bluuebunny/crossref_metadata_2025 huggingface dataset:
A Proposal Regarding Standard Terminology of Sonar Transducers
Authors: Horton J. W.
Published: September 2005

Two new quantities, a projector loss and a hydrophone loss, are described. These have been found useful in evaluating the performance of sonar transducers. Each is a true transmission loss. Each has been so specified that it is contiguous to other transmission losses normally identified with portions of a sonar system, including the water, which may adjoin the transducer. Convenient and simple relations between these losses and other characteristics of the transducer are noted.
A Theory of the Thickness Vibrations of a Barium Titanate Cylindrical Transducer
Authors: Globe Samuel
Published: September 2005

A transducer of barium titanate consisting of an infinitely long cylindrical shell of circular cross section and vibrating in a thickness mode is considered. The equation of motion is derived and solved for the steady-state condition. This solution is approximated by an asymptotic expression, valid when the wavelength is small compared with the radius, and from this asymptotic expression the coupled-circuit equations are derived. An expression is then obtained for the input impedance when the transducer radiates into water, from which expression the conventional equivalent circuit for a resonant system may be derived. This equivalent circuit is considered in two forms and its properties studied.
Increase of Low Frequency Absorption by Means of an Air Space between a Porous Material and its Perforated Facing
Authors: Callaway D. B. , Ramer L. G.
Published: September 2005

The increase of low frequency sound absorption of porous materials by means of perforated facings has been discussed by Bolt. Sepmeyer and Leonard have presented reverberation chamber measurements which show some discrepancies between theory and practice. Need for material with large absorption for a band of low frequencies led to an investigation of these discrepancies. Acoustic tube tests were made on two facings of 38-inch plywood with 12-inch holes, having 9 and 36 holes per square foot. The backing material was 3 inches of glass wool batt, which had an R/ρc of about 1.3. The first facing increased R/ρc to about 10, while the other facing resulted in values for R/ρc of about 4. With the first facing the peak absorption was only about 40 percent of the calculated value, and with the second facing about 70 percent. The increase in R/ρc indicated that the flow resistance of the sample had increased because the exposed area of the porous backing had decreased. It was decided, therefore, to investigate the effect of an air space between the facing and the backing material. It was found that the air space could be increased to reduce R/ρc to approximately that for the backing alone, and that this spacing gave values of absorption in good agreement with the calculated values. Design criteria for estimating the minimum spacing for facings with varying percent of open area will be presented.
A Study of the Acoustic Significance of the Amplitude and Phase of Harmonics Present in a Source of Sound in a Room
Authors: Robbins Joseph G.
Published: September 2005

This investigation is pointed toward obtaining some further measure of the acoustic properties of a room. First, a brief subjective study is made of listener response to sounds which have various degrees of modulation on their decay curves and to alternate sounds which differ only in the phase relation of one of the harmonics with respect to the fundamental frequency. Second, an objective study of the modulation of sound decay curves is carried out for rectangular rooms, with emphasis on the effect of the phase and amplitude of harmonics present in the source of sound. In this study, the sound source consisted of a Complex Wave Generator, a power amplifier, and a loudspeaker. The Complex Wave Generator provides output terminals for the fundamental frequency and for the first four harmonics at controlled phases and amplitudes. The power amplifier is a conventional resistance-coupled amplifier combined with a device designed to cut off the output at any predetermined phase of the fundamental frequency and with a minimum switching transient. The loudspeaker and the complete recording system are conventional.
Mechanism of Action of Ultrasound on Nerve Tissue
Authors: Fry William J. , Tucker Don
Published: September 2005

Further experimental work on the nontemperature effects produced by ultrasound in the central nervous system of frogs will be reported. Preliminary results presented at a previous meeting indicating that cavitation is not a contributing mechanism have been confirmed. The results of experiments designed to differentiate between the roles of particle acceleration, particle velocity, and acoustic pressure will be presented.
Absorption of Sound in Gases at Reduced Pressures
Authors: Parker J. G. , Adams C. E. , Stavseth R. M.
Published: September 2005

The absorption of sound in air is being measured for pressures in the region from one to ten mm of Hg and for frequencies in the neighborhood of 75 kc. Two rochelle salt transducers are mounted in a cylindrical tank which is surrounded by a water jacket for temperature control. The air in the tank has been dried and freed of CO2 by passing it through a tube containing CaCl2, then through a tube containing Ascarite, and finally through a dry ice trap. A triangular pulse of one-millisecond duration with sharp leading edge is applied to one of the transducers. With this shape of pulse, it is possible to distinguish the initial pulse from echoes which arrive one-fifth of a millisecond later. Measurements taken at atmospheric pressure show that for distances greater than 3 cm from the source, the pressure follows a 1/r relationship to within 5 percent. The second transducer is used to measure the sound pressure at various distances from the source. Its electrical output is first fed to a tuned amplifier with 3-db half-band width of 5 kc and then viewed on a cathode-ray oscilloscope. Data on absorption coefficients in air will be presented. Present plans include extension of the investigation to other gases.
The Grazing Acoustic Field of a Point Source Located at a Boundary
Authors: Lawhead R. B. , Rudnick I.
Published: September 2005

