bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of Acoustic Impedance by Short-Tube Techniques
Authors: Chrzanowski Peter
Published: September 2005

This paper deals with the techniques for measurement of acoustic impedance by a short-tube method. In this method, the unknown impedance is placed at one end of a cavity which is short in comparison with the wave-length of the sound. The sound field within the cavity is produced by a source, whose characteristics may be computed, and the field within the cavity is explored by means of a probe attached to a microphone. One technique is an adaptation of Flanders method in that the unknown impedance is obtained in terms of three electrical potentiometer readings of which two are for known impedances. In another technique, the volume velocity of the source is computed and the measurement of the impedance is determined from only one pressure reading within the cavity. The techniques and results are compared with long-tube impedance methods.
Some Observations on the Acoustic Impedance of Small Orifices
Authors: Bolt R. H. , Labate S.
Published: September 2005

The acoustic impedance of single and multiple small circular orifices is being studied as a function of (a) particle velocity in the orifice, (b) spacing between orifices, (c) orifice dimensions, and (d) eccentricity of orifice position in an impedance tube. The observations and analyses reported here are restricted to wave lengths much greater than the hole diameter. Sivian reported an increase in orifice resistance with particle velocity, but he found no variation of reactance. Using a precision impedance tube, we confirm the increase in resistance, but we find that reactance decreases significantly with increasing particle velocity. The impedance of an orifice is influenced by nearby orifices or bounding walls. Defining interaction impedance as the increase in impedance due to the presence of another orifice or a wall, we have measured and obtained approximate formulas for interaction impedance in certain simple cases. As two orifices in an infinite plate approach each other, the reactance of each becomes more mass-like. An orifice near a rigid plane wall interacts similarly with its image in that wall. For a single orifice centered in a circular (impedance) tube there is a correction for the interaction with the tube walls. As the orifice is moved away from the center of the tube, both the resistance and the reactance vary.
Reciprocity Calibration of Primary Vibration Standards
Authors: Thompson Sanford P.
Published: September 2005

Calibrations of vibration standards are usually made by methods inherently subject to large errors at frequencies greater than a few hundred cycles per second. It has been shown that linear and bilateral electromechanical transducers may be calibrated by the application of reciprocity principles, and that such procedures obviate: (a) assumptions about the internal construction of the standards, and (b) direct measurements of dynamical mechanical quantities. The construction of a set of primary standard transducers and the experimental techniques employed in their reciprocity calibration are described. Sensitivities obtained by reciprocity are compared with those obtained by optical methods of low frequencies. Accuracy of the results at higher frequencies is evaluated from the consistency of the reciprocity data with theory.
The Transient Response of Loudspeakers
Authors: Hardy H. C. , Hall H. H.
Published: September 2005

Transient response methods have been used for studying and evaluating loudspeakers at the Armour Research Laboratory since 1945. Other distortions, such as intermodulation, were found to give poor correlation with results from listening tests. The most convenient method for studying the transients has been to introduce an interrupted audio signal of varying frequency into the voice coil, pick up the effect in the sound field of the loudspeaker with a transient-free microphone, and connect this to an oscillograph for observation. Some of the techniques developed are similar to those described by Shorter. A Massa M-101 measurement standard microphone with resonant frequency above 20 kilocycles was found to be very suitable for this work. The use of a good anechoic room is also essential, since some of the pressure decay curves are similar to the reverberation decay of a room. An interruption rate of 20 per second was found to be convenient. Measurements with this experimental arrangement have shown that loudspeaker decay time (to 1/e) are often longer than 0.01 second. Since the system is three dimensional involving complicated resonances of the loudspeaker radiating elements, the on-transient is different from, and cannot in general be correlated with, the off-transient. The paper will be illustrated by several slides showing some of these phenomena and a short moving picture reel will be shown of the transients that occur in several types of loudspeakers as the frequency is varied. The chief psychological effect which can be correlated with the transient effects is the “cleanness” of response of the speaker. It should be emphasized that these effects are only one of the criteria of evaluation of a loud-speaker. Some other methods of evaluating speakers will be given in another paper.
A Simple Approach to Generalized Horn Theory
Authors: El-Mawardi O. K.
Published: September 2005

Conventional treatment of the input impedance of a chain of quadripoles whose characteristics differ progressively leads to complicated expressions that discourage generalization. More tractable relations are found by using a simple form of matrix algebra to isolate the effects of the characteristics of a typical quadripole and the scale factor that expresses the systematic variation of the component elements from one quadripole to the next. As the elements are made smaller, the behavior of the chain of quadripoles tends to that of a smooth line with continuously variable parameters. The analogy between electrical and acoustical systems makes these results directly applicable to the theory of flaring horns. The results obtained with this simplified procedure are in close agreement with those derived by Salmon.
Acoustics of Arches
Authors: Dorsey Herbert Grove
Published: September 2005

