bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiation of a Source in a Free Field near a Reflecting Plane
Authors: Cox Jerome R.
Published: September 2005

A free field above a perfectly reflecting, horizontal plane is a simple acoustic environment that closely approximates the actual environment of some sound sources. Compared to a completely free field, however, the radiation of sound from a source above a reflecting plane is more complicated because the size of the near field of the source is larger, the directivity pattern of the source is more complicated and the power radiated may or may not be doubled depending upon the number of wavelengths that separate the reflecting plane and the source. Assuming a sound source with an arbitrary, but fixed velocity distribution over its surface, the sound field above a reflecting plane has been examined. The extent of the near field was found to depend upon the directivity of the source in free field and the difference in the distances from the point of observation to the source and to its image. The special cases of sources with symmetry about a plane or an axis parallel to the reflecting plane have been studied in detail and some practical rules for estimating the extent of the near field will be given. For these same special cases the ratio of the intensity measured above the reflecting plane to that measured in a free field has been calculated for 13-octave, 12-octave, and octave bands of noise. The results of these calculations lead to practical procedures for measuring the total power radiated by a source above a reflecting plane.
Equivalent Network Models of Continuous Media and a Network Representation of Sound Radiators
Authors: Nielsen A. Kjerbye
Published: September 2005

Simple acoustical systems may often be treated as acoustical networks with lumped elements if the linear dimensions of the elements are small compared with the wavelength. At higher frequencies this circuit theory method becomes less practical, and it is then necessary to use a field theory where the medium is considered continuous. It is sometimes difficult to decide whether to use the circuit theory approach or the field theory approach, because the accuracy obtainable with the circuit theory and lumped elements is not immediately apparent. It is shown in the paper that much uncertainty may be resolved by careful analysis of the elements used in the circuit theory. This is demonstrated by means of a simple example. In the design of sound radiators it is often convenient to represent the radiation impedance as a lumped element network, and such representations are well known for simple sound sources. When an extended definition of radiation impedance is used it becomes possible to find a simple network representation of spherical sound sources of arbitrary order n. Examples are given for n = 0,1,2 … 7. A simple network representation is also possible for the input impedance of exponential and hyperbolic horns. The physical significance of these representations is discussed.
Scattering of Ultrasonic Waves in Gases
Authors: Mokhtar M. , Shehata M.
Published: September 2005

Ultrasonic beams of frequencies in the range of 80 kc/sec. to 1000 kc/sec. emerge from a rectangular slit of known adjustable width into air, oxygen, or carbon dioxide. The disposition of the field in front of the slit is studied by the aid of a hot wire anemometer, which gives the angle of divergence of the beam and the intensity distribution across the main and the diffracted beams.

The results obtained indicate that the laws of diffraction are valid so long as the gas is far removed from its anomalous dispersion region. CO2 in the range between 100 and 200 kc/sec. shows a high degree of scattering with the result that the emergent beam loses its definition completely and becomes very diffused.
The Intelligibility of Interrupted Speech
Authors: Miller George A. , Licklider J. C. R.
Published: September 2005

This paper concerns the effects of interrupting speech waves—turning them on and off intermittently or masking them with intermittent noise—upon their intelligibility. The effects were studied with various rates of interruption and with the speech left undisturbed various percentages of the time. Tests were conducted (1) with speech turned on and off in quiet, (2) with continuous speech masked by interrupted white noise, and (3) with speech and noise interrupted alternately, the speech wave being turned on as the noise wave was turned off, and vice versa.

(1) When the speech wave is turned on and off infrequently, the percentage of the message that is missed is approximately the same as the percentage of time the speech is off. When the interruptions are periodic and occur more often than 10,000 times per second, the interruptions do not interfere with the reception of the message. In the quiet it is easy to understand conversational speech so long as the interruptions occur more than 10 times per second.

(2) When continuous speech waves are masked by noise that is interrupted more than 200 times per second, intelligibility is independent of the interruption frequency and of the percentage of time the noise is on, provided the ratio of average speech power to average noise power is held constant. Interrupted masking noise impairs intelligibility least if the frequency of interruption is about 15 per second.

(3) When interrupted speech and interrupted noise alternate at frequencies below 10 alternations per second, the noise does not impair intelligibility. At higher frequencies of alternation the temporal spread of masking becomes appreciable.