The study is divided into two parts: (1) an experimental investigation of the nature of the field (both as regards phase and amplitude) of a point source where the bounding medium is an isotropic medium, of effectively infinite depth, which can be characterized by a complex propagation constant and characteristic specific impedance; (2) a theoretical and experimental study of the field when the bounding medium is characterized by a constant normal specific impedance. In the first part, the field in air, when the bounding medium consisted of Fiberglas, was determined at a number of frequencies. The propagation constants and characteristic impedances of the Fiberglas materials were measured and the field calculated from the existing theory. The results were found to be in good agreement. In the second part, a theoretical investigation indicates that the result can be put in the same form as that given in reference 1, provided then umerical distance be defined by W = [(i2k1R2)/(1 − Rp)2]Z12/Zn2, where Zn is the normal specific impedance, and the plane wave reflection coefficient be given by Rp = (Zn cosθ1 − Z1)/(Zn cosθ1 + Z1). A material satisfying the constant normal impedance boundary condition was constructed of drinking straws. The field was measured and compared with the theoretical field and they were found to be in good agreement.
Directionality Patterns for Acoustic Radiation from a Source on a Rigid Cylinder
Authors: Laird Donald T. , Cohen Hirsh G.
Published: September 2005

The directionality patterns produced by an acoustic source located on a rigid cylinder of infinite length have been investigated. Assuming that the source strength may be represented as a separable function of azimuth angle and axial dimension, expressions for these patterns are obtained using the method of steepest descent as applied by Silver and Saunders to the electromagnetic analog of this problem. It is observed that the pattern in a plane orthogonal to the axis of the cylinder is independent of the axial distribution of the source, and is, in fact, identical with the pattern given by Morse for a source of infinite axial extent. For the particular case in which the ratio of circumference to wavelength is 14, patterns in this plane have been computed numerically using an IBM-Card Programmed Calculator. Some properties of these patterns are qualitatively explained. Patterns in a plane containing the axis of the cylinder have also been investigated. Experimental patterns confirm the general features of the theoretical patterns and seem to indicate the validity of the theory.
The Influence of Boundary Conditions on Normal Incidence Transmission Loss through Thin Plates
Authors: Young James E.
Published: September 2005

Calculations have been made on the influence of certain boundary conditions on the transmission of sound through thin plates. It is assumed that the plates are finite and free at one pair of edges. The conditions imposed on the other pair are: clamped-clamped, clamped-supported, clamped-free, and supported-supported. The effect of internal resistance is introduced by a complex Young's modulus, Q̄ = Q(1 + iδ), in which δ is a loss parameter. Curves are plotted showing the variation of transmission loss vs frequency, with δ as a parameter for each set of boundary conditions. Above the fundamental frequency the curves can be brought into general agreement with the mass law for a range of values of δ. However, the range of values required to give agreement are not the same for the different boundary conditions. Extension of the theory to dissipative boundaries and the important problem of assessing the energy fed into such boundaries will be outlined.
Sensitivity to Changes in Intensity between Repeated Bursts of Noise
Authors: Pollack Irwin
Published: September 2005

The differential sensitivity for changes in intensity between successive bursts of white noise was determined as a function of the time interval between successive noise bursts. In general, there is a critical duration of 55 milliseconds between successive noise bursts: (1) above which, the differential threshold is constant and independent of the interval between successive bursts; and (2) below which, the differential threshold increases proportionately as the interval is decreased. The fact that an identical critical interval has been obtained by various independent measures of auditory persistence leads to an interpretation of the deterioration of differential sensitivity in terms of overlapping or interference of auditory persistence with the direct effects of stimulation.
Wave Effects in Vibration Mounts
Authors: Harrison Mark , Sykes A. O.
Published: September 2005

Theoretical and experimental studies of wave effects in vibration mounts have been made. From the standpoint of noise isolation, wave effects are important in the sense that the noise isolating properties of a mount are impaired by the presence of wave effects. The well-known lumped parameter treatment of vibration mounts predicts that the vibration isolation of a mount increases at 12 db per octave for frequencies well above the resonant frequency of the spring-mass system. This is true only when the wavelength of the elastic wave in the mount is large compared to the dimensions of the mount. Standing waves occur as would be expected and often impair the vibration isolation properties of the mount by as much as 20 db. For practical mounts, wave effects are often most deleterious in the audible frequency range 500 to 2000 cps. The theoretical and experimental treatments give good agreement and indicate various means for improving the vibration isolating properties of the mount.
A Difference in the Representation of the Two Ears in the Isofrequency Bands of the MES Auditory Area in the Dog
Authors: Tunturi Archie R.
Published: September 2005