At Washington, D.C. there are several bridge arches which are easily accessible and have interesting acoustical effects Many are over Rock Creek which runs through a National Park of that name. Where Massachusetts Avenue crosses it there is a fine span, 74 feet wide and of arch diameter of 148 fees, the entire semicircle being above ground. Because there are no trees or bushes near the base of the walls, which are smooth concrete, reflections are from acoustically smooth surfaces at the ground. If one stands close to the arch wall in early morning quiet one can hear two distinct echoes from a faint whisper or from a pin dropped a half-inch into a little paste board box. A louder sound, such as a hello or a hand clap, wil leasily give five to ten echoes at an average rate of about 2.5 per second with an air temperature of about 70°. Rayleigh's theory (confirmed by Sabine) about the whispering gallery in St. Paul's in London and the creeping of the sound around a curved surface seems confirmed again by experiments at this arch. Assuming that the sound travels close to the arch, the above figures give a velocity of about 1160 feet per second The layer in which the sound travels appears some 20 feet thick and the echo intensity dropsloft as you recede from the wall. For a whisper, a difference in two- or three-foot distance from the wall is noticeable. At the center of the arch you get three distinct simultaneous echoes, from the top and both sides. As you approach the wall, the first echo comes from the shortest perpendicular to the arch. At the P Street bridge the arch has its center far beneath the surface. The chord length is 95 feet and the height above the roadway is 35 feet. At the ends of the arch echoes can be heard for 5 seconds at the rate of 6 per second. For a limited area near the middle, echoes at twice the rate can be heard and they come so rapidly that they sound like a trill. One hears something between 50 and 80 echoes. A whistle of 350 cycles gives Rayleigh's “harmonic echoes” with surprising distinctness. All the arches make excellent “whispering galleries.”
Acoustic Properties of Snow
Authors: Watson R. B.
Published: September 2005

Many qualitative observations have been made on the quieting effect of newly fallen snow. This natural phenomenon must depend on the acoustical properties of such a layer of snow. While the physical constants for snow vary over a considerable range, and the data for obtaining some of the necessary acoustic constants are meager, the general behavior of a layer of snow as a sound-absorbing material may be estimated. Particular attention is paid to the equivalent absorption at grazing incidence since much of the atmospheric transmission of sound occurs at small angles to the snow surface.
New Results on the Propagation of Low Frequency Waves in Pipes
Authors: Atanasoff J. V. , Bobroff D. L. , Kolsrud E. R.
Published: September 2005

The experimental confirmation of the Hemholtz, Kirchhoff theory for the propagation of long waves in pipes has been subject to some doubt. This theory is complicated both by viscous drag and the irreversibility consequent to the transition from adiabatic to isothermal processes. An experimental method has been devised which eliminates the effects of thermodynamic irreversibility and permits a direct determination of the viscous effects. The Kirchhoff equations are recast in a form which, together with the analogy of the propagation of sound in pipes with the electric long line, permits a ready comparison of experimental results with the theory.
The Mechanical Impedance of the Surface of the Human Body
Authors: Franke E.
Published: September 2005

In order to compute the absorption coefficient of the human body for energy derived from various vibratory systems, the mechanical impedance of the body surface was measured through the frequency range, 50 to 500 c.p.s. The essential element of the measuring apparatus is a vibrating piston, the free end of which is placed in firm contact with the area of the body surface whose impedance is to be measured. The impedance of the body surface can be calculated from the measured amplitudes and phases of the piston when free and when in contact with the body. Measurements were also made by an alternative method in which a quartz crystal located between the piston and the body surface was used to measure the force applied. The mechanical impedance varies with the site of measurements. The absorption coefficient of the body surface for airborne sound was computed from the mechanical impedance measured by both of these methods. The absorption coefficient of surfaces overlying soft tissues is about ten percent at 100 c.p.s. and is approximately inversely proportional to the frequency.
Study of Thermoelastic Behavior of Metals by Vibration Measurements
Authors: Bordoni Piero G.
Published: September 2005

Elastic modulus is evaluated from measurements of sound velocity in thin rods of carbon and chromium steel, copper, and aluminum to investigate the effects of inhomogeneity, temperature, heat treatments, permanent stresses, and fatigue. The results are shown to support previous theories on thermo-elastic finite strains and plasto-elastic deformations, and to provide technical means to follow and control the effects of heat treatments.
Modern Schoolroom Acoustics
Authors: Yerges L. F. , Smith M. A.
Published: September 2005