The general features of these results are approximately the same whether the interruptions occur periodically or at random.
Torsional Vibrations of Coupled Cylinders
Authors: Kilpatrick Myra F. , Kilpatrick John E.
Published: September 2005

Equations have been derived to give the torsional vibrational frequencies of a system consisting of two identical cylinders joined axially by means of a third cylinder. Such a system corresponds satisfactorily to the experimental set-up frequently used to determine the modulus of rigidity of a sample. The relation of the frequency spectrum to the various parameters of the system is discussed in detail. Two sets of experimental frequency data are compared with the results of the theoretical equations.
Sound in the Theatre. II
Authors: Burris-Meyer Harold , Mallory Vincent
Published: September 2005

The application of electronic facilities to the control of sound in the legitimate theater has been subject of a long term research project subsidized by the Rockefeller Foundation, the Research Corporation, and the Stevens Research Foundation. Complete control of the auditory component of the show was achieved immediately prior to the war. A modular sound control system has been developed, capable of filling all needs of the legitimate theater and specialized requirements of other entertainment forms.
The Determination of Reverberant Sound Absorption Coefficients from Acoustic Impedance Measurements
Authors: London Albert
Published: September 2005

A method is described for utilizing normal absorption coefficient or acoustic impedance measurements to predict reverberant sound absorption coefficients. The average of coefficients for the six standard frequencies determined from acoustic impedance measurements agrees closely with the average reverberant coefficient, for cases where the material may be said to obey the normal impedance assumption. The normal absorption coefficients of some 26 different acoustic materials were measured at 512 c.p.s Using the method given in the paper, the predicted reverberant coefficient deviated from the measured reverberant coefficient by 0.05 or less for 18 materials, while in only 3 cases were the deviations greater than 0.10. The method should be particularly applicable to the problem of acceptance testing of installed acoustic materials.

In the theoretical development, best agreement with experiment was obtained by introducing a new kind of reverberant statistics which associates with each wave packet in a random field a scalar quantity equal to the square of the absolute value of the sound pressure in each packet, instead of the customary energy flow treatment. Also, it was found necessary to carry out the analysis using a concept of equivalent real impedance to replace the usual complex impedance.
The Perception of Speech and Its Relation to Telephony
Authors: Fletcher Harvey , Galt Rogers H.
Published: September 2005

The intelligibility of the speech received over a communication system is usually expressed in terms of one or another measure such as the vowel or the consonant articulation, the average speech sound articulation, the syllable articulation, the word articulation, or the sentence intelligibility. The present paper establishes relationships among several of these measures and the articulation index. Relationships based upon statistical considerations are compared with the results of observations. Functions are developed which permit the calculation of articulation index and hence of articulation for communication systems which include a wide variety of response versus frequency characteristics and of noise conditions, as well as several special types of distortion. Although the treatment is predominantly empirical, the functions and processes are closely related to various fundamental properties of speech and hearing. Four principal series of articulation tests are cited in detail, some of which have been described in published articles by various persons. The response and the noise, if any, are given for each of these cases and the observed articulations are compared with values calculated by the method here presented. The application of the computational method to the perception of speech by deafened persons is reserved for a subsequent paper. A “Foreword” to the present paper describes the historical importance of articulation tests in the Bell Telephone System.
The Design and Characteristics of a High Sensitivity Direct Current Operated Accelerometer
Authors: Lewis Robert C.
Published: September 2005

Some of the more important mechanical design details of a newly developed accelerometer are described, together with their bearing on the dynamic characteristics of the unit. The relationship of acceleration range, sensitivity, and natural frequency for the design is defined, and its properties of selectivity and stability are discussed. Various types of coupling circuits in present use with high and low impedance loads are diagramed, and the possibility of greater zero stability by the use of a low level carrier is noted.
A Dynamical Theory of the Cochlea
Authors: Peterson L. C. , Bogert B. P.
Published: September 2005

A hydrodynamical theory is given for a model of the cochlea. The model consists of two channels of varying but equal cross sections. The channels are separated by an elastic membrane with variable dynamical constants. The two channels are interconnected at one end, and the entire structure is rigidly enclosed except for two accessible areas corresponding to the round and oval windows. The equations of motion, continuity including the effect of the membrane, and appropriate boundary conditions are formulated. As a first step toward a complete analysis the non-dissipative case is considered. Numerical solutions are found using experimental data obtained by G. v. Békésy. Localization phenomena and phase velocities are found to be in broad agreement with experimental data.
A Review of the Absorption Coefficient Problem
Authors: Sabine Hale J.
Published: September 2005

In 1939 a symposium was held by the Acoustical Society of America on the measurement of absorption coefficients. In these and other papers, the reverberation chamber method of determining absorption coefficient was analyzed, and explanations for discrepancies between different chambers, and lack of agreement with other methods, were presented in terms of newly developed theory. Since that time, however, practically no progress has been made in the application of this theoretical work. The discrepancies remain unresolved, and the practical problems resulting therefrom have become increasingly serious.

Within recent months, interest in this subject has been strongly reactivated through the attention of three groups, namely, the American Standards Association, the American Society for Testing Materials, and the Acoustical Materials Association. By closely coordinated efforts, these groups are making a new attempt to develop methods of measuring and rating the absorptive properties of acoustical materials which will yield consistent results in the form most useful for practical engineering work and at the same time are grounded in well-established acoustical theory.

