bluuebunny/crossref_metadata_2025 huggingface dataset:
A Scale for the Measurement of the Psychological Magnitude Pitch
Authors: Stevens S. S. , Volkmann J. , Newman E. B.
Published: September 2005

A subjective scale for the measurement of pitch was constructed from determinations of the half-value of pitches at various frequencies. This scale differs from both the musical scale and the frequency scale, neither of which is subjective. Five observers fractionated tones of 10 different frequencies at a loudness level of 60 db. From these fractionations a numerical scale was constructed which is proportional to the perceived magnitude of subjective pitch. In numbering the scale the 1000-cycle tone was assigned the pitch of 1000 subjective units (mels). The close agreement of the pitch scale with an integration of the differential thresholds (DL's) shows that, unlike the DL's for loudness, all DL's for pitch are of uniform subjective magnitude. The agreement further implies that pitch and differential sensitivity to pitch are both rectilinear functions of extent on the basilar membrane. The correspondence of the pitch scale and the experimentally determined location of the resonant areas of the basilar membrane suggests that, in cutting a pitch in half, the observer adjusts the tone until it stimulates a position half-way from the original locus to the apical end of the membrane. Measurement of the subjective size of musical intervals (such as octaves) in terms of the pitch scale shows that the intervals become larger as the frequency of the mid-point of the interval increases (except in the two highest audible octaves). This result confirms earlier judgments as to the relative size of octaves in different parts of the frequency range.
Theory of the Haskell Organ Pipe
Authors: Jones Arthur Taber
Published: September 2005

A quarter of a century ago William Haskell developed an organ pipe which is like an ordinary open pipe with a smaller tube, closed at the top, inserted in it. This pipe gives the full series of harmonic partials usually obtained from an open pipe of length equal to the sum of the lengths of the outer tube and the inner one. The theory of the pipe is here developed and checked experimentally.
The Strike Note of Bells
Authors: Jones Arthur Taber
Published: September 2005

In view of a paper by Meyer and Klaes the author examines again certain arguments against the view that the strike note of a bell is a difference tone. The work of Meyer and Klaes is summarized, and four new tests are reported. The results of all four tests are opposed to the hypothesis that the strike note is a difference tone. In a very tentative way the possibility is suggested that in some bells the strike note may be the second partial tone of the bell. This is not true for the bells which the author has examined most carefully. For these bells he still believes that the strike note, aside from the octave in which it lies, is determined by the fifth partial tone of the bell. As to the octave, the work of Meyer and Klaes suggests that this may be determined by the difference tone from the fifth and seventh partials, this difference tone being produced by the nonlinear response of the auditor's ear. An appendix gives some information regarding harmonic overtones from tuning forks.
Direct Recording and Reproducing Materials for Disk Recording
Authors: Keller A. C.
Published: September 2005

Recently materials for direct recording and reproducing work have been improved so that they are now suitable for many uses. These materials, as they are available on the market, are classified chemically into five groups and measurements are given of frequency characteristic, surface noise, life, distortion, etc. These data have been taken with both lateral and vertical recording. A short aural demonstration was given of typical recording on materials of each group.
Sound Analysis
Authors: Hall Harry H.
Published: September 2005

Instruments for sound analysis may be grouped into five classes, graphic, resonance, heterodyne, stroboscopic, and diffraction analyzers. The operation of each type of instrument is briefly described. From the point of view of analysis sounds may be classified roughly into four groups: (1) Steady state sounds or sounds which may be maintained at constant fundamental frequency, constant intensity and unvarying quality for long enough to carry out the analysis; (2) Sounds which are essentially transient in nature; (3) Sounds which may be maintained constant on the average but whose intensity, frequency, and wave form are modulated at a constant frequency; (4) Noise, or sounds which are entirely random in form but which are continuously maintained. The sound spectra corresponding to each group are described and the application of the various types of instruments to the analysis of these spectra is discussed. It is pointed out that a graphic analyzer may not be used to analyze single waves taken from a short transient sound.
An Investigation of Subjective Tones by Means of the Steady Tone Phase Effect
Authors: Trimmer J. D. , Firestone F. A.
Published: September 2005

Two strong pure tones, of frequencies in harmonic relation, when heard together have a sound depending in loudness and quality on the phase relation between the tones. This dependence may be observed either by the method of beats or by the method of steady tones. The phase effect is closely linked with pitch perceptions contrary to Ohm's law of acoustics, with subjective tones, and in general with the nonlinearity of the ear—i.e., the way the ear responds to sounds of large amplitude. A careful quantitative study of the steady tone phase effect was made with frequencies up to 500 cycles per second. The results indicated a need for important revisions in the concept of aural nonlinearity. These revisions are summarized in the proposal of a definition for the previously undefined and promiscuously used term, “subjective tone.”
A Chromatic Stroboscope
Authors: Railsback O. L.
Published: September 2005