It was recently demonstrated that frequency was represented in the MES area by parallel overlapping bands of cortex rather than by points. Response curves for strychninized patches along a band revealed no differences in the intensity and frequency characteristics for the opposite ear. Response curves for the ear on the same side showed that the same frequency elements were used, but the intensity levels of the curves were elevated depending upon the position of the band tested. At the upper end of the band the intensity for the two ears was most similar, but at the lower end of the band the intensity for the ipsilateral ear was 40–80 db higher than for the contralateral ear. Intervening portions of the band showed intensity levels between the two limits. There appears to be an intensity representation along the band for the ipsilateral ear, but not for the contralateral. The gradient is about 0.2 mm/db. No information concerning the frequency and intensity of the tones can be found in the potential characteristics nor in the response or masking curves. From various evidence the only information present in the patches is whether an electrical change occurs or does not. These two conditions can be represented by 1 and 0. In the frequency axis of the area, 70 elements (0.2 mm or 0.1 octave wide) have been measured. Thus, 270 or 1025 pieces of information can be displayed on the area in the frequency dimension alone.
Cochlear Microphonics Following Noise Trauma
Authors: Tonndorf Juergen , Brogan F. A.
Published: September 2005

Guinea pigs were exposed to white or line-spectrum noise of 130 db for a period of thirty minutes. Cochlear microphonics (action potentials canceled electronically) were recorded before and immediately following exposure, and at intervals for at least three hours. By use of a high speed power level recorder it was possible to apply brief test signals and to complete tests rapidly. The impairment resulting from the noise exposure was evaluated from the gradient curves of cochlear response through 130 db and for ten frequencies between 180 and 6000 cycles. The curves indicated two distinct changes: (1) a response loss equal in quantity throughout the intensity range and (2) an increase in overload. These changes varied in magnitude independently: (1) while, in general, the response loss was dominant immediately following exposure, (2) it recovered at a more rapid rate than did the increase in overload. From most aspects the impairment was more severe in those animals exposed to white noise than in those subjected to line-spectrum noise. A theoretical explanation of the findings will be presented.
Effects of Various Types of Nonlinear Distortion upon the Intelligibility of Speech
Authors: Licklider J. C. R. , Held Richard
Published: September 2005

Articulation tests were conducted under uniform conditions (same talkers, same listeners, randomized block design of experiment) to compare the effects upon intelligibility of various types and amounts of nonlinear distortion. The types included peak clipping (both symmetrical and asymmetrical), center clipping, linear rectification, and parabolic (“square-law”) rectification. The results confirm previous reports that intelligibility is highly resistant to symmetrical peak clipping (80 percent word articulation with infinite peak clipping). They show that asymmetrical peak clipping is almost entirely without detrimental effect upon intelligibility until more than half the speech wave is eliminated (98 percent word articulation with half-wave rectification). In addition, the results provide a basis for comparing effects of odd and even harmonic distortion, for comparing the effects of noise that enters the system at a point ahead of the nonlinear circuit with effects of noise that enters at a point following the nonlinear circuit, and for deciding what are tolerable limits for nonlinear distortion in speech communication systems.
Equal Loudness Contours for Hearing by Bone Conduction
Authors: Watson Norman , Frazier T. V.
Published: September 2005

Earlier work by one of the present authors (N.W.) indicated that the equal loudness contours for hearing by bone conduction for normal-hearing listeners might be different in shape from similar contours for hearing by air conduction if both sets of contours were plotted in terms of level-above-threshold, referred to the respective thresholds. A bone-conduction “preferred” equal loudness contour for a listener with almost pure conductive loss had been found to be definitely different from the corresponding air-conduction equal loudness contours for normal-hearing listeners, being more sharply curved when all contours were plotted in terms of level-above-threshold, referred to the appropriate thresholds. Thus equal loudness contours for bone-conduction listening for normal-hearing listeners have been determined, using the electrodynamic bone-conduction vibrator and the air- and bone-conduction audio testing assembly at U.C.L.A., further developed and improved since its first presentation before the Society. Contours were determined for loudnesses corresponding to levels of 20 and 40 db above the bone-conduction threshold at 1000 cycles per second. The listeners' hearing was normal as determined by audiometric and otological examination. The contours, when plotted in terms of level-above-threshold, are definitely more sharply curved than the corresponding air-conduction equal loudness contours, plotted in terms of level-above-threshold (air conduction). A tentative explanation for and the implications of this difference is given for both normal-hearing listeners and those with pure conductive impairment.
Pitch Changes Produced by Bone-Transmitted Thermal Noise or Pure Tone
Authors: Frazier T. V.
Published: September 2005

