bluuebunny/crossref_metadata_2025 huggingface dataset:
The Measurement of Flow Resistance of Porous Acoustic Materials
Authors: Brown Richard L. , Bolt Richard H.
Published: September 2005

The flow resistance is one of the three physical parameters of a uniform, rigid, porous material which completely determine its acoustic behavior; for more complicated materials and sound absorbing structures the flow resistance gives at least some acoustic information. The specific flow resistivity or flow resistance per unit cube is given by the equation r = pAt/Vt, in which p is the pressure drop in dynes per cm2 across the thickness l cm of a sample of area A cm2 when a volume V cc of air flows steadily through the sample in a time t sec. An accurate method has been developed for obtaining r by direct measurements of the quantities p, A, t, V, and l. Using this method flow resistance measurements have been made on twenty-four materials over a wide range of velocities. The measurements show that for nearly all materials there is a variation of flow resistance with velocity. For most materials the resistivity is constant below 0.1 cm per sec. but increases considerably at higher velocities. It is shown that previously described comparison methods, which have the advantages of simplicity and speed, might yield results in serious error unless certain suggested precautions are observed. As indicated previously, a direct correlation is found between the steady state flow resistance values and the resistive component of acoustic impedance for several materials. Analysis of the data also yields additional information bearing on the basic physical processes involved in the absorption of sound in these materials. In addition, the measurement of flow resistance has practical applications, such as the rapid routine checking of uniformity of samples of porous materials.
Acoustic Filtration in Parallel Conduit Structures
Authors: Labaw L. W.
Published: September 2005

The filtration characteristics of five Quincke-Herschel tubes in series were measured in three ways: (a) by observing the ratio of the squares of the excess pressures at the end and beginning of the filter when terminated by a closed and open finite ending; (b) by observing the phase difference between these two pressures for the two cases in (a); and (c) by observing the true power transmission ratio with the filter terminated by an effectively infinite line. The experimental results are plotted with the corresponding theoretical curves. The number of experimental peaks in the transmission regions in (a) and (c), and the positions of the attenuation bands agree with the predictions of the theory. The values of the curve minima in the transmission bands are in excellent agreement up to 5000 cycles. The change of phase over the filter was also found to correspond closely to the theory.
Acoustic Filtration by Membranes
Authors: Labaw L. W.
Published: September 2005

This paper presents an experimental and theoretical study of the filtering action of a structure consisting of five stretched circular membranes equally spaced in an acoustic conduit, together with an experimental study of the effect on the transmission of puncturing the membranes with different sized holes. The stretched membrane filter is high pass, while the punctured orifices of sufficient diameter change the structure to a low pass type. This study shows that it is valid to assume plane wave propagation in the circular membrane filter up to 5000 cycles.
Sound Power Density Fields
Authors: Enns J. H. , Firestone F. A.
Published: September 2005

A graphical presentation of the streaming of acoustical energy near vibrating radiators and reflecting surfaces has been developed. The sound power density field from a simple source, a doublet source, a single point source before a totally reflecting plane (free as well as rigid), and a piston vibrating in an infinite fixed wall is considered analytically. The starting point of all computations is taken as the velocity potential resulting from the particular radiating source. Formulae are worked out to allow the computation of percent of total radiation flowing through a certain area. By interpolation of the graphs from these equations, the streaming or flow of energy for the several radiation fields is shown.
Edge Tones
Authors: Jones Arthur Taber
Published: September 2005

Edge tones are produced when wind from a slit strikes an opposed edge under suitable conditions. Two types of such tones are here distinguished. The first type is the one that has hitherto been most investigated. In it a gradual change of either wind pressure or distance from slit to edge gives several “stages” of tone, in each of which the pitch changes gradually, separated by sudden “jumps” from one pitch to another. The second type occurs at higher pressures, where the sheet of air that escapes from the slit is probably turbulent, and in this type the jumps in pitch are absent. In the first type the ranges of pitch between jumps are never much in excess of an octave, whereas in the second type there may be a continuous range over several octaves. Most of the tones now investigated were obtained with a brass wedge above a slit 0.7–0.8 mm wide. The wind escaped from the slit at velocities up to about 50 m/sec. The higher pitched tones were not studied, but only those running up to about 1900∼/sec. Tones of the first type are less simple than has usually been indicated, and they are described. Tones of the second type become practically simple as they fall toward inaudibility. A number of equations are tested to see which expresses best the relation between frequency N, slit-edge distance h, and the velocity U with which the wind escapes from the slit. It is concluded that the most satisfactory equation is Nh8 = jU, where s has, respectively, the values 1.00, 1.14, 1.22, 1.43, and j the values 3.9, 11.8, 24.0, 6.8, for stage I, stage II, stage III, and the second type of edge tones. In this equation h is in mm, and U in cm/sec. Various edge tone phenomena are given qualitative explanation on the basis of the pressure caused alternately on the two sides of the wedge by the injected air, together with the Bernoulli effect on the transverse velocity in the constriction between slit and edge. The velocity of the wind from the windway of one c2 organ pipe is measured, and found to be about 26 m/sec.
Propagation of Sound Waves in the Atmosphere
Authors: Gutenberg B.
Published: September 2005