A stroboscope is described which permits direct reading of any pitch with a precision greater than that of the ear. The readings are made in terms of musical scale notation instead of frequency.
An Empirical Formula for the Loudness of a 1000-Cycle Tone
Authors: Knauss Harold P.
Published: September 2005

The equation L = I(10−5/2I+1)−2/3 may be used to compute the loudness, L (millisones), of a 1000-cycle pure tone of intensity I (in units of 10−16 watts per square centimeter). This indicates that for low intensities the loudness is directly proportional to the intensity; for high intensities it is proportional to the cube root of the intensity.
Sound Propagation in Ducts Lined with Absorbing Materials
Authors: Sivian L. J.
Published: September 2005

In ventilator and exhaust systems it is desirable to provide a high degree of attenuation for audiofrequency waves while offering low resistance to continuous or slowly pulsating air flow. For that purpose ducts lined with absorbing materials are sometimes used. This paper deals with sound propagation, particularly with its attenuation constant, in such ducts. Two types of lining are considered: (a) nonvibratile, i.e., a lining in which there is no wave motion propagated in the direction of the duct axis; (b) vibratile, i.e., a lining admitting of such motion. Methods for computing the propagation constants in terms of the acoustic constants of the lining, are given. Some experimental data are presented. Comparison of observed and computed values indicates that the computational procedure is substantially valid up to a frequency at which the sound wave-length is about twice the internal duct diameter.
A Direct-Reading Pitch Recorder and Its Applications to Music and Speech
Authors: Obata Jûichi , Kobayashi Ryûji
Published: September 2005

An electrical arrangement was devised, by means of which rapid variations in pitch with time, such as occur in music or speech, could be directly recorded even in detail. The arrangement consisted of amplifiers, a wave form adjuster, an automatic volume control, and a frequency meter. The frequency meter itself is essentially the same as the two-tube thyratron inverter, which was first described by Hull as one of the applications of thyratron, and recently modified by Hunt. Efforts were made to utilize the frequency meter in conjunction with the wave form adjuster and the automatic volume control in order to make it suitable for indicating the rapid pitch variations, accompanied by large intensity variations, in music as well as in speech. The final indication of pitch is made in logarithmic scale, several applications of the arrangement to music and speech being given.
Theoretical and Experimental Investigation of the Transmission of Sound Over Reflecting Surfaces
Authors: Pierce George W. , Noyes Atherton
Published: September 2005

The paper presents an experimental and theoretical discussion of acoustic signaling between a sending station and a receiving station near a plane reflecting surface, and is applicable to fog horn and submarine signaling. The experiments were made with a small scale apparatus employing supersonic waves of a frequency of 67.5 kilocyles per second. It is shown that at grazing incidence the direct and reflected waves cancel, or partially cancel, as predicted by theory. The cancelation for propagation in air near a water surface, or other surface of high sound velocity and high density, is found to be restricted to angles very near to grazing incidence. On the other hand, theory predicts cancelation for much larger departures from grazing incidence if the reflecting surface has high density but low velocity; experimental curves for transmission over beach sand and certain other substances resemble such theoretical results very closely. Acoustic mirages caused by temperature gradients were studied experimentally, and their effects are shown to be of striking importance in modifying the above results.
Absolute Sound Intensity in Liquids by Spherical Torsion Pendula
Authors: Klein Elias
Published: September 2005

These experiments demonstrate the use of spherical radiometers to determine the energy density in an acoustic field. It is shown that the theoretical work of King is applicable to the measurement of radiation pressure in liquids. Considering the torsion pendulum as an absolute instrument, pressure microphones are calibrated as secondary standards.
The Position of the Vibrator in the Experiments of Melde and Kundt
Authors: Miller B. J. , Olsen L. O.
Published: September 2005

It is shown that the point of application of the obligatory motion in the experiments of Melde and Kundt is at a displacement “node” in the driven medium. The word “node” means a point of minimum amplitude in the standing wave in the tube or string. This result agrees with that of Rayleigh but is obtained in a simpler manner. The discussion is presented briefly to make the results more available as current textbooks indicate a nearly universal misunderstanding of these two well-known experiments.
The Velocity of Sound in Single Crystals of Bismuth
Authors: Focke A. B. , Lindsay R. B. , Wilks C. R.
Published: September 2005

The velocity of compressional waves in single crystals of bismuth of two orientations (P1 and P3) of the principal axis has been measured. The specimen to be tested (around 10 cm in length) is inserted between two longer steel rods, one of which is driven by a loudspeaker coil while the other has a free end. The velocity is computed from the observed distance between two nodes when all or part of the crystal is included between them. For the P1 and P3 orientations the velocities are found to be 2028 meters/sec. and 1541 meters/sec., respectively, and the corresponding Young's moduli come out to be 4.043 × 1011 dynes/cm2 and 2.245 × 1011 dynes/cm2.
Finite Solid Acoustic Filters
Authors: Lindsay R. B. , Focke A. B.
Published: September 2005