During the course of some experiments on listening by bone conduction, the author noticed an increase in the pitch of a pure tone in the middle audiofrequency region when a pure tone of lower frequency was sounded, and the qualitative results of several ensuing experiments were reported in a letter to the editor. The work to be described here dealt with the rise in pitch of a pure 2000-cps tone in the presence of a broad band of thermal noise and also in the presence of pure tones of lower frequency. All sounds were presented by bone conduction using the air and bone conduction audiotesting assembly developed at U.C.L.A. The results of extensive tests on one observer are presented as well as the results of pitch rise tests on several different listeners. All listeners had normal hearing as indicated by audiograms and otological examinations. It was found that the pitch of the 2000-cps tone rose with increasing level of the disturbing sound and that the greatest pitch rise occurred just before the pure tone was masked. Individual differences were evident among listeners. The effect on the pitch of the 2000-cps tone of clenching the teeth is discussed for one observer. A tentative explanation of the phenomena is given.
The Mechanical Impedance of the Human Mastoid and Its Relation to the Calibration of Bone Conduction Transducers
Authors: Franke Ernst K.
Published: September 2005

Knowledge of the compliance and the resistance of the skin overlying the human mastoid is important for the calibration of bone conduction hearing aids. Artificial mastoids for the testing of bone conduction transducers can be successfully designed and standardized only when they actually simulate the mechanical impedance of the human mastoid. The correct value of the impedance, then, can be achieved by a properly chosen “flesh simulating” pad. Because of discrepancies of about one order of magnitude in the measured values of compliance and resistance published so far, this question was taken up again. The impedance of the human mastoid was determined by methods previously developed to investigate the elastic and viscous properties of human tissue. The impedances of various subjects were measured. The compliance and resistance were calculated from the results and it is shown how they depend on certain parameters. The reasons for the discrepancies mentioned above are explained. The influence on the calibration of bone conduction transducers is discussed.
Experimentation on the Theory of the Reverberation Method of Measuring Acoustical Absorption
Authors: Hardy H. C. , Tyzzer F. G.
Published: September 2005

There is now a great deal of evidence that the approximation of randomness in reverberation rooms used for absorption measurements holds to a much greater extent than assumed in the classical analysis of the wave theory of sound in rooms by Morse and Bolt. Such factors as rotating vanes, averaging of measurements, diffusion due to wall absorption, and other elements, were not previously taken into account. Evidence of randomness includes (1) logarithmic decay curves linear over more than 60 decibels; (2) accurate random calibration of microphones at low frequencies; (3) measurement of correlation coefficients; and (4) fairly close fit between measured coefficients and those calculated by statistical theory from normal impedance measurements. The normal wall coefficient calculated by the perturbation theory given by Morse and Bolt gives coefficients as high as 6.00 for some materials while room measurements only slightly exceed statistical coefficients. If the random assumption holds, the chief source of error would appear to be the diffraction effects at the edges. The diffraction area correction for normal incidence has been derived from Morse and Bolt's treatment to be Se = S[1 + (λσL)/(4S)], where λ is the wavelength, σ the specific susceptance, L the length of free boundary, and S the area of the patch. It can be expected that the correction for random incidence would be greater. Using a percentage correction which is twice as great, good agreement is obtained between Riverbank coefficients and statistical coefficients over a wide range of frequencies and acoustic impedances.
Electronic Music Box
Authors: Kent E. L.
Published: September 2005

A new musical instrument is described and demonstrated. It demonstrates the versatility of electronic music by control of timbre, attack, release, loudness, legato, pitch, vibrato, and stereophonics. The instrument is keyed by a perforated roll of paper while all other variables are independently controlled by the operator. The instrument makes it possible for a composer or arranger to produce music without many of the conventional limitations of musical instruments and musicians. In other words, it is possible to produce music in the “Walt Disney” manner. The music box to be demonstrated is limited to a quartet of voices, but it will be accompanied by Mr. Charles Tennes at the console of a late model electronic organ.
Description of a New Swaging Machine Silencer
Authors: Hirschorn Martin
Published: September 2005

The solution to a very old and troublesome problem in the tubular manufacturing industry is described. In the past various attempts had been made to solve this problem by the installation of acoustical enclosures; however, these enclosures never had an effect on the actual source of the noise which is at a maximum pressure inside the swager machine inlets and outlets. The tubular swaging machine silencer, consisting of an acoustic filter type arrangement with sound absorbing liner and a special device for attenuating the noise transmission along the tube wall itself has successfully solved this particular problem. Actual noise level reductions of from 18 to 22 db have been obtained. In a typical case the noise level was reduced from 112 db to 94 db; the background factory noise level never dropping to below 94 db. The silencing equipment was designed so as not to interfere with the operating or maintenance efficiency of the equipment. A result of the installation has been a lift in the general morale of the personnel. Illustrations to be included are pictures of the installation itself, a general arrangement sketch, as well as charts of the frequency spectra before and after the installation.
Measurement of the Shear Impedance of Viscous Liquids by Means of Traveling Torsional Waves
Authors: McSkimin H. J.
Published: September 2005