The effect of humidity on the velocity of sound waves is investigated (Eq. (5), Table I). The radius of curvature for a ray of sound which is propagated in the direction of the wind is given [Eq. (13)] and discussed. The amplitudes of sound waves as a function of the distance are given [Eq. (17)], and the relative importance of the quantities involved is discussed.
Improved Low Frequency Horn
Authors: Klipsch Paul W.
Published: September 2005

Last fall this writer described two designs for a low frequency horn adapted to operate in the corner of a room. The data presented consisted of experimental and computed performance on an experimental model and the computed performance of an improved design. The present paper describes the measured impedance of the new design and shows that this new design is a practical answer to the requirements of a speaker of this type. Minor discrepancies between the computed and measured performance are explained. Such discrepancies as exist are more of academic than practical interest. The improved design possesses all the characteristics which are desirable in a device of this class. This speaker, which is about the size of an ordinary radio console, has good response down to 40 cycles and an average efficiency from 40 to 400 cycles of about 30 percent, and a maximum variation of efficiency in this frequency range of only 2 decibels. Its normal environment, a room corner, utilizes room space which is ordinarily wasted. Observers have commented favorably upon both the artistic use of the space in the corner of the room and the acoustic performance of the instrument. With its high efficiency it is suitable for large scale sound reproduction.
Applications and Limitations of Mechanical-Electrical Analogies, New and Old
Authors: Miles John
Published: September 2005

The general problem of mechanical-electrical analogies is discussed. The advantages of Firestone's mobility system over the older analogy is clearly demonstrated especially when used with the powerful methods of circuit analysis presented. Torsional and geared, as well as linear systems are discussed. It is shown that the choice of analogy to be used is usually one of convenience, but that certain systems intrinsically make only one analogy possible. It is further shown that the use of a mechanical-electrical analogy in a system involving both electromagnetic and electrostatic coupling is fundamentally impossible as the resulting circuit could not satisfy Maxwell's equations.
Filtered Thermal Noise—Fluctuation of Energy as a Function of Interval Length
Authors: Rice S. O.
Published: September 2005

Let a source of thermal noise be connected to the input of a band pass filter. Consider the energy which would be dissipated during the interval t1 to t1 + T if the output current were to flow through a resistance of one ohm. When T is held fixed and t1 regarded as a random variable, the resulting energies have a distribution whose average and standard deviation depend upon T. Here this dependence is studied. The standard deviation of the difference of the energies of two contiguous intervals, each of length T, is also obtained.
An Experimental Study of the Tone Quality of the Boehm Clarinet
Authors: McGinnis C. S. , Hawkins H. , Sher N.
Published: September 2005

Several theories of clarinet action have been developed, but have been based on rather fragmentary experimental data. The present paper offers such a description of the instrument as is essential to this particular study, and a consideration of the tone quality at the bell of the instrument in the three conventional registers. Special consideration is given to the harmonic structure of the tones and its variation throughout the pitch range. All pianissimo tones are extremely simple, consisting almost entirely of the fundamental. For this reason, the investigation of only the forte tones is necessary for the study of harmonic structure, since the mezzoforte tones represent a transitional stage. The harmonic structure simplifies in progressing from the lowest to the highest pitch with the extremes representing remarkable contrast. In the whole range of forty tones there are only five, blown forte, in which the percent of total energy in the fundamental exceeds fifty percent. The tone B4, which has the maximum energy, 88.2 percent, in the fundamental and no harmonics of comparable intensity, is the orchestral tuning note. The characteristic tone quality of the clarinet, often described as reedy or hollow, and most evident in the low register, appears to be due to the relatively strong odd numbered partial tones and to be determined mostly by the first eight partials.
Observations on the Vibrations of Piano Strings
Authors: Schuck O. H. , Young R. W.
Published: September 2005

The behavior of single piano strings struck in their normal positions was investigated through measurement of the frequencies of the partials and their amplitudes as functions of time after striking. Three-dimensional figures showing partial amplitudes versus time are presented. The frequencies of the partials of the tone were found to be those of the normal modes of vibration and are therefore referred to as the modal frequencies of the string. The higher modal frequencies are progressively sharp with respect to true harmonics of the fundamental; the sharpening follows approximately a square law with respect to mode number, to the extent that in one case the 15th partial had just about the frequency of a 16th harmonic. The resulting inharmonicity in piano strings is least in the two octaves below middle C; it rises sharply at the high end and more or less sharply at the low end, depending upon the type of piano. A correlation between inharmonicity and subjective tone quality rating is suggested. It is shown that the commonly observed “stretched” tuning (sharpening of the treble, flattening the bass) is a natural consequence of the inharmonicity.
Supersonic Measurements in CO2 and H2O at 98°C
Authors: Pielemeier W. H. , Byers W. H.
Published: September 2005

