bluuebunny/crossref_metadata_2025 huggingface dataset:
The Noise of “Choked” Jets
Authors: Powell Alan
Published: September 2005

The noise of a jet changes character after the pressure ratio exceeds the critical value appropriate to sonic exit velocity, the general roar being dominated by a loud “whistling” or “screeching.” Schlieren photographs show that sound waves of ultrasonic frequency are caused by the transition of the initially laminar boundary layer to turbulence, and also by this turbulence interacting with the shock waves of the flow, Larger disturbances have also been noted, involving both the jet stream and some of the air external to the jet, and these also give rise to sound waves which have been photographed it is these which are held responsible for the audible effects A two-dimensional study has shown the latter phenomenon to be enhanced, and it is shown how the system of disturbances is self-maintained by virtue of sound waves creating initially small disturbances at the jet exit. The directionality of the sound field has been predicted and found in agreement with experiment, and the dimensions of the motion are compatible with the suggested mechanism. The relation to edge-tones is pointed out and the mechanism indicated, a photograph of this phenomenon also being shown. Finally mention is made of how the characteristic noise of jets working above the critical pressure might be reduced, the suggested method having been found successful in practice.
Aerodynamic Noise in Wind Tunnel Diffusers
Authors: Mawardi Osman K.
Published: September 2005

The present paper presents the preliminary results of a program of study which has been started to study sounds of aerodynamic origin. The spectra of noise generated in a miniature blow down subsonic tunnel are compared with the results of a survey on wind-tunnel noise. The comparison indicates a substantial similarity between the spectra obtained and a tentative figure for the efficiency of noise conversion has been indicated.
A Remote Control Sound Level Meter and Analyzer
Authors: Scott H. H.
Published: September 2005

A combined sound level meter and analyzer has been developed for the U.S. Navy Bureau of Aeronautics, incorporating many novel features and meeting all pertinent ASA and military specifications. The main assembly, consisting of a combined sound level meter and octave band analyzer, is smaller and lighter than many ordinary sound level meters alone and is of a shape particularly adapted for placement in remote compartments of aircraft. This main assembly can be used as a complete sound level meter and analyzer by itself or it can be operated in conjunction with a remote control unit which can be mounted in the pilot's cockpit. All important functions of the sound level meter and analyzer can be controlled remotely from this unit, which occupies a volume of any 0.05 cubic foot. Although designed specifically for tests in single-seater fighter aircraft, the new instrument's versatility and compactness make it applicable to a wide variety of uses. The development is unclassified, and the resulting equipment is available on special orders to commercial customers as well as the U.S. Government.
Noise Level and Mechanical Stresses in Plastic Sound Records
Authors: Keller Ernest A.
Published: September 2005

Residual mechanical stresses in press-polished plastic film material used for embossed type sound recording are in many cases responsible for relatively high noise levels. Results of noise measurements of different plastic materials on a 400 grooves per inch recorder are presented. The relation between optically observed stresses and recorded noise level is given. Practical consequences are discussed.
Impedance Measurements on Small Orifices and Membranes
Authors: Caldwell S. A.
Published: September 2005

A method is described whereby measurements of the acoustic impedance of small orifices and membranes are made by comparison to a known impedance. The equipment required consists of a high impedance and a low impedance source of sound, a sound level meter, phase angle indicator, and a few easily constructed impedance references. The method is analogous to the electrical one where a known impedance is compared to an unknown using a constant voltage or constant current source and measuring the current or voltage. Data on the acoustic impedance of various types of cloth and metallic screens and membranes are given. Experimental results of impedance measurements of two orifices for various separations as well as for different shapes of orifices is also shown.
Distribution Characteristics of Cylindrical Radiators
Authors: May A. J. , Caldwell S. A. , Volkmann J. E.
Published: September 2005

Experimental data on the directional characteristics of a number of quasi-cylindrical sector radiators vibrating radially are given. The variables considered are those of sector angle, height, and frequency. The exact pressure distributions over equiphase surfaces in the vicinity of several of the horns are shown. These data are useful in predicting the directional characteristics of other similar radiators having different height dimensions. The accuracy of this application is also shown by measurements made on a similar radiator.
Propagation of Longitudinal Elastic Waves in a Solid Circular Cylinder
Authors: Harrison Mark
Published: September 2005

