bluuebunny/crossref_metadata_2025 huggingface dataset:
Flutter Noise in Suspended Hydrophones
Authors: Urick R. J.
Published: September 2005

Telephone wires have long been known to “sing” in a wind, creating aeolian tones attributed to periodic vortex shedding reinforced by the vibrational resonance of the wire. When a hydrophone is suspended from a cable in a water current, a similar quasi-sinusoidal noise, caused by cable flutter transmitted to the hydrophone, may be expected at the hydrophone terminals. Observations of what was apparently noise of this type were made in Chesapeake Bay at current speeds up to 1.3 knots. Strong sinusoidal noise voltages, at frequencies between 10 and 30 cps, depending upon current speed, were measured at the hydrophone terminals. The amplitude of this noise varied irregularly with cable length and cable tension but often was equivalent to a sound pressure level of +35 db above 1 dyne/cm2. Several remedies were tried and found effective: using a streamlined cable fairing, reducing cable tension by suspending the hydrophone by rubber bands just above its attachment to the cable, and using a hydrophone less sensitive to acceleration. Noise of this type should be expected in all low-frequency work with suspended hydrophones but apparently can be reduced greatly by one or more simple control methods.
Factors That Determine the Equivalent Noise Pressure, Free-Field Voltage Response, and Efficiency of a Transducer at Low Frequencies
Authors: Kendig P. M.
Published: September 2005

The physical properties and dimensions that determine the equivalent noise pressures of several different shapes of piezoelectric transducers are examined. Expressions for the equivalent noise pressure, free-field voltage response, and electroacoustical efficiency at low frequencies—those frequencies for which the free-field voltage response is flat—are obtained in terms of generally known or easily measured parameters. The results indicate that at low frequencies a well-designed hydrophone should have an equivalent noise pressure well below zero sea-state ambient. It is emphasized that the free-field voltage response is not an indication of a hydrophone's ability to measure low level signals but that either the efficiency or the equivalent noise pressure is an essential indication of this ability.
Sound Output of Nickel and Permendur Magnetostrictors under Static Compressive Stress
Authors: Whymark Roy R. , Witting James M.
Published: September 2005

Relative sound output of bar magnetostrictors has been measured as a function of polarization at atmospheric stress and at a compressional stress of about 300 atm. Frequencies both at and well below the resonant frequency of the bar have been used. Thin laminations of nickel or permendur are glued together to form bars whose ends are then ground parallel. Each bar is length resonant at about 23 kcps. Half-wavelength brass stubs with large flanges at their centers are soldered to the bar and loosely mounted in cylinders. Air can be forced between the flanges and cylinder walls to apply compressive stress to the bar. The displacement of the end of one of the stubs is observed. For nickel, compressive stress increases sound output at low polarizations and decreases sound output at higher polarizations. Representative values are these: a compressive stress of 300 atm increases the resonant amplitude by 15% at a polarizing field of 20 oe and decreases it 15% at 70 oe. The polarization at which 300 atm compressive stress does not alter the resonant displacement is 45.3 oe. Displacements at 60 cps and at resonance are related by a theory involving the magnetostrictive constant and the reversible permeability, and there is good agreement between this theory and experiments. (This research was supported by the Office of Naval Research.)
Use of Ultrasonic Surface Waves in the Determination of Residual Stress in Metals
Authors: Frederick Julian R.
Published: September 2005

In some early experiments in ultrasonic reflectoscopy [F. A. Firestone and J. R. Frederick, J. Acoust. Soc. Am. 18, 204 (1946)] it was observed that residual stresses in metals cause a change in the velocity of surface (Rayleigh) waves by a few tenths of a percent. This work has been extended and the measurement techniques improved. Results obtained on various aluminum and steel alloys under different states of stress will be presented.
Formants as a Voicing Cue in the Perception of /z/
Authors: Ingemann Frances
Published: September 2005

In earlier synthetic speech studies by researchers at Haskins Laboratories of the cues for the voiced-voiceless distinction of fricatives in initial position, hiss duration, intensity of the hiss relative to the vowel, separation of hiss from the vowel by a short period of silence, and the consonant-vowel transitions all had been shown to have an effect. In the present study, the addition of voiced formants to the hiss portion has been investigated. These formants are a rather obvious device for signaling voicing since a voice bar and often higher resonances as well can be seen clearly on spectrograms. In this experiment versions of the syllables /si/ and /zi/ were synthesized on PAT using the following variables: hiss amplitude: −5, −10, −15, and −20 db from the peak intensity of the vowel: voiced formant amplitude during fricative portion: −10, −20, and −30 db from the peak intensity of the vowel, as well as hiss without these formants; duration of the fricative portion: 85, 124, 170, and 213 msec; a 20-msec separation of the fricative portion from the onset of vowel transitions vs no separation; and vowel portions copied from spectrograms of spoken /si/ and /zi/. Low hiss intensity, short duration, no separation, and vowel formants from spoken /zi/ all tended to produce z judgments, but, as anticipated, the addition of voiced formants was the overriding cue.
An Approach to the Evaluation of Vocoded Speech
Authors: Meeker Willard F. , Smith Caldwell P. , Smith William R.
Published: September 2005