Measurements with CO2 at 98°C were made with the hope that the minor absorption peaks found earlier at 28°C might be made more prominent. At the higher temperature the CO2 holds more water vapor thus furnishing a longer range of relaxation times. The control and measurement of temperature and humidity were improved. Otherwise the procedure was that used at 28°C. The apparent change in relaxation times from 28°C to 98°C was not entirely the expected one and the minor absorption peaks were not as prominent as we hoped they would be. The velocity at high frequency and the change in velocity through the dispersion region and also the total absorption were found approximately as expected. The specific heat of CO2 calculated from our results agrees much better with the values calculated from spectroscopic data and with the most reliable calorimetric results than do the values obtained by some earlier investigators who used the velocity or the absorption of sound waves.
An Improved Telephone Receiver Analysis
Authors: Fay R. D.
Published: September 2005

The method of determining the performance of electromagnetic telephone receivers from input measurements has been extended and improved. It is shown that not only may an actual receiver at any one frequency be correctly represented by a simple piston-phone but also the the position of the equivalent piston may be chosen arbitrarily, say at the mouth of the receiver. The characteristics of the equivalent piston-phone for all positions of the piston are easily found graphically from a single, simple diagram constructed from data which are readily obtained from a few input measurements. The accuracy of the method has been tested experimentally by making independent measurements at the output end of the receiver. The results check closely.
The Damping of Large Amplitude Vibrations of a Fluid in a Pipe
Authors: Binder R. C.
Published: September 2005

Helmholtz and Kirchhoff developed a theory for the damping in space of the small amplitude vibrations of a fluid in a rigid cylindrical tube. Their work assumes laminar motion, and gives values which are very low for moderate and large pressure amplitudes. The present paper modifies the Helmholtz-Kirchhoff theory by the addition of an eddy viscosity term. This modified theory gives fair agreement with measured values of the damping of large pressure amplitudes for air and carbon dioxide.
The Reflection of Sound due to a Change in Cross Section of a Circular Tube
Authors: Miles John
Published: September 2005

The reflection and transmission coefficients due to a change in cross section of a circular tube are determined by calculating the exact pressure distribution in the vicinity of the discontinuity. The fluid medium in the tube is assumed to be ideal in the sense that the pressure distribution satisfies the wave equation. In contrast to the simpler theory which assumes a tube which is small compared to the wave-length, the following theory is subjected only to the assumption of an ideal fluid.
Supersonic Transmission at Oblique Incidence Through a Solid Plate in Water
Authors: Smyth J. B. , Lindsay R. B.
Published: September 2005

The selective transmission of oblique supersonic waves through a solid plate immersed in water is studied theoretically and experimentally over a frequency range from 1.0 to 3.0 megacycles. Three cases are considered: (1) transmission for angles of incidence less than the critical angles for both dilatational and shear waves; (2) transmission for angles of incidence between the two critical angles; (3) transmission for angles greater than both critical angles. In the frequency region studied good agreement is found between the measured transmission, using the torsion disk method, and the theoretical values. Particular interest attaches to case (3). Study is also made of the supersonic transmission through a stratified medium consisting of a set of solid plates immersed in water. The acoustical Bragg law is investigated and verified experimentally.
Pressure and Field Response of the Ear in Hearing Aid Performance Determination
Authors: LeBel C. J.
Published: September 2005

Since hearing aid receiver acoustical output is commonly measured by an artificial ear wherein a microphone replaces the eardrum, the measurement yields data only on nominal pressure at the eardrum. A correction factor must be applied to the artificial ear output to relate it to the free field performance of the ear (which is the norm). In other words, substituting pressure operation for free field operation of the human ear changes its frequency response, and this change must be allowed for in determining the corrective characteristics of a hearing aid. The variation is computed and is then compared with subjective test results. The sum of this correction and one for the obstacle effect of the human body wearing the hearing aid transmitter are then determined and it is concluded that such an over-all corrections so great that it cannot be neglected in determining or comparing hearing aid characteriistics. The over-all correction is then applied to representative instrument characteristics.
Young's Modulus of Elasticity of Fibers and Films by Sound Velocity Measurements
Authors: Ballou J. W. , Silverman S.
Published: September 2005