The propagation of longitudinal elastic waves in a solid circular cylinder is a problem of long standing. If flexural and torsional waves are excluded, then it is found there are three distinct types of propagation. These three types are referred to as the Young's modulus, the shear type, and the dilatational type. For the last two types there are an infinite number of modes. The Young's modulus type of propagation is characterized by the fact that as the radius to wavelength ratio a/λE approaches zero, the velocity approaches (E/ρ)12 for the zero mode. The shear type is characterized by the fact that as a/λs becomes large the velocity approaches (μ/ρ)12. The dilatational type of propagation is characterized by the fact that as a/λd becomes large the velocity approaches [(λ+2μ)/ρ]12. Only the zero mode for the Young's modulus type has been previously discussed. The higher modes for the shear and dilatational types exhibit cut-off frequencies below which there is attenuated propagation. As the frequency approaches the cut-off frequencies from above, the phase velocity becomes infinite and the group velocity approaches zero. As a/λE becomes large, the velocity for the Young's modulus type approaches the Rayleigh surface wave velocity. The shear and the dilational types differs from the Young's modulus type in one significant way; there is no zero mode for the dilational type, so that unattenuated propagation does not exist below the lowest cut-off frequency. Applications to ultrasonics, crystal systems, and noise isolation problems are discussed.
The Reversal of Acoustic Streaming Near Boundaries
Authors: Andres J. Milton , Ingard Uno
Published: September 2005

Starting with the nonlinear boundary layer equations for a viscous incompressible fluid, a perturbation calculation is carried through with the intention of investigating terms of third and fourth order. The investigation is motivated by the belief that these higher order terms might give a clue to the behavior of acoustic streaming flows as the amplitude of the exciting oscillations is increased. The calculations are rather laborious and involve several simple extensions of the perturbation technique. The resulting expression for the stationary flow is applied to several examples including the flow near a cylinder and is seen to indicate a reversal in the direction of circulating flow when the intensity reaches a certain level, in agreement with experimental observations.
The Attenuation of Repeated Shock Waves in Tubes
Authors: Werth Glenn C.
Published: September 2005

Measurements have been made of the attenuation of high amplitude sound waves which have assumed the sawtooth-repeated shock form. The range of sound amplitudes was from 0.08 to 0.30 atmosphere peak to peak. The fundamental frequencies used were between 400 to 1200 cycles per second. A siren was the source and a 18-inch barium titanate movable probe microphone the receiver. Both 1- and 112-inch tubes were used. When reciprocals of the sound pressure amplitudes are plotted against the number of wavelengths, straight lines result in accordance with the theory of Rudnick. The slopes are considerably less than those predicted by theory and this will be discussed.
The Scattering of Sound in a Turbulent Medium
Authors: Kraichnan Robert H.
Published: September 2005

The scattering of a sound wave in a medium undergoing shear flow confined to a finite region is investigated under the assumption that the total velocity field is everywhere small compared to the velocity of sound. Formulas are obtained for the angular distribution and frequency distribution of the scattered wave in terms of the four-dimensional Fourier transform of the shear velocity field. The cross sections for the scattering of a plane wave of frequency ω by a shear flow of given scale and spatial structure go typically as ω4M2, where M is a characteristic Mach number of the flow. The coupling between the shear and longitudinal velocity fields has a tensor character such that the scattering vanishes at 180° and at 90°. The spectrum of the scattered sound wave is very sharp in the forward direction and becomes broader at larger scattering angles. Explicit expressions for the cross sections are obtained for the case of scattering from a region of isotropic turbulence. When the frequencies of importance in the turbulence are small compared to the frequency of the incident sound wave, the average differential scattering cross section can be expressed directly in terms of the energy spectrum of the turbulence.
Recent Developments in the Selection of Candidates for Training in the Discriminative Detection of Auditory Signals
Authors: Pepinsky Abe
Published: September 2005

The uncanny ability of some musicians in detecting and differentiating acoustic signals has been frequently noted. The present study reveals that attitude toward music, even in the layman, can likewise play an important role in the selection of personnel for such tasks. Furthermore, it is shown that certain other vocational interests are highly correlated, both positively and negatively, with that of the musician. Intelligence and personality factors are given due consideration. Eighteen such vocational fields, as well as intelligence and personality factors, have been found to be significant at the one percent level, and fifteen more at the five percent level. Evaluation and interpretation of these findings point toward new sources of candidate material to be used in the discriminative detection of auditory signals.
Variability of the Absolute Auditory Threshold: A Psychophysical Study
Authors: Herman George
Published: September 2005