Many teachers are faced with conducting classes in buildings that were obsolete thirty years ago. Others are or soon will be teaching in new schools which are in no way modern in some of their basic construction details. A classroom, laboratory, or auditorium is fundamentally a shelter in which the basic functions are seeing and hearing. Daylight provides satisfactory seeing a large part of the time. In spite of this, about $1.50 per square foot is the average investment for auxiliary lighting, while satisfactory acoustics is often completely overlooked or is the target of severe economy measures. Architects today are often eliminating or seriously compromising on the acoustical treatment expenditure while spending lavishly for less important materials. Eighty-five percent articulation will usually result in satisfactory hearing for adults because the context supplies the transitional ties. For children, however, especially the very young, each word and every syllable is important. Excessive reverberation time or poor sound distribution may create conditions which seriously handicap the students and teachers alike. Modern medical methods have shown that poor eyes and poor hearing are often responsible for backward children who are really average or above average in mental ability. It is inexcusable to add to their natural handicaps those of poor architecture and poor engineering. Good acoustics is as fundamental as lighting and ventilation, and much more attention should be paid to this phase of school design.
Building to an Acoustical Optimum New Mutual-Don Lee Broadcasting Studios
Authors: Carruthers W. W. , Loye D. P.
Published: September 2005

The acoustical design and construction of the new Mutual-Don Lee Broadcasting Studios in Hollywood were done under novel and very favorable arrangements. First, a careful check was made of the optimum acoustical characteristics to which it was decided to design the studios. Two of the most satisfactory broadcasting auditoriums on the West Coast were acoustically measured, and information regarding an auditorium in the East was obtained. From these data as well as comments regarding the excellence and shortcomings of these auditoriums, the optimum characteristics for the new studios were determined. Mutual-Don Lee engineers had selected these auditoriums on the basis of listening observations over an extended period of time; and a more detailed study was made of one of them in connection with a series of 26 orchestral broadcasts conducted by the studio. During the course of construction, acoustical measurements were made several times for the purpose of “tailor making” the acoustical characteristics of the studios. On the basis of the measurements, it was found that only a few minor modifications in acoustic treatment were necessary, such as changing the mountings of the Acousti-Celotex materials in order to change the low frequency absorption characteristics as desired; and slight adjustments in amounts of materials were made in the case of some of the studios. The results are close agreement between optimum and finally measured acoustical characteristics, and very satisfactory broadcast programs. A demonstration recording of an orchestra broadcast from one of the four major studios will be given.
Report on International Conference on Sound Transmission and Noise, Conducted by Acoustics Group of the Physical Society of London, July 14–16, 1948
Authors: Beranek Leo L.
Published: September 2005

Thirty-eight papers from representatives of seven nations were presented. Particular emphasis was placed on the acoustic properties of practical partitions and floor structures in apartment houses. Papers were also given on the measurement of noise and vibration and applied acoustics. International standards for measuring air- and structure-borne attenuation were proposed and tentatively agreed to by representatives of at least three nations. A review of several of the more important papers, illustrated by slides, will be given. The formation of a French acoustical socity, “Groupement des Acousticiens de Langue Française. 24 Rue Bertrand. Paris, 7, France” with Professor Y. Rocard as president and an early reactivation of the Akustische Zeitschrift were announced informally. Among those presenting papers were: W. Furrer of Switzerland, E. Meyer of Germany, C. Kosten of Holland, P. Chavasse and F. Canac of France, and A. King, R. Dadson, P. Parkin, and W. Allen of England.
Some Factors Governing Frequency Response in Magnetic Recording
Authors: Wallace R. L.
Published: September 2005

A medium of finite thickness is assumed to be sinusoidally magnetized along the dimension of travel and uniformly magnetized in a cross section perpendicular to this dimension. A detailed computation of the field distribution both inside and outside the medium permits an evaluation of the effect of thickness of the medium and of spacing between the reproducing head and the medium.
Non-Linear Effects in Acoustic Resonators and Orifices
Authors: Bolt R. H. , Labate Samuel
Published: September 2005

Classical analyses of acoustic elements involve the assumption that the frictional force on a plug of air moving in a constriction is proportional to velocity. Similarly, the effective masses owing to the plug of air and its radiation impedance are assumed to be independent of velocity. We find cases in which these linearity assumptions are violated with sound pressure levels as low as 70 or 80 db. Acoustic impedance measurements on orifices show that their reactance decreases and resistance increases as the sound level is increased above some minimum value which depends on the orifice dimensions. Correspondingly, in a resonator the resonance frequency increases and the Q decreases. The absorption coefficient may either increase or decrease with increasing sound level, depending on whether the specific resistance ratio (R/ρc) is less than or greater than unity. This variation of absorption has been observed in reverberation measurements on two arrays of resonators tuned at 300 and 500 c.p.s., respectively. For these cases the decay curves are concave downwards and the “instantaneous absorption coefficient” increases more than twofold as the sound level drops 30 to 40 db below its initial value. We have analyzed these effects theoretically by inserting non-linear terms into a differential equation and solving by a perturbation method. This analysis shows that a variation in resistance is always associated with a variation in reactance and vice versa. Simplified formulas deduced for special cases agree quantitatively with experimental results.
Automatic Transducer Response Recording with a Standard Comparison Microphone
Authors: Reitz Lewis P.
Published: September 2005