A method for measuring the modulus of elasticity of fibers and films by sonic means is described and the results of an exploratory study of the modulus in various natural and synthetic fibers and films are given. The method is based on the equation for the propagation of longitudinal waves in elastic materials, which expresses the velocity of propagation in terms of the elastic modulus and density of the medium. The procedure involves measuring the sound velocity and calculating the modulus, using the proper density value. The modulus so calculated is characteristic of a dynamic, short period measurement, since a sound frequency of approximately 10 kilocycles per second was used. The range of moduli found was from 1×1010 to 50×1010 dynes per square centimeter. Correlations of moduli with other physical properties are given.
Curved Quartz Crystals as Supersonic Generators
Authors: Labaw L. W.
Published: September 2005

The supersonic beam radiated from a one-centimeter radius X-cut quartz disk is examined experimentally as a function of the spherical curvature of the crystal, at a frequency of 1110 kilocycles. The crystals of large curvature produce a definite focusing action as has been reported from an earlier investigation. The experimental data show that it is possible to obtain a much greater excess pressure amplitude close to the generator from a curved crystal than is possible from a flat crystal having the same area and thickness using the same input power. The quartz plates of small curvature do not produce a marked focusing action but give a larger supersonic amplitude at large distances from the generator. The flat crystal gives the smallest angle of spread of the supersonic beam. The resonant frequency for any of the five crystals studied did not differ by more than 20 kilocycles (2 percent) from that to be expected from the thickness.
Transmission, Reflection, and Guiding of an Exponential Pulse by a Steel Plate in Water. I. Theory
Authors: Osborne M. F. M. , Hart S. D.
Published: September 2005

In this paper the problem of the interaction of an underwater explosion wave with a steel plate is discussed. Particular attention is given to those aspects of the problem in which the plate can be considered as an elastic, two-dimensional wave guide. The phase velocities of the more important modes of the plate are evaluated as functions of frequency. They are used to derive the properties of the precursor, an oscillation which precedes the explosion wave as it travels along the plate. The results of the theory are compared briefly with experiment. A more detailed discussion of the experiments will be given in a second report. The methods used in this report are also applicable to the evaluation of the phase velocities of the modes of an electromagnetic-wave guide. The propagation of a transient, such as an explosion wave, down a wave guide, presents the interesting mathematical problem of the evaluation of a contour integral of a function of a complex variable defined implicitly. No rigorous solution for this problem as yet exists.
Wave-Front Determination in a Unidirectional Supersonic Beam
Authors: Labaw L. W.
Published: September 2005

An experimental method has been developed to determine the loci of equal phase in a high frequency compressional wave beam in liquids without utilizing standing wave patterns. The wave front in a supersonic beam may thus be determined simultaneously with the relative excess pressure amplitude. The electric signal generated by the piezoelectric action of a Rochelle salt microphone which has the phase of the compressional wave in the liquid is combined with a small electric signal of the same frequency having the phase of the voltage impressed across the quartz generator. The result of this superposition is an approximately sinusoidal spatial variation of the signal generated by the microphone as the latter is moved in the direction of propagation of the beam. The wave front may be traced by following a maximum of this sinusoidal variation as the microphone is moved at right angles to the direction of propagation. The experimental data, taken around 1200 kilocycles, indicates the accuracy obtainable by the method.
A Working Standard for Sound Pressure Measurements
Authors: Massa Frank
Published: September 2005

This paper discusses the necessary requirements to be met in an ideal sound pressure measurement standard. It also describes the design and shows the characteristics of a new microphone that meets these requirements to a greater degree than do existing units which are generally available for use as laboratory standards. The new microphone is stiffness-controlled and has an acoustic impedance approximately equal to 0.001 cc of air throughout the entire audio-frequency range up to 20 kc. The structure approximates rigid cylinder 58″ diameter × 1516;″ long and is linear to sound pressure measurements up to several million dynes/cm2.
Application of the Wave Theory of Room Acoustics to the Measurement of Acoustic Impedance
Authors: Harris Cyril M.
Published: September 2005

A method is presented for the measurement of acoustic impedance of large areas of material, at low frequencies, under actual mounting conditions and at various angles of incidence. It is then used to check the assumption in the wave theory of room acoustics that the boundary conditions for the sound field can be expressed in terms of a normal acoustic impedance which is not a function of angle of incidence. The impedance of material covering a wall is computed from two of the room's acoustic properties, the decay constants and frequencies of the normal modes of vibration. In this way the impedance of a large area that vibrates as a panel may be measured where determinations by methods using small samples are not applicable. By using large areas, the average impedance of a number of small samples may be obtained at once. The new method has the advantage that a direct and simple check of this value of the impedance may be secured by measurements of pressure distribution of the normal modes.
Gradient Microphones
Authors: Olson Harry F.
Published: September 2005