Distortions peculiar to speech analysis-synthesis devices, such as the vocoder, introduce new problems in the determination of performance standards. Intelligibility can be determined in the laboratory, but standardized methods for the measurement of the characteristics of speech quality and talker recognition have not yet been established. The researcher or engineer has little to guide him in the inevitable compromises involved in the development and design of such devices and in the selection of optimum data rates. In an effort to provide such guidance, a survey was first made of certain potential military users. Speech samples obtained by filtering, telephone transmission, and vocoder processing were presented for judgments regarding intelligibility, speech quality, talker recognition, and adequacy for normal use. This paper presents the results of these surveys, along with data regarding intelligibility and talker recognition obtained in the laboratory. The speech samples used in the survey will serve as a basis for comparison in laboratory judgments to determine approximate boundaries for those parameters affecting the characteristics considered.
Human or Machine?—a Study of Voice Naturalness
Authors: Kersta L. G. , Bricker P. D. , David E. E.
Published: September 2005

Critical examination of human voiced speech indicates that both sustained vowels and connected speech often show considerable variability in the spacing of successive glottal pulses. Such fluctuations are too rapid to be reproduced by a low-frequency parametric pitch signal as classically used in vocoders. What is the perceptual significance of these fluctuations? A preliminary investigation of this question utilized sustained vowels spoken by two talkers. The fluctuations in these utterances were removed by digital computer processing. These samples were used, together with the originals, to form A-B pairs, each pair containing one “machine” and one “human” utterance. Subjects were presented with the A-B pairs and asked to identify the one made by the human. Naive subjects were able to perform almost perfectly in this task. Since rapid period-to-period fluctuations were the distinguishing characteristic of the human samples, this result is evidence for the role of such fluctuations in voice naturalness.
Speech Detection in Noise
Authors: Veneklasen Paul S. , Christoff Jerry P.
Published: September 2005

Exploratory work to optimize the signal-to-noise ratio and the wearability of speech detection devices for use in high-altitude flight has led to quantitative determination of the relative capabilities of head contact devices in relation to acoustic detectors. One of the discoveries of the exploratory work was that an accelerometer fastened to an upper front tooth will detect highly intelligible speech and discriminate strongly against external noise. Data is presented on the acceleration spectrum of the teeth as a result of speech and external noise. The signal-to-noise ratio of the tooth microphone is compared to other speech reception systems under the same conditions. Refinement of the attachment of the accelerometer to the teeth is discussed. (This research was supported by U. S. Air Force contract.)
Review of the Noise Generation of Rockets and Jets
Authors: Eldred Ken
Published: September 2005

The recent emphasis on the study of jet-noise generation has resulted in a large body of data giving the acoustical power, spectra, and directivity of the noise radiated from jets which vary in nozzle diameter from 12 to 40 in., in velocity from 500 to 6500 ft/sec, and in total temperature from 525 to 4500°R. This paper examines the relationships between some of these acoustical data and the various accompanying jet parameters in an effort to define similarity laws which apply to all jets studied. These empirical correlations give a normalized power spectrum which is applicable for both subsonic and supersonic fully expanded flows. The resulting similarity characteristics are shown to extend the usefulness of a Lighthill parameter to the prediction of the noise of supersonic flows. (Supported by U. S. Air Force, Wright Air Development Division, Bio-Acoustics Branch.)
Measurement of Rocket Engine Noise
Authors: Keast David N.
Published: September 2005

The measurement of rocket engine noise has become a part of many recent missile and space vehicle development programs. The circumstances under which these measurements must be made, and the type of information desired, present the acoustician with several unique problems in acquiring and interpreting his data. This paper suggests a general approach to the measurement and analysis of rocket engine noise and illustrates this approach with examples of problems which have been encountered in the past. The general approach suggested is intended to provide not only the desired data, but also sufficient information about the instrumentation and measurement techniques employed so that possible sources of error in the data may be evaluated. This approach includes a detailed laboratory evaluation of the transducer systems to be employed, evaluation and calibration of the complete data acquisition system installed at the test site, and methods for reducing the possibility of human error by measurement personnel. Various steps in the processing and analysis of tape-recorded data, and the eventual estimation of the causes and magnitudes of errors in the measurements, also are discussed.
Determination of Ultrasonic Absorption Coefficients of Liquids by Thermoelectric Probes
Authors: Dunn Floyd
Published: September 2005