A short periodically repeated train of torsional waves is transmitted along a glass or metal cylindrical rod. After reflection from the free end, these waves are sent back to the quartz crystal which serves as both transmitter and receiver. The phase shift and added attenuation caused by immersing the rod in the test liquid are measured by means of a special balancing arrangement, and yield a calculation of the impedance presented to the rod surface. From an analysis of wave propagation both in the rod and in the liquid, one can calculate the characteristic shear impedance of the liquid, and the dynamic viscosity and stiffness. Illustrative data for polyisobutylene liquids with static viscosities up to 2000 poises are given for the frequency range 25–150 kc. High frequency data (5–25 mc) for the same liquids obtained by a method previously reported on are correlated to the present work.
Reflection from an Inhomogeneous Medium
Authors: Heller G. S.
Published: September 2005

The reflection coefficient, at oblique incidence, for a plane wave encountering a medium in which the wave velocity decreases exponentially has been computed by the author and presented in an earlier paper. In this paper, the reflection coefficient for several other values of ω/g, ω being the angular frequency and g the velocity gradient at the start of the exponential variation, will be given as a function of the angle of incidence. The positive exponential gradient has been treated and shows total reflection at all (except normal) angles of incidence, as one would expect from ray theory. A more realistic positive velocity gradient which remains finite yields a critical angle which depends on the entire velocity structure. The positive gradient in an absorbing medium is also being studied.
The Acoustics Design of the Large Council Chambers, Permanent Headquarters of the United Nations, New York
Authors: Newman Robert B.
Published: September 2005

The design of large parliamentary chambers for conferences conducted in many languages presents a number of new acoustics design problems. Not only must the acoustic environment be “comfortable,” but it must be possible to hear clearly a person speaking in any part of the delegates area, either directly or as simultaneously translated into any of four or five languages. The design and isolation of the many booths for translators, radio, and press surrounding these rooms was the subject of much study. The general isolation of noise from surrounding service areas and outdoor sources, such as traffic on the East River and in the air, were also considered. Special attention was given to the coordination of the sound reinforcing system with the acoustic design of the chambers. Preliminary articulation tests have been conducted in these rooms and the results are presented.
The Reflection from Irregular Surfaces
Authors: Young F. Mansfield
Published: September 2005

The problem of the reflection of acoustical waves from an infinite wall, having prescribed irregularities or a prescribed distribution of acoustical material upon it, is formulated in terms of an integral equation. By taking the Fourier transform of the equation it can be reduced to a form easily handled by numerical techniques. Expressions for scattering and absorption cross sections can be obtained without actually solving the equation. Much information of a qualitative nature can be obtained from inspection of the equation. An “index of diffusion” is introduced which is a measure of the effectiveness of a given surface in breaking up the reflected wave. The application of the above methods to simple scattering surfaces (e.g., strips and pillars) will bez discussed.
A Corner Loudspeaker-Enclosure Combination
Authors: Baruch J. J. , Lang H. C.
Published: September 2005

A corner loudspeaker-enclosure combination is described which embodies a modification of the acoustic phase inverter. The design incorporates an array of perforations to provide the inductance. Control of the Q is achieved by varying the size and shape of the array. An electrical analog has been built and tests indicate good agreement between the analog response and the measured response of the loudspeaker. The loudspeaker to be demonstrated has a response flat to within ±4 db from 45 cps to 13,000 cps and occupies a volume slightly larger than one-half cubic foot.
A Compact, Versatile Filter-Type Sound Analyzer
Authors: Von Recklinghausen D. , Scott H. H.
Published: September 2005