The response-frequency and directional characteristics of gradient microphones from zero to nth order have been obtained. Higher order unidirectional microphones have also been developed. The directivity increases with the order which means corresponding discrimination against random sound. The accentuation of the low frequency response, when the distance from a small source is less than a wave-length, increases with the order and the wave-length. This feature may be used to obtain high discrimination against distant sounds in a close talking microphone. The amount of discrimination increases with the order of the microphone.
Acoustic Intensity Distribution from a “Piston” Source
Authors: Williams A. O.
Published: September 2005

An approximate method for computing the theoretical supersonic acoustic field from a plane piston, recently presented by the author and L. W. Labaw, is extended to cover a larger region and is applied to concave pistons as well. The resulting equations are given for a certain range of frequencies, piston sizes, and distances from the source. The excess acoustic pressure can be evaluated from them, along the beam axis and for a narrow region around it. It is shown that the assumed nature of vibration of the piston does not matter very much unless the source is sharply curved. The locus of maximum excess pressure is not, in general, near the center of curvature. Labaw's recently published data on the acoustic fields of curved crystals are analyzed in the light of the present equations. The agreement in general is satisfactory. However, it seems that his plane crystal was faulty or else that curved crystals must produce several times the acoustic intensity of plane ones with the same applied voltage.
Simplified Flow Resistance Measurements
Authors: Leonard R. W.
Published: September 2005

A simplified apparatus for the measurement of the flow resistance of porous acoustical materials is described. The method used is absolute, not comparative. An improved technique for the preparation of the samples is discussed.
The Analysis of Plane Discontinuities in Cylindrical Tubes. Part I
Authors: Miles John W.
Published: September 2005

The effect of a plane discontinuity on a plane wave propagated in a cylindrical tube of arbitrary cross section is calculated by considering the higher order modes excited at the discontinuity. In carrying out the calculations, a transmission line analogy is used and the effect of the discontinuity at a distance is represented by a capacitance placed at the discontinuity. In Part I, the equations of motion for the propagation of a small disturbance in a cylindrical tube are assumed at the outset and are shown to yield the two-dimensional wave equation, the solutions to this equation constituting an infinite set of modes, in addition to the plane wave usually treated in the literature. Following a discussion of these solutions, the analogy between propagation of sound and an electrical transmission line is established, and it is shown that each mode requires a separate transmission line. The effect of the higher modes excited by a plane discontinuity may be represented by a lumped capacitance, and this capacitance is given by a variational expression which gives a systematic method of calculation yielding an upper bound to the true answer. For the case of a window, a variational principle is produced which gives a lower bound to the true answer. In Part II, this method will be applied to a window in a rectangular tube, to a change of cross section in a rectangular tube, to windows and changes of cross section in circular tubes and to the calculation of resonance in certain types of cavities. The ordinary reflection and transmission coefficients will also be correlated with the theory of Part I. To the author's knowledge this is the first rigorous treatment of the above problems with the exception of an earlier paper calculating the reflection due to a change of cross section in a circular tube (reference 1).
A Small Acoustical Tube for Measuring Absorption of Acoustical Materials in Auditoriums
Authors: Loye D. P. , Morgan R. L.
Published: September 2005

A small acoustical tube is described, which is suitable for measuring absorption coefficients of acoustical materials over a limited frequency range. This instrument is useful for checking the acoustical characteristics of materials after they have been applied to the walls of theaters, studio review rooms, and other auditoriums.
The Driving-Point Impedance of an Infinite Solid Plate
Authors: Jones R. Clark
Published: September 2005

In the design of mechanical filters whose function is, to prevent the transmission of vibration from one structure to another, it is necessary to know the impedances of the structures between which the filter is to be connected. In many of the cases which arise in practice, the impedance may be estimated from a knowledge of the driving-point impedance of an infinite plate. An expression is obtained for the impedance of an infinite plate of constant thickness when connection is made to the plate at a single point. The impedance may be written in the form 8ρh2v, where ρ is the density, h the thickness of the plate, and v a velocity approximately equal to the velocity of shear waves in the material. It is shown that the driving-point impedance is equal, except for a constant factor, to the impedance of a mass equal to the mass of a disk cut from the plate whose radius is the mean proportional of the thickness and the wave-length which corresponds to v. The results are exemplified by applying them to the case of a steel plate.
Toward a Theory of Intervals
Authors: Irvine Demar B.
Published: September 2005

