennsylvania State College will be particularly noisy in the ultrasonic frequency region where certain low level acoustic measurements are to be made. This potentially noisy equipment includes such items as a deaerator, an energy dissipator and a pressure control system which are located in a water loop, external to the main tunnel section. To effectively isolate this loop acoustically from the remainder of the tunnel where the acoustic measurements will be made, it has been considered desirable to design an acoustic filter which can be inserted at the junctions of the loop with the tunnel. This filter should serve the multiple function of reducing the sound transmission through the pipe walls and through the water in the pipes while permitting the passage of relatively large volumes of water at small loss of pressure.

The problem of isolation in the pipe path is a fairly common one and essentially requires the use of sound attenuating gasket material. However, the isolation of the water path is more complex in that water must be allowed to flow while sound is attenuated. In this case a honeycombed structure of a pressure release material is used.

This paper discusses the hydraulic consideration of the problem and presents the results of acoustic transmission measurements on the filter components. The measurements include: 1. The frequency characteristics of the acoustic filter under free field conditions, 2. The transmission properties of the isolating gaskets mounted in the pipe flanges, and 3. The effective response of the filter and isolation gaskets combined.
Variational Principles in Acoustic Diffraction Theory
Authors: Levine Harold
Published: September 2005

The diffraction of a plane harmonic sound wave by an aperture in an infinitely thin rigid plane screen is investigated theoretically. Variational principles for the diffracted spherical wave amplitude at large distances from the aperture are derived. In the first of these, the stationary property is exhibited for the class of functions comprising the normal derivative of the aperture velocity potentials, whose distributions are governed by a generally insolvable integral equation. The second involves functions which characterize the discontinuity in velocity potentials at the screen (or their deviation from infinite screen distributions), and are specified by another integral equation. A comparison of the two variational principles is given, which indicates that their over-all agreement following use of approximate functions, is a measure of the accuracy obtained. The plane wave transmission cross section of the aperture is related to the imaginary part of the diffracted amplitude in the direction of incidence and is cast in stationary forms. Particular attention is given to the low and high frequency behavior of the various forms of cross section, including comparison with Kirchhoff theory predictions.
Reciprocity Pressure Response Formula Which Includes the Effect of the Chamber Load on the Motion of the Transducer Diaphragms
Authors: Hawley M. S.
Published: September 2005

The reciprocity pressure response formula is derived from the equations of motion of the electroacoustic systems involved in reciprocity measurements in which only two of the three transducers are coupled at a time to the chamber. The formula includes the acoustical impedances of the three transducers. When it is assumed that the acoustical impedances of the transducers are large relative to the chamber impedance an approximate formula is obtained which includes the acoustical impedance of only one of the transducers, the one whose response is given by the formula.
The Masking of Pure Tones and of Speech by White Noise
Authors: Hawkins J. E. , Stevens S. S.
Published: September 2005

This report presents the results of a study of the monaural masking of pure tones by white noise at eight sensation levels (SL) from 20 to 90 db. The observed values of masking were employed to determine two basic functions:

(a) The critical band width of a masking noise, i.e., the ratio, in decibels, between the level of a pure tone and the level per cycle of the noise that is just able to mask the tone.

(b) The function relating the amount of masking to the effective level of the masking noise.

With the aid of these two functions, a set of contours was constructed to represent the masked threshold for pure tones heard monaurally against background of white noise having an ideal flat spectrum at the ear of the listener.

A study was also made of the masking by white noise of speech in the form of continuous discourse. Measurements of the threshold of detectability and of the threshold of intelligibility were made at the same eight noise levels used to mask pure tones.
Loudness Patterns—a New Approach
Authors: Munson W. A. , Gardner Mark B.
Published: September 2005

In the past loudness patterns have been based on the masking effect of one sound on another. For complex sounds having distributed energy spectrums this method appears to be valid. For sounds with single frequency components the method is thought to be in error due to the formation of beats and modulation products between the primary tone for which a pattern is desired and the probe tone which is used to determine the pattern details. To avoid these difficulties in the present tests, the probe tone was presented after the primary tone was turned off. The resulting residual masking patterns differ in a number of important respects from patterns based on the simultaneous masking procedure.