In the present paper, the general problem is outlined from the viewpoint of the acoustical materials manufacturer. Directions of further development in theory and of experimental attacks are suggested.
Classical Viscosity in Tubes and Cavities of Large Dimensions
Authors: Bogert B. P.
Published: September 2005

A method for the calculation of viscous losses in acoustic wave guides and cavities is described, similar to that used by Carson, Mead, and Schelkunoff for electromagnetic waves. The loss in a plane wave in a round tube is discussed and the results agree with those previously obtained by others. The attenuation for two higher modes in hard wall guides is computed, as well as the decay constants for a cylindrical cavity for longitudinal and pure radial modes.
Influence of the Relative Radial Thickness of a Ring on Its Natural Frequencies
Authors: Buckens F.
Published: September 2005

The flexural vibrations of a ring in its plane are studied by taking shear effect into account, and it is proposed to compute the deviation from the classical frequency formula, which is only valid for very small ratios of the thickness to the diameter. In a second chapter the possibility of extension is included, and in this procedure the frequencies of extensional vibrations are obtained as a by-product.

It is seen that the shear effect plays a predominant role in the deviation from the classical formula, and is more important than curvature effects and the rotational inertia. Extensibility influence is found to be very small.

An interesting comparison can be made with the similar influence of secondary factors on the flexural vibration's frequencies of a straight beam.
Evaluation of the Magnetostrictive Properties of Hiperco
Authors: Sussman H. , Ehrlich S. L.
Published: September 2005

The magnetostrictive properties of Hiperco, a magnetic alloy of iron, cobalt, and chromium, were investigated to determine its suitability as a material for electromechanical transducers. The relations between the magnetic and magnetostrictive characteristics of a material and its performance as a transducer element are reviewed and criteria for its evaluation established. The properties of Hiperco, measured after a suitable heat treatment had been determined, are discussed in relation to these criteria and a comparison made with nickel and Permendur. For operation at magnetic remanence Hiperco is shown to be as good as Permendur for hydrophone applications, but inferior to nickel and Permendur when used as an underwater projector at high power levels. When polarized above remanence maximum value of 0.17 for the electromechanical coupling coefficient is realized and operation as a projector is improved. Performance data are given for a small underwater transducer constructed of Hiperco operating at remanence.
On the Non-Specular Reflection of Plane Waves of Sound
Authors: Twersky Victor
Published: September 2005

The non-specular or scattered reflection of plane waves of sound by various rigid, non-absorbent non-porous surfaces composed of either semicylindrical or hemispherical bosses (protuberances) on an infinite plane is analyzed. Exact solutions for the problem of the single boss and a plane wave at an arbitrary angle of incidence are derived through consideration of a cylinder or sphere and two simultaneously incident “image waves.” Finite patterned distributions of such bosses are then treated and the far field solution obtained subject to the restriction that the secondary excitations of the various bosses be neglected. (The equivalent problems for cylinders and spheres are also considered as well as the second-order solution, for the cylindrical case, which takes into account the interaction of neighboring elements.) These solutions are found to contain the characteristic Fraunhofer terms for a grating or lattice. The asymptotic solutions for the single bosses (Kr≫1, Ka<1) are then extended to consider both finite and infinite uniform random distributions. The solutions for the finite distributions are found to contain the characteristic Fraunhofer terms for similarly shaped apertures. The solutions for the infinite distributions (of semicylinders or hemispheres) are found to be remarkably similar when expressed in terms of the volumetric departure from the plane per cm2 of distribution. Some extensions and ramifications of the results are also considered.
Short-Time Autocorrelation Functions and Power Spectra
Authors: Fano R. M.
Published: September 2005

The reciprocal relations between autocorrelation functions and power spectra, known as Wiener's Theorem, axe extended in a modified form to the case of experimental results obtained by means of filters with finite time constants. If the short-time autocorrelation function φt(τ) and power spectrum Gt(ω) are properly defined, it is found that φt(τ)=eα|τ|2π ∫ −∞∞Gt(ω)cosωτ dωGt(ω)= ∫ −∞∞e−α|τ|φt(τ)cosωτ dτ where 1/α is a time constant. These equations may be used to relate the autocorrelation-function representation of a speech wave to the corresponding spectrographic representation.
On the Acoustics of Coupled Rooms
Authors: Harris Cyril M. , Feshbach Herman
Published: September 2005