The purpose of this study was to explore the variability of the auditory threshold under experimental conditions, for comparison with the known variation in repeated clinical audiometric tests of the same subject. The literature on the reliability of audiometric methods is surveyed briefly. An experiment is reported in which the absolute threshold for pure tones of 1000 cycles per second was studied. Nine normal ears were studied in one to five sittings each. Each sitting involved ascending and descending presentation of the stimulus tones. Intensity was varied in one decibel steps. The subject was required to signal whether he heard each tone. The proportion of “yes” responses at each intensity level did not differ significantly between ascending and descending series. Psychometric functions were constructed by fitting the phi-gamma function to the data. Functions were computed for each sitting of each subject and for all sittings of each subject. No significance could be attributed to differences among the sittings of any subject. A psychometric function was computed which represents the total subject group. This function is the cumulation of a normal distribution with a standard deviation of 6.72 db. From this function a theoretical distribution of the average results of the type of series used in clinical audiometry is derived. The standard deviation of this distribution is 5.57 db for either ascending or descending audiometric series. The dispersion of audiometric tests of the same subject as reported in the literature is in reasonable agreement with the theoretical prediction, if allowance is made for the use of several series at each frequency in the clinical test. It is concluded that the fitting of the phi-gamma function is a satisfactory procedure for the study of the psychometric function for absolute auditory thresholds. This function (for a single subject) does not vary significantly over a period of several weeks. It is concluded also that the variation in clinical audiometric tests of the same subject is attributable to the variability of the subject's sensory behavior, rather than to the nature of the clinical test. The variability which has been observed over periods of several months is demonstrable in much shorter periods.
Wave Motion in the Cochlea Resulting from Bone Conduction
Authors: Zwislocki Jozef
Published: September 2005

It is generally known that, except for the difference in frequency characteristic, bone-conducted sound produces the same sensation as airborne sound. On the other hand, the transmission of acoustic vibration to the cochlea is quite different. The air-borne sound enters through the oval window and the bone-conducted sound causes compressions and dilatations of the whole inner ear. It does not seem obvious that the vibration pattern of the cochlear partition is the same in both cases; yet Békésy has shown that bone-conducted sound can be entirely compensated by airborne sound. This indicates that the behavior of the cochlear partition is the same, independent of the way in which the sound is introduced into the inner ear. Several investigators have used Békésy's findings, which have been confirmed in other experiments, to draw conclusions regarding the mechanics of the cochlea. The classic resonance theory and the so-called telephone theory have seemed to provide the best explanation; however, these theories disagree with other experimental results. Consequently there has been no completely satisfactory solution to the problem of cochlear behavior in bone conduction. It is shown in the present paper that the cochlear partition vibrates in the same way whether stimulated by airborne sound or by bone-conducted sound, independent of the factors determining the pattern of vibration. The cochlear partition behaves in the same way even when the vibration is delivered through an artificial opening placed somewhere along the wall of either the scala vestibuli or the scala tympani.
Voice Identification of Speakers
Authors: Pollack I. , Pickett J. M. , Sumby W. H.
Published: September 2005

Whereas the ability of a listener to identify a single aspect of a sound (e.g., the frequency of a tone) is disappointingly poor, a listener's ability to identify combinations or “patterns” of sounds (e.g., words of the language) is remarkably good. In an attempt to examine identification performance for a situation somewhat between these two extremes, we have chosen the identification of a speaker's voice (in contrast to the identification of the content of his speech). Among the factors which affect identification of a voice are: the size of the class of possible voices, the duration of the speech sample, the voicing or nonvoicing of the speech sample, the frequency range of the speech sample, and the interference of other voices presented at the same time. In general, for the testing conditions employed (recorded speech samples with 2 to 16 possible speakers), the efficiency of informational transfer (information transmitted relative to information presented) of a speaker's voice is independent of the size of the class of possible voices. The information transmitted increases as a growth function of speech sample duration such that each additional 0.1 second duration increases the informational transmission by an average of 30 percent of the portion available; the growth in informational transmission with increased voice sample is about 3 times as rapid with normal voiced speech as with whispered nonvoiced speech; speaker identification is strongly resistant to low slope, high or low pass filtering. Very large individual differences among listeners are obtained in the identification of combinations of speakers. This, in conjunction with the important role of speech sample duration, suggests that there is a large range in ability to encode complex stimulus patterns for quick identification.
An Underwater Acoustic Intensity Probe
Authors: Samsel R. W. , Henry G. E.
Published: September 2005

A problem of long standing in acoustic research is that of determining the sound intensity at some point in a body of liquid. The usual approach has been to make a sound pressure measurement at the point in question. One may then convert to intensity, using a known or assumed value for the acoustic impedance. The foregoing method has several drawbacks. Accordingly, we have undertaken to measure sound intensity directly, by observing the rate at which sound degenerates into heat in a small body of low Q solid material, placed in the liquid at the point in question. The pellet is made small, and its supporting stem very thin, in order to disturb the sound field just as little as possible. A particular objective has been to make the dimensions of the pellet small compared to the wavelength, under which condition one may properly apply lump-constant theory in analyzing the acoustics of the system. With our present probe, this desirable condition prevails at all frequencies below about 200 kcps.
The Velocity and Attenuation of Sound in Sand-Water Mixtures
Authors: Hoyer W. A. , Mifsud J. F. , Nolle A. W. , Runyan W. R. , Ward M. B.
Published: September 2005