This paper is intended to start inquiry into the question: Do chords, considered as fused klang effects, have individual “tone colors” analogous to the timbres of different instruments? As a first step, only the qualities of two-tone intervals are considered. Sine waves representing simple tones (without overtones) are superposed, by graphic means, so as to produce characteristic composite wave patterns for each of the commonly used intervals. Study of the resultant curves would seem to suggest that: (1) Each interval has a characteristic composite sound-wave-pattern. (2) The length of the composite wave pattern is equal to a number of cycles of the lower frequency expressed by its number in the interval ratio. (3) The composite wave patterns of dissonant intervals are longer and more complex than those of consonant intervals. (4) Comparison of the curves of an interval and its inversion shows little outward resemblance. (5) Intervals may be grouped together into “families,” according to the lengths of the composite wave patterns. These groupings differ somewhat from those usually recognized by harmonic theory. (6) An interval may be said to have a “frequency” equal to the number of complete cycles per second of the composite wave pattern characteristic of that interval. The lower interval frequencies are more dissonant. (7) A study of the configuration of the composite wave patterns shows how it is still possible for the ear to identify the component pitches in a given interval. (8) There seems to be graphic evidence of the physical existence, in the composite wave patterns, of frequencies corresponding to the difference tones.
The Portrayal of Visible Speech
Authors: Steinberg J. C. , French N. R.
Published: September 2005

This paper discusses the objectives and requirements in the portrayal of visible patterns of speech from the viewpoint of their effects on the legibility of the patterns. The portrayal involves an intensity-frequency-time analysis of speech and the display of the results of the analysis to the eye. Procedures for accomplishing this are discussed in relation to information on the reading of print and on the characteristics of speech and its interpretation by the ear. Also methods of evaluating the legibility of the visible patterns are described.
Visible Speech Cathode-Ray Translator
Authors: Riesz R. R. , Schott L.
Published: September 2005

A system has been developed whereby speech analysis patterns are made continuously visible on the moving luminescent screen of a special cathode-ray tube. The screen is a cylindrical band that rotates with the tube about a vertical axis. The electron beam always excites the screen in the same vertical plane. Because of the persistence of the screen phosphor and the rotation of the tube, the impressed patterns are spread out along a horizontal time axis so that speech over an interval of a second or more is always visible. The upper portion of the screen portrays a spectrum analysis and the lower portion a pitch analysis of the speech sounds. The frequency band up to 3500 cycles is divided into 12 contiguous sub-bands by filters. The average speech energy in the sub-bands is scanned and made to control the excitation of the screen by the electron beam which is swept synchronously across the screen in the vertical direction. A pitch analyzer produces a d.c. voltage proportional to the instantaneous fundamental frequency of the speech and this controls the width of a band of luminescence that the electron beam produces in the lower part of the screen. The translator has been used in a training program to study the readability of visible speech patterns.
Fluctuation Phenomena in Room Acoustics
Authors: Maa Dah-You
Published: September 2005

Two types of fluctuation phenomena are discussed theoretically, utilizing the concept of normal modes of aerial vibration in the room. The fluctuation noise due to the random motion of air molecules is found to be a property of air alone and independent of the room. On the other hand, the fluctuation during reverberation depends on both the room dimensions and its reverberation characteristics. Practical formulae of the magnitudes of the fluctuations are presented, which are in good agreement with the experimental data obtained previously. The astonishing gift of the blind in estimating the room size with surprising accuracy is explained, and further investigations on the problem of room liveness as related to fluctuation phenomena as well as its effect on the acoustical design of a room are suggested.
The Design and Construction of Anechoic Sound Chambers
Authors: Beranek Leo L. , Sleeper Harvey P.
Published: September 2005

Data on the performance of five different types of structures for use in echo-free (anechoic) chambers are presented. The best one of these structures is shaped like a wedge and manufactured from glass fibers held together by a binding agent. When mounted in the room, the wedges are spaced out several inches from the walls, and the dihedrals of adjacent units are turned through 90°. Generalized specifications for the optimal design of structures made from glass fiber wedges are presented in terms of either (a) lowest frequency at which 99 percent or better absorption is desired or (b) maximum depth of treatment which may be installed in the room. The application of these specifications to two rectangular rooms is shown and inverse square law measurements performed in the two completed chambers are presented. In the larger chamber the deviations are within ±0.3 db out to 10 feet and ±1.0 db out to 30 feet from a point source of sound. In the smaller, the deviations are within ±1.0 db out to 10 feet.
Effects of Finite Baffles on Response of Source with Back Enclosed
Authors: Nichols Rudolph H.
Published: September 2005

The effect of a finite circular baffle on the axial response-frequency characteristic of a central sound source in which sound from the back side of the diaphragm is completely suppressed was found to consist of an orderly series of peaks and valleys. The magnitudes and characteristic frequencies of these peaks and valleys depend upon the diameter of the baffle and the distance at which the response is measured. Experimentally determined values of the frequencies agree with values computed on the hypothesis that the phenomenon is one of interference between sound which reaches the measuring microphone directly from the source and sound which travels radially along the baffle and is “reflected” from its edge to the microphone. This assumption also accounts qualitatively for the increase of magnitude of the peaks and valleys with increasing distance from the baffle. As might be expected, baffles which are not symmetrical with respect to the sound source produce an irregular effect on the response.
A Just Scale for Music
Authors: Jones Arthur Taber
Published: September 2005