In many acoustically coupled systems the methods of geometrical acoustics do not apply. Reverberation formulas as ordinarily used would lead to incorrect results. This paper approaches the problem of coupled rooms from the “wave” point of view, treating the coupled rooms as a boundary value problem in obtaining an approximate solution. The results explain some discrepancies noted by earlier researchers between experiment and predictions from geometrical acoustics; for example, the dependence of absorption in a room on the position of the open area which couples the room to an adjacent one. For the case where the window area which couples one room to another is comparable in size with the partition which separates the rooms, the effect of the partition will be least when it is at a particle-velocity node. For the case where the window area is small compared with the partition which separates the two rooms, the effect of the coupling window depends on the square of the unperturbed pressure at the window. Thus the effect of the window varies with position and is least at a pressure node. Experimental data on isolated modes of vibration of a coupled system are given which check the results predicted by this application of the wave theory.
The Calculation of Vowel Resonances, and an Electrical Vocal Tract
Authors: Dunn H. K.
Published: September 2005

By treating the vocal tract as a series of cylindrical sections, or acoustic lines, it is possible to use transmission line theory in finding the resonances. With constants uniformly distributed along each section, resonances appear as modes of vibration of the tract taken as a whole. Thus, the fundamental mode of the smaller cavity may be affected considerably by a higher mode of the larger; and in addition, higher resonances are found without postulating additional cavities. This is an advantage over the lumped constant treatment, where it is necessary to postulate a different cavity for each resonance, and where the interaction terms in the equation do not include the higher modes of vibration. Under the distributed treatment, dimensions for each vowel may be taken from x-ray photographs of the vocal tract. The calculations then yield at least three resonances which lie in the frequency regions known for the vowel, from analyses of normal speech. Dependence of the different resonances upon the different cavities is discussed in some detail in the paper.

An electrical circuit based on the transmission line analogy has been made to produce acceptable vowel sounds. This circuit is useful in confirming the general theory and in research on the phonetic effects of articulator movements. The possibility of using such a circuit as a phonetic standard for vowel sounds is discussed.
Changes in Pitch of Tones of Low Frequency as a Function of the Pattern of Excitation Produced by a Band of Noise
Authors: Egan James P. , Meyer Donald R.
Published: September 2005

The excitation pattern of a tone is markedly altered by a masking stimulus. It should be expected that the pitch of the masked tone, as well as its threshold and its loudness, would be affected. In the present investigation the effect of a narrow band of noise (90 c.p.s. wide, centered at 410 c.p.s.) upon the pitch of pure tones of low frequency was measured in a series of experiments. Subjects matched the pitch of a tone heard in the presence of a band of noise to the pitch of a tone heard alone. These two tones, one partially masked and the other not, were presented to the subject in repeated succession. The listener first matched these tones for loudness. He then adjusted the frequency of the unmasked tone until it was the same in pitch as the masked tone. The pitch change was investigated at a number of frequencies and of loudness levels.

Systematic changes in pitch were observed. The presence of the noise raises the pitch of a partially masked tone whose frequency is immediately above those of the band of noise. This change in pitch occurs even though the intensity of the masked tone is greater than that of the unmasked tone. In contrast, the presence of the masking noise lowers the pitch of a tone whose frequency is just lower than those of the band of noise, but this change is largely accounted for in terms of the intensity difference required for an equal loudness match. An attempt is made to account for the changes in pitch in terms of a place theory.
Boundary Layer Attenuation of Higher Order Modes in Rectangular and Circular Tubes
Authors: Beatty Ralph E.
Published: September 2005

The boundary layer attenuation caused by viscous and thermal losses of all higher order modes of propagation in rigid walled rectangular and circular tubes has been calculated using the concept of boundary layer admittance introduced by Cremer. In the case of the rectangular tube Cremer's admittance formula for obliquely incident waves is applied directly in conjunction with Morse's duct formulas to obtain the attenuation of (ny, 0) and (ny, n2) modes. The boundary layer admittance of higher order modes in circular tubes is obtained from an extension of Cremer's method for plane waves. An electromagnetic wave-guide technique for calculating “skin” losses is applied to determine the attenuation of the asymmetric (m, n) circular tube modes. It is shown that the symmetric (0, n) circular tube modes are the least attenuated of all higher order modes in both rectangular and circular tubes; above a certain frequency the attenuation is less than the plane wave attenuation.
On the Diffraction of a Plane Sound Wave by a Paraboloid of Revolution
Authors: Horton C. W. , Karal F. C.
Published: September 2005

The problem of the diffraction of a plane wave by the convex surface of a paraboloid of revolution is solved. An infinite set of equations is obtained for the coefficients of the pressure fields inside of and outside of the paraboloid. For the particular case of a rigid paraboloid the coefficients of the series for the scattered field can be obtained explicitly. The ratio of the total pressure at the nose of the paraboloid to the pressure of the incident wave is shown as a function of the shape of the paraboloid for a plane wave whose wave front is perpendicular to the axis of revolution. The same quantity is plotted for the case of a sphere whose radius of curvature is equal to that of the paraboloid at the nose.
Volume Viscosity and Compressibilities from Acoustic Phenomena
Authors: Lu Hoff
Published: September 2005