The advent several years ago of a pocket-size Sound Level Meter meeting all ASA standards created an immediate demand for a complementary instrument to analyze sound. Such a unit is now available, based upon the Dynamic Noise Suppressor circuits. The resulting unit is contained in a small leather case, similar in appearance and weight to a home-type motion picture camera, and is provided with a shoulder strap which allows the instrument to be used while the operator is walking about. This greatly facilitates analytical noise surveys, since no cumbersome equipment is involved which has to be placed upon a table or on the floor. The Analyzer meets all proposed ASA standards and, in addition, has an adjustable band width varying from one-half octave upward in steps of one-half octave. One control switches the pass-band but the high and the low frequency cutoffs may also be adjusted separately. The frequency range exceeds proposed ASA requirements by one octave at both low and high frequency limits. In addition, the instrument includes a calibrated attenuator giving, with the indicating meter, a range of 66 db. The indicating meter meets the ASA ballistic requirements, and an output circuit is provided so that the instrument may be used as a general-purpose filter. While designed particularly for use with the Type 410-B pocket-size Sound Level Meter, the Analyzer may be used with practically any other Sound Level Meter or suitable signal source. The filters may also be cut out of the circuit entirely so that the instrument may function as a general-purpose amplifier or audiofrequency voltmeter with a calibrated attenuator. The range of the amplifier exceeds 20 to 20,000 cycles.
Propagation of Sound through Solids
Authors: London A. , Orlacchio A. W. , Bach R. L. , Epstein D.
Published: September 2005

A method of measuring the dynamic properties of resilient materials under static loads is described. The method is useful over a large part of the audiofrequency range. The experimental set-up consists of an electromagnetic loudspeaker pot and a rigid voice coil which is supported above and below by two disks of the sample material. The motion of the voice coil assembly is determined by an electrostatic variable capacity vibration detector. The ratio of the input to output voltage is measured in both magnitude and phase. The dynamic properties of samples can be determined either by making measurements for several values of the mass of the moving system or by the use of a resonance technique. Results obtained on several samples of rubber and a mechanical spring using both types of determination are presented. Apparatus similar to that described above is used to measure the transmissibility of several commercial shock amounts over a frequency range from 8 to 4500 cps. The results are presented graphically and are compared with results obtained by another group working on the same problem. Theoretical problems concerning low and high pass filtration of plane, compressional elastic waves are discussed for a system consisting of a vibrating driving mass coupled to a compound transmission line terminating in a rigid base.
Self-Excited Hydrodynamic Oscillators
Authors: Bouyoucos John
Published: September 2005

Self-excited sound sources for use in fluids are discussed with principal attention given to the Pohlmann whistle. Although certain applications of this source have been reported in the literature, little has been available concerning its properties. The Pohlmann source employs a fluid jet impinging upon the end of a rectangular steel blade simply supported at points on the nodal lines of its fundamental mode of vibration. It is assumed that the system of vortices characteristic of edge-tone production arise at the blade edge and exert upon it a periodic transverse force. When the period of this force is equal to the period of blade resonance (or is one of its odd subharmonics) the blade is set into strong vibration. The source strength, radiation properties, frequency range, and applications are described.
Some Design Considerations for High Frequency Anechoic Tanks
Authors: Weinstein M. S.
Published: September 2005

By an extension of the techniques of Lindsay the reflection from a semi-infinite attenuating medium is derived for the case of oblique angles of incidence. The results are applied to the design of a water tank each wall of which is assumed to be covered by a sheet of absorbing material. The optimum physical properties of the absorber required to make the tank anechoic in the megacycle region are then determined theoretically. Experimental measurements at 3.35 megacycles/sec indicate that the properties of tan rho-c rubber are very close to optimum. With a 14-inch-thick sheet backed by aluminum the reflection coefficient is about −30 db at normal incidence and rises to −20 db at an incident angle of 60°. Beyond 60° the rubber is of little value in reducing the reflectivity. The results obtained are in good agreement with those expected from the theoretical treatment when modified for reflections from the back surface and the phase difficulties previously described by the author.
The Analysis of Magnetostrictive and Piezoelectric Transducers by the Mode Impedance Method
Authors: Rosato Frank J.
Published: September 2005

The basis of this method, which is particularly applicable to continuous systems, is as follows: the dynamic configuration of the vibrating element is expressed in terms of the generalized coordinates of the system and the appropriate mode functions. Next, the kinetic and potential energies of the system are expressed in terms of the generalized coordinates. Application of the nonhomogeneous Lagrange equations then leads to the dynamical equations of motion, one in each of the generalized coordinates. For continuous systems, the number of dynamical equations is necessarily infinite. In this theory, the magnetostrictive (or piezoelectric) induced stresses are considered as part of the generalized forces; the remaining generalized forces take into account the reaction of the fluid medium on the vibrating surface and other forces acting on the system. The magnetostrictive (or piezoelectric) equation which relates the total magnetic induction (or total electric polarization) to the external magnetic (or electric) field and the induced strain, is then employed to obtain the electromechanical equations of the transducer. These equations may then be represented by electromechanical networks, consisting of two input electric terminals and an infinite set of output mechanical terminals. The final results may be obtained to any degree of approximation desired.
Lamb's Problem with Internal Viscosity
Authors: Newlands Margery , Knopoff Leon
Published: September 2005