Calculation is made of the transmission of harmonic compressional waves through a finite solid filter structure consisting of a metal rod loaded at equal intervals with heavy metal collars. The theory is then tested experimentally for the special case where the finite structure (consisting of 6 sections) is terminated by a finite rod with a free end. The structure acts as a low pass filter with a cut-off at about 2000 cycles and no further transmission up to 12,650 cycles. The transmission in the transmission band shows a series of peaks whose positions, with one exception, are accurately verified experimentally.
Acoustical Output of Air Sound Senders
Authors: Devik Olaf , Dahl Helmer
Published: September 2005

The use of the Rayleigh disk for measurement of extremely great sound intensities is discussed, and it is stated that the Rayleigh disk method is applicable also in this case. The Rayleigh disk may even be used in the open air as part of a pointer instrument when properly shielded, thus giving a means of directly measuring the actual sound intensity (in watts/cm2) of intense sound fields. The measurement also involves a determination of the direction of propagation of energy. The calculation of the total flux over the opening of a sound sender, giving the actual acoustical output in watts, is in that way very much simplified. As a secondary instrument a crystal microphone, calibrated by comparison with the Rayleigh disk instrument, was used. When used in extreme sound fields it was directly connected with a rectifier galvanometer. Reports are given of measurements of the total output of a membrane sender and of that of a diaphone. In the first case measurements were also made of the different kinds of losses occurring both in the electrical and the acoustical oscillation circuit, while the vibration losses were determined from the power balance and vibration measurements. Oscillograms of sound wave form are given and discussed.
The Vibrations of Symmetrical Plates and Membranes
Authors: Colwell R. C. , Friend A. W. , Stewart J. K.
Published: September 2005

The mathematical theory of vibrating membranes or plates, which is usually applied to squares and rectangles, may also be used for triangles provided the equations are plotted on axes which are not rectangular. These triangles are then fitted together to produce many sided symmetrical plates. Thus the Chladni patterns are found mathematically for polygonal membranes or plates. If one angle of the triangle is made very small, the plates are practically circular so that these equations also give the patterns on circular plates (approximately) for special cases. The triode valve oscillator which drives the plates is calibrated accurately so that the frequency of the plates is found at the time the patterns are formed. A table gives a comparison between the observed and calculated frequencies.
Resonance in Certain Non-Uniform Tubes
Authors: Jones Arthur Taber
Published: September 2005

When a solid rod is gradually lowered into an open organ pipe the pitch grows lower and then higher. The pipe and contained rod are equivalent to two tubes of different cross sections joined end to end. The equation for the frequency of such tubes is not new, but its consequences appear heretofore to have been examined only for cases where the narrow part of the tube is moderately short. The change in pitch may be surprisingly large, and agrees fairly well with that calculated.

Consideration is also given to a tube which consists of two cylinders joined by a flaring connector. For the one tube examined experimentally, the frequencies of the first five normal modes of vibration are found to be in satisfactory agreement with their theoretical values.
A Tubular Directional Microphone
Authors: Mason W. P. , Marshall R. N.
Published: September 2005

A tubular directional microphone is described which consists of a pressure type microphone coupled to an acoustic impedance element composed of a large number of tubes whose lengths vary by equal increments. The function of this variation in length is twofold. First, the multiple resonances of the individual tubes occur at intervals so close together that the net effect of the bundle is that of an acoustic resistance over a fairly wide frequency range and so does not impair the high quality of the attached microphone. Second, high directivity is secured, because for sound incidence other than normal each tube introduces a different path length with phase cancellation resulting in a composition chamber between the microphone and the ends of the tubes. The theory of operation is summarized and data are presented to show the performance of the instrument which is in fair agreement with the theory.
Investigation of Room Acoustics by Steady-State Transmission Measurements. I.
Authors: Hunt Frederick V.
Published: September 2005

It should be possible to deduce the necessary acoustical information concerning the bounding surfaces of a room from steady-state measurements of the transmission between a source and a receiver appropriately placed in the room. This problem is being studied by using a small model chamber yielding discrete normal frequencies in the range from 200 to 1500 cycles. A small high impedance sound source having substantially constant output over this frequency range was developed and comprises a multiple capillary formed by packing a brass tube with parallel round wires, the sound from a dynamic receiver passing down the canals between the wires. Analysis and experiment indicate that the total absorption in the chamber for a given mode of vibration can be measured by observing the width of the resonance curve at the half-intensity height. By selection of different modes of vibration, absorption coefficients are obtained as a function of both frequency and angle of incidence, the latter including grazing incidence. The decay rates predicted by these measurements have been checked experimentally. Methods are discussed for the extension of these results to large scale rooms by an appropriate statistical analysis.
Multiple Coil, Multiple Cone Loudspeakers
Authors: Olson Harry F.
Published: September 2005