The automatic recording of microphone and loudspeaker response is accomplished by continuous comparison to a standard reference microphone. A Twin Recorder, using two separate servomechanisms, is used in a variety of combinations for loudspeaker and microphone measurements. Methods are discussed for recording of both loudspeaker and microphone frequency responses with either constant input or constant output in a matter of seconds. In making microphone response measurements, accurate curves can be made in a moderately reverbrant and inexpensive room, since constant sound field is maintained at the microphone position regardless of the interference of echoes. This method has special significance for laboratories not possessing anechoic test rooms, who need make rapid and accurate microphone tests. Equipment for performing these recorded measurements will be demonstrated and typical curves will be presented which have been recorded on this instrument.
The Calibration of Microphones to One Hundred Kilocycles per Second
Authors: Koster H. F. , von Gierke H. E. , Oestreicher H. L.
Published: September 2005

The free field calibration of condenser microphones to 100 kc was investigated experimentally and theoretically. Several Western Electric 640AA condenser microphones were calibrated using the free field reciprocity theorem. The response of these transducers exhibits peaks and dips above the fundamental natural frequency due in part to vibration of the diaphragm in higher modes as is expected theoretically. The reciprocity theorem is, however, valid irrespective of the mode of vibration. A special condenser microphone with a fundamental frequency greater than 100 kc and set in a spherical housing (11 mm in diameter) was developed. It was calibrated by the reciprocity method and found to have a smooth response to 100 kc. This calibration agreed satisfactorily with that obtained by the electrostatic actuator method after theoretical correction was made for diffraction by the spherical housing.
Measurement of Shear Elasticity and Viscosity of Liquids by Means of Ultrasonic Shear Waves
Authors: Mason W. P. , McSkimin H. J.
Published: September 2005

It has long been suspected that if liquids were sheared sufficiently rapidly they would exhibit a shear elastic effect as well as a viscous effect. This supposition was verified recently by one of the writers by employing a torsionally vibrating crystal and measuring the mechanical loading for the shear wave by observing the increased resistance at resonance and the change in the resonant frequency. By this method it was shown that long chain polymer liquids had shear configurational elasticities in the order of 107 dynes/cm2. The use of a torsional crystal is limited in frequency to about 2 to 3×105 cycles on account of the small sizes resulting. In the present paper the range of shear wave measurements in liquids has been extended up to 60 megacycles by observing the effect on a series of shear waves in a fused quartz rod, of terminating the rod by a thin layer of a liquid. The shear wave in the rod is altered in magnitude and phase by the boundary layer impedance of the liquid. By observing the reflection loss and the change in phase caused by the liquid layer, a measure is obtained of the shear impedance of the liquid. By employing a fused quartz rod for which the shear wave strikes the reflecting surface at an angle from the normal of about 79 degrees, the effect of the shear wave impedance on the boundary is greatly enhanced and a more accurate measurement obtained. Applying both the torsional crystal and high frequency shear wave techniques to polyisobutylene and polystyrene liquids, it is shown that there are two relaxation frequencies in these liquids. At frequencies under 100 kc, the shear stiffness is in the order of 3×107 dynes/cm2, while in the high megacycle range it has increased to 5×109 dynes/cm2. The low shear elasticity appears to be associated with a composite motion of rotation and translation that allows a configurational change to occur from the most probable shape. When the shear stress is removed, the molecule quickly returns to its most probable shape. This results in a low shear stiffness. At high frequencies this motion cannot take place and the shear stiffness is determined by single potential wells, and the value approaches that for a crystal.
Determination of the “Better” Ear in Bilateral Impairment of Hearing
Authors: Silverman S. R.
Published: September 2005

In clinical practice it is important to know which is the “better” ear in bilateral impairment of hearing. This information is highly significant where surgical procedures and the use of hearing aids are contemplated. Orthodoxly, the threshold of acuity is the prime criterion in selecting the better ear. It is shown that we also need to know the capacity of the ear to discriminate speech at supra-threshold intensities since an ear which has relatively better acuity may have worse discrimination than the opposite ear. It is also pointed out that surgical procedures and hearing aids may improve acuity but they cannot be expected to improve discrimination.
Masking at Various Levels of the Auditory System
Authors: Gross N. , Kemp E. , Lowy K. , Thurlow W.
Published: September 2005