The determination of ultrasonic absorption coefficients of high-absorption liquids by thermoelectric probes is accomplished by exciting the sound source (in contact with the liquid) electrically to emit acoustic pulses of accurately reproducible envelopes and time durations. The temperature in the immediate neighborhood of the thermocouple junction is transiently increased, principally because of the action of the viscous forces acting between the thermocouple wires and the liquid. The peak amplitude of this transient thermoelectric emf is proportional to the acoustic intensity in the plane wave field in the immediate neighborhood of the junction. The space gradient of the acoustic intensity is obtained, and the value of the absorption coefficient is readily computed from this data. Results obtained with several oils in the 100 Mc/sec frequency region will be presented. (This work was supported by the Office of Naval Research.)
Measurement of the Distortion of Finite Ultrasonic Waves in Liquids
Authors: Lutsch A. G. , Ryan R. P. , Beyer R. T.
Published: September 2005

Pulses of ultrasound of finite amplitude are generated in liquids by an electrical system of 2.5-Mc frequency and an air-backed x-cut quartz crystal of the same fundamental frequency. After passage through the liquid, the signals are received on a 2-in. diameter x-cut quartz crystal with a fundamental frequency of 25 Mc. The distortion is measured either by a Fourier analysis of the received waveform or by measurement of the various harmonics by a tunable amplifier. Photographs of the waveform at different distances from the transducer illustrate the formation of the sawtooth wave. Comparison is made between the measured harmonic content and that computed theoretically. [The work was supported in part by the Office of Naval Research and by a National Research Council Fellowship to one of the authors (A.G.L.).]
Infrasonic Waves in the Atmosphere Associated with Geomagnetic Disturbances
Authors: Chrzanowski P. , Young J. M. , Greene Gray , Lemmon K. T.
Published: September 2005

Experimental observations at Washington, D. C., indicate that infrasonic pressure waves are generated in the atmosphere during geomagnetic storms. Certain of these waves are characterized by (a) periods between 20 and 300 secs, (b) trace velocity over the earth's surface higher than the local speed of sound, (c) consistent diurnal-directional properties, with azimuths changing from the NE to the NW during the progress of the day. Observational data on these traveling waves are presented and discussed in relation to magnetic field fluctuations and other accompanying phenomena.
Determination of the Acoustic Impedance of a Viscoelastic Fluid in a Circular Tube
Authors: Thurston George B.
Published: September 2005

An experimental study of the behavior of the acoustic impedance of some viscoelastic fluids in circular tubes has been carried out. Some of the fluids concerned are petroleum oil, silicone oil, glycerine, and water. It is found that the shear rigidity of the fluids has a pronounced effect on the acoustic impedance. Under appropriate conditions the elastic effect serves to change the acoustic reactance from an inertance type to a compliance type. The experimental determinations are correlated with a previously published theoretical treatment [G. B. Thurston, J. Acoust. Soc. Am. 32, 210 (1960)]. The experimental behavior and theoretical description are in substantial agreement. (This work was supported by the Office of Ordnance Research of the U. S. Army.)
Torsional Horns
Authors: Pyle Robert W.
Published: September 2005

Investigation has shown that tapered solid elastic rods display hornlike behavior for torsional waves. These have been dubbed “torsional horns.” The propagation of torsional waves in torsional horns is governed by a typical small signal wave equation which, for certain horn contours, can be particularized to a torsional analog of the Webster horn equation. Solutions of these equations for various horn contours will be shown, and experimental results will be presented showing good agreement with the theoretical predictions. (This work was supported in part by the Office of Naval Research.)
Diffraction of a Dipole Field by a Rigid Cone
Authors: Yildiz M. , Mawardi O. K.
Published: September 2005

The general solution for the diffraction of a dipole field by a semi-infinite rigid cone has been obtained. The method similar to that reported in some previous work [M. Yildiz and O. K. Mawardi, J. Acoust. Soc. Am. 32, 921 (1960)] makes use of spectral representations of the Green's function pertinent to the problem. The diffracted sound field has been derived for an arbitrary orientation of the dipole axis. The functional dependence for the distribution of the sound pressure on the surface of the cone has been discussed in detail.
Perturbations in Vocal Pitch
Authors: Lieberman Philip
Published: September 2005

An acoustic analysis of perturbations in pitch, that is, sudden irregularities in the glottal excitation rate, during connected speech, is described. The effects of different vocal tract configurations, over-all sentence intonation, and different speakers on these discontinuities are noted. The degree of accuracy with which these perturbations must be located in time for the retention of natural quality and talker identification is discussed.
On the Limitations of the Decision-Theory Approach to System Optimization
Authors: Van Meter David
Published: September 2005

The assumptions of statistical decision theory are examined, and its domain of application delimited. Two common criticisms of the usefulness of the theory are discussed: (1) the argument that there are classes of inference problems outside the proper domain of the theory which are particularly important at present in view of our need for increased understanding of signal and noise processes in the sea, and (2) the argument that even within its proper domain the theory is of questionable value because costs and prior probabilities appropriate to the context of application are difficult if not impossible to justify.
Space-Time Decision Theory of Acoustic Field Processing
Authors: Stocklin Philip L.
Published: September 2005