A consideration of the direct radiator loudspeaker shows that to obtain uniform response, relatively high efficiency and adequate power handling capacity over a wide range requires a large diameter, rugged diaphragm and heavy coil at the lower frequencies and a relatively light weight vibrating system at the higher frequencies. This paper describes a number of direct radiator loudspeaker systems which satisfy these requirements: the double coil, single cone loudspeaker; the single coil, multiple cone loudspeaker; the double coil, double cone loudspeaker; the multiple coil, multiple cone loudspeaker.
Velocity of Sound in Air
Authors: Pielemeier W. H.
Published: September 2005

Determinations by capable investigators of the velocity of sound in dry air at 0°C range from 330.6 to 331.9 m/sec. Miller's value (331.36±0.08) m/sec. is probably the best for low frequency (explosive) sounds. Part of this range of values may be due to dispersion. The CO2 and the O2 contents of air are sensitive to water vapor in this respect. The dispersion in air manifests itself as a small excess over the velocity values calculated for given humidities and temperatures by means of Miller's classical formula. These calculated values of V for a given temperature are approximately a linear function of the vapor pressure. These and the observed values are plotted. Other curves show how the dispersion frequencies in CO2 and O2 depend on the humidity. The “observed” value of V0, the velocity of sound in dry air at 0°C, calculated from measurements at a given frequency and humidity by Miller's formula depends somewhat on these existing conditions.
Mechanical and Electrical Analogies of the Acoustical Path
Authors: Saxton Harold L.
Published: September 2005

The infinite acoustical path is treated as a smooth mechanical transmission line, using Firestone's method of drawing schematic diagrams. The method is adopted of translating thermodynamical, hydrodynamical and collision processes into purely mechanical terms by solving for the circuit elements from their definitions. When representative T sections have been obtained for paths having successive single causes of sound absorption, these are compounded into a T section for the case where all these causes of absorption are present. The electrical analogy follows at once from the mechanical. Input mobilities and complex velocities for the several cases are derived.
Some Aspects of the Theory of Room Acoustics
Authors: Morse Philip M.
Published: September 2005

An exact solution for the decay of sound in a rectangular room is obtained; assuming that each wall is uniformly covered with absorbing material, which may differ from wall to wall. It is concluded, from recent experimental measurements, that the boundary conditions for the sound field are correctly expressed in terms of the effective normal impedance of the wall material. The sound is analyzed into its component normal modes of vibration, and the reverberation times and frequencies of the different normal modes are calculated as functions of the wall impedances and their phase angles. Curves are given for these quantities for a wide range of the parameters involved. The effect of the absorbing material in distorting the sound field is shown, and several other interesting points are brought out in the discussion: that waves which travel “parallel” to a wall are absorbed by the wall, but are not absorbed as much as are waves striking at more oblique angles; that it is sometimes possible to increase the reverberation time for a standing wave by decreasing a wall's effective acoustic resistance; etc.
Effect of an Absorbing Wall on the Decay of Normal Frequencies
Authors: Bhatt N. B.
Published: September 2005

The acoustical damping of individual normal modes of sound in a model rectangular chamber, due to absorbing material on one wall, was measured experimentally by two different methods. Using a “constant output” source at one of the resonance frequencies the reduction in intensity level due to insertion of the absorbing material was measured; a “steady-state” measurement. The reverberation time for each normal mode was also measured; a “transient” measurement. The two sets of data, reduced to the same units, checked fairly well. The combined results were then used to verify some of the predictions of the recent theory of room acoustics. The intercorrelations between these data, and the comparisons with directly measured values of the normal acoustic impedance of the absorbing material show that the theory is verified, within the rather large experimental error of the measurements.
The Loudness of Continuous Spectrum Noise and its Application to Loudness Measurements
Authors: Brittain F. H.
Published: September 2005

A sound having a spectrum evenly and continuously distributed over the audiofrequency band is proposed as a reference sound for subjective noise tests in order to permit of subjective determinations being made in any surroundings instead of only in a heavily damped room as at present. Examples are given. The relation between loudness of a single pure tone and a continuous spectrum sound is given, together with some observations on the relative sensitivity of various parts of the cochlea.
Filtration of Oblique Elastic Waves in Stratified Media
Authors: Lindsay R. B.
Published: September 2005