A review of experiments on suppression of the electrical response to one sound by another will be presented. Recordings at the level of the cochlea, first neuron, nerve stem, and colliculus demonstrate that the phenomenon is probably due to several different mechanisms which will be discussed in succession.
A Movie—Some Physical Aspects of Conduction Deafness
Authors: Perlman Henry B.
Published: September 2005

The movie is designed to demonstrate how the acoustic oscillations of the conduction mechanism may be affected in some pathological conditions. Various parts of the conduction apparatus in fresh human cadaver specimens are viewed stroboscopically while oscillating to a loud low frequency sound (180 cycles per second). Lesions are produced that affect the mass or stiffness of the mechanism. Observed and measured differences in the vibration amplitude resulting from these types of lesions help to explain why high or low tone deafness may predominate in some middle ear diseases.
Loudness Patterns—A New Approach
Authors: Munson W. A. , Gardner Mark B.
Published: September 2005

When a sound enters the ear it sets up a vibration pattern along the basilar membrane. This vibration pattern initiates nerve pulses which travel along the eighth nerve to the brain, where a second pattern of neural activity results. Two general methods may be used to study these patterms: (1) the objective measurement of cortical potentials by the electro-physiologist, and (2) subjective measurements such as masking and fatigue effects. The present paper is based on the latter procedure in which the results of loudness and fatigue tests have been combined to give patterns of cortical activity in terms of millisones per kappa-band. Use of fatigue patterns permits employment of a low level probe tone which overcomes the difficulties formerly experienced with artifacts in masking patterns due to beats and distortion products.
Mutual Problems of the Physicist, the Physiologist, and the Audiologist
Authors: Kinney Charles E.
Published: September 2005

This paper will point out some of the physical and physiological stumbling blocks that are confronting the practicing audiologist. One or two examples of how these problems are being attacked in teaching centers where there is planned cooperation between these departments.
Focusing Radiators
Authors: O'Neil H. T.
Published: September 2005

An approximate theory has been derived describing part of the sound field due to a concave spherical radiator, vibrating with uniform normal velocity. The diameter of the circular boundary is assumed to be large relative to the wave-length and large relative to the depth of the concave surface. The theory describes the distribution of sound pressure, particle velocity, and intensity along the axis of symmetry and in the vicinity of the focal plane. It is shown that the ratio of the intensity at the center of curvature to the average intensity at the radiating surface is nearly equal to (2πh/λ)2, where h is the depth of the concave surface and λ is the wave-length. This ratio can be made very large by suitable choice of dimensions. The diffraction pattern in the focal plane is nearly the same as the diffraction pattern of a flat circular piston at large distance from the piston, if the piston and concave radiator have boundaries of equal radius. The theory has been compared with experimental data for a 5-mc concave quartz crystal, described previously by G. W. Willard and J. F. Muller. The calculated and experimental data are in reasonable agreement when allowance is made for the non-uniform distribution of normal velocity of the crystal.
A Direct Reading Micro-Displacement Meter Having Zero Mechanical Impedance
Authors: Arndt J. P.
Published: September 2005

Equipment has been built for measuring vibratory displacements of very small mechanical elements such as phonograph styli and piezoelectric crystals. It employs a probe of small dimensions so that virtual point measurements may be made. The probe does not contact the point under measurement and therefore imposes no mechanical load. The variation in capacitance between probe and vibrating surface is used to measure the displacement. Through the use of a built-in calibrator, the sensitivity may be adjusted electrically for direct meter reading of vibratory displacement without resorting to precise adjustment of condenser plate spacing. Displacement amplitudes of less than 10−6 cm may be measured. The output signal corresponds accurately to the displacement both in magnitude and in phase over a wide frequency range, so that complex vibrations are portrayed accurately on a cathode-ray oscilloscope. The equipment has been calibrated by four independent methods, including a reciprocity method, with close agreement.
The Discriminability of Interaural Phase Relations In Two-Component Tones
Authors: Licklider J. C. R. , Webster J. C.
Published: September 2005

Previous experiments have shown that the subjective sound and location of a low frequency tone depend to a considerable extent upon whether the sound waves arrive at the two ears in phase or out of phase. For pure tones of high frequency, however, the interaural phase relation appears not to be an important factor. The question is, do these statements hold also for complex sounds? In particular, is the binaural mechanism entirely “phase deaf” at high frequency? Using the simplest of complex sounds—two superposed sinusoids of equal amplitudes—we have explored this question by determining the frequency limits within which changes in interaural phase relations give rise to changes in auditory experience. We find that if the frequency of either of the two sinusoids is below 1400 c.p.s., reversing the phase of that component in one ear produces a change in the subjective sound. If both of the sinusoids are above 1400 c.p.s., a similar reversal of one of them is discriminable under either of two conditions: (1) if the two sinusoids are close enough together in frequency for beats or roughness to arise; (2) if the subjective difference tone is audible and below 1400 c.p.s. With a frequency difference of less than 10 or 15 c.p.s. the subjective sound ceases to beat in the center of the head and starts to move back and forth from one ear to the other when the interaural phase difference of one of the two sinusoids is switched from 0 to 180 degress. With higher frequency differences, the subjective change is from a rough or beating complex in the center of the head to two rough or beating complexes, one at either ear. These observations indicate that, even at high frequency, interaural phase relations play some role in the localization of complex sounds, even though they may not provide a basis for the localization of pure tones.
The Discontinuous Growth and Decay of Sound in a “Long Corridor”
Authors: Miller Harry B.
Published: September 2005