A phenomenological theory of volume viscoelasticity formulated, resulting in an equation recently used by Hall. Application of this equation to the dispersion and absorption of sound in fluids is extended to the whole range of frequencies. Results of calculations of the volume viscosity and the instantaneous and relaxational compressibilities for gases and liquids from certain available absorption data are given. The bearing of this theory on the classical theory of hydrodynamics is pointed out.
Some Effects of Vibration on X-Ray Diffraction by Crystals
Authors: White J. E.
Published: September 2005

A technique is described which makes use of Bragg reflection of x-rays to give a “picture” of the vibration pattern on the face of a crystalline plate. The dependence of this pattern on the direction of the x-ray beam is illustrated, and a comparison between dust patterns and x-ray patterns for a vibrating bar fairly well establishes that the local curvature of the crystal is responsible for the observed increase in x-ray reflection. Reference is made to quantitative measurements on statically bent quartz and to an approximate theory treating the effect.
Acoustic Filters as Ear Defenders
Authors: Zwislocki J.
Published: September 2005

Ear wardens in form of ear plugs acting as acoustic low pass filters are described. They allow a part of speech frequencies to pass through into the ear and in this way improve the speech intelligibility in comparison with other ear plugs. Nevertheless their protective action against noise was proved to be sufficient.
The Opening Time and Opening-Quotient of the Vocal Cords During Phonation
Authors: Tarnóczy T. H.
Published: September 2005

The vowel sounds are produced by vibration of the vocal cords and are modified by the filtering effect of the resonant cavities. Thus, in the case of voiced speech sounds and of the sounds of singing, two factors play a part in the final shaping of the sound picture: the cord-tones and the resonators. The present paper deals with the cord-tones and with the peculiarities of vibration of the vocal cords.
Spectra and Loudnesses of Modern Automobile Horns
Authors: Callaway D. B.
Published: September 2005

As one phase of a noise survey of the Chicago area, measurements were made of the acoustic output of commonly used automobile horns. Over-all sound levels on the axis at three feet ranged from 108 to 125 db. Loudness levels ranged from 125 to 140 phons. Fundamental frequencies of all horns ranged between 160 and 380 cps. Two types had approximately harmonic overtones, with large amplitudes below about 2000 cps and smoothly decreasing amplitudes at higher frequencies. A third type, with the most unpleasant sound, had inharmonic overtones, groups of which were greater in amplitude than the fundamental. Sound from a pair of horns at various distances was measured both inside and outside a closed automobile. The over-all level outside was 88 db at 50 feet and 74 db at 300 feet, with corresponding loudness levels of 105 and 82 phons. The over-all level inside was 60 db (loudness level, 72 phons) at 50 feet and 50 db (loudness level, 54 phons) at 300 feet. Filtering out overtones above 1200 cps improved the quality of horn sounds markedly, reducing the loudness level inside the automobile by four phons at 50 feet, but only one phon at 300 feet. Since the typical horn is louder than necessary at close range, use of a low pass acoustic filter on automobile horns appears desirable.
Ultrasonic Absorption in Liquids
Authors: Moen C. J.
Published: September 2005

A reverberation method has been used to measure the absorption of sound at frequencies down to 140 kc in a number of liquids of widely different properties. For glycerol the experimental result agrees well with that predicted by classical theory. For benzene and water good agreement is found with the values calculated using the modified theory employing the values of second viscosity determined by Liebermann. The absorption in carbon disulfide agrees well with Ouang's result. For these four liquids the absorption is found to be proportional to the square of the frequency, but results indicate that toluene has a relaxation frequency of the order of 105 sec−1.

The reverberation method is especially useful at the lower ultrasonic frequencies, where loner wave-lengths and lower absorption render methods used at higher frequencies less reliable or unusable.
Ultrasonic Absorption Measurements in Aqueous Solutions of Magnesium Sulfate
Authors: Smith M. C. , Barrett R. E. , Beyer R. T.
Published: September 2005

Experimental values are reported for the sound absorption coefficients in aqueous solutions of magnesium sulfate over the frequency range 3 to 43 megacycles. Measurements were made by an electrical method at the lowest frequency and by the pressure balance method at higher frequencies (12–43 mc). Absorption coefficients are reported as a function of frequency, concentration, and temperature. An excess absorption (relative to water) is found at the higher frequencies which cannot be interpreted by the present theory.
Effects of Reflected Signals and Electric Pick-Up at an Ultrasonic Microphone
Authors: Williams A. O. , Keck Winfield
Published: September 2005