Previous work has been done on the problem of the transmission of plane compressional elastic waves normally through a stratified medium consisting of alternate plane parallel layers of two different substances. Such a structure acts as a low-pass elastic wave filter. The present paper extends the analysis to the case of oblique transmission. This proves to be comparatively simple when the layer substances are both fluid. The transmission and attenuation (i.e., reflection) bands are found to depend on the angle of obliquity in a characteristic way. When one of the layer substances is solid, the problem is more difficult. Account must here be taken of the fact that compressional waves in the fluid layers give rise to both dilatational and shear waves in the solid layers. The result is somewhat more complicated than in the previous case but the structure still turns out to be a low-pass elastic wave filter for angles of obliquity less than the critical angle. As before the transmission and reflection bands are a function of the angle. In the special case in which the thickness of the solid layers is small compared with that of the fluid layers one obtains the elastic wave analog of the Bragg reflection law for x-rays, that is, one gets reflection approximately for the wave-lengths λn = 2l/n ⋅ sin θ, where l is the distance between successive solid layers and n is integral.
Normal Modes of Vibration in Room Acoustics: Experimental Investigations in Nonrectangular Enclosures
Authors: Bolt Richard H.
Published: September 2005

Existing theories of room acoustics have long been recognized as incomplete. The inadequacy is particularly disturbing in the lack of agreement between absorption coefficients measured in different laboratories. Also, the acoustic behavior of small rooms cannot always be prescribed with certainty. Several investigators have shown the necessity of analyzing a room as an assemblage of normal modes of vibration in a three-dimensional continuum. The present paper reports preliminary studies of one aspect of the “wave” approach: the influence of boundary shape on normal modes of vibration. An experimental method reported previously, (reference 25) has been improved and further developed for exploring the distribution of sound in small reflective enclosures of various shape. Standing wave patterns have been plotted for the lowest four or five modes of vibration in each of three models: a parallelogram in plan, a trapezoid in plan, and a model with an “alcove” or coupled space. A number of normal frequencies have been measured in each model, and the nature of the frequency distributions has been studied. The results have been summarized with respect to their significance for room acoustics. In particular, it appears that rooms of irregular shape still possess discrete normal modes and standing wave patterns as pronounced as those which occur in simple shapes.
Microphone Efficiency: A Discussion and Proposed Definition
Authors: Massa Frank
Published: September 2005

This paper proposes a definition for microphone efficiency, together with a discussion for justifying the points of view presented. Up to the present time, the question of microphone efficiency has been generally ignored and in its place has grown the use of microphone sensitivity, which is the voltage output for a particular value of sound pressure actuating the unit. Although this latter quantity may be quite satisfactory for commercial use, the author feels that from a scientific point of view, we should be interested in the absolute ability of a microphone to absorb acoustic energy from a sound field and convert it into electrical energy: this ability he defines as efficiency. A relation for microphone efficiency is derived and a family of curves are computed which show the efficiency of a microphone as a function of its size, impedance, and sensitivity. One obvious conclusion that can be deduced from the paper is that only a tiny fraction of the acoustic energy intercepted by an average microphone is actually converted into electrical energy. This means that we are yet very far from the theoretical limit and, consequently, great possibilities still exist for fundamental research in the improvement of microphone efficiency.
The Propagation of Shock Waves in Air. I
Authors: Thompson L. , Riffolt N.
Published: September 2005

From a consideration of conditions at the boundary of a source of shock in air, and of conditions at great distances, a formula of the Riemann type has been derived for the velocity of propagation of a finite pulse. The formula includes an additional constant which provides the necessary flexibility to represent data obtained throughout the fields of sources consisting of detonating charges of explosives, and of other sources. The observations of wave displacements have been summarized in terms of an integral for “reduced” times, from which the constants of the function for velocity are immediately available. All distances are defined with reference to an equivalent dimension of the source, and the characteristics throughout the velocity field are obtainable from the characteristics of the source. A table is given for the velocity of a condensation pulse, for representative boundary velocities, at various distances from the source out to points at which the velocity has decreased approximately to its asymptotic value (the normal velocity of sound). Results obtained by Wolff and Burlot for very large sources are shown for comparison on a plot of reduced times. Preliminary results obtained with piezo-gauges of experiments to determine relative pressures at the head of the wave are given in comparison with theoretical gauge pressures obtained by formulas derived in Part II.
The Propagation of Shock Waves in Air. II
Authors: Thompson L.
Published: September 2005

Using the Rankine-Hugoniot equation of condition and Hugoniot's velocity of a discontinuity in a gas, formulas are obtained for density and pressure at the head of a shock wave which are referred to velocity of the wave as a parameter. Gauge pressures are defined in a form considered to represent the observations of pressure obtained in Part I. The function is used to calibrate the gauge, and a comparison of results by Rayleigh's pressure function is included. A discussion is given of the appropriate ratio of specific heats for condensation cycles so extremely short in duration as those of intense shock waves, with references to the literature bearing on the subject of the accumulation of molecular excitational energies in short intervals of time.
On the Auditory Significance of the Term Hearing Loss
Authors: Steinberg John C. , Gardner Mark B.
Published: September 2005