The space-time decision theory of acoustic fields is developed from an examination of the conditions necessary for uniform and nonuniform spatial sampling. Basic transformation relations between spatial sampling and direction sampling are given. From this sampling theory, the monochromatic space likelihood ratio, the broadband space likelihood ratio, and the general space-time likelihood ratio are developed. Examples of the general theory include the point, blurred point, and ring signal fields and several (spatially) statistical noise fields. Superdirectivity and the spatial matched filter are related to the space-time likelihood ratio, and the paper concludes with a brief discussion of necessary and complete information inherent in an acoustic field.
A Study of Community Background Noise
Authors: Clark W. E. , Clark V. A. , Pietrasanta A. C.
Published: September 2005

A field measurement program was performed to determine the background noise levels as a function of time and location for a number of selected community areas in one city. Measurements of background noise were obtained according to a randomized sampling schedule. Concurrent observations were obtained of street traffic flow and other significant noise sources. This paper discusses variations in mean background noise spectra with time and location and short term fluctuations in noise levels. Inferences are drawn for the number and disposition of necessary field measurements to establish background noise levels with desired precision. The degree of correlation between background noise levels and street traffic flow is discussed and related to a previously reported procedure for estimating the noise from vehicular traffic [J. Acoust. Soc. Am. 29, 779(A) 1957]. Comparisons are drawn between observed aircraft noise and background noise from all other sources. [This work was supported in part by the U. S. Air Force under Contract Nos. AF 33(616)-3335 and AF 33 (616)-3938.]
Minimal Rules for Synthesizing Speech
Authors: Liberman A. M. , Ingemann Frances , Lisker Leigh , Cooper F. S.
Published: September 2005

In attempting to synthesize speech by rule, one must take account of the fact that the perceptually discrete phonemes are typically encoded at the acoustic level into segments of approximately syllabic length. It is, therefore, not possible to synthesize speech by stringing together prefabricated phonemes. By taking advantage of knowledge about the acoustic cues for speech perception, however, one can write rules for synthesis in terms of the constituent phonemes plus a few rules of combination. Thus, the number of rules can approximate the number of phonemes rather than the number of syllables. Indeed, one can reduce the number of rules still further by writing them in terms of subphonemic dimensions, viz., place and manner of articulation and voicing. Several complicating factors make it impossible to achieve an ideal minimum. First, rules must be added to take care of certain prosodic and positional variations. Failure to do so not only affects naturalness, but also impairs intelligibility, even at the level of segmental phonemes. Second, it is necessary in a few special cases to have different rules for a single consonant phoneme (or dimension) before different vowels. This reflects the occasionally complex relation between phoneme and articulation on the one hand and sound on the other; presumably, this complication would not affect the rules of synthesis for an articulatory model.
Thresholds for Interaural Difference in Intensity
Authors: Mills A. W.
Published: September 2005

The just noticeable interaural difference between the sound pressure levels of similar stimuli presented simultaneously to both ears was measured by the method of single stimuli as a function of frequency, sensation level, and interaural phase. The stimuli were either 1-sec tone pulses, 1-sec pulses of bands of noise, or a 0.2-msec click. For each kind of stimulus, thresholds were determined individually on 7–11 observers. When the stimuli are pure tones with no interaural difference in phase, the threshold for interaural difference in intensity is maximal at about 1000 cps (approximately 2 db at 50 db SL). Two octaves above or below this frequency the threshold is about half as large. If, however, the tones are presented 180° out of phase, the maximum at 1000 cps does not appear. When octave bands of noise are used, the threshold is the same whether the interaural phase is 0, 180°, or random, and there is no maximum at 1000 cps. When a click is presented out of phase, the just noticeable interaural difference in intensity is about half as large as for the same click in phase. For pure tones the threshold is a tilde-shaped function of the sensation level (10 to 90 db) with a maximum at 30 db and a minimum at 70 db. For a click, however, the corresponding function is caret-shaped with a maximum at about 50 db SL.
Individual Differences and Inconclusive Experiments in Psychophysics
Authors: Tanner Wilson P.
Published: September 2005

Psychoacoustic experiments are usually performed with a small number of observers. Sometimes the experimental results are inconclusive and not reported. Results which appear conclusive do not readily yield to replication. Examination of “inconclusive” results in the light of conclusive data suggests that individual differences between observers may account for the inconclusiveness encountered in some experiments and the nonrepeatability of others. These differences are not mere differences in level of performance but differences in pattern of performance, sustained over a series of experiments. Observers who perform well detecting pure tones have been observed to perform poorly detecting samples of noise, and vice versa. A similar reversal has been observed between short- and long-duration tones. Frequently performed experiments involving the detection of tones of unknown frequency indicate that the uncertainty sometimes has a detrimental effect, sometimes does not, and sometimes gives a mixed result. A single parameter to account for these results is suggested. The crux of these differences appears to be that the hearing mechanism is not fixed, and that the way in which it is variable differs between observers. The individual differences are discussed with a view toward a procedure for characterizing these differences in a consistent manner.
Analysis for a 3 kMc Ultrasonic Liquid Interferometer
Authors: Stewart James L.
Published: September 2005