The “long corridor” referred to is actually a mechanical transmission line in the form of a helical spring. Compressional waves in the audiofrequency range are sent down the spring by an electromechanical driver and received at the far end by an electromechanical pick-up. Reflection occurs at each terminal allowing standing waves to build up. Oscillograms have been obtained for frequencies below 100 cycles, showing the discontinuous nature of the “reverberation” which is produced in such a “long corridor.” Eyring's mechanism, using image sources, is shown to fail to explain the oscillograms. A new mechanism is presented which seems to explain all cases, including that of non-normal modes.
A Pulse Phase Comparison Method for the Measurement of Sound Velocity in Semisolid Substances
Authors: Richards J. R. , Pember E. J.
Published: September 2005

Ordinary methods of sound velocity measurement are not easily applicable to soft semisolid material such as animal tissues and gels. For this purpose a pulsed phase comparison device has been developed whereby a pulse of sound transmitted through the sample is compared in phase on an oscilloscope with the transmission excitation voltage. Wave-lengths, and thence velocities, are determined at a frequency of 5 megacycles by compressing the sample a measurable amount so as to alter the relative phase through an integral multiple of 360 degrees. Gateing amplifiers serve to eliminate multiple transmission paths, and a high driving pulse power permits highly absorbent sample material to be employed. Measurements on liquids of known velocity have indicated an accuracy of about 1 part in 1500. Preliminary data on animal tissue indicate that at the above frequency the velocity in these substances is from 3 to 10 percent greater than that in a 1 percent saline solution at the same temperature.
The Attenuation of Sound in “Synthetic Sea Water.”
Authors: Leonard R. W. , Combs Paul C. , Skidmore Leslie R.
Published: September 2005

Attenuation measurements made in solutions containing the principal inorganic constituents of sea water indicate that the absorption in frequency range below 400 kc is due primarily to the salts in solution and not to marine life. It is further indicated that MgSO4 plays a dominant role in the observed absorption.
Self-Reciprocity in a Plane Wave Sound Field
Authors: Simmons B. D. , Urick R. J.
Published: September 2005

An electroacoustic reciprocity parameter is derived for a transducer which transmits and receives plane sound waves. The parameter is shown to have the value J = 2A/ρc, where A is the area of the diaphragm, ρ is the density, and c the velocity of the medium. The plane wave parameter has possible application to the calibrations of transducers at megacycle frequencies by the self-reciprocity method. As illustrated by an example using the pulse-reflections technique, it is simply necessary to measure the impedance of the transducer and the ratio of the amplitudes of the driving and the reflected received pulse as observed on an oscilloscope screen. A correction is necessary for the absorption of the fluid. The validity of the method is dependent upon the extent to which plane waves determine the sound field between transducer and reflector.
Non-Linear Distortion in Dynamic Loudspeakers Due to Magnetic Effects
Authors: Cunningham W. J.
Published: September 2005

Two sources of non-linear distortion in a dynamic loudspeaker are considered in this discussion; both are related to the magnetic characteristics of the driving mechanism. The first type of distortion arises due to a force of attraction between the voice coil, carrying a current, and the iron of the field structure. This force varies as the square of the current and produces second harmonic distortion. The force may be related to the space rate of change of self-inductance of the voice coil as it moves in the air gap. The magnitude of the distortion produced in this way may be several tenths of one percent, and is greater for low frequencies and large currents. This distortion may be reduced by proper proportioning of the voice coil and field structure, and by using a short-circuited winding on the field structure. The second type of distortion arises due to non-uniformity of the magnetic field in which the voice coil moves. This effect is well known qualitatively, but equations are given here for its quantitative evaluation. These equations indicate that the distortion characteristically is less than one percent, and is greater for large amplitudes of motion. If the voice coil is centered in a symmetrical field, only odd-order distortion is produced. If the field is not symmetrical about the voice coil, even-order distortion is also present. This distortion may be reduced by proportioning the voice coil and field structure so that the mean field in which the coil moves remains as constant as possible.
A Continuously Adjustable Filter for Audiofrequencies
Authors: Tisdale Glenn E.
Published: September 2005