When a small microphone is used in an ultrasonic beam, the direct acoustic signal is mixed with electric pick-up and reflected signals. The use of pulses may not entirely remove these interferences. The mixing process is analyzed here for a particular situation and experimental evidence is adduced to support these conclusions: (1) that the main response pattern of the microphone shows space variations of both half the acoustic wavelength and the full wavelength; (2) that the former variation dies out at sufficiently great distances from the source at a rate showing that a single echo path (source-microphone-source-microphone) is responsible along with a constant electric signal; and (3) that the three signals can be resolved analytically.
A New Expansion for the Velocity Potential of a Piston Source
Authors: Carter A. H. , Williams A. O.
Published: September 2005

The Rayleigh surface integral, giving the velocity potential for a plane piston source surrounded by an infinite rigid flange, reduces to a line integral when the coordinates are suitably chosen. As shown by Schoch, for points within the geometrical cylinder whose base is formed by the piston surface, the line integral is expressible as a plane wave term plus a “perturbation” integral. For external points, a different integral results. In the present work, these two complementary expressions are evaluated for a circular piston, as series of half-integral order Hankel functions in kz and polynomials in x/a; k is the propagation constant, a the piston radius, z the axial and x the radial coordinate of a field point. The resulting rigorous equation (valid for points not on the piston surface) converges for any value of ka, provided z>a. For large values of kz, where asymptotic formulas apply, the expression assumes a particularly simple form. Sample calculations have been made for ka = 10, z = 10a and ka = 50, z = 50a. Also, an approximate expansion has been derived which may be more useful than the rigorous result in paraxial regions.
On the Acoustical Radiation of an Emitter Vibrating Freely or in a Wall of Finite Dimensions
Authors: Pachner Jaroslav
Published: September 2005

An acoustical radiation field excited by an emitter vibrating freely or in a wall of finite dimensions is considered as superposed by two fields. The first of these is that one where the same emitter is vibrating in an infinite wall and the second is that which (i) causes the resultant field in the free part of the plane going through the wall to vanish, and (ii) has a normal derivative which vanishes on the surface of the emitter and of the wall. While the first field may be considered as known from other papers, the second is computed from an integro-differential equation that follows from Rayleigh's formula. The equations expressing the velocity potential distribution and those deduced from it are written in an abstract form by means of the Dirac bra-vectors, ket-vectors, and linear operators represented by the corresponding matrices. This method of solving the given special diffraction problem of a scalar wave may be used for any mode of vibrations of the emitter and for any shape of the wall, but the computation becomes far easier if the wall is circular. It may be applied also for any wavelength of the radiated sound, but the functions expressing the dependence on the azimuthal angle and containing the Legendre associated functions of the first kind converge faster, the longer the wavelength in comparison with the dimensions of the wall. Numerical calculations are not given. They will be adequate for the difficulty of the problem, i.e., very tedious; but they can be done, especially with the help of modern electronic calculating machines.
The Error Due to Masking in the Measurement of Aural Harmonics by the Method of Best Beats
Authors: Egan James P. , Klumpp Roy G.
Published: September 2005

The method of best beats has been employed to estimate the intensities of aural harmonics and of combination tones. It has been generally assumed that the listener hears best beats when the exploring tone produces in the cochlea a disturbance that is equal in magnitude to that of the aural harmonic or of the combination tone being measured. However, as the experiments to be reported will show, when the tone to be measured is near the absolute threshold or is partially masked, the most prominent beats will be heard when the intensity of the exploring tone exceeds that of the unknown tone. Consequently, since aural harmonics and combination tones are partially masked, their intensities will be overestimated when the method of best beats is used. The present paper concerns the magnitude of this error of overestimation.

A procedure is presented by which a better estimate of the intensity of an aural harmonic or of a combination tone may be made. When the tone being measured by the method of best beats is well above threshold, the listener hears beats over a wide range of intensities of the exploring tone, and the error of over-estimation is small. However, when the tone being measured is near threshold, there is a small range of audible beats, and the error of overestimation is of considerable magnitude. It is therefore possible to correct for this error by determining the range of intensities of the exploring tone over which beats are audible as well as the intensity of the exploring tone required for best beats. Application of this procedure to the measurement of a second aural harmonic is illustrated.

An explanation is given of the fact that the intensity of a tone near its absolute threshold will be overestimated by use of the method of best beats. This explanation is formulated in terms of the relation of the minimum and the maximum of the envelope of the beating complex to the listener's threshold for the tones that beat.
A Strain-Gauge Type Artificial Mastoid
Authors: Carlisle R. W. , Pearson H. A.
Published: September 2005

An artificial mastoid for testing bone conduction receivers is described. It consists essentially of a stiff metal bar which has a fundamental resonant frequency above the measurement range, with a strain-sensitive translating element fastened to the under side and a compliant pad on top.