The equivalent electrical circuit for a variable path liquid interferometer employing an X-cut quartz crystal in the gap of a reentrant cylindrical 3 kMc electromagnetic cavity resonator will be presented. This will be employed in an analysis of the optimum design of the cavity-crystal transducer system and associated microwave circuitry to determine the change in the radiation impedance as the acoustic path is varied. The analysis of the atter to yield values of the acoustic velocity and high attenuation in the liquid and reflection at the liquid-solid interfaces and the design of the acoustic interferometer will also be discussed.
Comparison and Discussion of Some of the Current Ideas and Methods for Obtaining Microwave Ultrasonics
Authors: Truell Rohn
Published: September 2005

Some of the current methods of generating and using ultrasonic waves in the range of 1000 mc will be discussed together with speculation on some of the uses of such microwave ultrasonic energy.
Shallow Water Sound Propagation Studies in an Intermediate Scale Model
Authors: Scrimger J. A.
Published: September 2005

A one-half square mile of constant water depth of about 10 ft is situated close to Pacific Naval Laboratory in the Esquimalt Lagoon and forms a natural intermediate scale model tank for shallow water propagation studies. Bottom velocities and layering depths have been obtained from a seismic survey. The dependence of sound level of a cw signal on transmitter depth at range to depth ratios of 30, 45, 60, 75, 90, 105 has been obtained for the first three modes of propagation. The received sound level from a transducer moved at constant depth towards the receiver has been recorded with acoustic wavelength and transducer depth as parameters. Instrumentation to (a) permit rapid measurement of vertical temperature profiles from a remote position, and (b) to record surface wave amplitude at the Tx and Rx position has been installed. Fluctuations in level of received cw signals appear well-correlated in frequency with surface waves. The dependence of the percentage of fluctuations on (a) surface wave amplitude, and (b) acoustical wavelength has been determined. A wave packet experiment, using the bow wave from a boat to produce the sound fluctuations, throws some light on the mechanism involved.
Propagation of Narrow Band-Limited Noise in a Layered Wave Guide
Authors: Clay C. S.
Published: September 2005

The propagation of narrow band-limited noise in a layered wave guide was studied experimentally and compared with theory. The experiments were made with a noise source that had a band width of about 1/15 of the center frequency. The depth of water in the area was about 2λ and the water was over a thick layer of unconsolidated sediments. These experiments were made in the area studied by Tolstoy as reported in J. Acoust. Soc. Am. 30, 340 (1958). Normal mode theory for a simple harmonic point source in a layered wave guide was extended to include a band-limited noise source. Experimental acoustical pressure levels as a function of source distance are compared with the theoretical pressure levels. (This work was supported by the Office of Naval Research.)
Preliminary Investigation of Internal Waves and Underwater Sound
Authors: Lee Owen S.
Published: September 2005

A preliminary study of underwater sound in the presence of internal waves is presented. The medium through which the sound travels and the internal wave are idealized but both approximate summer conditions in shallow water off the coast at San Diego, California. The ray paths, sound intensity field, and the transmission anomaly field were determined by machine computation and the data are shown graphically. The results indicate that internal waves can cause 22 db change in sound intensity over a distance of about one-half the internal wavelength. Future experiments at sea and additional computations which are based on this study are briefly discussed.
Designing Auditory Perspective into Live Musical Presentation
Authors: Veneklasen Paul S.
Published: September 2005

Musical concert performances can be made more interesting and intelligible for audiences by enhancing auditory perspective. The musical meaning can be dramatized by stage placement of instruments or groups taking cues from the musical or textual context. Often the identity or distinction between voices can be enhanced by special localization or relative liveness. Examples are given from orchestral and oratorio literature.
Acoustic Čerenkov Effect
Authors: Mawardi Osman K.
Published: September 2005

It is well known that the energy of a moving screw dislocation becomes infinite in a relativistic fashion as its velocity approaches the velocity of transverse elastic waves. There are nevertheless dislocation-like solutions which move with supersonic velocities. The radiation from such dislocations, a true equivalent of an acoustic Čerenkov effect, has been examined. The role played by these dislocations to diffusion theory has also been studied.
Absorption of Finite Amplitude Waves
Authors: Westervelt Peter J.
Published: September 2005

The nonlinear absorption of waves has been calculated by a method which does not require the solution of the nonlinear wave equation. The method appears to be useful for waves of arbitrary configuration. (This work has been supported by the Bio-acoustics Branch of the Wright Air Development Center, U. S. Air Force.)
Propagation of a Pressure Pulse in a Compressible Flow
Authors: Powell Alan
Published: September 2005