A comparison between the loudness of a primary tone, as evidenced by the magnitude of its residual masking pattern, with the results of loudness judgment tests was made. This was done by replacing the physical scales of pressure level and frequency by the subjective scales of loudness and position. A reasonably good check of computed and measured loudness values was obtained. Patterns for a 1000-cycle tone were measured to show how the loudness of the standard reference tone is distributed and how this distribution changes as the level of the tone is increased.
The Relation between Localization and Intelligibility
Authors: Hirsh Ira J.
Published: September 2005

The binaural masked threshold for speech depends upon the relation between the interaural phase angles of the speech and those of the noise. When these phase angles are the same, the threshold is high, and both speech and noise appear to be in the same place. When the interaural phase angle of the speech is reversed relative to that of the noise, the threshold is low and the speech and noise appear to be in different places. These relations have been clearly demonstrated with earphones, and they suggest that in free-field listening the threshold of intelligibility might be affected by the relative locations of the sources of speech and of masking noise. It was found that when the azimuths of the sources of speech and of noise are changed relative to each other, the threshold of intelligibility changes by small but consistent amounts. When the sources are close together, the threshold is high; when the sources are far apart, the threshold is reduced.

Although this relation is partially confounded by the effect of azimuth on the sound pressure levels at the ears, the factor of localization appears to play a significant role, especially when two ears are used and when the head is allowed to move. In order that the hard-of-hearing may take advantage of these effects, they must have a hearing aid with two separate microphones mounted near the ears and connected each to a separate earphone.
Levels and Spectra of Transportation Vehicle Noise
Authors: Bonvallet G. L.
Published: September 2005

In the recent past a program was initiated to survey vehicle, traffic, and industrial noise in the Chicago area. The phase on noise of vehicles has been completed. The investigation included street, elevated, and subway cars; diesel, steam, and electric trains; and motor buses, trucks, and automobiles.

Measurements were made of over-all and octave band levels. Inside of the vehicles, flat network over-all levels ranged from 85 db in a new “L” car to 95 db in subway cars. The readings in the 400–800 c.p.s. band ranged from 68 db in an automobile to 91 db in subway cars. Outside of and close to vehicles, the flat network over-all levels ranged from 78 db for automobiles to 94 db for subway trains. Observations in the 400–800 c.p.s. band ranged from 66 db for automobiles to 87 db for subway trains.

Variations in the over-all levels inside of vehicles ranged from ±1 to ±5 db. Variations the 400–800 c.p.s. band ranged from ±2 to ±5 db. Outside of the vehicles, variations ranged from ±2 to ±6 db in both the over-all and 400–800 c.p.s. band levels.
The Acoustical Impedance of Closed Rectangular Loudspeaker Housings
Authors: Meeker Willard F. , Slaymaker Frank H. , Merrill Lynn L.
Published: September 2005

Direct-radiator loudspeakers are often mounted with the back of the diaphragm working into a completely enclosed space. Conventional theory states that when the maximum linear dimension of such an enclosure is small compared with the wave-length, the pressure is uniform throughout, and the acoustical impedance presented to the loudspeaker is -j/ω(V/ρc2), where V is the enclosed volume. Although it has not been clearly established how small an enclosure must be before it is “small compared with the wave-length,” the foregoing expression is generally used, at low audiofrequencies, to calculate the acoustical impedance of closed loudspeaker housings.

It is shown here that while the acoustical impedance of a closed rectangular housing is capacitive at very low frequencies, it passes through zero as the frequency increases and becomes that of an inertance as the frequency of the first normal mode is approached. For a typical housing 11 in. × 22 in. × 22 in., the frequency at which the impedance presented to a very small speaker passes through zero is in the vicinity of 70 c.p.s.; at this frequency, the maximum linear dimension of the enclosure is less than one-seventh of the wave-length.

These results are obtained by using the methods given by Morse for determining the pressure distribution in a room. A point-source loudspeaker is assumed and the pressure at the source is calculated as the summation of the pressures due to the normal modes of the enclosure, losses being neglected. Measurements of the pressure within the enclosure support this analysis.

From measurements of the pressure distribution over the surface of the loudspeaker diaphragm, it may be deduced that the magnitude of the acoustical impedance which the enclosure presents to the loudspeaker diaphragm and the frequency at which the impedance becomes zero depend upon the dimensions of the loudspeaker diaphragm as well as the dimensions of the enclosure.
The Velocity of Compressional Waves in Rods of Rectangular Cross Section
Authors: Morse R. W.
Published: September 2005

Solutions for compressional waves in an isotropic rod of rectangular cross section are formulated from the general elastic equations of motion. Two general modes of propagation are constructed which satisfy the boundary conditions for the free vibrations of a long rod for which the width 2d is considerably greater than the thickness 2a. The thickness mode is shown to have an infinite phase velocity (cut-off frequency) at that frequency for which the half wave-length in an infinitely wide rod would be equal to 2d. The phase velocity of both of these propagation modes is shown to approach that of Rayleigh surface waves when the wave-lengths are much smaller than the lateral dimensions. The calculated thickness mode cut-off frequencies are compared with measurements reported earlier and the agreement is shown to be good. A comparison between the predicted and measured phase velocity dispersion is made for various values of a/d. The agreement is found to be satisfactory as long as d is several times greater than a.
Phase Distortion of Acoustic Pulses Obliquely Reflected from a Medium of Higher Sound Velocity
Authors: Arons A. B. , Yennie D. R.
Published: September 2005