For a number of years hearing impairment has been expressed by the audiogram, which is a measure of the hearing loss for sounds which are just audible to the deafened ear, with the implication that impairment for threshold sounds indicates the impairment for sounds above threshold in the range of interest to the deafened. It is shown that the audiogram is not an accurate indication of hearing impairment for above threshold sounds in cases involving nerve deafness, when the loss in loudness sensation is taken as the criterion; but when the capacity to interpret speech sounds, as indicated by articulation tests, is the criterion, the audiogram does give an approximately correct indication of the hearing impairment. The relation of the audiogram to experiences in interpreting the speech of everyday life and the relation of hearing loss to absolute ear sensitivity are given.
Interaction Impedance of a System of Circular Pistons
Authors: Klapman S. J.
Published: September 2005

Curves have been calculated for the total radiation resistance and reactance of a single circular piston as a function of frequency when another circular piston is vibrating in the same infinite plane, the distance of separation of the two pistons being a parameter. The curves shown are for equal velocity amplitudes and for phase differences of 0° and 180°. These results enable one to calculate the impedance of a piston when there are present an arbitrary number of pistons in arbitrary phases.
The Absorption of Sound by Small Areas of Absorbing Material
Authors: Pellam John R. , Bolt Richard H.
Published: September 2005

An indirect experimental check is now obtained for the calculated “shape factor” for an absorbing strip of material in a reflecting wall, reported previously The effect of the strip on each of the component “partial waves” in the incident plane wave is studied by placing the strip in a special enclosure, the shape of which allows the partial waves to be studied individually as isolated normal modes of vibration. A semicylindrical room is employed; its concave wall tends to “focus” sound upon the center of the opposite, flat wall on which the absorbing panel is centered. If the wave equation is set up in elliptic cylinder coordinates with the edges of the panel as foci, the boundary conditions are readily satisfied. The theory is strictly applicable to a semielliptical room only, but holds well in the semicircular for strip widths up to one-third the diameter. The experimental enclosure has a 14-inch radius, a “thickness” of 4 inches between the parallel walls, and is constructed with highly reflective metal surfaces. The selective damping of various normal modes is investigated for strip widths from about one to ten inches, for several values of normal acoustic impedance. The results substantiate the calculated values of “shape factor” which were given in the previous paper.
Variable Boundary Impedance for Room Acoustics Investigations
Authors: Bolt Richard H. , Brown Richard L.
Published: September 2005

A method has been developed for obtaining a wide range of impedance values, both as to magnitude and phase angle. In one form, a variable impedance unit consists of (1) a steel face with perforations of variable area (obtained by overlapping identically drilled plates), (2) a “resistance” element—a layer of porous absorbing material with appropriate flow resistance, (3) a “mass” element—a solid board through which small channels are drilled at uniform spacing over the whole surface, and (4) a “stiffness” element—a cavity of variable depth backed by a rigid steel plate. The behavior can be predicted approximately by analysis of an equivalent electrical circuit. A unit with face 8×8″ has been built and calibrated, at several frequencies, using a straight acoustic impedance line of the same sectional area. The useful range appears to include phase angles from −60° to +45° and magnitudes of z/ρc from 0.4 to 10, for frequencies between 250 and 800 c.p.s. Normal incidence “absorption coefficients” of 99 percent have been obtained. In using this type of impedance structure with non-normally incident sound it is found necessary to break up the backing cavity into cells of transverse section small compared with a wave-length. This modification minimizes tangential components of particle velocity and renders the specific acoustic impedance at the face essentially independent of angle of incidence and of the area of the surface, and yields a marked increase in “average absorption coefficient.”
Non-Uniform Acoustical Boundaries in Rectangular Rooms
Authors: Maa Dah-You
Published: September 2005

An exact solution is presented for the behavior of sound in a rectangular room whose walls are covered non-uniformly with acoustical materials. The sound field in the room is first decomposed into normal modes of vibration. By assuming the characteristics of an acoustical material to be representable by its specific acoustic admittance, the normal frequency, decay constant and pressure configuration of each normal mode of vibration are determined as functions of the room dimensions and the distribution of acoustical materials in the room. An approximate theory is also presented, applicable to cases in which the diffracted reflections from the walls can be neglected. It is shown that the effect due to diffraction can be superposed on the result from the approximate theory. Experiments were performed in confirmation of the theoretical results. Applications of the theory to practical problems, such as measurements on acoustical materials, acoustical design of rectangular rooms, etc., are discussed. The possible extension of the present theory to the case of nonrectangular rooms is discussed.
Acoustic Impedance and Sound Absorption
Authors: Morse Philip M. , Bolt Richard H. , Brown Richard L.
Published: September 2005