A nonlinear one-dimensional treatment of the propagation of a pressure pulse through a channel carrying a compressible flow is given. The method consists of summing the multiple linear reflections of increasing order, the “transmission” and “reflection” coefficients then being given in the form of power series whose terms depend only on the entry and exit Mach numbers of the mean flow. The series fails to converge under certain circumstances, but there is nothing corresponding to this in the equivalent results obtained from “before” and “after” steady-state considerations. Both methods are shown to fail at a sonic throat, where it is suggested that the assumption of one-dimensional perturbations is no longer tenable.
Magnetohydrodynamic Wave Guide
Authors: Mawardi Osman K.
Published: September 2005

The propagation of Alfvèn's wave in an ideally conducting wave guide filled with a plasma is studied. It is predicted that the Alfvèn's waves are coupled to conventional acoustic (compressional) waves which propagate with an altered sound velocity. The behavior of these waves is reminiscent of the longitudinal and shear waves in a rod and an analog of the Poisson's ratio is the ratio of the magnetic pressure in the plasma to the collision pressure in the gas.
Vibration and Sound Radiation of Plates
Authors: Greene David C.
Published: September 2005

A plate, suspended on thin wires, has been vibrated by an electromagnetic vibration generator. From this experiment, these measurements were obtained: the amplitude distribution; the driving-point velocity as a function of frequency; and the sound pressure radiated as a function of frequency. The measurements were repeated after the plate had been heavily damped with an asphaltic coating. The velocity was found to decrease with distance from the driven point. The rate of decrease became greater when either the damping or the frequency was increased. The driving-point impedance and the radiated sound pressure proved to be practically independent of the frequency and of the damping of the plate. These results are in agreement with the theory.
Excitation of Finite Plates by Variable Incidence Sound
Authors: Dyer Ira , Watters B. G.
Published: September 2005

The vibration of finite plates excited by sound waves is of importance in such problems as the transmission loss of walls and the fatigue of aircraft panels. Schoch, and, more recently, Powell have given wave theories for the excitation of finite plates. The connection between these theories is shown, and the use of Schoch's asymptotic “mass law” is discussed. Laboratory measurements made on finite panels are presented as functions of frequency (mode configuration) and of angle of incidence. These measurements are compared with theory. It is shown that a finite plate behaves not only as a frequency filter but also as a space filter. The response averaged over both frequency and spatial angle is also obtained. [This work was supported in part by the Wright Air Development Center under Contract AF 33 (616)-6340].
Limits of Ear Protection
Authors: Veneklasen Paul S.
Published: September 2005

Recent experience using improved muff type ear protectors confutes earlier experience, that in a high noise field use of earplugs under receivers improves intelligibility. Careful investigation following this lead strengthens the theory: that the attenuation of aural protective devices is limited by vibration of the skull in the sound field, which is most prominent in the frequency region of the first flexural mode; that the skull vibrates with respect to a relatively immobile protector; that the attenuation limit will be increased by making the protector as light in weight and large in volume as possible and coupling it to the head over a large contact area and as rigidly as possible.
Measurement of the Acoustical Impedance of the Guinea Pig's Ear
Authors: Mundie J. R. , von Gierke H. E. , Rebbin W. R.
Published: September 2005

The complex impedance of the guinea pig's ear at the tympanic membrane was calculated from measurements of the sound pressure in front of the tympanic membrane and in the closed bulla. In order to arrive at the accurate quantitative values, it was necessary to calibrate probe tube microphones with respect to amplitude and phase and to determine the air volume in the bulla. Advantages and limitations of this method are discussed. Data are presented showing impedance as a function of the frequency from 100 through 9000 cps. These data show the impedance decreasing inversely proportional to frequency from 100 through 2000 cps, and to equal the impedance calculated for an air-filled chamber having a volume the same as the bulla. These relationships disappear above 2000 cps, and following a minimum, the impedance reaches another maximum between 3000 and 5000 cps. The frequency at which this peak occurs varies with the animal. The lowest impedance of 50 to 100 dyne-sec/cm2 appears between 4000 and 7000 cps, again varying with different animals. The effects on the impedance of intensity of the stimulus, static pressure, anesthesia, and death are discussed.
Response of Cochlear Models to Aperiodic Signals
Authors: Tonndorf Juergen
Published: September 2005

The responses of the model to various aperiodic signals were analyzed by taking high-speed motion pictures of the resulting “traveling bulge” and by recording the accompanying particle motion within the cochlear fluids. Bekesy's earlier results were confirmed in many respects. Inward stapedial displacement caused the basilar membrane to bulge into scala tympani and vice versa. The displacement amplitude of the traveling bulge with distance along the partition represented the combined effect of the signal spectrum (with its low-frequency emphasis) and the (steady-state) response pattern of the model to its own natural frequency. Therefore, amplitude within the proximal portion of the partition was relatively higher than indicated by the spectrum alone. The velocity of propagation of aperiodic signals along the partition was higher than that of periodic signals. This is characteristic of capillary waves where group velocity > phase velocity. In gravity waves this relation is reversed. When impulses were repeated periodically (rate≪natural frequency) a pair of large, slowly revolving eddies appeared which followed well-defined pathways. This is in contrast to eddies caused by sinusoidal signals which soon become diffuse as each newly generated harmonic forms its own sub-eddy within that of the fundamental. [Research supported in full by U. S. Air Force contract AF 41(657)-148, monitored by School of Aviation Medicine, U. S. Air Force Base, Texas.]
Fabric Acoustimeter
Authors: Veneklasen Paul S. , Jorgensen Finn
Published: September 2005