Lamb's diffraction approach to theoretical seismology is re-examined for materials exhibiting a viscosity of the Stokes type. The response of a homogeneous viscoelastic half-space to a localized normal impulse at the surface is treated. Following Lapwood, the method of Sommerfeld contour integration is used for the evaluation of the integrals which describe the resultant motion at a remote point in the surface. The computations demonstrate the dispersive nature and the damping of the wave motion and the dependence of these quantities upon the parameter αr/ν involving the velocity of compressional waves, the focal distance, and the kinematic viscosity. By using a block of microcrystalline wax, an attempt has been made to demonstrate these results in the laboratory; comparison is made between experiment and theory, and a tentative value is derived for the viscosity of this wax.
A One-Dimensional Seismic Wave Model
Authors: Evans J. F. , Hadley C. F.
Published: September 2005

Helical wire springs have been used as one-dimensional wave models. Sections having predetermined acoustic impedance contrasts can be wound, and then attached together. The springs are suspended horizontally. Means are described for introducing various forms of mechanical pulses into the model and for detecting and recording the resultant motion at any desired position along the model. Interfering modes of wave motion are minimized by proper design and uniform winding. Such a model has been used for experimental study of a number of plane wave propagation problems, including the verification of the formulas for the reflection and transmission of plane waves at vertical incidence, pulse propagation through a section of tapered acoustic impedance, the reflection of a pulse by a thin layer of relatively large acoustic impedance contrast, and the change in form of a pulse with time as it travels through a medium of low but finite absorption. It is concluded that the helical spring model can be used as an analog computer for many problems in plane wave propagation in applied seismology.
Some Observations on Traveling Waves from Explosive Sources
Authors: Lawrence P. L. , Dobrin M. B.
Published: September 2005

Seismic propagation studies made in the Delaware Basin of West Texas have disclosed unusual waves traveling along near-surface beds. The area is characterized by alternating high and low velocity layers near the surface, with a thin high velocity cap over much of the region investigated. Physical characteristics of the recorded waves have been correlated with this layering. Only those events arriving earlier than Rayleigh waves will be discussed here. Shortly after the first compressional arrival, waves are observed having apparent velocities equivalent to those of shallow refraction markers and with periods which may be predicted, assuming that waves leaving the refractors at the critical angle are reflected back into the refractors by interfaces closer to the surface. Later arriving, long period events are tentatively identified as refracted shear waves. A group of waves arriving just prior to the Rayleigh waves is observed only where the high velocity cap is present and underlain by a low velocity zone above the water table. These waves are not recorded when the source is appreciably below the low speed layer. They are considered to result from normal mode transmission of compressional waves with the low speed zone acting as a wave guide.
The Electro-Acoustic Sensitivity of Cylindrical Ceramic Tubes
Authors: Langevin R. A.
Published: September 2005

Increasing use is being made of cylindrical barium titanate tubes as underwater acoustic transducers. An exact derivation is given for the low frequency acoustic sensitivity of such tubes. The sensitivity and capacity are used to provide equivalent circuit representations of such tubes for a variety of boundary and polarization conditions and for a number of current titanate materials.
Design Techniques for a High Frequency Transducer with a Conical Wide-Beam Directivity Pattern
Authors: Lane A. L.
Published: September 2005

It is frequently essential for a transducer to have a smooth, wide-beam directivity pattern with negligible side lobes. If, in addition, this transducer must operate in the fractional megacycle frequency region, then conventional designs, such as small plane radiators or curved surface mosaics, are not satisfactory. As a result of an extensive investigation, it was found that a barium titanate spherical shell sector properly designed will meet these difficult wide pattern and high frequency specifications. This paper outlines some of the design considerations required to adapt a ceramic shell into a transducer with the desired performance. The problems discussed include shell size, included angle of the spherical shell, inside diameter to thickness ratio, and the uniformity of the thickness dimension. Various baffle techniques are also examined in detail. Shell diameters ranging from two to fifteen wavelengths are considered as to their effects on the shape of the surface displacement distribution, and on the directivity pattern. Although this paper presents a solution to a specific transducer problem, the design techniques discussed are applicable to transducers having other frequency and pattern requirements.
Electrokinetic Low Pressure Detectors
Authors: Packard Robert G.
Published: September 2005

For the detection of sinusoidal pressures of low amplitude and low frequency, electrokinetic detectors have been constructed and tested. The detector consists of a Lucite cylindrical body, the ends of which are sealed with flexible polystyrene diaphragms, a capillary unit separating the body into two chambers, two platinum electrodes, and two vacuum filling plugs. The detectors are vacuum-filled with distilled water and tested over the frequency range of 20 to 350 cps. Amplitude and frequency response curves are given. The capillary elements consist of beeswax, sealing wax, sulfur, and paraffin. These detectors with capillary elements behave as low pass filters with a low Q.
Experimental Studies of Jet Noise
Authors: Hubbard Harvey H. , Lassiter L. W.
Published: September 2005