The velocity and attenuation of sound in four sand-water mixtures were experimentally determined at frequencies of 0.2 mc, 0.5 mc, and 1.0 mc and the results compared with values of velocity and attenuation calculated from theory previously derived by R. W. Morse, in which the sand-water mixture is represented as a bundle of tubes of small cross section. The equivalent diameter of these tubes is assumed to be proportional to the average diameter of the sand particles, and is determined from dc flow measurements. The agreement between the experimentally determined velocity and the velocity calculated from theory is within the accuracy of the experiments. However, the experimental attenuation is smaller by a constant factor than the attenuation predicted by theory. The four sands used in the sand-water mixtures were graded sands of uniform particle size. The average particle diameters of the graded sands are 0.66 mm, 0.32 mm, 0.16 mm, and 0.081 mm. The ultrasonic velocity and attenuation in the sand-water mixtures were experimentally determined by the use of a pulsed-signal echo-ranging system.
Acoustical Characteristics of Underwater Jet-Edge Systems
Authors: Brackenridge J. Bruce , Nyborg Wesley L.
Published: September 2005

It has been shown that underwater whistles, of a type wherein a fiat jet is projected on a sharp edge, may be very effective for certain purposes. Nevertheless, workers have been discouraged from further exploitation of these devices by the complicated nature of their characteristics. As one varies the parameters of such a whistle, its output may vary rapidly in frequency and amplitude, the generator jumping from one mode of oscillations to another in a seemingly chaotic fashion. In hopes of finding some regularity in this behavior a study has been made of the output spectrum of a reed-type whistle as a function of its parameters. It is found that, at least, some aspects of its characteristics may be understood surprisingly well if one compares the whistle sound with the associated edge tone, obtained by replacing the thin blade normally used with a heavy, rigid nonvibrating wedge. Characteristic “resonance” effects take place as the frequency of an edge tone stage nears a natural frequency of the blade, the sequence of events being very similar to results for jet-edge-resonator whistles in air.
A Large Area Condenser Type of Transducer
Authors: Aamodt T. , Harvey F. K.
Published: September 2005

A condenser type of transducer is described featuring a nonstretched, large area diaphragm designed to vibrate in phase over its entire surface. The operating elements consist of an aluminum foil front electrode cemented to a 332-inch foam rubber sheet mounted on an aluminum back plate. The whole assembly is fitted in a picture frame 18 inches square and protected with a perforated metal face plate. As a microphone the open circuit sensitivity is about −57 dbv (re 1 dyne/cm2), and the frequency response is flat within ±3 db from 200 to 5000 cycles. Directional characteristics taken with the microphone, using a distant source, show a close agreement with the values calculated for a flat surface with all elements moving in phase. It appears suitable for applications where the talker remains at a distance from the microphone and where an ordinary nondirectional transducer would introduce serious room reverberations.
A Miniature Microphone for Transistorized Amplifiers
Authors: Bauer B. B.
Published: September 2005

A new magnetic microphone has been developed for use in hearing aids and communication and similar equipment which requires a small, light, medium-impedance microphone. The new microphone has a diameter of 1 inch and a thickness of 38 inch, and it weighs 9 grams. The sensitivity is high enough to override the relatively high noise level of transistors in the frequency range most useful for voice transmission.
Power Requirements for Home Music Systems
Authors: Tanner R. H.
Published: September 2005

This paper examines the Home Music System as an extension of the broadcasting or recording chain, and by applying the same criteria and factors of safety, a figure is reached for the desirable output power for a Home Music Amplifier. From this engineering approach, constructive suggestions are made regarding other components of the system, with a view to arriving at an efficient and economical result.
CBS Television City Audio and Communication Facilities
Authors: Chinn H. A. , Monroe R. B. , Palmquist C. A.
Published: September 2005

CBS Television recently placed in operation its new Television City headquarters in Hollywood. This new and ultramodern plant is built around a flexible and expansible architectural master plan that provides for orderly future expansion to twenty-four studio units together with expansion of all supporting activities. The recently completed first phase of construction provides a modern studio building housing four large studios, each exceeding one-quarter acre in area. Together with an adjacent three-story service building, the plant incorporates all the technical production, shop, and Office facilities to completely support CBS Hollywood television operations. Part I is entitled “Description of the Audio Facilities,” and Part II is “Description of the Communication Facilities.”
The Frequency Dependence of Acousto-Electrochemical Effects
Authors: Saunders Frank , Yeager Ernest , Dietrick Harry , Hovorka Frank
Published: September 2005