Theoretical analysis predicts the occurrence of phase shifts when harmonic waves are reflected from interfaces of higher sound velocity (such as the sea bed in underwater sound propagation) at angles exceeding the critical angle of total reflection. Since the phase shift depends only upon the acoustic parameters of the media forming the interface and upon the angle of incidence and is independent of the frequency of the incident wave train, one would expect pulses of arbitrary shape to be subjected to distortion upon reflection.

The expected shape of the pressure wave reflected from a semi-solid bottom is derived for the specific case of exponentially decaying shock waves produced in underwater explosions, and the theoretical predictions are found to agree well with the observed pressure-time curves of the first and successively higher order reflections. The theoretical analysis assumes acoustic plane waves incident upon an interface between two fluid media. Experimental records were obtained at relatively great distances from the charge so that finite amplitude effects were negligible and the radius of curvature of the wave front quite large.
Measurements of the Ultrasonic Cries of Bats
Authors: Griffin Donald R.
Published: September 2005

The ultrasonic sounds emitted by bats have been analyzed with a system sensitive to frequencies from 1 to 150 kc. These sounds are used by the bats to detect obstacles by means of their echoes; and they consist of pulses about two milliseconds in duration. Most of the measurements were made with the common little brown bat, Myotis l. lucifugus; with this species the sound pressure at 40 to 55 kc, measured 5 to 10 cm from the animal's mouth, averaged 60 dynes/cm2 (109 db on the conventional scale of sound pressure levels). The highest recorded intensity was 173 dynes/cm2 (119 db). The frequency of the ultrasonic sound falls during each pulse by about one octave; the average frequency at the peak amplitude was 47.8 kc, while the average at the beginning of the pulse was 77.9 kc, and at the end 39.1 kc. Low frequency waves (about 10 kc) accompany the pulse, but their amplitude is a very small fraction (1/100 to 1/1000 or less) of the peak sound pressure at ultrasonic frequencies. The envelope form is variable; the emission is directional with most of the energy concentrated into the forward direction; and the pulses are commonly repeated at rates of 20 to 30 per second.
Frequency Response Fluctuations in Rooms
Authors: Bolt R. H. , Roop R. W.
Published: September 2005

The frequency irregularity of a steady-state transmission curve is defined as the sum of the differences, in decibels, between adjacent peaks and valleys divided by the frequency interval over which the summation is made. An approximate theory of frequency irregularity for arbitrarily selected speaker and microphone positions in rectangular rooms is presented. Experimental curves of frequency irregularity vs. frequency and absorption are given for three small rooms (V<2500 cu. ft.). Comparison of these results with theoretical curves shows reasonable agreement for rectangular rooms. In particular, a predicted frequency region of maximum irregularity is observed. The frequency irregularity observed for an irregularly shaped room is significantly lower than that for a rectangular room of comparable volume throughout the region of maximum roughness.
Pulse Statistics Analysis of Room Acoustics
Authors: Bolt R. H. , Doak P. E. , Westervelt P. J.
Published: September 2005

Many sounds of speech and music more nearly resemble pulsed wave trains than abruptly terminated continuous sounds as used in reverberation measurement. It is therefore not surprising to find that two rooms can differ markedly in acoustical quality even if they appear identical under reverberation analysis which ignores details of short transients.

This paper introduces a pulse statistics point of view which takes immediate account of the pulse-like nature of common sounds. Fundamentally, the method consists in examining the response of the room to a short pulse. The walls are replaced by an array of image sources (simple images if the walls are hard, or appropriately modified if there is absorption). These image arrays are then considered statistically.

From this approach one can derive such classical quantities as reverberation time and mean free path. One can also analyze the detailed nature of discrete reflections including interference effects, and thus obtain an average correlation between room geometry and the character of its pulse response.