An electronic instrument has been developed to operate as a filter with cut-off frequencies continuously adjustable over the audio spectrum. The circuit consists of a low-pass and a high-pass network in the form of four-terminal impedances. These may be used singly or connected in tandem to give band pass action. A single control selects the cut-off frequency of each network over a range of 10 to 1, while a decade switch extends the over-all range to 100 to 1, or more. The low pass section provides a pass band flat within 1 db below the cut-off frequency. Attenuation past cut-off is at the rate of 18 db per octave down to the noise level of the circuit. The high-pass section provides a pass band flat within 2 db above the cut-off frequency. Suppression is more than 32 db for the first octave below cut-off, and the attenuation continues to increase down to the noise level. Noise is at least 60 db below a signal of one volt. The voltage attenuation in the pass band of the complete filter is about 6 db. The principle of operation of the instrument is based on active feedback networks using only resistance and capacitance. The variable elements are ganged resistors, whose values may be in error by several percent without affecting the performance described. Because there are no high gain stages in the circuit, distortion and noise problems are reduced to a minimum.
On the Propagation of Sound in Narrow Conduits
Authors: Mawardi Osman K.
Published: September 2005

The analysis of the propagation of sound in narrow tubes has usually been restricted to shapes yielding tractable mathematical expressions. A great number of practical applications do not fall within these categories and await a solution. An approximate solution of sufficient accuracy for narrow tubes of arbitrary shape has been derived. The derivation has been applied to a multiple capillary tube formed by filling a conduit with circular wires, a device often used as a high acoustical impedance. The theoretical predictions check satisfactorily with the experimental results. It is believed that the results of the analysis will be useful to other similar applications.
The Least Discriminable Intensity for Random Noise
Authors: Harris J. Donald
Published: September 2005

When a continuous white noise is increased in intensity for one second every fifth second, a subject can be asked to judge when such increases occur. A device using moving tape to record a subject's responses largely eliminates invalid judgments. Under these conditions, the just noticeable increase was measured at each of several sensation levels from 3 to 50 db. A surprisingly slight loss of discriminability is found as the over-all sensation level decreases from moderately loud to very weak. Differences between the present data and previous experiments can, however, probably be attributed to differences in the manner of computing zero sensation level. The just noticeable increase which is the largest theoretically possible is inferred to be something less than 3 db, reasoning from the psychophysical curve for the noise vs. no-noise judgment. The just noticeable difference is found to be somewhat smaller than by the above method when the subject is forced to judge whether the second of two short noises is louder or softer than the first. It is possible that this second method drives the subject more nearly to his physiological limit. If so, it should perhaps receive special attention.
The Acoustics Department of the Juilliard School of Music
Authors: Robin Harry L.
Published: September 2005

In recognition of the ever increasing dependence by musicians upon acoustical engineering techniques, an Acoustics Department was established in the Juilliard School of Music in 1947. This Department has the following functions: 1. To conduct courses in musical acoustics for degree and diploma students. 2. To record student and faculty performances and all School concerts. 3. To supervise the technical aspects and production of the broadcasts of School concerts. Some observations on the work of the Department are made and discussed.
Comparison of Performances of the Same Melody Played in Solo and in Ensemble with Reference to Equal Tempered, Just, and Pythagorean Intonations
Authors: Nickerson James F.
Published: September 2005

A study was made of solo and ensemble performance of the same musical material as related to systems of intonation postulated by certain acoustical, musical, and psychological theories. In particular, it was desired to check earlier findings that unaccompanied performance and listener preferences approximate Pythagorean intonation and to extend a similar line of investigation to ensemble performance. Solo and ensemble performances by 24 well-trained string quartet players were recorded from which stratified random samples of tones were obtained for frequency analysis. This analysis was made through the use of 16-mm sound-on-film loops with a chromatic stroboscope (Stroboconn). The results confirm earlier findings for unaccompanied melodies and indicate that Pythagorean intonation is also most typical of ensemble performance. This tendency appears to dominate any “cultural conditioning” which may exist for equal-tempered intonation.
San Diego County Fair Hearing Survey
Authors: Himes H. W. , Webster J. C.
Published: September 2005