In addition to the well-known just major scale, Robert Smith described another that he regarded as equally harmonious. The second scale is like the well-known one except that its first two intervals are interchanged. It is now shown that this scale may be obtained from three consecutive minor triads, in a manner similar to that in which its well known counterpart is often obtained from major triads. A comparison and experimental test of the two scales shows little to choose between them. Sometimes one is preferred, and sometimes the other. A satisfactory scale should include the pitches from both.
The Velocity of Dilatation and Rayleigh Waves in Metal Bars
Authors: McMillen J. Howard
Published: September 2005

Single elastic waves in rock and metal specimens were studied by partially immersing the specimen in water and observing the body waves in the water by means of spark shadowgrams. The waves were started by impact from a fast moving steel sphere. Waves in the water were observed to come from the dilatation body wave and the Rayleigh surface-wave in the solid. Using the slope of the wave front in water, the dilatation wave velocities were measured for cold-rolled steel, yellow brass, and an aluminum alloy, as well as for two rock specimens, a marble and a diabase. These agreed well with the velocities calculated from the elastic constants. The velocity of the first wave or pulse was found to correspond to the velocity of a dilatation wave in an infinite medium, while the wave velocities which were measured when the bar was in resonant oscillation were observed to have values characteristic of a dilatation wave in a narrow bar. The Rayleigh surface wave velocities were measured in steel, brass, and an aluminum alloy and showed good agreement with the theory. None of the surface waves was strong enough in the rock specimens to be observed. Spark shadowgrams were also made, showing that a Rayleigh wave is produced when a dilatation wave passes through an interface at nearly normal incidence. Rayleigh waves were also observed to produce a second Rayleigh wave at a square corner, demonstrating that the Rayleigh wave has a component of displacements which is parallel to the surface.
Refinements in Supersonic Reflectoscopy. Polarized Sound
Authors: Firestone Floyd A. , Frederick Julian R.
Published: September 2005

One form of simple circuit for the generation of the short duration high frequency voltage trains and in the supersonic reflectoscope is shown. The required band width of the system, including the crystal, is discussed. Techniques for the radiation of longitudinal, shear, and Rayleigh waves are set forth. Either longitudinal or shear waves can be used to establish various modes of thickness resonance through a plate for measuring velocity of propagation or thickness. Shear waves have some of the properties of polarized light, for instance, double refraction has been measured by three methods in elastically aeolotropic solids and a technique for the direct indication of the amount of elastic aeolotropy developed. By means of a quarter-wave plate, circularly polarized sound can be produced.
Jungle Acoustics
Authors: Eyring Carl F.
Published: September 2005

The study of jungle acoustics was carried out during the wet season in Panama. Measurements permit the following conclusions to be drawn: Within a jungle the temperature and wind velocity gradients are so small that the sound refraction they produce may be neglected for all practical purposes. Humidity increases the transmission loss at high frequencies and field measurements of the loss agree with laboratory values reported by others. Terrain loss, measured in db, between any two specified distances from the sound source is defined as the transmission loss between these points less that caused by the geometrical divergence of the sound beam. Terrain loss in the jungle was found to increase linearly with distance. The terrain loss coefficients, measured in db per foot, were measured for various types of jungle and were found to be a function of frequency and of the density of the terrain, the density of terrain being measured by the difficulty of penetration and the distance a foreign object may be seen. The level of the ambient noise in the wet season jungle is very low especially for the quiet periods between animal calls. At night the low frequencies decrease as the light breezes cease and the high frequencies increase as the insects begin their nocturnal chorus. A jungle is a difficult place in which to judge the direction of a sound—a probable error of 20° is to be expected. The error is found to be smallest when the sound comes from a direction near the axis passing through the two ears, and in the range studied the error decreases as the sound source moves farther away. Reverberation and scattering are the cause of part of the error of judgment, but an improved technique of listening suggested may increase the observer's accuracy.
Reflection of Sound Signals in the Troposphere
Authors: Gilman G. W. , Coxhead H. B. , Willis F. H.
Published: September 2005

Experiments directed toward the detection of non-homogeneities in the first few hundred feet of the atmosphere were carried out with a low power sonic “radar.” The device has been named the sodar. Trains of audiofrequency sound waves were launched vertically upward from the ground, and echoes of sufficient magnitude to be displayed on an oscilloscope were found. Strong displays tended to accompany strong temperature inversions. During these periods, transmission on a microwave radio path along which the sodar was located tended to be disturbed by fading. In addition, relatively strong echoes were received when the atmosphere was in a state of considerable turbulence. There was a well-defined fine-weather diurnal characteristic. The strength of the echoes was such as to lead to the conclusion that a more complicated distribution of boundaries than those measured by ordinary meteorological methods is required in the physical picture of the lower troposphere.
The Propagation of Sound in an Inhomogeneous and Moving Medium I
Authors: Blokhintzev D.
Published: September 2005