The translating element is a slab of activated ceramic, which is essentially invariant to humidity and temperature changes. The compliant pad generally used is Koroseal No. 74, which simulates the flesh over the mastoid prominence reasonably well. Koroseal No. 15 simulates flesh better but is less rugged. Calibration methods are discussed.
The Wavelength of a Spherical Resonator with a Circular Aperture
Authors: Levine Harold
Published: September 2005

The fundamental wavelength of a spherical resonator with a circular aperture is calculated. The result, λ/2π = R(2πR/3a)12(1−0.14324a/R+0.08455(a/R)2), where a denotes the aperture and R the sphere radius, is the correct form of an expansion as far as terms of relative order (a/R)2, inclusive. A procedure for approximating to the wavelength of a resonator with arbitrary shape is also described.
Sound Scattering of a Plane Wave from a Nonabsorbing Sphere
Authors: Hart Robert W.
Published: September 2005

Sound scattering from a sphere of arbitrary size is treated theoretically when the acoustic properties of the sphere are near those of the surrounding medium. Closed form analytic expressions are found for the reflectivity and total cross section. These expressions become exact only in the limit as the ratio of the densities and ratio of the speeds approach unity, but it is shown that for ratios as large as 5/4 and probably as large as 3/2 the approximate reflectivity and total cross section compare favorably with results of exact calculation. Calculations of reflectivity and total cross section, which would require weeks if made from the exact solution of the wave equation, may be completed in a few hours using the approximate closed form expressions.
On the Reflection of a Spherical Sound Wave from an Infinite Plane
Authors: Ingard Uno
Published: September 2005

The reflection of a spherical sound wave from a wall with the boundary conditions expressed in terms of a normal impedance independent of the angle of incidence is treated. It is shown that the integral for the reflected wave can be evaluated exactly in closed form under certain conditions. The solution given for an arbitrary normal impedance involves a slight approximation of the integral. The reflected wave is brought into a form such that it can be considered originating from an “image source” having a certain amplitude and phase. Graphs for determining this amplitude and phase are given in terms of a “numerical distance,” which depends on the normal impedance and the position of the field point. Pressure distributions around point sources for different wall impedances are shown. The limitations in simulating plane wave conditions at a boundary and the corresponding effect on free field methods of measuring acoustic impedance are discussed.
Absorption of Sound by Resonant Panels
Authors: Sacerdote Gino G. , Gigli Antonio
Published: September 2005

Report on research of resonant absorbers; the experiments have been carried out with sound waves by normal incidence on the panel, and by random sound. The measured values are compared with the theoretical ones.
The Propagation of Sound in Carbon Dioxide Near the Critical Point
Authors: Anderson N. S. , Delsasso L. P.
Published: September 2005

The expression for the normal velocity of sound in a fluid becomes indeterminate at the critical point since the coefficient of elasticity tends to zero and the specific heat at constant pressure approaches infinity. An expression has been obtained for the isentropic slope which does not become indeterminate at the critical point. Observations were made with a Pierce type variable path acoustic interferometer placed completely inside a pressure tank with provisions for external control. The isothermal variation of velocity and absorption was measured as a function of pressure at 572 kc in carbon dioxide. Above the critical temperature it was found that, as the pressure increases, the velocity decreases steadily, goes through a sharp minimum, and rises abruptly. Below the critical temperature a discontinuity in velocity was observed between liquid and vapor. The velocity in the liquid at 29°C was 208 meters/second, and in the vapor 179 meters/second. The velocity variations away from the critical point agree with previously published results. The attenuation decreases steadily with increasing pressure and shows an abrupt rise as the critical point is reached, which would be expected from critical fluctuation scattering. Using isotherm data two values of the velocity at the critical point were calculated, 154 and 146 meters/second. The measured value was found to be 141.6 meters/second.
A Method for Determining the Propagation Constants of Plastics at Ultrasonic Frequencies
Authors: McSkimin H. J.
Published: September 2005

A pulse technique particularly suited to dissipative materials is described for measuring attenuation and phase shift constants of plastics, using either transverse or longitudinal waves in the frequency range of 5–50 Mc.

A thin wafer of the material under test is placed between two identical fused silica buffers; and waves generated by quartz crystals at the ends of the assembly are transmitted simultaneously through the specimen in both directions. Comparison of transmitted and reflected components by means of a special balancing circuit provides information from which the complex propagation constant can be calculated, and hence dynamic rigidities and viscosities.

Illustrative data for polyethylene and Nylon are given.
The Influence of Interaural Phase on Masked Thresholds I. The Role of Interaural Time-Deviation
Authors: Webster Frederic A.
Published: September 2005

When a narrow band of noise, of constant level (the signal) is masked by a wider band of noise (the masker), the amount of noise required to drown out the signal often depends significantly upon the interaural phase relations of signal and masker. Thus, more masking noise is required if the signal is 180° out of phase at the two ears than if it is in phase: provided that the masker remains in phase. The out-of-phase signal, in other words, shows an advantage in audibility. The advantage is very striking for a signal near 250 cps; and falls off progressively toward 1400 cps, above which it is small. At all frequencies between 250 and 6000 cps, the level of masking, for a signal in phase at the ears, remains relatively constant.