Theoretical curves for acoustic impedance vs. frequency, obtained by solving the equation for wave motion within the material, are compared with recently measured values of the impedance of various sound absorbing materials. The satisfactory agreement in most cases measured indicates that that for homogeneous materials the effective values of flow resistance, porosity and material density are nearly constant over the frequency range 200 to 6000 c.p.s. This makes it possible to express the whole acoustic behavior of these materials by three constants. The behavior of two nonhomogeneous materials is also discussed. The relation between impedance and the various types of absorption coefficient is discussed, and specific formulas for the coefficients are given in terms of the acoustic impedance. It is indicated that in rectangular chambers, when the sound does not have a random diffusion throughout the room, the decay curve for pressure squared in db, vs. time in seconds, is a broken line, with the initial slope corresponding to a coefficient for normal incidence, and the final slope corresponding to a coefficient for grazing incidence. If enough scattering objects are inserted to make the diffusion of sound complete, so that no standing wave is all grazing or all normal to any surface, then the decay curve is a straight line with slope corresponding to the coefficient defined by Sabine. A formula relating this coefficient to the acoustic impedance is given. The Acoustical Materials Association values of absorption coefficient, used in practice, are compared with the curves of normal and Sabine coefficient, computed from the experimental curves for impedance. It is apparent that above 2000 c.p.s. the A.M.A. values equal the computed Sabine coefficients, indicating sufficient diffusion of sound in the reverberation chambers at these frequencies. Below 500 c.p.s. the A.M.A. values correspond more closely to the normal coefficient, indicating an insufficient diffusion of sound, and that the initial slope of a broken line decay curve is being measured. In these two ranges of frequency, therefore, one can compute the A.M.A. coefficients from the impedance. It is suggested that field coefficients for large auditoriums correspond to the computed Sabine coefficient for lower frequencies than do the A.M.A. coefficients. A simple formula is given for the broken line decay curve in a rectangular room with no diffusion, and is checked by experimental curves. A case of partial diffusion is discussed, where the decay curve is intermediate between the theoretical curves for complete and for no diffusion.
The Absorption of Sound in Carbon Dioxide
Authors: Leonard Robert W.
Published: September 2005

The absorption was measured by a direct method over frequencies between 22 and 112 kc. A microphone responding to the sound pressure is moved away from a piston source located in a flat surface. The output of the microphone is amplified and recorded photographically. The resulting pressure-distance curve yields the pressure attenuation coefficient. The measurements were made in carbon dioxide which was carefully dried by passing it through phosphorus pentoxide. The results of the measurements bear out the theoretical work of Bourgin. The lowest value obtained for the frequency at which the absorption per wave-length μ is a maximum was 30 kc. In order to account for the maximal value of μ obtained experimentally, it is necessary to assume that both the symmetrical valence vibration and the deformation vibration are effective in producing the absorption, and in addition that the second harmonic of the deformation mode also participates in the absorptive process. Velocity measurements made with the same apparatus have shown a reasonable agreement between the dispersion and absorption in carbon dioxide.
Results of the World's Fair Hearing Tests
Authors: Steinberg J. C. , Montgomery H. C. , Gardner M. B.
Published: September 2005

The hearing test for musical tones which formed part of the Bell System exhibit at the New York and San Francisco Fairs is described. Average hearing acuity and the frequency of occurrence of various amounts of hearing loss are given as functions of age and sex. The relation of hearing acuity to place of residence, economic status and certain other factors is discussed briefly. Data from the two Fairs are used to determine the distribution of hearing acuity in the United States population, subject to certain stated limitations. Accuracy of the test is discussed in relation to ability of visitors to understand the test procedure, disturbing effect of background noise, and calibration of the test equipment. Certain results of the survey are expressed in terms of ear canal pressure and equivalent free field intensity, and on this basis a comparison is made with the results of other surveys of hearing.
A Theory of Tracing Distortion in Sound Reproduction from Phonograph Records
Authors: Lewis W. D. , Hunt F. V.
Published: September 2005

General formulas are derived, as an expansion in powers of the amplitude and its derivatives, for the motion of a stylus of any shape sliding on the warped groove surfaces representing any arbitrary recorded signal. These formulas provide simple expressions for the second- and third-order harmonic distortion of pure tones, and are also applicable for the computation of cross-modulation products for two- or many-tone signals, including a continuous recorded spectrum. Typical computations for these cases are presented and a general criterion is established for the maximum r.m.s. distortion for an arbitrary steady-state signal. The computations provide a basis for discussing the advantages of lateral-cut over vertical-cut, the effect on distortion of “pre-emphasizing” the recorded spectrum, and the efficacy of rerecording techniques.
Elastic Wave Filtration in Nonhomogeneous Media
Authors: Lindsay R. B.
Published: September 2005