In this paper the wave equations for the propagation of sound in an inhomogeneous and moving medium are established (Section I). In Section II special cases are considered and a generalization of Kirchhoff's theorem (Huygens' principle) is given for a moving medium. In Section III the general equations of acoustics are considered in the approximation of geometrical acoustics. And finally, in Section IV the equations are generalized for the case of a medium containing a salt solution (sea water).
The Propagation of Sound in an Inhomogeneous and Moving Medium II
Authors: Blokhintzev D.
Published: September 2005

In the present paper several applications of the theory developed in Part I are set forth. Section I deals with the propagation of sound in a turbulent medium, Section II with its propagation through a shock wave.
Development of Midget Earphones for Military Use
Authors: Pearson H. A. , Mundel A. B. , Carlisle R. W. , Knauert W. F. , Zaret M. E.
Published: September 2005

In order to meet the military requirements for a headset that could be worn under the MK-1 helmet, midget hearing aid receivers were redesigned for this application. Data are presented showing characteristics of the original design adopted and of subsequent improvements resulting in improved performance. Some of the methods for securing adequate performance in quantity production and methods for 100 percent inspection of frequency characteristics are described.
The Directional Characteristics of a Free-Edge Disk Mounted in a Flat Baffle or in a Parabolic Horn
Authors: Slaymaker Frank H. , Meeker Willard F. , Merrill Lynn L.
Published: September 2005

In the attempt to obtain a sharp beam of supersonic sound from a parabolic horn, it became apparent that the sharpness of the beam was a function of the mode of vibration of the diaphragm exciting the horn. Experiments showed that the sharpest beams with the smallest side lobes were obtained when the diaphragm was radiating most of the energy toward the side walls of the horn rather than toward the mouth. Experiments with a free-edge disk for a diaphragm related the wide-angle radiation to the two-nodal-circle mode of vibration of the disk. Comparison between directional patterns measured with the disk in a flat baffle and the corresponding calculated patterns confirmed the correlation between the wide-angle radiation and the two-nodal-circle mode. Further analysis indicated that similar radiation patterns could also be obtained from one- and three-nodal-circle modes of vibration. In the analysis, the differential equation for the vibration of a thin plate is solved for a free-edge circular disk assuming circular symmetry and considering a driving force applied at an arbitrary circle concentric with the disk. The dynamic curve of the disk is given for any frequency in the form of a series of the normal modes of the disk, and the directional pattern of the radiation from the disk mounted in an infinite baffle is calculated. The calculations have been so systematized and tabulated that the dynamic curve and directional pattern for any disk and frequency can be calculated quite readily. The analysis is sufficient to predict the disk diameter and thickness required for the one-, two-, or three-nodal-circle modes for any frequency, when the disk is mounted in a flat baffle and when the maximum radiation is placed at an arbitrary angle from the axis. On the basis of the analytical predictions, a disk has been made to give the required wide-angle radiation when vibrating with one nodal circle instead of with two nodal circles as discovered experimentally.
The Pressure Distribution in the Auditory Canal in a Progressive Sound Field
Authors: Wiener Francis M. , Ross Douglas A.
Published: September 2005

The variation of the sound pressure along the auditory canal was determined experimentally on a number of subjects, male and female, placed in a progressive sound field. This was accomplished by insertion of a small flexible probe microphone at various positions along the auditory canal. The subjects were placed in front of a loudspeaker in a room free from acoustic wall reflections. The free sound field at the subjects' location was essentially that of a plane progressive wave. The measurements were carried out over the significant range of audiofrequencies for various orientations in azimuth of the subjects with respect to the sound source. The sound pressure at the eardrum is found to be greater than the free-field pressure. The average ratio of these two quantities is a function of frequency, and reaches values of about 20 db in the vicinity of 3000 c.p.s. The human ear is thus an effective acoustic “amplifier.” The increase in sound pressure at the eardrum over the free-field pressure is caused by a combination effect of diffraction by the head and pinna and resonance in the auditory canal. The measurements of the sound pressures at several other positions along the auditory canal serve to separate these two phenomena to a certain extent and to furnish additional information about the pressure distribution. Most of the data were obtained with a group of male subjects, but measurements on a few women did not show any marked discrepancies.
Physics and Physiology of Acoustic Trauma
Authors: Rüedi L. , Furrer W.
Published: September 2005

Acoustic strains that are physically quite different always produce similar functional damage to the ear. The well-known so-called “4000 c/sec. dip” is based on a functional peculiarity of the cochlea. There ensues the possibility of constructing a universally efficient ear defender which does not essentially impair the auditory capacity.