If sound waves impinge on either an electrode at which gas is being liberated by electrolysis or an electrode surrounded by a porous covering in a dilute electrolytic solution, an alternating potential of the same frequency is developed on the electrode relative to the bulk of the surrounding solution. The frequency dependence of these effects has been determined under various electrochemical conditions with pulse-modulated ultrasonic waves at frequencies from 200 to 1000 kc/sec. These measurements indicate a decrease of the order of 20 db in these effects over this range with no abrupt transitions. The mechanisms involved in these effects are discussed in light of the new data. The use of these effects in the forming of high frequency probes is also demonstrated.
Some Observations on the High Frequency Absorption of Magnesium Sulfate-Water Solutions
Authors: Barrett R. E. , Bever R. T.
Published: September 2005

In a recent issue of the Journal, Hall analyzed some data on ultrasonic absorption in a solution of MgSO4 in water, which the authors published several years ago. Hall noted the appearance of a second peak in the curve of αλ vs frequency, occurring at about 20 Mc. The authors have re-examined these and other data taken in these solutions and have made some additional measurements at several concentrations and temperatures. The appearance of this second peak is confirmed, although the measurements are not of sufficiently high accuracy to fix the dependence of relaxation frequency upon concentration.
Ultrasonic Propagation in the Liquid-Glass Transition Region
Authors: Litovitz T. A. , Lyon T.
Published: September 2005

This paper deals with the measurements of ultrasonic absorption and velocity in diphenyl pentachloride in the liquid-glass transition region at frequency from 7.5 to 52.5 Mc. Earlier measurements at higher temperatures have shown this liquid to behave as a typical associated liquid with a value of αobs/αshear equal to 3.3. At temperatures below 10°C (viscosities exceeding 107 poise) the normal shear and compressional viscous effects have relaxed and have given rise to a constant term, K1, in the absorption coefficient per cm. In addition this absorption coefficient has a term which varies linearly with frequency. This type of loss has been noted by Mason et al, in highly viscous polymers and is attributed to a hysteresis effect. In this glassy state, therefore, the absorption coefficient per cm(α) can be represented by the following expression, α = K1 + K2f. The frequency independent term K1 decreases exponentially with decreasing temperature. Tentative results indicate that K2, the hysteresis constant also decreases exponentially with decreasing temperature. The K1 term has the steeper slope indicating that a larger activation energy is associated with this process than with the hysteresis effect.
A Directional Acoustic Receiver as a Space Filter
Authors: Hills Robert
Published: September 2005

The performance of a directional acoustic receiver can be treated with the same mathematics used in treating electric filters. The output of a scanning directional receiver is related to the distribution of sound sources to which it is listening in the same way that the signal out of an electric filter is related to its input signal. The “space frequency” response of a directional receiver is determined by the physical construction of the receiver. In many cases it is possible to improve this response by the use of an appropriate electrical equalizing system on the output of the receiver. After a review of the general concept of space filtering and its particular application to receiving systems, some of the problems encountered in designing optimum equalizing systems will be discussed.
Transformer Couplings for Equivalent Network Synthesis
Authors: Bauer Benjamin B.
Published: September 2005

It may be shown that regardless of the type of analogy chosen (“direct” versus “inverse” or “mobility”), topological disconformity will be encountered in synthesizing the equivalent network of an mechano-acoustic array. A method is described employing transformer couplings for getting around this difficulty. As a result, it becomes possible to construct equivalent circuits of mechanical arrays in a simple and straightforward manner by the method of direct analogy. Similar benefits are achieved for the method of inverse analogy in the synthesis of equivalent circuits of acoustic arrays. Equivalent networks for certain mechano-acoustic arrays which cannot be conventionally synthesized become feasible through this method.
Stereophonic Effects in the Reproduction of Music from Disk Records
Authors: Lindridge Charles D.
Published: September 2005

To obtain stereophonic effects in the reproduction of recorded music a listener must have not only the equivalent of three-dimensional sight to give him sound in depth, but also the equivalent of two-dimensional sight to give him sound from different directions. When music is recorded binaurally these effects are obtained in such manner that they accurately convey information regarding conditions which existed at the time that the music was recorded. When reproduction is from binaural disk records, however, other second- and third-dimensional effects which do not convey any information regarding these conditions may possibly be present and enhance the binaural effect so that the listener is not so well informed regarding the actual conditions. These other effects, conveying untrue information, can be used to produce stereophonic effects when music is reproduced from monaural records. It is concluded that an arrangement making use of these effects with reproduction from monaural records is usable for the purpose of obtaining an illusion of the actual presence of the musicians under some conditions, although the illusion created is that of a possible relative location of the musicians and not the actual relative location of the musicians when the music was recorded.
An Artificial Voice and Miniature Anechoic Chamber for Hearing Aid Measurements
Authors: Carlisle R. W.
Published: September 2005