Previous work has indicated that when harmonic compressional waves pass through a stratified medium consisting of a series of alternating layers of two different substances, which may be either fluid or solid, the transmission is selective. In other words the medium acts as an acoustic filter with alternate transmission and attenuation frequency bands. This analysis of the transmission characteristics of the medium was based on the assumption that the transition in acoustical properties (i.e., density and sound velocity) from each layer to the next is abrupt and effectively discontinuous. The purpose of the present investigation is to examine the consequences of assuming that the transition in question is a gradual one. The interesting theoretical result is that for a gradual transition in which there are no discontinuous changes in either the acoustical properties or their gradients the stratified medium passes all frequencies. In order to assure attenuation for a gradual transition one must assume at least discontinuity in the gradients of the acoustical parameters. Certain special cases and their consequences are investigated.
The Analysis of Pulses by Means of the Harmonic Analyzer
Authors: Shankland Robert S.
Published: September 2005

An important group of nonperiodic phenomena, including the transmission of acoustic pulses in air and solid materials, the passage of pulses through amplifiers, etc., requires the evaluation of the Fourier integral to give the complete spectrum, both amplitudes and phases, of the components making up the pulse. The analyses of such problems are important in studies of the starting and stopping of sounds, especially as these greatly influence the tonal characteristics of musical instruments. The solution of these problems can be carried through directly and with ease by charting the pulse to several scales of abscissae and analyzing the graphs with the Henrici analyzer which is capable of giving thirty components. The detail in the resulting analysis depends simply on the number of curves drawn to different scales which are traced. The method is also of general use in other problems where the Fourier integral appears, including the analysis of radiation pulses, x-ray and electron diffraction patterns, and the flow of heat.
The Influence of Certain Atmospheric Conditions upon Sound Transmission at Short Ranges
Authors: Eagleson Halson Vashon
Published: September 2005

A formula was derived giving sound intensity as a function of temperature, humidity and barometric pressure. A series of measurements was made of the intensity of sound from constant sources at short fixed distances. Readings were taken in all sorts of weather that can be found in Indiana. It was found that in general the experimental values agree with those calculated from the formula to within five percent.
A New Microphone Providing Uniform Directivity over an Extended Frequency Range
Authors: Marshall R. N. , Harry W. R.
Published: September 2005

A new microphone is described which consists of a moving coil pressure element combined with an improved ribbon pressure gradient element to give a cardioid directional characteristic. The theory of operation is reviewed, and consideration is then given to variations in directivity caused by diffraction, separation of the elements, and disparities in their phase and response characteristics. It is then shown how these variations are largely eliminated by equalization in the electrical circuit so that the resulting directivity is practically independent of frequency throughout the range from 70 to 8000 cycles. The use of a moving coil pressure element makes high efficiency possible, while the design of an unusually rugged ribbon element provides a marked reduction in noise due to air currents. Several useful directional patterns in addition to the cardioid pattern are provided in the new microphone, and the theory and merits of these patterns are presented. Finally some of the results which were obtained in field trials of the new microphone are di-cussed.
The Intensive Difference Limen in Audition
Authors: Dimmick Forrest L. , Olson Ruth M.
Published: September 2005

In spite of its interest for acoustics, the intensive difference limen for sound has not been determined over a comprehensive range of frequency and amplitude. In the present experiment we have determined the difference limens for discrete intensive discrimination at five frequencies ranging from 128 cycles to 1000 cycles with intensities from 0.02 bars r.m.s. pressure to 3.4 bars (−65 db to −15 db). Both the physical conditions and the psychological judgment have been carefully controlled. The data yield a value for Δ db of 2.4 db in the middle range of intensities (−35 db to −15 db or 35 SL to 70 SL.)
Frequency Distribution of Normal Modes
Authors: Roe Glenn M.
Published: September 2005

The formula obtained by Maa for the frequency distribution of the normal modes for a rectangular enclosure has been verified by a more direct computation and extended to apply to a cylinder, a sphere, and to a number of derived shapes. In all cases it is found that the number of normal modes with frequencies less than ν is given by 4πVν3/3c3+πAν2/4c2+⋯, where V is the volume and A the surface area of the enclosure. The application of this result is illustrated by a calculation of resonance response and rate of decay of sound in a cylinder.
A Sound Measurement Room Utilizing the Live End-Dead End Studio Principle
Authors: Stahl M. D. , Louden W. C.
Published: September 2005

The requirements of a suitable sound test room for the measurement and analysis of sound produced by household appliances are discussed. It is shown that neither the highly reverberant room nor the highly absorptive room will meet all requirements due to the room resonant response of the former and the high sensitivity to directional effects of the latter. A sound room is described which makes use of a reflective floor, absorptive walls and a highly absorptive ceiling with the microphone placed near the ceiling. The sound source is moved in a reciprocating manner and a sound diffuser is rotated slowly during measurements to average out the space pattern effects. A high degree of sound reduction through the walls is achieved by massive construction and a large reduction of transmitted vibration is effected by a coil spring suspension. The test room has been used over an extended period of time and has been of great assistance in sound analysis investigations directed toward sound reduction on electric cleaners.