A recorded hearing test similar in part to the Bell Telephone Laboratory's World's Fair Test was given at the San Diego County Fair. In addition to the absolute pure tone thresholds for the five-octave frequencies 440 c.p.s. through 7040 c.p.s. a white noise masking threshold was found for 880 c.p.s. and 3520 c.p.s. A report was made by each of the approximately 3700 participants as to age (within 10 year groupings), sex, musical training, noise environment, and known hearing difficulty. Statistical breakdown of hearing losses as a function of sex and age agreed in general with the earlier study. Functional relationships indicate that (a) musical training was negatively related to hearing loss especially with the older age groups, (b) noise environment was positively related to hearing loss at 3520 c.p.s. for males and (c) declared (known) hearing difficulties correlate with actual hearing losses. Masked thresholds, in general, show trends similar to those of the absolute thresholds in relationship to the above factors. However, these trends are not as pronounced as those shown by the absolute thresholds. Subsequent analyses and work on masked and absolute thresholds on naval recruits will supplement these results.
The Sounds of Disease-Carrying Mosquitoes
Authors: Offenhauser W. H. , Kahn Morton C.
Published: September 2005

In the summer of 1947, the authors made recordings of a number of species of mosquitoes in West Africa. The equipment used to make the recordings is described, and some of the records obtained are listed. In the summer of 1948, the authors went to Cuba where they recorded the rounds of the female Anopheles albimanus mosquito. The recordings were played back in the Husillo Swamp there for the purpose of calling male mosquitoes of the same species. Relatively large numbers of mosquitoes were killed in the sound-baited trap. It is believed that this was the first time that a sound-baited trap has been used successfully for catching mosquitoes. Mosquito sounds are quite distinctive; gross differences occur and are readily detected both by listening and by wave analysis. In general, male sounds seem higher pitched than female; examination of sound spectrograms suggests that the difference is due in considerable degree to the differences in harmonic emphasis. All fundamental sounds seem to occur in the center of the sonic range (from 300–1000 c.p.s.); all mosquito sounds are rich in harmonics. All mosquito sounds are warble-modulated; some at a single vibrato rate (in the order of 5 c.p.s.), others at a double rate (with the higher rate in the order of some 5 times the lower). In some tones, some harmonics are rather completely interrupted or pulsed, while the fundamental remains quite undisturbed. Warble amplitudes are usually quite large. The fundamental pitch often drifts; in one case, for example, them was a 25 percent increase in fundamental pitch in as little as 0.05 sec. Generally speaking, mosquitoes do not respond to sine-wave tones; this may explain why the tones they generate are complex. Mosquito sounds are low in energy level. A rough measurement has been made of the total sound power output of the insect whose sound was used for sound baiting the trap used in Cuba. The power was in the order of 10−13 watt. With such a low power level, it is difficult to obtain high signal-to-noise ratios as we know them in conventional high quality sound recording. Despite this, it has been possible to turn out recordings frequently with as much as 50-db signal-to-noise ratio. The usual operating nuisances of microphonics, hum induction, noise, and the like are encountered in aggravated form.
The Acoustic Gallstone Detector
Authors: Thurston E. G. , Walker Eric A.
Published: September 2005

A serious problem during a gallstone operation is to determine if all the stones have been removed, and, if not, where they are located. They may be in the bile ducts, in the gall bladder, or even in the liver itself. An electro-acoustic instrument to assist in this determination has been devised. It consists of a very small transducer mounted on the end of a slim metal rod. This is connected through a cable to a standard amplifier-loudspeaker system. The characteristics of the system are such that when the dilator touches healthy tissue no sound is emitted; but on striking a stone, a ringing sound is emitted. Another variation of the probe uses a Bakes dilator mounted in a handle containing the crystal. Still another variation which is being prepared is to be used for locating kidney stones.
Universal Phonograph Styli
Authors: Reid John D.
Published: September 2005

The advent of “slow speed” records with small grooves has created mechanical problems in phonograph reproducers in that the needle size and in some cases the tone arm weight has to be adjusted as well as the speed of rotation of the record. This leads to undue complexity of operation. Several solutions to the problem in the form of universal styli which will fit both standard and small grooves will be presented and the relative merits of each compared.
Levels and Spectra of Noise in Industrial and Residential Areas
Authors: Bonvallet G. L.
Published: September 2005

At the Cleveland meeting of the Acoustical Society, in November, 1948, some data were presented on the noise in and near transportation vehicles. These were taken as part of the work on the Chicago Noise Survey. The survey includes investigation of noise conditions in industrial zones and in residential areas, and some data on these now are available. These consist of over-all noise taken with a sound level meter, and spectra taken by the use of an octave band filter unit. The noise conditions vary with the season, and the data on industrial noise, up to the present, were taken when factory doors, windows, and other openings were not open to the extent they are in warm weather. Further investigation will be made in this respect. It is difficult to interpret residential area noise levels since the noise sources may be traffic conditions rather than industrial noise. However, data have been analyzed and the attempt has been made to obtain meaningful information. Slides of characteristic over-all and octave band levels will be presented. Study of the data indicate that the average spectrum is peaked below 300 c.p.s., and the sound energy per cycle. drops about 9 db per octave toward the higher frequencies.