Idealized experiments in a hard-walled rectangular room are employed to illustrate the essential features of this approach. A point source emits an exponential damped 3600-c.p.s. wave train of about 2 msec. duration. The received signals are recorded logarithmically on an oscillograph and the system is calibrated for quantitative results. Several dozen discrete reflections can be measured and correlated with calculation. The pulses merge into a more or less continuous background after a time that is calculated and confirmed experimentally. Detailed differences arise according to the positions of the source and microphone in the room.
An Equivalent Circuit for a Vibrating Beam Which Includes Shear Motions
Authors: Trent H. M.
Published: September 2005

Often, the lower normal modes of slim but complicated structures can be computed with sufficient accuracy if the computations are based upon a tapered continuous beam in which the stiffness and inertia of the beam match those of the structure at a large number of points along its length. Shear motions play a dominant role in these calculations. Even with the continuous beam approximation, the numerical computations involved are lengthy and time consuming. An equivalent electric circuit for the beam makes it possible to substitute model experiments for lengthy calculations. It is here shown that such an equivalent circuit can be constructed which is made up of lumped passive components. Given such a circuit, it is then possible to determine experimentally the gross responses of the structure to an arbitrary excitation.
On Ultrasonic Scattering by Polycrystals
Authors: Huntington H. B.
Published: September 2005

The scatter in phase which occurs when a beam of ultrasonic radiation transverses polycrystalline medium is here treated as a stochastic process. This approach leads to an attenuation from polycrystalline scattering proportional to the grain size divided by the square of the wave-length. Whereas prior treatments of this subject are valid for the wave-lengths either much smaller or much larger than the grain size, the method used here holds best over the intermediate frequency range.
The Apparent Source of Speech in the Mouth
Authors: Hawley Mones E. , Kettler A. H.
Published: September 2005

Investigation of the sound pressure on the axis of the mouth indicates the existence of a point which may be represented as the source of speech sounds. The location of this point is dependent upon the particular speech sound, the intensity with which it is uttered, and upon the frequency components under consideration. Using young, average, male speakers, this apparent source of speech was located for 18 frequency bands covering the portion of the spectrum which contributes to speech intelligibility in each of 38 fundamental speech sounds.
Sound Measurements for Psychophysical Tests
Authors: Munson W. A. , Wiener Francis M.
Published: September 2005

The results of psychophysical tests on the auditory sensory system usually show much more variability than experiments involving only physical measurements of sound waves. This has resulted in the general impression that the variance of measurements of the auditory threshold and other psychoacoustic measurements is largely due to the inability of the observers to give consistent responses. This paper reports the results of a series of threshold measurements made under carefully controlled conditions and accompanied by search tube measurements of the sound pressures in the ear canals of the observers. The small variances obtained under certain conditions suggest that the large variability oftentimes experienced in psychophysical work is partly due to inadequate techniques for measuring the levels of the stimuli that activate the sensory system.
The Relative Output from Magnetostriction Ultrasonic Generators
Authors: Leslie F. M.
Published: September 2005

An approximate theoretical treatment of the output from the simplest type of ultrasonic generator in the form of a laminated bar is first developed. The dumbbell generator is shown to be very similar to the simple bar type and the previously obtained relations are then employed for determining the relative output in terms of its face and neck dimensions.
A Continuously Variable Filter
Authors: Fant C. Gunnar M.
Published: September 2005

A heterodyne filter for the speech frequency range is described. It can be used as a band pass filter of continuously variable low and high frequency cut-offs adaptable for any desired transmission band in the frequency range 40–4000 c.p.s. and with more than 60-db attenuation and with cut-offs of a maximal slope of 1 db per c.p.s.

Band width and low frequency cut off are varied by two separate controls. Band elimination filtering can also be obtained with continuously variable mid-frequency but with band width restricted to either 300 or 800 c.p.s.

The variable filtering is performed by single sideband transmission with successive filtering in two modulation units and demodulation in a third unit. Distortion products from impurities in transmission and modulation are kept at a −60-db level and random noise at a −85-db level, relative to a single test tone.

The filter was designed for articulation tests and general speech research purposes and also as a wave analyzer of continuously variable band width, 45–4000 c.p.s. in the frequency range 40–2000 c.p.s. The maximum permissible speed in analysis of sustained sounds, governed by the inverse square of band width law, is discussed with regard to the quality of the analyzing band pass filter.
Accelerometer Calibration Technique
Authors: Tyzzer F. G. , Hardy H. C.
Published: September 2005

A technique is described for the dynamic calibration of accelerometers or other vibration-measuring instruments. For each calibration frequency the accelerometer is attached to a free-free bar driven electromagnetically at a known sinusoidal displacement. Displacement amplitudes are measured by means of strain gauges which are calibrated optically. The accelerometer output can be conveniently compared in magnitude and phase with the displacement as indicated by the strain-gauge voltage. Instruments have been calibrated at frequencies between 400 and 2000 c.p.s. The error in determining phase angle was about ±2 degrees. The error in the optical measurement of displacement was about ±0.1 × 10−3 cm. The percent error, however, could be made fairly small by calibrating the strain gauges at large displacements of the bar. For instance, the error in calibrating a 200-g accelerometer at 1000 c.p.s. was about ±2 percent.