An electro-mechano-acoustic instrument, known more intimately as the “Fuzz Feeler,” has been developed in cooperation with the U. S. Department of Agriculture to provide a quantitative measure of the properties known in the industry as the “handle” of fabrics. The noise spectrum of the combined vibration and noise produced when fabric samples are rubbed together is measured.
10-Millisecond Mechanical Audio-Frequency Delay Line
Authors: Ashley George H.
Published: September 2005

A new type of delay line has been developed for the audio-frequency range which provides delays of the order of 10 msec. The delay medium is basically a stretched string with two important modifications. First, the actual form of the delay medium is a thin 116-in. wide tape; second, the motion of the tape is twist rather than transverse. Tape with a thickness of about 12 mil is stretched until the velocity is 100 meters per second. The input transducer is a constant displacement moving conductor with the input current path defined by a short slit in the end of the tape and with a magnetic field across the width of the tape. Twist of the tape provides angular modulation of light beams which are reflected onto silicon solar cell pickups. The tape is sandwiched between two plates with variable spacing to provide an air viscosity termination. The time delay is uniform to within 0.1% over the frequency range of the delay line and the amplitude response is uniform to 5000 cps. Attenuation versus delay is negligible for metal tapes, but quite appreciable for plastic tapes above 1000 cps. (The work reported here was performed at Lincoln Laboratory, a technical center operated by Massachusetts Institute of Technology with the joint support of the U. S. Army, Navy, and Air Force.)
New Portable Sound and Vibration Analyzer
Authors: Faran James J.
Published: September 2005

A new portable, battery-powered sound and vibration analyzer has been designed. It includes two amplifiers connected in cascade which are made frequency selective by means of resistance-capacitance feedback networks of the “Wien Bridge” type. These frequency-selective amplifiers can be synchronously tuned for “narrow-band” response or stagger tuned to produce a one-third-octave band width. The “narrow-band” response is 3 db down ±4% from the center frequency and 40 db down at one octave. The one-third-octave response is 34 db down one octave from the center frequency. Far from the center frequency both of these response characteristics continue to fall off at a rate of 12 db per octave. An “all-pass” response is also provided to permit easy initial measurement of the amplitude of the entire input signal. The analyzer covers the range from 2.5 cps to 25 kcps. The circuits are almost entirely transistorized, except that some subminiature vacuum tubes are used because of the high impedance of the Wienbridge network for the lower frequency ranges. Inclusion of a handle guarantees portability.
Some Acoustic Correlates of Word Stress in American English
Authors: Lieberman Philip
Published: September 2005

English noun-verb pairs, differing principally in their stress patterns, were spoken by native speakers of American English. The fundamental frequency and the relative amplitude and duration of the stressed and unstressed syllables were determined and correlated with the aurally perceptible stress patterns. Certain “trading” effects in the importance of these parameters with different speakers are discussed.
Results Obtained from a Vowel Recognition Computer Program
Authors: Forgie James W. , Forgie Carma D.
Published: September 2005

As a first step toward a general speech recognition computer program, a program has been developed to recognize ten Eng.ish vowels in isolated words of the form /b/—bowel—/t/. Input to the computer was real time spectral data obtained through the input system described at these meetings in May, 1958, by Forgie and Hughes. The program was developed from the study of spectral data from five speakers. The program was tried on new speakers, the errors analyzed, and the program modified. The program, as finally evolved by this process, first determines the rough location of the first two formants. The remaining confusions are resolved by combinations of the following, as required: (1) position of F3, (2) the slopes of F1 and F2, (3) an estimate of pitch, and (4) the energy in the frequency band between F1 and F2. Whenever the over-all power is above an arbitrary threshold, each 6-msec scan of the filtered data is classified as one of the ten possible vowels. A tally at the end of the word determines the final decision. The over-all recognition score for 21 subjects (11 male and 10 female) is 89%. Modifications of the program took place on the first 16 subjects only. The score for the last five speakers was 82%. (Lincoln Laboratory is operated by Massachusetts Institute of Technology with joint support of U. S. Army, Navy, and Air Force.)
Time Compensation for Speed of Talking in Speech Recognition Machines
Authors: Olson Harry F. , Belar Herbert
Published: September 2005