There is described an enclosure for a standard loudspeaker mechanism which provides a compact sound source or artificial voice. With a size slightly larger than a human head, the frequency range is 100 to 7000 cps. A simple and reproducible arrangement of damping felt gives unusual flatness. This artificial voice can either be used in free space or with a small chamber arranged as an acoustic labyrinth, for testing microphones and hearing aids. In the chamber, a calibrating microphone is mounted opposite the test microphone for continuous monitoring. Simple RC compensating networks are utilized to equalize the sound source for either free-space or labyrinth measurements.
The Uniaxial Microphone
Authors: Olson Harry F. , Preston John , Bleazey John C.
Published: September 2005

A small unidirectional microphone has been developed with the following features: maximum sensitivity along the axis of the microphone; a high ratio of electrical output to size; a sharper directivity pattern than a cardioid; a directivity pattern that is independent of the frequency; a blast proof vibrating system. The high discrimination which this microphone exhibits to sounds which originate from the sides and rear makes it particularly suitable for long distance sound pick-up in radio, television, sound motion pictures, and sound reinforcing systems.
Electronic Sound Absorber
Authors: Olson Harry F. , May Everett G.
Published: September 2005

The electronic sound absorber is so named because it absorbs sound or reduces the sound level by means of an electronic transducing system as contrasted to conventional sound absorption by direct conversion from acoustical to heat energy. The electronic sound absorber consists of a microphone, amplifier, and loudspeaker connected so that for an incident sound wave the sound pressure at the microphone is reduced. Thus it will be seen that electronic sound absorber is a feedback system which operates to reduce the sound pressure in the vicinity of the microphone. The sound pressure in the neighborhood of the microphone can be reduced 10 to 25 decibels over a frequency range of three octaves in the low frequency portion of the audiorange. The electronic sound absorber may be used to reduce the noise over a small volume, that is, spot type noise reducer, or it may be used with an acoustical resistance to obtain a high order of sound absorption in the low frequency range.
On the Design of Acoustic Resonators
Authors: Ingard Uno
Published: September 2005

Absorption and scattering from resonators in a free field as well as in walls are discussed. Considering losses due to viscosity, heat conduction, and radiation, it is shown that there exists an optimum value for the resonator volume at which the resonance absorption is maximum. The effect of damping the resonator by means of a screen in the aperture and in particular the dependence of the damping on the distance between the screen and the aperture is analyzed. Nonlinear effects on the absorption and the resonance frequency are also included.
Attenuation by an Absorptive Patch in a Duct
Authors: Young James E.
Published: September 2005

A short patch of absorptive material in a duct may provide greater attenuation per unit length of treatment than a long continuous lining of the same material. The increase in attenuation arises from wave scattering at the ends of the patch and consequent diffraction of additional energy into the absorptive material. A theoretical analysis based on a variational principle has been carried out for certain configurations that have also been studied experimentally. The theoretical results agree in the limiting cases with lumped constant analysis for a very short patch and with continuous duct analysis for a long treatment. Measurements were made in a rectangular duct of cross section 5×8 inches, terminated by an absorbing wedge and driven by a plane wave source. Absorptive patches extending the full width of one wall, and flush with the plane of that wall, were inserted in lengths of 1 foot to 4 feet. Two types of material were used: a homogeneous porous layer 2 inches thick, and a resonant structure consisting of a perforated facing, a resistive cloth, and a backing air cavity subdivided into small cells to insure a normal impedance independent of angle of incidence. Measurements were made from 100 to 1000 cps. Continuous plots of sound pressure along the duct were obtained from which could be determined the attenuation along the patch and the constants of the standing waves caused by scattered components. The measured attentuations range from 8 to 45 decibels depending on patch length and frequency. The results are in close agreement (within 3 percent in db) with theory, for strip lengths greater than three times the width of the duct. A more detailed theory applicable at shorter lengths is discussed.
Audiofrequency Attenuation of Vibration Isolation Methods
Authors: Samsel R. W. , Plunkett R.
Published: September 2005

One of the most useful methods of noise reduction in machinery is vibration isolation. Unfortunately, this is not always as successful as the theory presented in books on vibration might indicate. Where this analysis might show 80 db attenuation at 1000 cps, one is lucky to get 10 db. The reason for this must be sought in the various mechanical impedances of the system—the transfer impedance of the mount, the driving point impedance of the foundation and the impedance looking back into the machine. Our analysis of these factors will be presented here and some experimental measurements discussed. These measurements indicate, at least qualitatively, why the attenuation of typical mounts is as low as it is.
A Narrow Band Speech Transmission System Employing the Recognition of Phonetic Elements
Authors: Dudley Homer , Davis K. H.
Published: September 2005