The mixing region of a jet is observed to be a complex noise generator. The noise produced is highly directional and is a function of the jet size, velocity, density, and turbulence level. Experimental studies of the noise from a series of model subsonic jets ranging in diameter from 0.75 to 12.0 inches in diameter are compared with data for a turbojet engine. The noise generated by this engine was found to be closely related to that generated by the simple model jets, and an empirical relation is given to allow the extrapolation of available jet noise data to other operating conditions. In addition, an attempt is made to correlate the noise data with some aerodynamic measurements inside the jet, and a brief discussion is given of recent experiments with devices designed to alter the jet noise spectrum. Data are presented in a form convenient for comparison with Lighthill's theory.
Sound Radiation Patterns of Gas Turbine Exhaust Stacks
Authors: Wells R. J. , Crocker B. E.
Published: September 2005

Radiation patterns have been calculated for the case of wide-band noise, assuming a negligibly low gas velocity. Both shape of stack and bandwidth of analyzing equipment are considered. Theoretical results are compared with data measured on models and on a prototype exhaust system.
Noise Control for Ground Operation of the F89 Airplane
Authors: Veneklasen P. S.
Published: September 2005

A noise control project has been completed by Northrop Aircraft, for the ground testing of the F89 airplane. Phases described include the prediction of performance, acoustical design considerations, correlation of acoustical and thermodynamic requirements, general construction, acoustical tests during construction, noise survey around the exposed airplane, and neighborhood survey around the completed installation. This noise suppressor is comparatively simple and inexpensive, requires no water cooling even for afterburner operation, and has proved adequate in terms of neighborhood relief and protection of operating personnel.
Some Typical Problems Encountered in Practical Noise Control Work
Authors: Hirschorn Martin
Published: September 2005

The field of practical noise control engineering is inevitably a very wide one. Each problem has its own individuality and must be treated as such. The lack of standard equipment or standard solutions for the many diverse problems encountered requires special engineering for practically each job, however large or small. A brief description of some interesting problems encountered and how they were solved, including before and after noise level spectra where available, will be given in reference to the following subjects: (1) noise and vibration control equipment for a motor generator set; (2) an air discharge silencer for a roof exhauster fan; (3) silencing equipment for an air conditioning cooling tower; and (4) a large soundproof cell for a cutting machine.
Method for Measuring Noise of Ventillating Fans
Authors: Reynolds J. L. , Wilson K. E. , Beranek L. L.
Published: September 2005

A method has been developed for measuring the acoustic power output in watts and the frequency spectra of ventilating fans. Both the inlet and the exhaust noise are determined as a function of back-pressure. The fan is ideally connected to a circular section of duct about equal in diameter to the fan diameter and five to ten diameters in length. Three to six diameters from the input end of this duct an air straightener is located. The acoustic measurements are made with a condenser microphone enclosed in wind screen located four to eight diameters from the inlet. The outlet end of the duct couples through an exponential connector to an anechoic wedge structure two feet square. This is backed by a plenum chamber containing an array of holes that can be progressively closed to produce increased back-pressure. Sound pressure level measurements are made in frequency bands and converted to acoustic watts per band. The inlet noise is measured outdoors on a hemispherical surface to get the directivity pattern and the acoustic power for each frequency band. Engineering formulas for applying the data to prediction of the noise produced in rooms are presented, including the effects of duct bends, duct losses, and the distance from the ventilating grille in the room.
An Accurate and Simple Means of Extending the Range of Underwater Sound Measurements to Include Infrasonic Frequencies
Authors: Beatty Louis G.
Published: September 2005

The extension of hydrophone sensitivity measurements to frequencies below 40 cycles per second is usually done by reverting from free field measurements to a stiffness controlled system. This requires an elaborate set-up not obtainable at most laboratories. The method described presents by comparison a relatively inexpensive and simple means for hydrophone sensitivity measurements in the infrasonic range. Limitations are discussed, and data are given showing the comparison of the method to a stiffness controlled system for instruments of various sensitivity levels.
Optical Calibration of Accelerometers
Authors: Edelman Seymour , Jones Earle , Thompson Moody C.
Published: September 2005

At relatively low frequencies optical calibration of vibration pick-ups has been used extensively in the past. Direct measurement of displacement by means of a microscope simultaneously with measurement of open circuit voltage from the accelerometer affords a simple and accurate means of determining the response of an accelerometer, if the motion is sinusoidal with time and if the displacement is large enough to measure accurately. This paper presents various expedients used to extend the frequency range of this method. Some of the points discussed include mounting and construction of the microscope, construction of the vibration exciter, use of stroboscopic methods for measuring the displacement, and use of a photoelectric method for measuring the displacement. Sources of error and the means used to minimize them are discussed.