A simplified version, but nevertheless complete system, of a phonetic typewriter which types in response to words spoken into a microphone has been described [H. F. Olson and H. Belar, J. Acoust. Soc. Am. 28, 1072–1081 (1956)]. One of the many extensions of the original system is a means for compensating for the effects of the speed of talking. Instead of transferring information into the spectral memory in fixed time increments, as was done in the original machine, the transformation of information is determined from a correlation with significant changes in the speech spectrum with respect to time. In the improved machine, coincidence detectors compare the current with the preceding speech spectrum. No information is fed into the spectral memory until a significant change has been detected in the speech spectrum. The system provides an automatic time compensator for variations in the speed of talking in a speech recognition machine.
Sound Scattering in an Inhomogeneous Medium in the Case of a Kolmogorov Distribution of Patch Sizes
Authors: Skudrzyk Eugen
Published: September 2005

In an acoustically inhomogeneous medium, such as the air or the sea, the sizes of the scattering patches are always distributed continuously. The patches vary from a maximum size determined by the geometry of the fluid to a minimum size prescribed by viscosity and heat conductance. The Kolmogorov equilibrium law seems to give a very realistic description of the dimensions of the scattering patches, though this law was originally derived for homogeneous turbulence only. The scattering integrals are evaluated for such a distribution of patch sizes. The results show backward scattering to be mainly determined by the patch sizes that have a diameter equal to the wavelength, whereas forward scattering is mainly produced by larger patches up to a certain size determined by the wavelength and distance. (The integrals have logarithmic infinities at the corresponding space-wave numbers of the patch distribution.) The results also show that backscattering radically limits the range of propagation at low frequencies to a value that depends on the distance from the nearest boundary.
Atmospheric Attenuation of Sound from an Elevated Source
Authors: Sabine Hale J. , Burkhard M. D.
Published: September 2005

Experimental data were given in a previous paper at the October, 1957, meeting of the Acoustical Society of America on the effects of weather parameters on the attenuation of sound propagated from an airborne source to ground. These results have been analyzed in greater detail and improved procedures have been developed for predicting attenuation. The residual attenuation, defined as that in excess of spherical divergence and molecular absorption, does not vary systematically with any of the measured weather parameters for source elevations of 10° or higher and for altitudes above a few hundred feet. Its average value in this region may be expressed by α0=0.11f12 db/1000 ft, where f is the frequency in cps. At lower source elevations and altitudes, the attenuation is increased by an amount depending on frequency, source elevation, wind velocity and direction, and temperature gradients. Comparison of upwind with downwind propagation for source elevations of 2°, 5°, 10°, and 15° shows a significant difference only for 2°. The statistical data for upwind propagation are compared with theory applicable to shadow formation. (This research was supported by Wright Air Development Center.)
Pressure Disturbance on the Ground due to a Low-Flying Aircraft
Authors: Cheever Richard N. , Oestreicher Hans L.
Published: September 2005

An analysis of the pressure disturbance at ground level produced by the lift of an aircraft in subsonic, level flight is performed for various altitudes and velocities. The form of this disturbance for the case of an aircraft operating at altitudes very large compared to its wingspan is well known, but it will be shown that, in the case of low altitudes with compressibility effects taken into consideration, the form of the disturbance differs quite markedly from that predicted by the high-altitude approximation. In some cases, the peak pressures turn out to be five or six times larger than those predicted by the high-altitude approximation. A frequency analysis of the pressure disturbance shows that most of the energy is contained in the sub-audible frequency range, even when the aircraft is traveling at relatively high velocities. A graphical illustration of the analytical results is presented using the B-52 let bomber as an example.
Second Low-Temperature Peak in the Internal Friction of Aluminum
Authors: Lax Edward , Filson Daniel H.
Published: September 2005

A peak in the internal friction of aluminum due to dislocations has been found well below the temperature of the usual Bordoni peak. Measurements were made from 26 to 130 kc using a reverberant decay method, and from 265 to 1075 kc using a pulse-echo method, yielding a relaxation peak that varied with frequency over a temperature range at 21°K to 31°K. The activation energy of the peak was found to be 0.024 ev. (This research was supported in part by the Office of Naval Research.)
Scattering of Acoustic Energy from Solid Metal Spheres in Water
Authors: Hampton L. D. , McKinney C. M.
Published: September 2005

This paper presents the results of an experimental investigation of the scattering of acoustic energy from solid aluminum and brass spheres in water when the scattered energy is measured at large distances. Polar diagrams of the distribution in angle of the scattered energy for a range of values of ka from 4.1 to 57 for steady-state conditions are presented. Measurements of target strength are presented for steady-state conditions and also using pulses short enough to measure only the surface reflection. The measured target strengths are compared to theoretical values. The variation of the backscattered pulse is shown as a function of frequency. The data indicate that the scattering of sound waves from such spheres cannot be described by formulas based on the assumption of a rigid sphere. (This work was partially upported by Bureau of Ships Contract NObsr-72627.)