In a companion paper, one of the authors discusses the automatic recognition of phonetic elements of speech by a device which continuously correlates the measured frequency spectrum of incoming speech with a set of reference spectra. After such recognition, one can transmit the resultant information over lines of very narrow band width for speech synthesis by a reverse process. In a circuit which has been built to study this method of transmission, the number of phonetic elements which is recognized is limited to ten, thus requiring transmission only of information as to ten on-off signals, between sending and receiving terminals, plus a narrow band channel for transmitting pitch information. This information is used to control hiss and buzz energy sources in a synthesizer which is similar to that of the vocoder. A demonstration of the operation of this system will be given using recordings.
The Recognition of Synthetic and Natural Vowels
Authors: Hirsh I. J. , Reynolds Elizabeth G.
Published: September 2005

Listeners identified five different versions of each of nine simple English vowels. Synthetic vowels were obtained from a magnetic recording of the output of an electrical analogue of the vocal tract, designed by K. N. Stevens of M.I.T. The natural vowels were sung or spoken by an adult male. One synthetic version started and stopped gradually, while a second consisted of only of a steady-state portion bounded by vertical scissors cuts of the tape. Similarly, one of the sung versions was started and stopped naturally, while the other consisted a middle portion cut out of a longer sustained rendition. In addition, the vowels were spoken, rather than sung. Among these five types of vowels, the order of recognizability was: (1) spoken vowels, (2) sung vowels with natural starts and stops, (3) sung vowels with artificially abrupt boundaries, (4) synthetic vowels with gradual starts and stops, and (5) synthetic vowels with sharp boundaries. The superior recognition of natural vowels over that of synthetic vowels, with similar transient conditions, points to the necessity for change that is inherent and uncontrollable in the natural vowel but is absent in the synthetic vowel. These results are limited to isolated vowels, without any consonantal environment.
Spectra of some Stop Consonants
Authors: Halle Morris , Smith Caldwell P.
Published: September 2005

Although considerable data exist describing the spectra of vowel sounds, little information is available with regard to the consonants. Such information is obviously of great importance to all who are interested in the study of the acoustical properties of speech. A study has been made of the spectra of some of the stop consonants, using a refined measurement technique that permits precise separation of the consonant from its context. From a catalog of syllables recorded on magnetic tape, loops of tape were prepared, each containing a single syllable. A tone burst was added immediately preceding the syllable, to provide precise timing of both an oscillographic display and the segmentation circuitry. By observing the filtered and rectified speech wave on a long-persistence cathode-ray tube, it was possible to identify the characteristic vowel and consonant patterns, as well as to separate the burst from the aspiration in the stop consonants. Using visual and aural observation of the speech signal as a guide, it was also possible to precisely adjust timing circuits so as to switch the speech signal and the measuring circuitry on and off during that interval of the speech sound for which a spectrum was to be ascertained. Some of the results obtained by these measurements, as well as details of the measuring device, are presented.
Zero Crossing Intervals of a Sine Wave in Noise
Authors: Young F. Mansfield , Grace James C.
Published: September 2005

The probability distribution of zero-crossing intervals has been calculated for signals consisting of a sine wave and filtered random noise. The distribution function obtained holds for a wider range of conditions than that found by S. O. Rice and agrees with Rice's result in the range of validity of that function. Experimental results will be presented which cover a wider range of conditions than do the theoretical calculations.
The Masking of Tones by White Noise as a Function of the Interaural Correlation for the Noise
Authors: Jeffress Lloyd A. , Blodgett Hugh C. , Deatherage Bruce H.
Published: September 2005

In a previous paper the authors have described the effects upon the masking of a tone by white noise of increasing the interaural time delay to one ear for the noise by amounts in excess of one cycle of the masked tone. The smaller masking level difference that occurred when the delay was one and one-half cycles as compared to one-half cycle was attributed to a reduction of the interaural correlation for the noise. In the present study the interaural correlation is reduced by adding uncorrelated noise at the two ears. The relation between the interaural correlation and the masking level difference is presented.
Pitch Shifts as They Relate to Hearing Losses Caused by Simultaneous Masking
Authors: Webster J. C. , Schubert E. D. , Reger S. N. , Schultz M. , Rhinehart F.
Published: September 2005

A method was developed for getting a continuous and running record of pitch matches between the two ears over a frequency extent of one octave. The automatic recording audiometer, usually referred to as a “Bekesy” audiometer, measures hearing loss (amount of attenuation) as a continuous function of frequency. By using the automatic frequency and paper drive features of the Iowa version of the “Bekesy” audiometer but using the subject's adjustment of a second oscillator, instead of an attenuator, to drive the recording pen, a continuous tracing of pitch matches is available. A one-second tone from the motor driven (standard) oscillator (controlling the paper drive) is presented to one ear and a two-second tone from the subject's (variable) oscillator (controlling the pen writing mechanism) is presented alternately to the other ear. Between successive presentations of the standard tone its frequency is shifted continuously upward in increments of 0.77 percent. The listener, who controls the frequency of the variable tone, is instructed to keep the two alternating tones matched in pitch. In this manner 90 distinct pitch matches are made in each octave and a permanent record of the frequencies of the two oscillators is available. Ten music students made these pitch matches over the six-octave interval from 150–9600 cps. In the ear in which the standard tone was heard various types and amounts of hearing loss were simulated by masking noises. Results indicate a correlation between the slope of the hearing loss and the amount and direction of the pitch shift. The pitch tends to shift away from a region of hearing loss to a region of no, or less hearing loss. Downward shifts in pitch were observed but were never as marked as the upward shifts.
Information and Absolute Pitch
Authors: Ward W. D.
Published: September 2005