An important property of physical stimuli that produce an audibility-advantage is an interaural phase-deviation caused by the signal. When the signal is added to a corresponding masking component, the combined pressure-wave, on the average, leads in one ear and lags in the other. The resulting phase-deviation depends upon the relative levels and phase-positions of the masking and masked components. It is maximum when these components are 90° apart. The interaural time-deviation thus produced depends also upon frequency. If the frequency is increased, the level of the masking component must be decreased in order to hold the time-deviation, associated with the different frequencies, constant.

The present experiments showed that, with the out-of-phase signal, the decline in level of masking, as a function of frequency, is just such as to hold the interaural time-deviation (for the 90° position) constant at about 125 microseconds. Tonal signals tend to show a decreasing critical time-deviation with increasing frequency.
Dispersion of Velocity and Anomalous Absorption of Ultrasonics in Nitrogen
Authors: Zmuda Alfred Joseph
Published: September 2005

By means of the interferometer, the velocity and absorption of ultrasonic waves in nitrogen were measured at the frequency of 2.992 mc/sec, in the pressure range of 2.09 to 76 cm of Hg at a temperature of 29°C. A dispersion of velocity was found ranging from 354.3 meters/sec to 364.4 meters/sec. The ratio αexp/αclass dropped from 1.40 to 1.32, and the corresponding value of Cv/R dropped from 2.50 to 2.08.

Theoretical values for the change in velocity and in the absorption ratio, calculated by applying the equations for the exchange of energy between translational and vibrational degrees of freedom, show good agreement with the observed values. The increase in velocity and the decrease in αexp/αclass is interpreted as due to the slow exchange of energy between the translational and rotational degrees of freedom. The relaxation time for rotation was found to be 1.2×10−9 second.
Measurements on an Acoustic Wave Propagated Along a Boundary
Authors: Lawhead R. B. , Rudnick I.
Published: September 2005

The sound field of a point source located at a plane boundary was measured both with respect to its amplitude and phase characteristics. This was done for a very high impedance boundary and one composed of material which is relatively absorbing. The fields were calculated, the calculations for the latter material being based on measured values of impedance and propagation constant, and found to be in good agreement with those measured.
The Effect of Temperature Inhomogeneities in the Ocean on the Propagation of Sound
Authors: Liebermann Leonard
Published: September 2005

Horizontal small-scale temperature inhomogeneities in the ocean have been measured with the aid of a highly sensitive, rapid-response thermometer mounted on a submarine. The mean size of these inhomogeneities is of the order of 60 cm; a typical average temperature variation is 0.05°C. The physical features of the inhomogeneities which determine acoustical effects are shown to be described by the autocorrelation function; a specially constructed computing machine was used to calculate this function from the temperature records. The calculated backward scattering (reverberation) resulting from these inhomogeneities is given as a function of frequency. In the region 10 to 100 kc the calculated scattering is, in general, less than that which is actually observed in the ocean, suggesting that biological material is the major source of scattering in the ocean. A calculation of the attenuation of transmitted sound in the ocean resulting from scattering shows that this effect is small compared with absorption at the higher frequencies. Refractive effects of the inhomogeneities result in another acoustical phenomenon: local warping of the wave fronts produces intensity enhancement or diminution at the receiver. These effects quantitatively explain rapid acoustic fluctuations (scintillations) which are observed in the ocean.
On the Threshold and Loudness of Repeated Bursts of Noise
Authors: Pollack Irwin
Published: September 2005

The loudness and threshold of an interrupted white noise (of constant sound-time fraction) was studied over a wide range of interruption frequencies. White noise has the useful property that, when interrupted, no spectral changes result in the white noise spectrum (in the frequency range passed by a dynamic earphone). Both at the threshold and at equal-loudness, less energy is needed for an interrupted noise than for a continuous noise. In many cases, an interrupted noise (sound-time fraction of 0.45) sounds louder than a continuous noise of the same burst amplitude (but of greater energy). There is a broad minimum, in the intensity required at equal loudness, for interruption rates between 2–10 per second. A conceptual formulation to encompass the results is presented.
Sensitivity to Differences in Intensity between Repeated Bursts of Noise
Authors: Pollack Irwin
Published: September 2005

Thresholds for the detection of a decrement in noise intensity between repeated bursts of noise were determined as a function of the duration of the interval between successive bursts. The results indicate a critical duration (55 milliseconds) between successive noise bursts: (1) above which, the differential threshold is constant and independent of the interval between successive bursts and (2) below which, the differential threshold increases proportionately as the interval between successive bursts decreases. Since an equivalent critical interval has previously been obtained by several different independent measures of auditory persistence, the observed deterioration of differential sensitivity is interpreted in terms of the overlap or addition of auditory persistence with the direct effects of stimulation.