Although the ability of most listeners to identify (and name) sinusoids presented in isolation is relatively poor, certain individuals are exceptionally proficient at this type of task. These people are said to possess “absolute pitch.” The present paper deals with a series of information-theory-type tests applied to one such observer. The results imply that for this subject over 6 bits of information are transferred via pitch, i.e., that more than 70 different frequencies in the 50 to 4500 cps range can be consistently identified (10 times out of 10). Maximum information transfer is apparently reached when the stimuli are approximately one semitone apart. While this performance is somewhat below the subject's expectations, it is several orders of magnitude above the 5 categories (2.3 bits) recently reported by Pollack for the average listener.
The Attenuation of Magnetoacoustic Waves
Authors: Leonard R. W.
Published: September 2005

This paper reports the results of a study of the damping effect of a transverse magnetic field on the attenuation of longitudinal waves in a conducting bar of circular cross section. The attenuation is measured by a decay method. The bar is excited by an electrostatic actuator and the motion of the bar during decay is picked up by the polarized actuator electrode, amplified and recorded on a level recorder. Measurements at frequencies between 16 and 80 kilocycles and in fields up to 10 kilogauss have been made. The net increase in decay rate at 40 kilocycles is about 1 decibel per second for a 1.25-cm diameter aluminum alloy rod in a field of 10 kilogauss. The attenuation due to the magnetic field seems to depend on the square root of the frequency. No change in phase velocity has been observed.
The Effect of Reduced Transmission Time of Side-Tone upon Rate of Utterance
Authors: Peters Robert W. , Davidson G. Donald , Black John W.
Published: September 2005

Earlier studies established that delayed side-tone was accompanied by a retardation in the rate of talking. These results prompted the view that airborne side-tone, being experienced subsequent to “bone-conducted” side-tone, might be a retarding influence in the rate of talking. This assumption was tested experimentally. In a series of studies in which the transmission time minimum of the air channel was reduced, with signal level at the ear held constant, statistically significant increased rates of oral reading were observed. Each study involved 18 male subjects. Sets of measurements were secured from both throat and lip placements of microphones feeding acoustically isolated receivers.
Field Measurements of the Attenuation of Sound in the Low Audio Range
Authors: Delsasso L. P. , Leonard R. W.
Published: September 2005

The fluctuations in the attenuation constants for sounds in the open in the frequency range 125 to 1000 cycles/sec have been determined for a series of meteorological conditions. Laboratory measurements for comparable pressure, temperature, and humidity conditions in the range 1000 to 6000 cycles/sec have been made. Although the fluctuations in the field measurements are large, general agreement is observed at the overlapping frequency of 1000 cycles/sec. Possible causes for the differences are proposed. This work was partially supported by the U. S. Air Force.
The Compound Diffraction Projector
Authors: Hoodwin L. S.
Published: September 2005

A public address type driver unit loaded with a reintrant horn on one side of the diaphragm for reproducing the bass frequencies and with a small straight horn on the other side is described. Owing to the small size of the high frequency radiator, considerable improvement in high frequency dispersion is achieved. The high frequency response also is considerably improved over that of a conventional reintrant horn. The two throats of the two horns can both be designed with optimum impedance for the frequency range they are to reproduce.
Applications and Suggestions for Research Concerning Acoustical Problems in Medical Engineering
Authors: Hilliard John K.
Published: September 2005

This paper discusses some of the research and development already undertaken or being considered in the fields of heart and circulatory diseases, neuro-surgery, shock or blast effect on animals, impairment of hearing from military and industrial high level noise, and communication aids to surgery.
Gun-Shot Reinforcers and Synthesizers
Authors: Hathaway J. L. , Lafferty R. E.
Published: September 2005

The sound of a gunshot in a studio is difficult to reproduce. In the home it often sounds more like the snap of a finger than the roar of a gun. The factors responsible are described, together with a means of circumventing their combined effect. A shot may be electronically reinforced with great realism through the use of simulated reverberation. This is keyed on automatically by the initial impact and then decayed gradually to create the effect of a longer shot, which in turn produces a louder sound. Methods are also described for generating several special effects, such as the distant roar of cannon fire, the chatter of machine guns, and the ricochet whine of bullets.